12 volt LEDs on 24 volts, the easy way....

boogiethecat · June 01, 2006, 07:12:06 PM

I posted this on BNO and received a request to post it here also. So word-for word, here ya are...

Since this keeps coming up and it also is apparently difficult for some to grokk, here's a simple way to
figure out what resistor to hook in series with your 12 volt LEDs to run them on 24 volts. Kind of a "seat of the pants" method that'll work fine...
You'll need a voltmeter.

Go down to radio shack or equivalent, and purchase these resistors (or as near as you can get)

all 5 watt:

100 ohms (two pieces)
240 ohms (1 piece)
470 ohms (1 piece)

Start by hooking all four resistors in series (in a string). Resistance adds when you hook resistors in series, so you'll have an overall resistance of 910 ohms to start with.
Hook the mess in series with your LED, and hook it all up to 24 volts.

Now take your voltmeter and measure the voltage across the LED lamp. It's unlikely that with 910 ohms the voltage will be higher than 12 volts, but if it is you need more resistors.

If it's lower than 12 volts which it probably will be, try shorting out one of the 100 ohm resistors and measure again.
If it's still too low short out the other 100 ohm resistor and measure again.

If it's still too low, unshort the 100's and short out the 240 ohm. Still too low? Short the 100's out again one by one.

Still too low? Unshort the 100's and the 240's, short out the 470, and start at the beginning again with the 100's.

The idea is to slowly decrease the resistance by shorting and unshorting various combinations of these resistors until you finally get 12 volts or so across your LEDs. Then what resistance is left in the circuit is what you want to use permanently.
Once you've figured out the resistance by totalling the values of the ones you have in the circuit (the ones that are not shorted out) you can go back to the store and get a single resistor close to that value and you're done.

As far as the wattage of the resistor, you can calculate it using ohms law, or you can simply run the thing for a while and if the resistor doesn't burn your finger, it's ok. My guess is that a 5 watt resistor will probably take care of most any LED fixture although single-LED marker lights will probably only require 1/2 watt resistors.

Just as a reminder, if you purchase a 5 watt 12 volt zener diode as Richard so brilliantly suggested in another thread, that's all you'll need (per LED lamp) *instead of* resistors, and it'll pretty much take care of itself and your LED without all the fiddling.

Hope that works out easier than using ohms law and current meters for all you non-electronic types..

sergei · June 01, 2006, 08:30:51 PM

I've used 12v LEDs for over a year without problems. I chose 12v over 24v because they are easy to find.(most auto or truck parts stores) If one should burn out or get broken, i'm back on the road quickly and they are short dollar. I thew ohms law out the window, used 1156 automotive bulb as the resistor, again easy to find and simple to connect.

boogiethecat · June 02, 2006, 04:50:05 AM

I'm afraid I'm gonna have to scold you for that attitude sergi... Actually I think instead, I'll thank you for posting, and use your post as a great example of how not to run 12 volt leds on 24 volts, and why....

Sometimes solutions like yours will work- athough if your use of a bulb for a resistor causes the LED to see more than 12 volts, which it very likely will, It probably won't hurt the LEDs themselves but the resistors inside the LED fixture will get much hotter than they're supposed to and can/will fail. And something else you're probably not aware of, a light bulb is only about 1/4 of it's resistance when it's cold.. so whenever you turn your LED on with a lightbulb as the voltage dropping device, you're giving that LED a giant wallop of what is known as "inrush current". That, over time, can easily compromise the LEDs as well as the internal resistors.

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~

So if you've stuck a light bulb in series with your lights to make them work on 24 volts, and they occasionally burn out, what's the point of using them at all. You've just made things equally as unreliable as straight lightbulb systems if not moreso.

My take: 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 poor engineering. A burned out LED could be the beginnings of an accident, and throwing ohms law out the window because using an 1156 bulb "works" is not a valid reason for doing a half assed engineering design on a safety related system.

Sorry, you should take a moment to either learn ohm's law, use my "seat of the pants" workaround or something like it, or probably go back to light bulbs. 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's worked "for over a year without problems"

pvcces · June 02, 2006, 07:18:36 PM

I agree that light bulbs make lousy ballasts for the reasons given in the prior post. It's only a matter of luck if they turn out to be servicable for a while, and luck should not be a primary way of dealing with safety items.

For what it's worth.

Tom Caffrey
Suncatcher
Ketchikan, Alsaka

oldmansax · October 01, 2006, 04:24:55 PM

I am going to ask a dumb question here. If MCI used 12V head light bulbs in series on a 24V system, why can't I used two 12 LED lights in series on a 24v system? I am not being augmentative, I just want to know.

ChuckMC8 · October 01, 2006, 05:04:05 PM

If MCI used 12V head light bulbs in series on a 24V system, why can't I used two 12 LED lights in series on a 24v system?

Tom, you can, but if one bulb blows, you wouldn't have any headlights.

boogiethecat · October 01, 2006, 05:20:30 PM

OMS, as long as the 12 volt LED fixtures that you're putting in series are identical, it'll work fine. And they don't usually blow out so you won't have anything to worry about
G

Nick Badame Refrig/ACC · October 01, 2006, 06:38:31 PM

Your Question:

[If MCI used 12V head light bulbs in series on a 24V system, why can't I used two 12 LED lights in series on a 24v system?]

I just replaced a sealed low beam headlight in my mci and the other one was still lit.....

MCI uses a dedicated 12v curcuit for the headlights.

As far as the LED's, I purchaced the correct size marker led's in 24v from IBP. and for backup, I just baught a couple extras.

Nick-

DavidInWilmNC · October 01, 2006, 06:54:11 PM

Quote from: Nick Badame Refrig. Co. on October 01, 2006, 06:38:31 PM
Your Question:

[If MCI used 12V head light bulbs in series on a 24V system, why can't I used two 12 LED lights in series on a 24v system?]

I just replaced a sealed low beam headlight in my mci and the other one was still lit.....

MCI uses a dedicated 12v curcuit for the headlights.

Actually, many older MCI's did not have 12 volt headlights from the factory. There was a service bulletin (not sure exactly what it's called) some years back for converting them to run in series. Mine are like this. There's a couple of big diodes in the compartment below the driver's seat and a wire in the driver's control panel that connects to the 12 volt center tap ONLY to keep the remaining bulb lit when one burns out. Luckily, there's a big '12V' stamped on my headlight bezel, else I wouldn't have known that they were converted.

David

buswarrior · October 01, 2006, 10:28:21 PM

Hello.

Back when this LED tailight stuff got started....

Some of the offerings were purported to be able to handle a wide voltage span, 5 to 30 volts.

Now when new stuff comes out, it often has a ton of over-engineering in it, then the sales take off, and then they engineer it down to cheaper production....

I do know of a set of Dialight 7" red stop/tail lights, rated for 12 volts, that have been running without apparent problem for quite some time on 24 volts in an MC8.

Plumbing an incandescent bulb in line defeats the whole purpose of the LED lighting: dramatically lower power consumption and phenomenally longer lifespan.

For those of us who are all thumbs when it comes to electronics, the use of two 12 volts LED fixtures in series sounds like a GREAT idea and easily done by many more of us less skilled with the solder gun...

Adding more lighting to the rear isn't a bad thing, fer shure!

happy coaching!
buswarrior

oldmansax · October 02, 2006, 01:29:59 PM

So, just to clarify, two 12V LEDs in series powered by 24V will work fine, as long as they are identical bulbs and one does not burn out; right? I will not have violated Mr. Ohm and my bus wil not turn into a smoldering pile of metal and rubber.

DrivingMissLazy · October 02, 2006, 03:06:25 PM

You are absolutely, positively 100% correct. Based on the extremely long life of the LED's, one will probably not burn out in your lifetime of use.
Richard

Quote from: oldmansax on October 02, 2006, 01:29:59 PM
So, just to clarify, two 12V LEDs in series powered by 24V will work fine, as long as they are identical bulbs and one does not burn out; right? I will not have violated Mr. Ohm and my bus wil not turn into a smoldering pile of metal and rubber.

gumpy · October 03, 2006, 05:26:53 AM

Well, maybe....

I did exactly this with Maxxum truck LED clearance lights. I had to modify the light to add a ground wire, pretty simple and straight forward. I'm now rebuilding them, removing their guts and putting in my own board of LEDs and resistors becasue nearly every one has had individual and/or complete sets of the LEDs burn out. My suspicion is it has something to do with putting them in series, but I can't prove it because the voltages and currents are all valid.

Maybe Maxxum lights are just crap!

craig

oldmansax · October 03, 2006, 06:00:42 AM

Just when I thought it was safe to go to LEDs................

You have to confuse me with FACTS!!!!! LOL

gumpy · October 03, 2006, 06:14:52 AM

Yeah, I know. I'm just a troublemaker.....

Now, if I were to do it again, I'd buy the standard 12 volt LEDs (which I did) and I'd install them using the tail light signal coming off my toad relay board (http://www.gumpydog.com/bus/MC9_WIP/Electrical/Tail_Light_Converter/tail_light_converter.htm) to power them. Unfortunately, I have so many burned out, that it makes more sense to repair them than it does to buy new ones, and since I'm building my own guts, I might as well make them 24 volt compatible.

BTW, of all the LED lights I've built for my bus, not one single LED has burned out. Must have done something right, contrary to some opinions when I initially built them.

Oops, that's not entirely accurate. I do have a couple LEDs in one side light that I accidently shorted when I was showing them off, and I also have a string in one of my porch lights that are not working. Again, probably shorted against the bus sides as the clearance behind them is pretty tight.