upgrade LED Trailer lights for 24V Use

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

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  1. seadz

    Thread Starter New Member

    Jun 8, 2015
    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.
    back.JPG front.JPG
    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.

    Thanks in advance,
    sorry for the non technical English.
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    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

    AAC Fanatic!

    Jan 29, 2010
    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.

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

    AAC Fanatic!

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

  5. Wendy


    Mar 24, 2008
    As I read this thread it is indeed automotive.

    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.

    This thread is against the AAC forum rules, Chapter 6, as seen here:

    6. Restricted topics. The following topics are regularly raised and however are considered “off-topic” at all times and will results in Your thread being closed without question:

    • Any kind of over-unity devices and systems
    • Automotive modifications
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    • LEDs to mains
    • Phone jammers
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    • Transformer-less power supplies
    This can be found in our Terms of Service (ToS)

    Automotive modifications of any kind are strictly forbidden. Therefore, this thread will be closed.

    Please try to understand the reasons behind this action, and feel free to browse and use the forums.

    You might find answers to your questions in one of these forums:


    Another good forum that shares many of the same membership is http://www.electro-tech-online.com/ . Good luck.

    WBahn, May 19, 2015 Edit Delete IP Report
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