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Flyback Diode

Discussion in 'Electronic Basics' started by Michael, Feb 16, 2007.

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  1. Michael

    Michael Guest


    How do you go about choosing a suitable flyback diode for a 1.8kW 36V motor?
    Does it have to be able to handle above the motor's stall current of 50A?


  2. You might get by with a smaller diode, but you would have to
    heat sink it very well to cover the worst case situation.
    At stall, something approaching 90% of motor current will
    pass through the diode.
    You could parallel the two halves of this one for less than
    $6 from Digikey:

    This one costs twice as much but has more margin (both
    voltage and current):
  3. Michael

    Michael Guest

    Thanks John,

    I'm actually in the UK and it seems Farnell don't sell that one, however I
    used those spec's as a guide and would I be ok using this one:

    I realise the lack of datasheet isn't helpful, but it seems to 'fit the

  4. It looks very similar, except that it is packaged in a
    larger, fully insulated case, that is convenient to heat sink.

    This data sheet may be pretty close:

    My only concern would be that the diode would have to handle
    the voltage applied to the batteries at full charge. But if
    you go with a higher voltage diode for a larger safety
    factor, there, you will probably have to put up with higher
    forward voltage drop, also.
  5. Phil Allison

    Phil Allison Guest

    "John Popelish"

    ** Define the term " at stall " - John !!!!!!

    Remember - Ambiguity is ERROR !!!

    ......... Phil
  6. Phil Allison

    Phil Allison Guest

    "John Popelish"

    ** That is an ambiguous definition.

    The "stall current " of a motor is the current drawn at the voltage supplied
    with the rotor held - it is typically a very large number and the motor
    will not sustain it for more than a few seconds.

    The OP has falsely equated " motor's stall current " with rated, full load
    current ( ie 1800 /36 = 50) .

    Maybe his PWM controller has a built in current limit of 50 amps average -
    if so he needs to say that.

    Otherwise confusion reigns.

    ........ Phil
  7. Stall (as I am using it here) is motor standing still while
    being driven at full rated current.
  8. Michael

    Michael Guest

    I was told by an electrical engineer that when the nameplate on the motor
    states "Voltage 36V Current 50A" this is the stall current.

  9. Michael

    Michael Guest

    Cheers John,

  10. Phil Allison

    Phil Allison Guest


    ** LOL !!

    NEVER believe anything some dumb as dog shit sparky tells you !!

    That 50 amp figure is the max running current the motor can sustain - needs
    to be spinning fast to cool itself too.

    ........ Phil
  11. I think those ratings would be the normal load, full speed
    voltage and current ratings.

    Motors driven by constant voltage (at name plate voltage)
    sources have stall currents that may be 8 to 10 times rated
    current, which would not be a safe stall current for very
    long, and may even degauss a permanent magnet motor's
    magnets, almost instantaneously.

    But if you are driving the motor with a PWM drive, it may
    have a built in current limit function that drops the
    average voltage to the motor to limit the current if it
    exceeds some settable value, and a good current to set that
    limit might be the full rated current.

    Do you know if your PWM drive includes a current limit?
  12. Michael

    Michael Guest

    Could you please explain what you mean by 'normal load'? I would have
    thought that could be just about anything......

    No it doesn't - I'm building the drive :)

  13. I am referring to the full rated load (the torque the motor
    can produce at name plate current) assuming that it is
    getting enough cooling to keep it at a safe temperature.
    The torque a permanent magnet or shunt wound motor produces
    is proportional (roughly) to the armature current.
    The normal range is between no load current (the current it
    takes to spin the motor with no external load) and full
    rated current that drives full rated torque.
    Lets hope you decide to add this feature. It protects not
    only the motor, but the PWM components, battery and wiring,
    as well. The current sense might be based on the voltage
    drop across a current shunt (a low value series resistor) in
    series with the motor, or some magnetic field sensing
    mechanism (like a hall effect sensor) that reacts to the
    magnetic field around a motor lead, produced by the motor
  14. Michael

    Michael Guest

    Thanks John,

    I'm thinking about using a shunt resistor, with a voltage divider either
    side fed into a 741 setup as a diff amp. Would that cause any problems?


  15. If you use a shunt resistor, the full scale signal voltage
    is small, to begin with, just to keep the resistor power
    waste down, so adding a pair of dividers to that, to get the
    signal within the common mode voltage range of the 741 is a
    step backwards. I'm not saying it can't work, but it is a
    hard climb.

    An integrated high side sensor chip is handy for when the
    shunt is in the positive supply side of the circuit. They
    convert the small differential voltage to a current to
    ground, that will produce a much larger, but proportional
    voltage across a grounded resistor, for the current
    controller to use as a process measurement.
    Here is one example at random, that you can use to pull key
    words from, to look for more:

    Isolated Hall effect current sensors are even more
    convenient, since that don't involve the voltage of the
    current carrying conductor, at all.
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