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DC motor pwm, mosfet barbecue, etc.

Discussion in 'Electronic Basics' started by [email protected], Feb 14, 2013.

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  1. Guest

    Hi,

    I built a pwm circuit (200 Hz switching frequency) using 12V as a supply, also using around 12V to the gate of an IRF530 mosfet. I tested it with an automotive turn signal light (about 1 amp) and it worked just fine (without a heat sink on the mosfet).

    I then tried using it to control a 12V tire inflator/compressor (about 4 amps). The inflator operated fine, but then when I felt the mosfet I was surprised it was so hot it burned my finger.

    Yes I know I should use a heat sink, but I've also heard I should use a protection diode. How do I size this? Won't the power be dissipated (wasted) in the diode anyway, transferring the heat from the motor to the diode?

    Also, is 200 Hz the correct frequency to drive a 4A, 12V DC motor?

    If I increase the frequency, won't the voltage spikes from the motor coils be even higher?

    Thanks,

    Michael
     
  2. Guest


    Correction: "Won't the power be dissipated (wasted) in the diode anyway, transferring the heat from the MOSFET to the diode?"
     
  3. Guest

    On Thursday, February 14, 2013 10:00:29 AM UTC-8, Tim Wescott wrote:


    (snip)

    Ok, thanks, Tim.

    Should I seek a 4-amp diode, since I'm driving a 4-amp motor? What about the voltage? Would a 1N4001 do?

    It's all for a chemistry experiment, really. Just trying to grow some algae to make biodiesel, and shove some air (carbon dioxide) into the algae tank.

    Thanks,

    Michael
     
  4. Guest

    On Thursday, February 14, 2013 11:34:42 AM UTC-8, Tim Wescott wrote:

    (snip)

    Yup, sure is, thanks! When the chemistry gets boring that's when I start digging into the circuits.

    Thanks for the tips!

    Michael
     
  5. Jamie

    Jamie Guest

    At that freq, you could be saturating the coil and your apparent
    speed control that you may think you have is more mechanical than
    electrical.

    It is common to use ~8kHz

    with out knowing more about the motor you're driving, you would
    need to monitor heat output of the motor's core to determine if
    you are reaching too far for the PWM carrier frequency.

    Too many hz will cause heating and efficiency losses.

    Jamie
     
  6. Phil Allison

    Phil Allison Guest

    ** It's probably more than that.


    The inflator operated fine, but then when I felt the mosfet I was surprised
    it was so hot it burned my finger.

    ** Really ?

    Did you estimate the power dissipation using I squared R ?


    ** The correct name is "freewheeling diode" - it greatly increases the
    efficiency of the drive.

    The diode needs to be rated for at least half the max motor current.

    ** No.

    The best frequency is about 2kHz.

    Try a pair of 3A diodes ( 1N4502 ) in parallel.



    ..... Phil
     
  7. Pimpom

    Pimpom Guest

    Cheap and common diodes -
    BY399 (3A 100V fast recovery)
    1N5822 (3A 40V Schottky)

    Some time ago, I made a speed control for a motor of unknown
    characteristics except that it's rated for 24V 3A. I used an
    IRF540 at ~25kHz to avoid the possibility of an audible
    whine. It was part of a more complex whole and everything
    worked fine. The MOSFET was mounted on a plain 1.5mm
    aluminium sheet of approx 15 sq.in. and it barely got warm
    at full load. I used the BY399 because I didn't have a
    Schottky then.

    One factor which partly determines transistor dissipation is
    the gate drive current available, which becomes more
    significant as you increase the frequency. With a weak gate
    drive, it takes longer to charge up the gate and fully
    switch the MOSFET on. At higher frequencies, that transition
    period becomes an appreciable fraction of the switching
    cycle. During that period, the transistor spends some time
    between fully on and fully off states and that's when it
    dissipates the most power. I used about 0.3A drive current
    in my project. High-power high-frequency drives use amperes.
     
  8. Pimpom

    Pimpom Guest

    Typo: BY399 is rated for 800V, not 100V.
     
  9. Nobody

    Nobody Guest

    While conducting, it will need to carry the same current as the motor. To
    be on the safe side, it should be sized for the maximum current you will
    be supplying to the motor.

    If you're thinking of sizing it according to duty cycle, bear in mind that
    the diode's duty cycle will rise to 100% when you switch the motor off
    (even if you do so by removing the power; the energy stored in both the
    motor's inertia and its inductance has to go somewhere).
    The diode will dissipate power equal to the motor current multiplied by
    the diode's voltage drop (this is lower for a Schottky diode than a P-N
    diode) multiplied by duty cycle. This will be much lower than the power
    you're currently dissipating in the MOSFET.
    There isn't a "correct" frequency. Too low a frequency may produce audible
    vibrations. Too high a frequency and you have to start worrying about the
    effect of load capacitance on efficiency.
    No. Without a flyback diode, the voltage will be whatever it takes to
    force the MOSFET to conduct (i.e. its breakdown voltage). With a flyback
    diode, the voltage will be whatever it takes to force the diode to conduct
    (i.e. some fraction of a volt above the supply voltage, depending upon
    whether you're using a P-N diode or a Schottky diode).
     
  10. Phil Allison

    Phil Allison Guest

    "Nobody"

    ** Absurd drivel.

    ** So what ?

    The real issue is a VERY high RMS current flow in the motor and excess
    dissipation in the switch.

    With a sensible choice of frequency, motor current is nearly pure DC.



    ..... Phil
     
  11. amdx

    amdx Guest

    I have noted that most golf cart motor drivers run between 18khz and
    22Khz. Also seen it written that they went that high to get above the
    hearing range.
    Yes, I know these run 200 to 400 amps at 36 to 48 volts, and would have
    more commutations per revolution than a 4 amp motor. Hmm... so maybe my
    point is moot :) I'll let Phil tell me!
    Mikek
     
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