I have a friend who has asked me to help him sort out his truck lighting. It is a 24v truck with a 24-12v dc-dc converter for the body. It has 2 flashing lights, that when active make all the other lights on the body flicker. When I disconnect the flashing lights the other lights are bright and stable so it must be the cause or part of it. I have checked all the wiring, the supply is 12awg,then from a distribution block each load has 18awg cable. The converter is a 30amp rated model(I'd guess 20amp continuous which should be plenty for the modest lighting). I've added an image, I hope it is clear enough.

Any suggestions on the cause, and solution? I would prefer to keep things as original as possible as I have not looked up the driving regs on these lights yet. However I would consider an LED replacement.

Thank you for any help and information. I appreciate what all you people with knowledge do on these forums.enter image description here

  • 1
    Did you try with the flasher lights but without the flasher unit? That would tell you whether the culprit is the extra load or the flasher unit.
    – Damien
    Aug 1, 2015 at 18:52
  • 2
    The problem is that you're flashing incandescent lights, which have a huge inrush current, and monitoring with continuous LEDs, which have an instantaneous response to the voltage dip. Changing either would help. Adding some resistance in series with the flashing lights might help.
    – tomnexus
    Aug 1, 2015 at 19:42
  • Thanks for the comments Damien & tomnexus, I will certainly have to try some things out and see which is the best option for the regulations.
    – Frosty
    Aug 2, 2015 at 7:16

3 Answers 3


tomnexus comment is your culprit. Incandescent bulbs have a filament which has much lower resistance when cold than when it's been on for a fraction of a second. As a general rule, the resistance is 1/10 to 1/20 the hot resistance. https://en.wikipedia.org/wiki/Incandescent_light_bulb#Current_and_resistance, for instance, uses a number of 1/15.

When both flashers turn on, their total current will (briefly) be in the range of $$i =2 x 1.75x15 = 52 amps$$ and it's no wonder your converter can't keep up.

As a check, remove 1 bulb from the flasher and see if this doesn't improve the problem. I suspect you'll still get a noticeable flicker, but it should be better than what you're seeing now.

There are basically two solutions. Either replace your flashers with LEDs, or build a circuit which will relatively slowly ramp up the current to the flasher bulbs.

If tomnexus would care to write up his comment as an answer, his should be given precedence over mine.

  • Thanks WhatRoughBeast I didn't think it would be that high an amp level, I knew it would have a decent initial draw. Perhaps the converter can't keep up, he has replaced a few over the years. If it's legal I will just set up an LED flashing system for him. Is there anyway I could test the amp draw? My multimeter is only 10amp max. Would they be fast enough to show the draw? The flashing is about 300milliseconds on-off.
    – Frosty
    Aug 2, 2015 at 7:22
  • @Frosty - In order to test the draw, you'll need an oscilloscope. A multimeter is not fast enough. And a 10 amp meter will not handle it. As a matter of fact, you're not likely to find one that will. You would need to use an external shunt, and even then the meter just isn't fast enough.
    – WhatRoughBeast
    Aug 2, 2015 at 13:22
  • I did not think they would be fast enough. At this stage I won't bother with the oscilloscope, I'll just fix the problem. I'm sure at a later date it will get the better of me and I will though :P Thanks again.
    – Frosty
    Aug 2, 2015 at 15:44
  • Wow, 15 times is more than I thought. That'll definitely (briefly) overload the converter. This happens in all cars, but will be most noticeable on other LED lights; incandescent ones will just ride it out. If the cold/hot ratio is so large then a series resistor will make a big difference to the peak current.
    – tomnexus
    Aug 2, 2015 at 18:34

You could help the situation bu reducing the current spikes the other devices can see, they are obviously voltage sensitive so the dips are objectionable:

  1. First run independant wires for the flasher direct to the converter terminals to avoid the other devices having to see the volt drop in the flashing circuit wiring after the converter

  2. Then get a jumbo sound system capacitor and place it at the flasher unit location to supply the inrush current same as on a sound system bass unit

  3. Consider aternating the two flasher lamps by controlling a relay coil with the flasher and using the NO, NC contacts to alternate the load, current will be halved and will also be more constat as one switches of as the other turns on with only the one inrush current causing the voltage dip.

  4. Specifying a next size up converter as this will have more muscle



As you said, it has an electromechanical device for flashing. Maybe it's an relay that when turned off produces a spike (back EMF kick). Check if a freewheeling diode is installed (if possible) in parallel to relay's coil.

If the other lights are flashing with short but intense burst, then this EMF kickback is the reason, otherwise if flashing of other lights is following the flashing light then you have a volage drop: small cross sections of cables or simply the inverter can't deliver as much current.

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