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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

Pᴀᴜʟsᴛᴇʀ2
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3 Answers3

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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

schematic

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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.

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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.

Marko Buršič
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