Some regulars here are aware that I have been working on an LED conversion project for our coach. As of this afternoon that project is substantially complete. For the 110 V. side of the coach the bulbs came from my favorite campground - Camp Wallymart. For the 12 V. side it was considerably more complicated. I ended up building my own modules and documented the process here:
LED Project (http://www.rjevans.org/misc-personal/LEDProject.html)
very interesting. I am surprised there is nothing comercially available.
Nice work Bob,
A friend told me about a similar project he was working on. He mixed in a few yellow LED's with the whites and got a much warmer color without much reduction in light. I think the ratio was about 1 in 5.
Viento - there's some marine stuff available - the Sailor Sam's link has some good stuff. But its wicked expensive. I've got about $10 of cash cost in each module. Buying comparable luminosity commercially would be north of $100. My time is worth something but its not worth that much.
Len - we actually like the cold light. I know that's not the norm but we prefer it to the "warmer" light that we had from the halogens. We had them side by side in the sunshine ceiling in the kitchen for the past couple of weeks and found we preferred the LEDs. But for somebody who wanted the warmer light you are absolutely right, it wouldn't be hard to mix in some yellow LEDs. In my banks of 4 it would make the most sense to put 1 in 4 but it wouldn't be hard to rearrange the layout and there's other voltages available. If you found a 2.5 nominal voltage then you could have banks of 5 with 1 in 5 being yellow.
Oh gawd, here we go again.
Bob, your homemade board is nice and it may work, but I don't see any resistors in your concoction, and you are talking about adding LED voltages up to make them equal to your battery voltage... bad bad bad.
Why? Led's are current driven, not voltage driven as incandescents are. This appears to be a difficult thing for a lot of people to grokk, and I have to admit that if you don't have a concept of current sourcing, it's equally as hard to explain. But I'll try...
An Led's operating point (and it's destruction point) is defined by the current you put through it, NOT the voltage you put across it. An Led's voltage is a printed spec that is a result of the physics of the material it's made of and the color it is, (and it's temperature) and this does NOT mean that this is the voltage it requires. It's kinda like this- if you put the proper amount of current through it, and then measure the voltage across it, you'll see roughly the spec'd voltage. But the drop across the LED has absolutely nothing to do with how you should drive it. It's only an artifact of being properly driven, as a specification it's useful to a designer but it's NOT an operating voltage in the sense that it would be for an incandescent.
Case in point, your cool little boards. You mentioned that they draw .32 amps (320Ma) per board, and looking at your board, you have 4 "strings" of four leds on each board... that means each string "section" is drawing .32 amps / 4 = 80 milliamps per set of 4 leds. This means that each LED on your board has 80Ma going through it!~! Now if you look at your LED's data sheet, you'll see that it recommends an operating point of only 20 milliamps per led... this means that you are overdriving every led on your board 4 times more than the maximum recommended current!!! This is a major no-no design-wise...you are way overdriving every LED and that will cause early failure, or at worst possible burn/fire because they will likely be running WAY too hot. You need to take control of the current going thru each LED so it doesn't exceed 20ma...
How?
In short, to gain control of the current you are putting thru your led's you need a resistor in series with each "string" at best, or at least a resistor in series with your whole board. In your case, since your boards are already done, you probably can get by with putting a resistor in series with each board whose value makes the boards draw 80 Ma instead of 320. Yes your Led's will get a lot dimmer than they are, but they won't fail because they will no longer have 4 times their rated current being rammed down their poor little anodes!. As it is you're headed for disaster someday.
IF you were doing this board over again, you'd probably want to put 4 resistors on it... one for each "string".
To calculate the resistor, you'd want to stack up your LED's so that their total "specification" voltage is maybe 3-4 volts less than your vehicle's system, and then use ohms law to calculate a resistor that will drop the remaining voltage whilst limiting the current thru the led's to that given in their spec sheet as the proper driving current (volts across resistor/current=needed resistance). Then you again use ohms law to calculate the wattage requirement of the resistor so that it doesn't burn up (voltage across the resistor x current = watts of the resistor).
Being that your LEDs are typically 3.2 volts, I'd only put 3 in series instead of 4 (3x3.2=9.6v) and then calculate the resistor to soak up what's left between the 9.6 and your bus's 12-14.
Clear as mud?
Here's a webpage that tries to explain it.
http://white-leds.co.uk/led-wiring-guide.htm
A great calculator for all of this stuff is here:
http://www.theledlight.com/resistancecalculator.html
You can add up the voltages for the led's in the string and put the total in the "led volts" box and it will calculate the rest for you.
Quite nice....
Bottom line, your led's appear to work but their lifetime is going to be severely degraded by the way you've hooked them up. A resistor is
required to do this right, period....
...............................................
Philosophy:
Though I'm not a "don't do that- the sky will fall" kind of guy... and in general I love to go by "what works is what works"... isn't the whole idea of using LEDs mostly because they don't burn out? ~reliability~
My take: ALL lighting on a bus is for safety. If it's for safety, it should be done correctly, and head-in-sand is not an excuse for lack of understanding or poor engineering. A burned out LED could be the beginnings of an accident, be it a full on crash or only a stubbed toe in the dark, but throwing ohms law out the window because "it works" or you simply don't understand it is not a valid reason for doing a half assed engineering design on a safety related system.
So PLEASE Bob and all of you guys... take a moment to either learn ohm's law and how to drive your LED's correctly, or ask questions until you understand, or probably go back to light bulbs. Bob, your method appears to work, yes, but it's a bad design. I would NOT recommend others reading your post and trying it... even though your experience is that it works...
My apologies if I sound harsh... I'm not trying to scold, but I am trying to get you guys to understand that with LEDs there is more to it than just "making it work"... if you learn to do it right, you'll have a wonderful system. If not, you will likely have failures, or worse, accidents, fires, etc.
It sounds like I have a bad design. Which comes as no surprise since there was exactly zero time spent on design. I'll take my chances - they don't run "hot" - I'll check that with my digital thermometer today just for giggles but they don't run noticeably hot. If they fail too soon I'll have to deal with it.
FWIW, the .32 amp figure was arrived at by multiplying the 16 LEDs x the 20 mAmp draw. I'm not an electrical engineer (and don't want to be one either) but I think the correct calculation for my configuration would have been 4 x the individual draw. I've now looked at both the websites cited and they clearly DO take into account the voltage drop across the LEDs. There ain't no way around Ohm's law. I'm not convinced that this "problem" isn't akin the bumblebee. You know the story - engineers can prove that the bumblebee can't fly but he can't read the proof so he just goes happily bumbling along - flying.
Oh come on... did I just waste my evening typing all of that? This is one of those little things in bus-land that should be done right, or not done at all. There's nothing wrong with learning, nothing wrong with doing it right, nothing wrong with making a mistake and correcting it.
Just go get yourself a resistor, stick it in series and have a good design, and don't take your chances. You ARE overdriving them, and their temperature right now or any other day won't tell you a thing. It's the temperature of the LED die itself that matters and you can't get to that. And there's a million reasons that I won't waste my time typing that the temp. doesn't tell you anything useful.
Look... try this.. get yourself a handfull of ten ohm half watt resistors and stick one in series with your board, then measure the current. See what it is. If it's even close to 80ma, you're done. It'll probably be very close depending on your battery state.
My Guess is this- you have 4 led's that are 3.2 volts nominal at 20ma, this means 12.8 volts total and your bus is probably somewhere between 13.2 and 14.6... but since you have 4 strings you need 80ma to get the individual current correct.
Typing those values into the calculator I gave you above says you need roughly 20 ohms when your battery is high and 5 ohms when it's low (this is because your led string total is too close to your battery voltage... had you used three led's this range would be a lot less)
I would simply use a 10 ohm 1/2 watt resistor- it's a compromise but SO much better than nothing... your led's will like you and I will applaud you.
If you can't figure out where/how to get some resistors, I'd be happy to send some to you for free, in the interest of seeing your project be done properly. Just email me your address....
Cheers
Gary
Boogie,
I heard you loud and clear... Thank you for taking the time and will give a full report on my project when done.
I heard you loud and clear too. And I don't take well to being lectured without a complete explanation of the theory.
Boogie stated:
"get yourself a resistor, stick it in series and have a good design". Actually I still wouldn't have a "good design" because a "good design" with respect to LEDs would require a current limiting circuit. Since that isn't practical for most lighting applications we work in the realm of poor compromises and use resistors. The real issue with LEDs appears to be that the relationship between current and voltage isn't linear (IOW they behave as if they have a variable resistance) and that they break down with temperature. So the risk is that you get a runaway inside the LED that starts with a voltage spike and results in a non-linear decline in resistance which leads to higher current flow which leads to more heat ...........
I believe that I am operating in a safe range but time will tell. As I have already indicated I will monitor the temps to see if there is an indication of a problem. Is a better design possible? ABSOLUTELY. But there's a lot of bad designs in this world that function just fine, starting with the giraffe. Engineers don't like to admit that every design involves some kind of compromise and safety margin - each of us who designs our own equipment has to come to terms with where we are on that continuum.
Here's an exercise for you Boogie: plug the actual nominal voltage of my LEDs (3.3) into your calculator and tell me what size of resistor to use at my current coach voltage (13.0).
Quote from: bobofthenorth on February 23, 2009, 09:07:00 AM
I heard you loud and clear too. And I don't take well to being lectured without a complete explanation of the theory.
Bob,
I have to go along with Gary on this. To be fair to him, he did provide a link to the explanation of the theory, which is sort of second-nature to those of us who had to reduce the entire universe to Thevenin equivalent circuits for a whole semester.
I'll spare you any additional lecture, and simply say that I believe, if you have no resistors at all in your modules, that you will see LED failures in relatively short order. Every minute you are running those, more material is migrating across the junctions. I know you have a lot of work in these modules, and I'd hate to see you have to start all over -- even replacing one LED looks to be a bit of work (and, by the time one LED in a module fails, the others will have migrated so much material that their failure is imminent). I will go further to say that the only reason they are working at all (and did not fail immediately) is that the length of wire in the lighting circuit represents, at least, a small resistance, even if it is in the range of milliohms. BTW, relying on those few milliohms to drop the (varying amount of) leftover voltage in the circuit probably also means that you are dissipating a lot of unnecessary power in the wiring.
I'm not sure I agree that this is any kind of safety risk. Even incandescents burn out, and any heat generated even by overdriven LEDs is not nearly the fire danger that the halogens you replaced presented. I'd just hate to see all your hard work (and the cost of your LEDs) go down the tube in a matter of just a few months, when, properly limited, those LEDs should last the lifetime of the coach.
FWIW.
-Sean
http://OurOdyssey.BlogSpot.com
Thanks Sean
Bob, my apologies if I appeared to be harsh.
Bob, I don't need the excercise, I've been an electronic designer for 40 years. And I guess I have to ask you please don't start preaching to me about stuff you obviously don't have a clue about. I didn't write you to lecture you. I wrote you inform you and help you understand something that you and many others obviously don't.
To respond to your last post:
For what it's worth, a resistor IS a "current limiting circuit".
>Since that isn't practical for most lighting applications we work in the realm of poor compromises and use resistors<
No Bob, the use of resistors is the way it's done in the industry. When you get into cellphone backlights and higher powered devices, then some electronic current limiting becomes necessary-usually using switcher topology- to save energy. But for almost any "common" led you'll find a resistor behind it.
>The real issue with LEDs appears to be that the relationship between current and voltage isn't linear<
No Bob, the real issue is that LED's are manufactured with a VERY SPECIFIC maximum operating CURRENT and you or anyone else in the world should design to meet that spec. It has absolutely NOTHING to do with an LED's linearity etc.
>I believe that I am operating in a safe range but time will tell<
...no Bob, you're not. Without current limiting of some sort, there is no "safe range"...
I think you should take some Led taillights apart and look inside. Good designs, using resistors.
All I can offer at this point is, I certainly hope that you don't apply the same mentality to
tightening lug bolts...
I'm done....
Gary, I saved your information for further studying. Thanks for taking the time to do that, I'm sure it wasn't easy.
I had not thought of any LED projects, but with this information I might consider a small one.
~Paul~
Bob,the arrangement looks bitchin but these guy's are right,I know ,I did a similar design for a set of foot pegs on the M/C a few years ago.The end result was pre mature failure ,a simple test should solve any dispute you might have,hook them up to a steady power source equaling your coach's and leave them hooked up,and note the length of their performance.Hope this helps and all works out for ya.
Van
Hi Bob,
About a year ago, I purchaced 16- G4 LED clusters from Salior Sams to replace my 10 watt halogen bulbs in my coach.
2 problems I had. 1- the G4 clusters didn't fit very well in the puck housings. 2- I couldn't get used to the light that they emmited.
Needless to say, I removed them all and went back to the halogens.
Good Luck
Nick-
We found the same Nick - the so-called replacements didn't fit in the puck housings. I did some considerable alterations involving the hot glue gun, the soldering iron and a trip to WallyMart. We now have what I think are fairly attractive replacements for the pucks.
(https://busconversionmagazine.com/forum/proxy.php?request=http%3A%2F%2Fwww.rjevans.org%2FRJEvans%2FBusStuff%2FPuckReplacements.JPG&hash=e39173f0804cf1573242e4d6e9410db38ee2100b)
Update time:
After considerable reading and consulting off-line with some people who are capable of explaining the subject matter it appears that my "design" will fail. Time will tell when it will fail, maybe tonight for all I know. I opened this discussion by admitting that I didn't know what I was doing and I still don't although the list of what I don't know has got a little bit shorter. Since there never was any design talent applied to this project the fact that I came up with a bad design should surprise nobody, least of all me. I'm prepared to run the risk of imminent lighting failure in exchange for economical short term lighting but I am also working to actually design a circuit board which will incorporate some resistors.
The great thing about internet forums in general and this one in particular is the free exchange of knowledge. Sometimes you have to wade through the chest thumping and loud proclamations of "you can't do it that way" to get to the core material. The mark of intelligence is the ability to speak to your audience and that is particularly obvious on forums. Nevertheless there is a tremendous amount of intellect (and sometimes imagination) brought to bear on a particular problem if people are willing to share - witness Ace's electric thread and Doug's or my engine threads.
The frustrating part of forums is the chest beating and scare mongering that sometimes occurs. In my case I'm replacing halogen bulbs that run hot enough to literally kindle a fire. Suggesting that my LEDs somehow pose an incremental safety risk is just silly. That kind of foolishness always causes me to discount everything else a particular poster may have to offer right out of the gate, sometimes at my own peril. Stay tuned for an update on the imminent failure of my current modules, or just watch out your window for the fireball depending on your inclination.
My offer still stands to send you some 10 ohm, 1/2 watt resistors. If you include just one of them in series with each of your boards, I think that your failure rate will probably go close to zero.
Kind regards
Gary
Quote from: bobofthenorth on February 24, 2009, 09:32:52 AM...I opened this discussion by admitting that I didn't know what I was doing and I still don't although the list of what I don't know has got a little bit shorter...
Hey Bob,
You might see my name coming up a few times on this board whenever LEDs are discussed. I'm a geek, and work in the electronics industry (currently working as a contractor at Nv*dia, job is ASIC Qualification Engineer). I'm interested in your project mostly because you went through the trouble of building a PCB for the LEDs. While I do agree with Boogie about the minimum safety added by a current limiting resistor in series, I don't like them.
This isn't to say that I'm disagreeing with Boogie, I prefer an alternative in LED driver ICs. One of the problems I have with current limiting resistors is that they aren't very smart - if the voltage changes, it doesn't keep the current constant to the LEDs which can cause visible dimming, color shift, or shutoff of your LED string. Driver ICs watch the current drawn by the string and adjust the voltage it is supplying to the LED string to keep the current constant. Even among regulators, you can have differences - switching regulators tend to be more complicated than linear regulators, but they also can have much higher efficiency and better stability over a wider range of system voltages. A way to describe a linear regulator is like a variable resistor, while a switching regulator is like a light switch that gets the supply around the area you want it to be, then relies on the attached circuit to even out the supply.
Take for example this part from Linear Technologies (http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1094,C1768,P86296,D26674). It claims a 94% efficiency around the 24-30VDC range.
The complexity with LEDs is that yes, they need a certain forward voltage to start conducting - but once they do start there is only the the size of the PCB traces, the die-connection-wires, and the silicon's thermal runaway characteristics that will limit the current (with an abrupt "pop" I might add, or at least the escape of some smoke). Since you don't want to be an Engineer, I can bring up the water analogy for this example - but for LEDs it gets more fun... ;)
Imagine that the doped electron gap in the LED die (the part of the silicon which determines which side is +/- or Anode/Cathode respectively) is actually a small ditch. On one side of the ditch is a kid with a hose, on the other is a bucket. Now, depending on how much that kid opens the valve on that hose only so much water will come out. If he doesn't open it very much, most of the water drops right down at his feet and barely even gets into the ditch (nowhere near the bucket...). If he opens it a bit more, the water might spray halfway across the ditch, but still not make it into the bucket. If he opens it just the right amount, the water goes all the way across the ditch, and makes it into the bucket.
By crossing the "ditch", electrons in the LED jump the gap in the die and this act will emit light at a specific wavelength due to the chemical "doping" the LED manufacturer used for the process - for white this is usually a form of blue - with a white phosphor coating (like in a florescent tube) that reacts to that color and emits white (it can be a mix of RGB, or blue and yellow, but this is becoming less common).
Now here is the fun part... Yeah okay, now you got the water across the ditch, but the valve can open more - but if you overshoot the bucket, you're wasting water. For LED's this waste is heat, and as every person who's been around electronics can tell you - heat is bad m'kay? That heat in the die has to go somewhere, and the LED manufacturer has only designed the LED to be able to get rid of so much at a time (this gets worse if the temperature around the LED is high, as there isn't as much difference in temperature to "lose" heat to...).
Now I understand where Boogie is coming from... Do you have kids? What profanities would you be yelling out your back door if you told your kid to fill the bucket and he was spraying it all over the place - especially in California where they are talking about water rationing or jacking up our rates? All of a sudden it becomes important to hit that bucket huh... Just the same, seeing your kind of circuit drives us engineers nuts and we don't know what to do with ourselves ;D.
Well, an LED driver IC is like a computer controlling the valve on that hose, with a sensor to see if the water is landing short or long, and can correct the opening of the valve base on what it measures. A current limiting resistor is like a valve with a fixed opening, so if the city cranks up the pressure or it has a sudden drop, your water may miss the bucket (and may not even cross the ditch causing the light to go out, or will completely overshoot the bucket causing the LED to burn up). But either of those devices is really better than just taking what the city provides and blasting it across the ditch blind.
Back to the Linear part I mentioned above - it would make your circuit a bit more complex than the board you designed, but the fact that you showed that you are willing to play around with this and are bold enough to build boards, shows me that you could learn a little more and successfully put some form of driver IC on the board to get a nice reliable piece for yourself.
-Tim
Now we're getting somewhere and you have rifled in on my concern about the fact that a resistor in the circuit may regulate the current but it still doesn't limit it with fluctuating voltage. IC sounds expensive - maybe that's just more evidence of my lack of knowledge but couldn't I accomplish more or less the same effect more economically with a zener diode? The whole point of this exercise is to come up with a light module at a reasonable cost.
Thank you Tim for a clear explanation and for taking the time to understand what my concern was.
Hey Bob,
No problem. The Linear part and all of the required auxiliary parts can be had for less than $8 per fixture. The Zener solution is one way, it's like a linear regulator of sorts but is more for regulating the voltage of a load, not the current. There is an additional complication that I spared you from, in my previous description - LED's have a "dynamic resistance". Therefore, the voltage required to "jump the gap" can climb slightly as the die gets warmer. This is why current regulation is the way to go for LEDs in illumination applications (really in any application, but more so in illumination). By giving a regulator free-reign to control the voltage, so long as the current stays correct, this takes care of that problem.
I did a quick search, the Linear part can be had in single quantities for $6.38 at Digikey.com (http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LT3756EMSE%23PBF-ND). The other parts are a few cents each. After that it's just a matter of building a board around it - the circuit for driving your LED strings should be about the same physical size as two of your LEDs (and remember, since it doesn't put out light - it can be on the back of the board to save space).
-Tim
Quote from: bobofthenorth on February 24, 2009, 04:58:01 PM
Now we're getting somewhere and you have rifled in on my concern about the fact that a resistor in the circuit may regulate the current but it still doesn't limit it with fluctuating voltage. IC sounds expensive - maybe that's just more evidence of my lack of knowledge but couldn't I accomplish more or less the same effect more economically with a zener diode? The whole point of this exercise is to come up with a light module at a reasonable cost.
Thank you Tim for a clear explanation and for taking the time to understand what my concern was.
Everything Tim's written is, of course, correct. But, FWIW, virtually every commercially available LED fixture for this type of use utilizes a simple resistor for current limiting.
Resistors are a few pennies; driver chips are several dollars. Where the latter really shine is in applications where the source voltage varies within a wide range. For example, to deliver a relatively constant brightness flashlight given battery voltage that cascades downwards from, say, 3v all the way to 1.5v -- a 50% drop. Or to build multi-volt fixtures, such as marker lights than can be used on both 12v and 24v nominal systems. Or, as Gary wrote, to extract the most output with the least amount of energy, such as cell phones, music players, and the like, where miniature battery longevity is a paramount concern and the extra price of the part is relatively minor in comparison to the total package.
When voltage varies within a narrow range, such as in a nominal 12v system (where voltage typically ranges from about 12.2 to 14.0, or only about 15%), a resistor is a perfectly acceptable solution, and it's definitely more cost effective.
If you choose your LEDs and resistors properly, the modules will work across this entire range, and there will be almost no noticeable difference in brightness. In addition to saving a few bucks on every module, you'll have less complexity in the solution (and less board real estate as well).
Again, if you take apart almost any off-the-shelf 12v LED fixture (or bulb-replacement module), you'll find resistors, not driver chips.
FWIW.
-Sean
http://OurOdyssey.BlogSpot.com
Quote from: Sean on February 24, 2009, 05:59:50 PM
Again, if you take apart almost any off-the-shelf 12v LED fixture (or bulb-replacement module), you'll find resistors, not driver chips.
Sean, shame on you.
Allow me to paraphrase:
"Again, if you take apart almost any off-the-shelf RV you'll find truck chassis not Prevosts or Neoplans."
Sean, I think we both need to walk away from this one & move on... you can take a horse to water....
Gary
Quote from: bobofthenorth on February 24, 2009, 08:06:31 PM
...
"Again, if you take apart almost any off-the-shelf RV you'll find truck chassis not Prevosts or Neoplans."
...
Bob, I'm a practical guy. I had gotten the impression somewhere along the line that you were, in part, making your own modules to save some money, in addition to the other benefits. I was simply pointing out that there is a place where driver chips are cost effective solutions, but this application is not it.
If what you want is the whizziest solution, irrespective of cost, then by all means go ahead and get the driver chips. They are certainly effective. But many other folks are following this thread, and, with all due deference to Tim, some folks are looking for simple and inexpensive, and I felt it needed to be pointed out that,
in this application, resistors present a cost effective solution that is very nearly as good.
The comment regarding construction of off-the-shelf fixtures was for the benefit of the many who may not realize how they are made.
-Sean
http://OurOdyssey.BlogSpot.com
Bob,these are the folks that helped me out with my project
http://www.hitechledproducts.com/product_info.php?cPath=26_38&products_id=196&osCsid=2b4b9094e985a6682a644841b7e6f2eb
And you are right Sean but I couldn't resist the analogy. And I still have to ask in the context of this community - is it too radical a concept to look for an elegant solution that is also cost effective?
Hey Bob,
If you want "elegant", one of the key criteria is "simple". Thus if you are seeking elegant, go for the resistor. If you want efficient/stable go for the switcher or another regulator.
I believe that Sean and I both took different approaches with your question (one of the great things about this board, is you get so many different right answers). I was running under the assumption that the reason you were going with LED fixtures was the desire for low-power-draw/high-overall-efficiency in place of a hot/high-power-draw halogen - I had made a little leap based on the fact that you had gone into building circuit boards for your project. Sean has stated that he was under the assumption that:
Quote from: Sean on February 24, 2009, 09:34:06 PM...I had gotten the impression somewhere along the line that you were, in part, making your own modules to save some money, in addition to the other benefits...
He also undoubtedley read the part of your post where you stated that you were not an Engineer, nor had any intention of trying to become one.
Now I have nothing but greatest respect for Sean and Boogie - and the resistor is one of many right answers. For each assumption, our respective solutions are correct. They do compete, but this is actually one of the parts of Engieering that most people miss - the balancing act. There is always a cost/performance(reliability) trade-off, and a design-timeline/complexity trade-off, and you have to know your limits.
Something you should know about me is that I spend a lot of money on my projects because I want them to be "just-so". It's a bit of pleasure for me to voluntarily remove some of those trade-off constraints that are typically found in commercial engineering (this is after all MY damn bus!!!). The benefit to me is that I know a lot about how things work and can make them do exactly what I want them to, with any parts combination I want, upfront-cost be damned.
Commercial products are constrained by these limitations however - and yes like Sean said, you will find resistors in the better-selling fixtures, because they are cheaper to build and can be sold for a better margin.
Take a look at your application - are you running these lights from solar on a deep-cycle until they are dead in order to squeeze out as much power as possible? If yes, then a switcher or some other form of driver IC is for you,. If however you are plugged into shorepower and running a genny, and never run your batts below 95%, don't waste the time with having to learn good circuit design, just use a resistor (the math is way simpler, which would satisfy the "not make you an engineer" criteria).
The whole "shame on you" thing? Even if it was in good fun, after I read it (mind you with exactly zero knowledge of your personality - which is a hazard with a BBS post) I was a bit put-off.
With respect to the "chest thumping" and "you cant' do it that way" comment and the:
Quote from: bobofthenorth on February 24, 2009, 09:32:52 AM...Suggesting that my LEDs somehow pose an incremental safety risk is just silly. That kind of foolishness always causes me to discount everything else a particular poster may have to offer right out of the gate, sometimes at my own peril...
Look when someone tells you not to stick your hand in a blender and you yell at them because you know it's not plugged in - it doesn't mean that they are spitefull and just trying to posture, they may be thinking about training you not to put your hand in the blender (next time you may forget and it might be plugged in). I find it's best not to make a habbit out of dodging bullets - I'd rather be off the range.
-Tim
.... and when someone tells you not to put your hand in a mixing bowl they're just being silly. When somebody says something stupid they can expect me to call them on it.
You guys need to lighten up considerably. Anyone here who couldn't read the humour in the "shame on you" comment needs to step back and assess why they couldn't see it.
Tim you have opened my eyes as to some of the broader possibilities. My goal is 2-fold: One to keep the cost under what it would cost me to buy LED lighting, if what I wanted was even available. Two to let us live off the grid for extended periods of time. Since your first post I've done a lot of reading about ICs and current limiting diodes and all it has accomplished so far is to open my eyes to how little I know. What I have gleaned out of my reading so far is exactly what you just said - that the IC will be the most efficient way to drive my LEDs - I'm not yet sure whether the difference in theoretical efficiency is material in our situation. And I'm also not sure what kind of an IC solution is possible without a bunch of support circuitry (read $$$). It also appears that there are other simple current limiting circuits that I might be able to implement for relatively little cost. In the meantime I've got 47 ohm resistors in front of most of my famous modules with a couple left as test cases.
Quote from: bobofthenorth on February 25, 2009, 05:03:20 AM
And you are right Sean but I couldn't resist the analogy. And I still have to ask in the context of this community - is it too radical a concept to look for an elegant solution that is also cost effective?
No, it's not radical. But, hey, didn't you write this:
Quote from: bobofthenorth on February 23, 2009, 05:37:05 PM
... But sometimes things can get over-engineered and it is better to go the simple route. ...
? ;)
Quote from: bobofthenorth on February 25, 2009, 12:22:05 PM
... to let us live off the grid for extended periods of time. ... the IC will be the most efficient way to drive my LEDs - I'm not yet sure whether the difference in theoretical efficiency is material in our situation.
Bob, it's actually pretty easy to calculate exactly how much energy will be "wasted" by using resistors instead of more advanced methods.
Since we care (presumably) only about energy consumption when on batteries, and not when on shore or generator power, we start with an average system voltage of around 12.4 volts. In practice, your battery voltage will run from about 13.2 down to about 11.8 (loaded). With the three-led strings we discussed, with a 200-ohm resistor in series (which is worst case -- the four-LED strings with smaller resistor would be more efficient, if they worked at the lower voltage), you will be dropping 2.5 volts (12.4 - 3x3.3) in the resistor, for a power consumption of .03125 watts.
Multiply .03125 times the number of modules you will have illuminated at any given point, then multiply by the number of hours used, to get watt-hours.
For example, if you have 20 of these modules on for five hours each day, that would be 20x5x.03125 or 3.125 watt-hours. On a 12-v battery bank, that works out to just about one quarter of an amp-hour; half that on a 24-volt system (less whatever inefficiency is involved in the 12-to-24 conversion). Remember, too, that this is an upper limit on the "waste" -- driver ICs also impart operating losses to the system, just smaller ones.
In two full weeks of use at that rate, that's only about three and a half amp-hours. Of course, plug in whatever numbers make more sense to you -- I'm just guessing at your usage rate.
HTH,
-Sean
http://OurOdyssey.BlogSpot.com
UPDATE
I've now had three out of eight of the Lights of America (professionally engineered) 110 volt LED bulbs fail. Meanwhile my little modules continue to light the bus every evening and (so far) haven't let out even one little puff of smoke, let alone lit us on fire. And that includes the ones with no resistors as well as the ones with the 47 ohm resistors and the ones with the 10 ohm resistors. In addition one of the bulbs I bought from Sailor Sam's (at marine pricing) is intermittently going dim. Usually if I slap it lightly or turn it off and back on it comes back. So the store bought bulbs are currently failing at a rate of roughly 1 bulb per month. I'm not smart enough to calculate MTBF but that seems high to me.
I "built up" 6 of these beauties running off of 12.6 volts. I didn't use a resistor but simply figured the voltage drops needed! They all failed within a month.
Thank you, thank you, thank you.
John
PS: Boogie, You can give me information that I might possibly find help full wrapped in any amt of sarcasm, any degree of "talking down" or just plain rudeness you might see fit. No need to parse your words, walk on eggs or consider my ego. Let er rip. SHOW ME THE MONEY,errrrr, give me the info. Thank you for all the info you contributed this time and all the other times. AND, that goes for the rest of you.
You are all going to come out of this as still beer drinking buddies and this place amazes me.
John
Quote from: bobofthenorth on April 08, 2009, 11:29:42 AM
I've now had three out of eight of the Lights of America (professionally engineered) 110 volt LED bulbs fail. ... In addition one of the bulbs I bought from Sailor Sam's (at marine pricing) is intermittently going dim.
Bob,
FWIW, I was an early adopter of the Truck-Lite LED tail lights (back when they were giving them a "lifetime" warranty). Out of four turn signals and four brake lights, I ended up replacing seven lights within two years. (The replacement models did
not come with a lifetime warranty -- I think Truck-Light learned a lesson there.)
You will almost always get better quality by building it yourself. I think the manufacturing tolerances were just too loose (and the packaging technology too new) for these items. The circuit engineering was probably fine.
The LoA stuff, BTW, is particularly mass-market cheap. Many complaints about them, including back when they focused more on compact fluorescents. Any product can be cost-reduced to the point of unreliability :)
-Sean
http://OurOdyssey.BlogSpot.com
Quote from: JohnEd on April 08, 2009, 03:42:00 PM
I "built up" 6 of these beauties running off of 12.6 volts. I didn't use a resistor but simply figured the voltage drops needed! They all failed within a month.
You might want to look hard at what you did different then because I built 32 of them and none of them have failed. And I use mine every day. I'm not pretending mine are going to work forever - in fact after the flaming I have taken here I expect them to fail every night when I flip the switch on. But so far they have kept on keeping on.
Bob -
Started reading this thread tonight, and didn't realize it until your update that it started back in February!
OK. . . so much for me paying attention - NOT!
OTOH, I learned something tonight about LEDs, which is what these message boards are all about - sharing ideas and solutions to the complex task of converting a coach.
So even if there were a few rough spots in the conversation, to me it was well worth it.
Therefore, thanks to you, Gary, Tim, Sean and the others who all contributed their share to this thread.
FWIW & HTH. . .
;)
Bobn O N,
I don't know either. So many variables: mfr, light, V rating, my being arithmatic and spelling challanged....who knows? :-[ :P ;D
I am going to build some more though. What do you suggest for a source for the LED's. I liked the idea of adding in some yellow light. Unlike you, I don't care for that intense white light. They must have different temps of white so that you don't have to mix colors.
Sorry about the flaming, really I am.
Thanks,
John
Quote from: JohnEd on April 09, 2009, 10:21:05 AM
... I liked the idea of adding in some yellow light. Unlike you, I don't care for that intense white light. They must have different temps of white so that you don't have to mix colors. ...
John,
White LED's are not really sold by "color temperature" (although you could certainly calculate it).
It is, in fact, technically challenging to make a "white" LED. Colored LED's are purely monochromatic -- they emit light of exactly one wavelength. Since "white" light is a combination of many colors, yet the physics of an LED allow it to produce only one wavelength, white LEDs are made one of two ways:
1. Combining three diode junctions in the same package, one Red, one Green, and one Blue, with intensities adjusted so that the output appears white. These LED's are very bright, use a lot of power (relatively speaking), and are quite expensive. If you wanted to build a lamp with, say, nine LEDs in it, it would be cheaper and easier (and you'd have a much greater degree of control) to use three blue, three green, and three red LEDs of appropriate intensity, rather than nine of this type of white LED. Above nine LED's, you can adjust not only the intensity but also the number of each color LED to tweak the color temperature of the light output.
2. Using a monochromatic Blue junction or an ultra-violet junction with a yellow phosphor coating. The light emitted by the diode is blue or UV, and the phosphor is excited by this wavelength and re-emits at a yellow wavelength. In the case of blue junctions, the visible emission is a combination of the blue and yellow, making white with a bluish tint. This is the most common (and least expensive) type of "white" LED. Again, if you are going to make a multi-LED fixture, you can adjust the color temperature by adding red or yellow LEDs as needed.
The technology to generate broad-spectrum ("white") light from diodes is improving all the time. Today's white LEDs are available much warmer than those of a few years ago. When we built
Odyssey, we put no LEDs in, other than some floor-level "night lights," because Louise absolutely can not stand the too-cool color temperature. In our next home, however (which we expect, at this point, to be a boat), we may very well have a great many LED fixtures, because the color temperature will have improved (in some cases due to manufacturers simply slipping a couple of reds into the fixtures).
Hope that helps.
-Sean
http://OurOdyssey.BlogSpot.com
Quote from: JohnEd on April 09, 2009, 10:21:05 AM
I am going to build some more though. What do you suggest for a source for the LED's. I liked the idea of adding in some yellow light. Unlike you, I don't care for that intense white light. They must have different temps of white so that you don't have to mix colors.
Hiteq LED source (http://www.hiteqelectric.com/proddetail-led.asp?linenumber=419)
That's where I got the ones I am using. He seems to have changed his minimum lot to 1000 units - I was able to start out with less than that. If you want a smaller quantity to play with contact me offline and we will work something out.
I'll probably get shot down for the next statement but it appears to me that the
voltage of the LEDs is a function of the colour output. If that is true you might be able to use the
voltage of the LED as an indicator of the colour of the light output. But note: I never said that was true or a rule - its just something I have noticed and not pursued.
Bob,
Can you get more of the boards you had made up?
Thanks
Skip
Quote from: bobofthenorth on April 09, 2009, 11:57:32 AM
I'll probably get shot down for the next statement but it appears to me that the voltage of the LEDs is a function of the colour output. If that is true you might be able to use the voltage of the LED as an indicator of the colour of the light output. But note: I never said that was true or a rule - its just something I have noticed and not pursued.
Bob, you are partially correct.
The light color depends on the underlying technology (read: semiconductor materials) used in the LED, and that same selection of materials also determines the voltage drop. But voltage alone is not a reliable predictor of color output; a 3-volt part, for example, could be green, blue, violet, UV, or white.
There is a nice chart showing wavelength, technology, and voltage for most common LED technologies about halfway down this article:
http://en.wikipedia.org/wiki/LED (http://en.wikipedia.org/wiki/LED)
-Sean
http://OurOdyssey.BlogSpot.com
Quote from: Sean on April 09, 2009, 11:26:47 AM
Quote from: JohnEd on April 09, 2009, 10:21:05 AM
... I liked the idea of adding in some yellow light. Unlike you, I don't care for that intense white light. They must have different temps of white so that you don't have to mix colors. ...
John,
White LED's are not really sold by "color temperature" (although you could certainly calculate it)...
Sean, that's not entirely correct... (I'll explain in a second)
Guys, just like every other product built out there - given the price and the quality of manufacture, there is a broad field for product reliability in any category. If you buy a Yugo vs a Toyota, you can expect it to be cheaper and not last as long...
The same holds true for LEDs. Different manufacturers have different processes, and different quality control schemes. The brand that I most prefer (and use for all of my LED lighting projects) is Lumileds (now part of Philips). They have been leaders in high power (not indicating) LED lighting for about a decade now. This is the brand LED that lit up the "ball" in Times Square this past New Years.
They take lighting so seriously, that they actually bin their LEDs into like-brightness and like-color part numbers. This means that if you buy a specific "bin" of their LEDs (even white), you can expect a high level of consistency between parts. These are also the guys that found out that LEDs prefer a "soft-start" (a slow ramp up in voltage at startup) to prevent stress on the bonding wires (the little gold wire that connects the LED Die to the pins).
Quote from: bobofthenorth on April 08, 2009, 11:29:42 AMIn addition one of the bulbs I bought from Sailor Sam's (at marine pricing) is intermittently going dim. Usually if I slap it lightly or turn it off and back on it comes back.
This is indicative of a bonding wire that has burned and has an intermittent "open" between the Die and the pin (the rest of the wire is held in place by the acrylic packaging). This is one of the three most common failure modes for LEDs: high-temp burnout of the die, and start-surge burning of the bonding wires, and finally discoloration (browning) of the acrylic near the die causing a severe drop-off in perceivable light output (this is considered a failure even thought the LED may appear to be outputting light still...).
What you mention above is exactly the kind of problem that Boogie, Sean and I have been warning about with cheap design and construction... As stated previously, even the corporate jobs can have corners cut.
The store bought assemblies are always going to be more expensive (monetarily), since they have to buy parts, add the cost of building it for it to be profitable for them, then the reseller has to inflate the price so that it is profitable for them. Usually the material cost is about 1/3 the final retail price. The reason I like to build most of my electronics myself (other than a sense of accomplishment) is that I can spend more of the money I would have spent on a finished product - on the actual raw materials to begin with.
I buy Lumileds Brand "Luxeon" LEDs from Future Electronics (their website to be specific). Each LED is about $1.50 to $4.50 depending on brightness and color. They have most of the Lumileds "bins", which means that you can buy "Warm" 3100K (this is about the color of a halogen light bulb), "Neutral" 4100K (this is about the color of sunlight at noon), and "Cool" 6500K with different light outputs (from 8.2lm up to 200lm per LED), in several different package types (the "Luxeon Rebel" is my current favorite - it is physically the size of the "bump" on the + side of a AA battery, but puts out up to 180lm at 700mA for cool white).
The cost may sound steep, but remember that the part that is sold by "hiteq" is only 2lm (1850mcd w/75deg beam angle = 2.402lm {source = http://led.linear1.org/lumen.wiz (http://led.linear1.org/lumen.wiz)}). To match the light output of one of my $5.44, 200lm LEDs (http://www.futureelectronics.com/en/technologies/semiconductors/lighting-solutions/high-power-led-emitters/white/Pages/1192327-LXK2-PWC4-0200.aspx) - you need to use just about 83 LEDs... (which works out to about $20 worth of the hiteq LED, and you need more space on the PCB to put more LEDs {bigger fixture}, and there are more points of failure, etc...). Plus on the power side, the one LED draws 1Amp at 3.75V, where as 83 LEDs draw .02 amps at about the same voltage - figuring power at 3.75 x 0.02 = 0.075watts, an you have to multiply this by 83... So it's 6.225 Watts for the hiteq part vs. 3.75 for the Lumileds Luxeon part for the same light output - which do you think is the better deal?
As I'm sure must be said, I am not compensated for talking about LEDs no matter what manufacturer... I speak my opinion for free and without influence. ;)
-Tim
Quote from: Tim Strommen on April 09, 2009, 04:07:00 PM
Sean, that's not entirely correct... (I'll explain in a second)
OK, Tim, second's up. Please explain what you meant. Other than Lumileds, who's binning their white LED's by color temperature?
-Sean
http://OurOdyssey.BlogSpot.com
Osram (they make the "Golden Dragon" LED)...
Wait a minute... I didn't think I had to come up with more than one!!! ;D
The cheesy 20mA LEDs are not really the best for general illumination.
-Tim
Quote from: Tim Strommen on April 09, 2009, 06:50:53 PM
Osram (they make the "Golden Dragon" LED)...
OK. But I note that folks like Vishay don't (although, if you hunt around, you might find most of their whites are 5500 Kelvin).
So I (mostly) stand by my statement. Perhaps modified to read "white LED's are
generally not sold by color temperature." Yes, you can find them that way, but most folks buying parts from common suppliers will not find a selection of temperatures, or even published information on color temps for the parts they are buying. FWIW.
I did see you said "not
entirely correct" so I'll let you slide :)
-Sean
http://OurOdyssey.BlogSpot.com
Quote from: Sean on April 09, 2009, 06:59:11 PMI did see you said "not entirely correct" so I'll let you slide :)
Phew! That was a close one ;D
-T
Tim...be careful here...I have read a lot of Sean's wifes comments in their blog...that girl is sharp...I will have to say I have Sean's back...I don't want her on mine! ;D
Just in case someone did not get the humor here...I am joking.
Actually I like the fact that there are two folk's here trading comments back and forth, one telling the other he might not be entiely correct and an all out war has not started! And we are all learning something.
I am also tickled that after everyone said BobOfTheNorths lights won't work long...they still are! ;D
Jack
Quote from: Tim Strommen on April 09, 2009, 07:14:45 PM
Phew! That was a close one ;D
Seriously, Tim, thanks for the correction. It's good to know there are at least a couple of sources for low-volume buys of color temperature-binned parts. If we can't get the temperature we want for our next batch of fixtures (I'd much rather buy off-the-shelf than have to build from scratch), I might just have to fire up the Weller.
-Sean
http://OurOdyssey.BlogSpot.com
Ulp me again... :)
Something no-one's mentioned...yet...
The "white" LED's aren't really white LEDs. There really is no such thing! But wait... there is...no there ain't... huh?
Well, a while back some smart guy (I think at Nichia) figured out that you could plop some phosphor on top of a blue LED and make it glow some other color than blue (the blue excites it) and brightly at that... from there, they found that a mixture of various phosphors of different colors created a goo that you could deposit on top of a blue LED to make it look white...
From there, the mixture of goo allows "white" led's to be "warm" "cold" "yellowish" etc.
If you look at any LED other than a white one with a prism, all you'll see is the same color as that LED is... ie red, blue, etc.
but if you look at a white LED with a prism you'll see a continuous band of colors just like you'd see if you look at the sun with the prism!
Just though some of you'd like to know although I'm sure it's old news to Sean and Tim (hi guys) but I can certainly see binning them (white ones) to color, as phosphor runs are likely to be slightly different from batch to batch...
Cheers
Gary
Quote from: jackhartjr on April 09, 2009, 07:53:53 PM
I am also tickled that after everyone said BobOfTheNorths lights won't work long...they still are! ;D
Jack,
I know you don't need an answer to this, but I want to write one anyway, because I think some folks may not really understand what some of us meant, way back at the beginning of the thread.
What I said back then was not that they
would not work "long," but rather that "I
believe[d] ... that [he would] see LED failures in
relatively short order." (Emphasis added for this post.)
The key here is the word "relatively," and it is this word that may be the source of confusion.
LEDs have an MTBF (roughly translated as "life expectancy" of between 100,000 and 1,000,000 hours, depending on type, environment, and application. (And, yes, I know that is an entire order of magnitude spread.) To put that in perspective, that means that a typical LED should last, illuminated continuously and operated under the conditions of voltage, current, vibration (or lack thereof), temperature and other parameters as specified by the manufacturer, for a minimum of 11 years and up to 110 years.
So if you shortened that life by, say, 99% due to exceeding specs, you could still expect the LED to last 1,000 hours, or nearly 42 days of continuous operation. I strongly suspect that Bob has not had his LEDs running for a full thousand hours, so the jury is out on whether or not the lack of current-limiting resistors will effectively shorten the lifetime. Bottom line is that if he ever has to replace them in his lifetime, then, yes, it probably did.
For comparison, BTW, the lifespan of a halogen bulb is from 2,000 to 5,000 hours. I've had halogens in my bus for five years and I've only replaced two bulbs out of dozens, which gives you a sense of how many hours even a full-timer puts on lamps. Most of us could easily compromise the design life of an LED by 95% and never know it because we just don't ever rack up that many hours. OTOH, if you play too fast and loose with the current specs, it will only take one momentary voltage spike, say from a twitchy alternator regulator, to take out every LED that happens to be turned on.
Hope that clarifies things a bit for all concerned.
-Sean
http://OurOdyssey.BlogSpot.com
I will go back and revisit Boogie's post on computing the resistor size. Is it the resistor that is the current limiting device?
Were I building a fixture that put out the equal lumen's of a 200 watt bulb, I imagine I would have some money invested. That 'current limiting IC" would seem to make sense even if it cost $6 if it would protect "every LED in the bus" from a voltage spike.
I understand the concept of stress caused by "rapid start up". How do you soften that shock? A resistor isn't a coil, if you follow my meaning, so that resistive component wouldn't address that surge problem. Right?
Thanks,
John
JohnEd, what do you think would happen if you used a capacitor across the LED with a current limiting resistor?
What I don't know is how slowly the power should be applied to the LED.
Tom Caffrey
Hey Bob and others, Christi & I are thinking about LEDs. Do you have the Pros and Cons on them. Do they put out enough lite for reading, etc...?
Cost, installation? The bset brands? Thanks, M&C
Well I've enjoy all this theory stuff.
One thing I did learn this month is that those little fluorescent light bulbs do not like being on a dimmer
light switch. I tried it none lasted over a month.
LEDs (a simpletons view of what has been discussed)
"it will only take one momentary voltage spike, say from a twitchy alternator regulator, to take out every LED that happens to be turned on."
Not really too worried about it. If you think about it (on my bus) the alternator goes to the coach batteries
then to a master switch to a solenoid switch then to the house batteries. between the batteries as the
"shock absorber" and over 60 ft of wire for the resistance and capacitance then down to 12 volts the voltage
is pretty stable.
Initial turn on spike of the unit...........tore apart my cheap led flash lights and found no fancy circuitry.
Been using them for 2 years now No big deal in my simple mind.
Just a FWIW
Skip
Michael:
The reason we did this was to reduce our power consumption prior to putting in solar panels. We had roughly 40 of those little teardrop Halogens in the bus. They did a better job of heating than they did of lighting and I say that only slightly tongue in cheek. Sean claims good life out of what I assume to be the same bulbs but that most definitely was not our experience. We've had the coach 5 years and I have bought over 40 of those bulbs in that time. I probably have 10 left in inventory for the few of them we have left in service but still I think we have likely replaced 40 bulbs over the 5 years. So they most definitely were not a reliable lighting source in our application.
Skip:
Thank you for the voice of common sense, a commodity that has occasionally been sorely lacking in this thread. The only time that I have any real concern about voltage with regard to my LED modules is when we equalize. Since we do that manually I can control what lighting is on while we do that. I'm not convinced there is a problem even during an equalization cycle but, if there is, then it is a controllable problem.
I'll repeat my caveat: I don't know how long these creations may last but so far they have dramatically outperformed the commercial units that they are running with. I haven't detailed all the commercial LED solutions that we have used but I can say that every one of them has failed to perform in some respect. We also tried a 12 volt fluorescent fixture. When it failed (fairly promptly) I replaced the tubes with some of my LED creations and just used the fixture as a housing for the LED lights. For what I have spent on the homemade LEDs c/w what I have spent on the commercial solutions I can afford to replace my LEDs in less than 10,000 hours, a lot less in fact.
I will however be trying some of the LEDs that Tim referred to. I have difficulty believing that they are really 80x as bright as the ones I am using but even if they are 10x then they would be worth the extra bux.
Quote from: poppi on April 10, 2009, 08:17:20 AM
LEDs (a simpletons view of what has been discussed)
"it will only take one momentary voltage spike, say from a twitchy alternator regulator, to take out every LED that happens to be turned on."
Please let me clarify my own comment: I said that IF you failed to properly limit the current, which merely requires resistance of the appropriate value, THEN a spike could take out all your LEDs. LED's have a non-linear voltage/current relationship -- once voltage rises even a little bit above the maximum rating for the part, current draw skyrockets, and it is the current which kills the part. Having some resistance in the circuit forces the voltage/current relationship closer to linearity.
Quote
....tore apart my cheap led flash lights and found no fancy circuitry.
Been using them for 2 years now No big deal in my simple mind.
You can't draw parallels between cheap flashlights and an alternator-, converter-, or charger-driven system.
If you take apart one of those LED keychain flashlights, you will find no parts besides the battery, LED, and switch. So where, you might wonder, is the resistor? The answer is: in the battery. Every battery has "internal resistance." You can easily calculate what this resistance is by short-circuiting the battery with an ammeter. I do not recommend you do this with your house batteries!
The tiny little button cells used in keychain lights have very high internal resistance compared to their output. In other words, even if you shorted the battery completely, the total amount of current it generates is very limited. So limited, in fact, that it is safe to drive an LED directly from such a battery.
No such thing is true about the electrical system on your coach. Even when running only on batteries, your battery bank is capably of generating huge amounts of current if shorted -- you probably could even weld with it. I always remove my watch and my ring when working on batteries for just this reason.
BTW, modern LED flashlights often use driver chips. I have a light I always carry with me that uses a single AA cell, with one of Tim's high-output Luxeon Rebel's in it. It is a very bright light. The only way to drive such an LED with a single 1.2-1.5v cell is to use electronics. But to put a price tag on that, the light cost me $20. Inexpensive ($2-$4) LED flashlights, which have to use two or three batteries to get the proper operating voltage, usually have a simple resistor in series with the LED.
-Sean
http://OurOdyssey.BlogSpot.com
My apologies for not including the If portion on the quote.
Though that brings back a quote my dad used to say
If the dog hadn't stopped to wet the tire he would of caught the rabbit.
:)
Take care
Skip
Quote from: poppi on April 10, 2009, 08:17:20 AM
"it will only take one momentary voltage spike, say from a twitchy alternator regulator, to take out every LED that happens to be turned on."
Not really too worried about it. If you think about it (on my bus) the alternator goes to the coach batteries
then to a master switch to a solenoid switch then to the house batteries. between the batteries as the
"shock absorber" and over 60 ft of wire for the resistance and capacitance then down to 12 volts the voltage is pretty stable.
The 60Ft of wire thing though is what can get you into trouble... if you switch the LED locally (at the near side of the 60ft wire), you'll get an inductive "kick" (spike) as the current starts running through the wire. The best way to counter this is to put a bulk capacitance at the switch to absorb the kick at the switch (both on/off). We use this same concept when dealing with hot-swap backplanes in the electronics industry.
Quote from: JohnEd on April 09, 2009, 08:52:08 PM
...I will go back and revisit Boogie's post on computing the resistor size. Is it the resistor that is the current limiting device?...
Hello John,
Yes, you are correct, the resistor is a current limiting device in this type of circuit. The is because the LEDs have a specified forward voltage (a voltage which must be overcome before it begins to conduct in volume), and after you add up all of the V
fwd of all the series LEDs there is only so much voltage left. This is where the resistor comes into play - it has a measured resistance against the passage of electrons for a given voltage, and it's effect on the flow of electrons is calculated by using the formula in Ohm's Law:
E
---
IxR
Simple math is all that is needed to pick a resistor with the correct "Ohm" value to allow a measured passage of current at a given voltage. However its reliance on the voltage across it is the weak point - if the voltage is not regulated before the circuit - the circuit is then subject to the fluctuations of the power supply (any line noise, surges, sags, etc.).
Quote from: JohnEd on April 09, 2009, 08:52:08 PM...I understand the concept of stress caused by "rapid start up". How do you soften that shock? A resistor isn't a coil, if you follow my meaning, so that resistive component wouldn't address that surge problem. Right?...
You can do this by putting an inductor in series with the current limiting resistor on the power supply side, and a large value capacitor in parallel to the LEDs - the inductor will resist the high speed change of the input voltage, and the capacitor will rob some of the current that is allowed to pass through the current limiting resistor, until the voltage matches the V
fwd of the LEDs (roughly). The capacitor in parallel with the LEDs should add a bit of safety to the power supply - but the math for doing this is a bit more involved than just picking out a current-limiting resistor.
The easiest way to take care of start surge is to get an LED driver IC that has Soft-Start built in (and a vast many of them do, LED driver manufacturers are staring to be told that if they don't have SS customers won't buy their part...). The rate at which the chip is ramped up to the desired current is typically controlled by picking the size capacitor you want (and there is easy math in the driver's datasheet to figure that out).
-Tim
P.S. Sean and Boogie are great guys, I've spoken with them off-line several times - I think we understand each other enough to know when we're joking around ;). -T
Hey there,
I thought maybe making a mechanical analogy of LED's, voltage, and resistors might help you all figure out what this is all about, and why current limiting is needed.
So I made this little drawing of a piston in a cylinder...
(https://busconversionmagazine.com/forum/proxy.php?request=http%3A%2F%2Fwww.heartmagic.com%2FledAnalogy.jpg&hash=447b67ed4cc8f4d446ff96dae18e05f884fe37b2)
Imagine that the LED is the red part in the drawing, and it won't glow until it gets some pressure applied to it by squeezing it. (pressure equals voltage in this adventure)
For this example let's say that the voltage rating of the LED is 2 volts. What this means is that anything less than 2 volts across it will not make it glow. That's all you need to know about the voltage.
For current, in this example LED current is equivalent to the squeezing force that the piston applies to the LED as it's being raised. The LED isn't solid, but it also isn't a marshmallow. It does have a little bit of give but if you squeeze it too much (ie put too much current thru it) it will squish and be destroyed. force = current...
For the resistor in this example, consider it a spring. The more it stretches, the more current is flowing...
..and a whimpier spring would be equivalent to a higher ohm value resistor...
Ok.Now consider drawing A and suppose that you start raising the piston up- for the sake of arguement let's say that instead of raising it with something weak like your fingers (equivalent to a little lithium coin cell in this context) it's being raised by a very strong thing, like a hydraulic system that would rather break things than be stopped. (equivalent to your bus's battery bank that would rather melt things when given the opportunity ..a short for instance)
So when the piston is at the bottom (0 volts) nothing happens.
As you raise it (ie raise the voltage across the LED) still nothing happens until the piston finally reaches the LED and touches it (the voltage finally becomes equal to the LED's rated voltage)
Now the LED starts to glow.
Looking at drawing B you can see that the LED is glowing (some current is flowing) but it is getting a little squished because the voltage is slightly more than 2 volts, although not enough to break it. As you further raise the voltage as in drawing C, you find that without too much more increase in voltage, the poor LED has gotten squished (equivalent to way too much current flowing thru it) and it's now a DED instead of a LED!! (ie it doesn't work anymore)
It didn't take much more than 2 volts to kill it because the hydraulic system squeezed it between the piston and the stop with mucho force. Equivalent to even a small, momentary voltage spike in your bus...
So as you can see, the difference between a happy LED and a dead one is a very small amount of voltage. This is exactly what happens when you simply add LED voltages up and hook em to a battery. They may work but it doesn't take much to kill them and there is little control of the current through them.
RESISTOR TO THE RESCUE
Now consider adding a "stretch me" kind of spring in the middle of the piston shaft, which is equivalent to adding a resistor in series with an LED. When pressure starts getting applied to the LED , the spring starts to stretch and keeps the hydraulic system from squishing the LED. Likewise a resistor adds some "springiness" to a circuit and limits how much current can flow. Like the piston system with a given sized spring, it will eventually stretch far enough that it can't limit the pressure anymore and the LED will get squished. Same with a fixed resistor, it will limit the current effectively but if you raise the voltage high enough, it will let enough current through to kill the LED.
In drawing D, again the voltage isn't enough to make the LED glow, and the spring isn't stretched at all (no current flowing).
In drawing E, the voltage is now up enough to make the LED glow, and the spring is a little stretched because the voltage is a bit higher than the LED's published voltage of 2V. In fact it's high enough to kill the LED if the spring wasn't there, but the spring stretches, gives the LED some slack and keeps the hydraulic monster from squishing the LED. (ie the resistor keeps the extra voltage from causing too much current to flow, and keeps the LED within its safe range of current)
Finally in drawing F, we've raised the voltage to 4 volts. Well enough to destroy the LED but since the spring is in there and it's stretched, (more current is flowing but not enough to kill the LED) everything is happy even at 4 volts.
So now as you can see, the difference between a happy LED and a dead one -when a spring is included in the system- is a fairly wide range of voltage instead of a very narrow range of voltage.
That's the magic of a simple resistor!
Last, in this analogy if you consider more resistance to equal a whimpier spring, you can start to grokk how it all works. More ohms/resistance = dimmer (but safer) led's....not as bright as with fewer ohms but a wider safety range voltage-wise. That's where the compromise comes in... with a resistor you can safely work over a 2:1 range and have reasonable brightness at either end of the range. Beyond that range, it's probably time for electronic limiters which can give you hundreds-to-1 voltage range. But on a bus, 12 volts plus or minus 6 is just fine with a simple resistor, and you probably won't see nearly that much swing anyway...
Oh heavens I hope that helps...
Cheers
Gary
Gary, (boogiethecat)
Thanks for the time and effort you put into your explanation.
I got to tell though I would never put individual springs on a piston.
I'd go with an acumilator and regulator inline so I wouldn't have to worry about uneven pressure
and keep it constant ;)
Skip
Heh! I guess we need accumulistors for this job!!! :)
Don't you mean capacitors? ;D
-Tim
PVCCES aka Tom,
The voltage across the led would rise with the charge on the cap and at the same rate. The circuit values would determine the time needed.(RC time constant..I think) Same result of putting a coil in series with the led. All of this is becoming to complicated as I expect to make a few different size "lights" composed of many leds. They just seem to be the only way to go. My problem in the past was the temp of the light and having to put 50 of them together to get the lumen's I wanted. It seems everything has changed and I am liking it.
Thanks for you thoughts,
John
Capacitors and inductors absolutely aren't necessary with LEDs, because LEDs don't mind being turned on and off rapidly.
Case in point, a lot of data is transmitted optically thru fibers using simple red or infrared LEd's modulated at high megahertz rates in music synthesizer systems (adat optical interface, etc) and in millions of other applications (your TV's remote being another example). Nothing special about the LEds used in these apps- you can modulate any LED from full on to full off in nanoseconds and they're happy.
(white LED's don't care either, but due to their glowing phosphors they don't "optically" go on and off instantly, but that's nothing that a busnut would ever need to care about either).
You're probably thinking of incandescent lightbulbs, whose filament resistance is something like 10 times less when they're off than when they're on. They have a heck of an inrush current while they're coming up to operating brightness. LEd's don't do that... ALL they care about is that they don't have too much current pushed through them, and that they aren't reverse biased beyond their specification (a static electrical spark hitting them before they are soldered in to a circuit for example)...beyond that they don't care much about anything else.
Cheers
G
Boogie,
That isn't my lie, I'm just repeating it. :P
Lots of talk in this thread about Soft Start features in current regulating devices. What am I missing? I know the resistor was a current limiting device albeit a poor one in some of the aspects of performance. I'm so confused ??? :'( :( They never said it would be this hard :-[
Thanks,
John ;D
John, it really isn't this hard. This thread has, in places, gotten blown way out of proportion. The current limiting resistor is the industry standard, it's simple, it performs well, and it's only the surface tension of a few people here who aren't engineers that is continually making this to appear difficult.
There are definitely places in life for active current regulators (switcher ic's etc) but in probably 95% of all LED applications on the planet, you'll find the lowly resistor doing the job effectively, simply, inexpensively and reliably. Electronic regulators are absolutely great when you're driving a luxeon 1 watt LED on 12 volts, because they require a few amps rather than a few milliamps. Resistors in that case would be a bad choice- they'd work but you'd be dumping 10 watts thru them and the heat would be hasslesome. But for strings of small 20Ma led's or individual ones, a simple resistor is the way to go.
If people want to use switching electronics to drive their reading light LEDs tail light LEDs, turn clicker LEDs etc, let em have at it. There's nothing more amusing than watching silly creatures bang their heads on a concrete wall when you can giggle knowing that they don't have to... :)
Ok off to melting some glass for the afternoon...
G
Quote from: bobofthenorth on April 10, 2009, 08:36:51 AM...I will however be trying some of the LEDs that Tim referred to. I have difficulty believing that they are really 80x as bright as the ones I am using but even if they are 10x then they would be worth the extra bux...
Here is the conversion from candelas to lumens ("q" is the beam angle):
Lumens = (Candelas x 2pi (1 – Cos (q/2) )
The Lumileds Rebel 100lm Cool-White LED P/N: LXML-PWC1-0100 (http://www.futureelectronics.com/en/technologies/semiconductors/lighting-solutions/high-power-led-emitters/white/Pages/8666695-LXML-PWC1-0100.aspx), has a beam angle of about 140 degrees. This is about 24200 millicandela (about 13x the brightness of the Hiteq LED for the same segment of beam angle - which is to say that if you poked a hole through some thick cardboard to block all of the light except for a 75 degrees beam {like the hiteq part}, it would be 13x brighter for that small cone, and in this example we'd be wasting the blocked light {more than half of the emitted light} that doesn't make it past the cardboard).
The Lumileds K2 TFFC 220lm Cool-White LED P/N: LXK2-PWC4-0220 (http://www.futureelectronics.com/en/technologies/semiconductors/lighting-solutions/high-power-led-emitters/white/Pages/1184142-LXK2-PWC4-0220.aspx), has a beam angle of about 140 degrees also. This works out to about 53200 millicandela (about 28x the brightness of the Hiteq LED for the same segement of beam angle).
The Lumileds "lambertian pattern" LEDs put more light over a wider area than the Hiteq parts.
The beam angle as well as the light output has the most to do with the usefulness of an LED - for a given work-surface you need so much light per square foot, and thus so many LEDs at a given height and direction to fully cover the surface with the appropriate amount of light for the task. The light can either be directly used from the LED device (with a narrow beam) which gives a light fixture the "thousand points of light" look, or you can use a wider-angle LED and some form of optics to direct the most light where it is needed. To place the LEDs closer to the work-surface, you need a wider beam angle. I don't personally prefer the "thousand points of light" look, I like my light fixtures to be smooth and even (less eye strain, sexier appearance). I use LCD backlight light-pipe material from 3M to even out the light in a very thin piece of plastic. You can also use lenses and reflectors that are made specifically for LEDs (Future Electronics has several different types made specifically for Lumileds LEDs).
-T
El BooGay,
Munch o grassy @$#. ??? ;D
You are gifted at turning a phrase. :)
John
The naysayers will be relieved to hear that 2 of my resistorless LED modules finally failed. Five months is not an acceptable lifespan but it is worth noting that nothing caught on fire and the cat was not injured. I've still got modules running with 10, 22 & 50 ohm resistors inline with them but I won't put up any more with no resistors. >:( ::)
Compared to the commercial bulbs I'm still well ahead of the game. I've replaced 5 of them in the same time span.
Hey, we're not happy/relieved that your resistor-less fixtures failed - just not suprised.
Quote from: Tim Strommen on July 19, 2009, 08:18:48 PM
Hey, we're not happy/relieved that your resistor-less fixtures failed - just not suprised.
You were always more of an another-way-sayer than a naysayer. ;D
we are glad the cat is ok
sometime the innocent get injured
Melbo