# upgrade LED Trailer lights for 24V Use

Discussion in 'The Projects Forum' started by seadz, Jun 8, 2015.

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Jun 8, 2015
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Hi Guys,

this should be an easy one but i can't seem to figure it out:

I've got these LED trailer lights that i bought from China (there's the problem ;-) that were described as 12v/24v compatible but after testing them on my regulated power supply on 24V they didn't last very long:

It's a very basic pcb design consisting of 16 LED's which are wired in series and parallel with some resistors and 2 diodes.
After researching it a bit it is obvious that this is designed around 12Volt input and would have never lasting long on the 24 volt motorhome it is intended for.

Does anyone know a quick way in changing the resistors and or diode so that i can withstand a constant voltage of 27Volt (that's what i got when the alternator is running)

See Attachment for pcb layout.

The pcb consist of 2 circuits where the constant voltage on the red wire would give you the "tail lights" which is limited by a resistor and the green light would bypass the resistor and provide the LED's with a higher voltage that gives you the "brake" lights.

So my first question is:
what is a safe voltage for 1 LED to have when i'm braking and my input voltage is 27Volts?

i tried to work it out and got this far:

R = E/I
E = Vbatt - Vled
I = 20mA

Vled = 2.1 volts
Vbatt = 27 volts

R=24.9/0.02 = 1245 ohms

The resistors that are on it at the moment are 100 Ohm and 570 Ohm in parallel creating effectively 85 Ohms over the circuit.

What resistors should i use to find the best range to drive these LED as a "tail light" and "brake light" circuit.

sorry for the non technical English.
Sietse

2. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
1,425
357
This may be classed as an automotive topic and might be closed shortly.

It doesn't look like the cleverest of designs. I would modify the tracks so that the central 4 LEDs could be driven at full brightness normally (i.e.20mA) and the brake input would illuminate all 16. For 20mA per LED You would require around 1k per 4x series LEDs for 27V operation.

Last edited: Jun 8, 2015
3. ### ericgibbs Senior Member

Jan 29, 2010
2,421
369
hi,
Assume 3V drop per White LED, as there are two in each chain thats 6V, so 27V - 6V = 21V.
Also there are two diodes in series with the 24V supply, so 21- 1.5V = 19.5V

So at 20mA per chain, thats 19.5/0.02 = 975R, change the resistors to 1K.
E

EDIT.
See that they in fact they are RED LED' so allow ~2V per LED

4. ### AnalogKid Distinguished Member

Aug 1, 2013
4,304
1,180
When calculating the current through each string, note that those 570 ohm resistors actually are 470 ohm.

ak

5. ### Wendy Moderator

Mar 24, 2008
20,735
2,499

The owners of All About Circuits have elected not to host discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

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