24v to 13.8v converters

richard5933 · February 18, 2019, 07:33:18 PM

Okay - I'm close to having everything on hand for the update to my headlight circuit. Four bright and shiny DC-DC converters. They take in 24v (actually anything ranging from 19v-36v) and theoretically output a constant 13.8v, which should be just right to keep the headlights at maximum bright. I will be using one for each headlight, keeping the same circuitry as the OEM design. These are rated for 40 amp each, so hopefully there is lots of head room.

Here's the question:

The instructions state the following...

It is recommended to use stable DC power for the input power supply If you use the pulsed DC power provided by the generator, please filter it with capacitor before connecting it to the converter.

Is this something I need to be concerned about? I am thinking that they are trying to avoid having the output pulse, causing things like LED lights to pulse. I'm going to use incandescent bulbs for now, which don't usually exhibit problems quite as much as LEDs. But, I really don't want to have my headlights pulsating.

Seems to me that the output from the bus alternator will be tempered quite a bit by the existence of the two 8D batteries in the circuit, so that by the time these converters get the input it will be pretty smooth. Or, am I misreading this and are they talking about installing the filter capacitors after the converters in the output circuit feeding the headlights?

If I should be installing capacitors, anyone able to tell me what size and where they go? Across the lines feeding the converters?

I'm about at the end of my knowledge base once we start talking about installing filter capacitors.

For reference, here is the link to what I'm installing: https://www.amazon.com/gp/product/B01LY8D7U0/ref=ppx_yo_dt_b_asin_title_o00__o00_s00?ie=UTF8&psc=1

bevans6 · February 19, 2019, 04:20:11 AM

The pulsing is AC ripple on top of the 28 volt DC that the alternator is putting out, at 6X alternator RPM. A battery will not smooth it out very much, but you won't see it with your eyes. Your devices are 40 amp, not 40 watt. If they give you a capacitor specification for automotive use, I'd add the cap. They are designed for true DC, from a battery or a solar system, but are also recommended for automotive use, so they should be fine. How many are you using? Typical halogen headlights are 65 watts each, about 5 amps, so four are total 20 amps.

richard5933 · February 19, 2019, 04:44:03 AM

Quote from: bevans6 on February 19, 2019, 04:20:11 AM
The pulsing is AC ripple on top of the 28 volt DC that the alternator is putting out, at 6X alternator RPM. A battery will not smooth it out very much, but you won't see it with your eyes. Your devices are 40 amp, not 40 watt. If they give you a capacitor specification for automotive use, I'd add the cap. They are designed for true DC, from a battery or a solar system, but are also recommended for automotive use, so they should be fine. How many are you using? Typical halogen headlights are 65 watts each, about 5 amps, so four are total 20 amps.

Thanks for the typo correction - I fixed that in my earlier post.

The OEM wiring has four separate circuits in place for headlights - one for each of the four lamps. I guess they took redundancy to the extreme, but I kind of like knowing that if something fails it just takes out one lamp, not two or four. I plan to use four of these converters, mainly to maintain the OEM wiring and keep things simple.

If I'm not going to see the pulsation, then perhaps I'm best leaving things be for now. However, after watching a few YouTube video (if they are accurate) the fix doesn't seem to complicated. Guys are using 6800 uF capacitors soldered across the pos & neg wires going to their LED headlights to fix LED flicker.

Could I do the same thing here, except put them in the lines feeding the converters? I found some 6800 uF 35v capacitors, and if that would help I can incorporate them into the harness I'll be putting together.

Jim Blackwood · February 19, 2019, 06:42:30 AM

I like to use water as a means of visualizing what electricity does. So if you have a piston pump feeding a hose it'll pulsate at the rate it's pumping right? Coming right out of the hose you can see that. But if you run it through a big bucket of water the pulsations just turn into small ripples. That's what a capacitor or a battery does. If you want it smoother than that you put a restriction in the line after the bucket, that's what a choke coil does. Then for smoother yet another bucket and the two buckets and the restriction make what's called a "Pi" filter, after the shape of the Greek letter for pi.

So, if you really want to get the ripples out you could install a choke coil in the line to the headlights. What value? Well now you're getting into the math. Obviously with heavy enough wire to handle the current, then enough turns to block the pulses.

Jim

neoneddy · February 19, 2019, 08:44:28 AM

I've been using this https://amzn.to/2TZgHwk with good results so far. It cleans the 24v to a nice 12-ish volts.

eagle19952 · February 19, 2019, 11:17:41 AM

So, $300.00 for converters for headlamps ?

richard5933 · February 19, 2019, 11:59:03 AM

Quote from: eagle19952 on February 19, 2019, 11:17:41 AM
So, $300.00 for converters for headlamps ?

I have four converters at about $35 each. Could probably have gone with the $20 ones, but wanted a bit of extra capacity for safety's sake. Whole project should be less than $200 when all is said and done.

Quote from: neoneddy on February 19, 2019, 08:44:28 AM
I've been using this https://amzn.to/2TZgHwk with good results so far. It cleans the 24v to a nice 12-ish volts.

Looks very similar to what I'm using, only would be large enough to provide for all four headlights. The OEM system actually has four separate circuits, breakers and all, so that each headlamp gets its own feed. I guess they didn't want one failure to take out both sides, and I liked that idea and chose to keep it.

Interestingly, that converter has the same warning as mine about pulsed DC. I'm assuming from your comment that the output is clean enough to run the headlights without noticeable flicker or pulsation?

Quote from: Jim Blackwood on February 19, 2019, 06:42:30 AM
...That's what a capacitor or a battery does. ...
Jim

I take from this that you're saying the batteries should be able to smooth things out enough to run the headlights without problems? (For some reason, in my mind I put batteries and capacitors in the same category. Maybe that's why I keep thinking that they'll handle the potential problem.)

chessie4905 · February 19, 2019, 12:05:44 PM

You are still going to have headlights that suck. But anyways, there a guy on Facebook that is parting out a 4905. Contact him for the plug and socket.

bevans6 · February 19, 2019, 12:28:23 PM

Incandescent or halogen lights do not turn on and off very fast. They will absorb any ripple on top of the DC and just stay hot, stay on, without any issues at all. LED's can certainly turn on and off quickly, but may or may not offer a problem - you can always add the caps if you run into a problem. Lots of LED's are just on or off, if they get enough voltage they just are on full blast, and a little more just reduces the current they draw. Others are sensitive to input voltage in terms of light output. I wouldn't add the caps at the input to the devices unless the manufacturer suggests so, they will be trying to smooth out the entire bus electrical system if you put them across the batteries directly. Lord only knows what other loads they might see. Which they might ignore, it's been 40 years since I took this stuff in school...

The original system apparently used resistors to drop the voltage. This means they were passing some significant power for resistors, and individual ones per light would be needed to accomodate what are fairly powerful lights, 65 watts each. Not to mention redundancy. I had a setup like that in my MCI, it sucked. One box per light. You could use one of your converters for all of your lights with 50% over-capacity, two converters, one for low beam and one for high beams, or one for left and one for right, but it's simple to just duplicate what you already have and use the four of them.

eagle19952 · February 19, 2019, 12:41:06 PM

I just followed the Amazon links, and may have mis-read that you are using 4 separate converters.

Jim Blackwood · February 19, 2019, 12:53:05 PM

Quote from: richard5933 on February 19, 2019, 11:59:03 AM
I take from this that you're saying the batteries should be able to smooth things out enough to run the headlights without problems?

Yeah, I would think so Richard, of course led's are more sensitive to fluctuations so it's hard to say for sure but I'd be pretty surprised if the alternator output caused a problem. In the first place it's 3 phase with a full bridge rectifier so you'll have some overlap on the sine pulses. IIRC that's at about the 45° point which is right at the RMS power point anyway, then those big batteries will tend to suck up the ripple. I think I'd try it and not worry about it unless I could see it.

Jim

richard5933 · April 17, 2019, 12:56:55 PM

Here's the completed converter panel ready for install once the final connector arrives. Old resistor panel to the left.

I did test these with engine running and saw no flicker at the headlights. Much brighter light output.

I'll post a lights-on photo once the install is done.

chessie4905 · April 17, 2019, 05:12:56 PM

They should work great for driving in daytime. Once you experience a good set of led headlamps, you'll throw all that in the can.

richard5933 · April 17, 2019, 05:41:33 PM

Quote from: chessie4905 on April 17, 2019, 05:12:56 PM
They should work great for driving in daytime. Once you experience a good set of led headlamps, you'll throw all that in the can.

Couldn't find a set of DOT approved LED lights that would fit in the factory buckets.

We all drove with halogen lights for many years. Since we don't do that much night driving in the bus at all, I'm not worried about the lack of LEDs. For finding my way on back roads, I've got a set of LED 'driving lights' mounted under the bumper. They throw lots of light, but it goes everywhere so they can't be used on the road.

chessie4905 · April 17, 2019, 06:10:48 PM

Yeah, I see vehicles stopped and checked for the DOT on headlights. Even inspection stations don't look. Good headlamp enclosures in the buckets are adjustable and have good light patterns, at the very least equal to what you have. If you get negative reaction from opposing drivers, adjust them down a little. Other drivers are your best critics. Everything on the road switching to leds isn't just for saving money or economy. They work better. Talk to a couple of truckers thet have them and get their opinion.