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Electric Bicycle Contoller

Discussion in 'Electronic Repair' started by Harvey, Dec 20, 2004.

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  1. Harvey

    Harvey Guest

    Sorry for the long question...
    I've been asked to look (and repair) an electric bicycle controller - it
    seems to be a reasonable straight forward design; power mosfet output with
    PWM drive, 24V supply, etc. etc. and so far I've managed to get it largely
    working and the motor control part of it seems OK.

    The only issue now is the relay - this connects the motor to the mosfest.
    I've bypassed this and the motor runs as it should do (as far as I can
    tell), BUT what I can't tell is exactly how or when this relay should be on.
    Its driven by a microcontroller (a Elan EM78P156EL if you really want to
    know), via a few transistors and various other bits, and I'm at a loss as to
    why the particular control pin on the micro isn't being driven.. either
    there's some fault detection in place that I've so far missed, or for some
    reason that particular output pin on the micro has failed. - I can either:

    * somehow figure out a way of switching the relay on ONLY when the motor is
    being driven - which is at a guess what is supposed to happen.

    * bypass the relay control and make the relay 'on' all the time.

    What I'm unsure about is how the circuit would cope with the back emf
    generated by the motor on downhill runs..there's a few big diodes near the
    mosfets which would probably soak the current, but not having any other
    experience with such items I'm unsure as to what the 'normal' way of these
    things working is.

    Any advice? -- Idealy not 'tell them to sod off' :) ....which is
  2. Harvey

    Harvey Guest


    Actually - describing it as a bicycle is probably a bit misleading, its more
    of a scooter, with the motor permanently connected to the rear wheel by a
    belt drive. There's no freewheel.
  3. James Sweet

    James Sweet Guest

    The relay is probably there as a safety device, preventing the motor from
    being driven in case the MOSFET shorts. Check the drive circuitry for the
    relay carefully, unless something is defective there then the I/O pin on the
    micro is bad, I've seen it happen before on a bread machine. You may be able
    to add something which will activate the relay when the MOSFET gate PWM
    kicks in.
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