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  1. #1
    Join Date
    Aug 2008
    near Rockhampton

    Default Brake controller induced voltage in wire

    Have an interesting issue on a truck I am wiring up for a trailer.

    The truck is 24V. The controller is a 24V-12V controller of a cheaper variety as the truck is of off highway use only.

    I also have fitted a 24-12V converter to go to the reverse light pin to act as an auxillary 12V supply to charge the breakaway battery and also charge a battery on the trailer that powers the hydraulic tipper.

    Both wires are 6mm in the same sheath and run from the cab to the rear of the truck (about 4m). Upon testing it magic smoke was released from the 24-12V converter. A power diode burnt up. Replaced it and all is fine.

    Isolating and testing the wire that goes from converter to rear finds that when the brake is applied a voltage is induced into the converter wire.

    What I believe is happening is the electric brake controller puts out a pulse of electricity as it is a pulse width modulator. This was confirmed by a handheld oscilloscope. This then induces the same voltage in the parallel wire which is the converter wire which the oscilloscope confirms. The voltage according to the oscilloscope is 17V peak. Which would explain why the converter diode burnt up.

    So I need to run a second wire on the opposite side of the truck to make sure the brake controller does not create problems.
    Gold, the colour of choice for the discerning person.

  2. #2
    Join Date
    Aug 2008
    near Rockhampton


    Well nope. That was not the problem so no wonder I could not find any information about it on the internet.

    The problem may be.

    Firstly the brake controller that is advertised as 24V input but 12V output is anything but. As they are PWM is just has the pulses further apart then the 12V versions but still puts out 17V or so.

    Now I do not know how trailers are supposed to be wired but this one is wired with an on board brake controller as well. It gets it's power from the pin 2 or reversing light, which I am told is generally used an auxiliary wire.

    So I had 13V going to one side of the brake controller and when I applied the brakes I had power from the service brake circuit going to the other side and the controller would have let both meet in the middle.

    As such the on trailer brake controller must have been fried from the higher voltage coming from the vehicle controller and it just went full on whenever the brake was applied.

    Also the brake signal voltage would have been 27V or so. It might have been that alone that fried it.

    I have disconnected the on trailer brake controller power lead and everything works as it should.
    Gold, the colour of choice for the discerning person.

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