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VERY simple dc motor drive circuit?

Discussion in 'Electronic Basics' started by John E., Mar 2, 2007.

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  1. John E.

    John E. Guest

    5vdc/670mA max miniature gear motor (load is such that current will be far
    below 670). I want to set speed with pot control and leave it (may replace
    pot with fixed R when best speed is discovered). Forward only, no reverse.
    Just need single transistor, if possible, to do the work. Will use wall wart
    p.s. for power.

    Speed accuracy, etc. not necessary. This is for friend's art exhibit.

    Google turns up an extraordinary number of patents, data sheets for quite
    bright ideas using controller ICs, H-bridges, digital controls with feedback,
    etc. Much more than I'm looking for.

    Your ideas or pointers to designs greatly appreciated.

  2. Tim Wescott

    Tim Wescott Guest

    Set up a 555 timer to be PWM controlled from the pot (there are circuits
    floating around, ask back here if you can't find one).

    Drive the transistor from the 555. If I wanted the World's Easiest
    Circuit I'd use an N-channel MOSFET and drive it slowly (100 to 1000Hz)
    -- that way I could go straight from the 555 output to the gate of the
    FET. If you use a 12 volt supply you won't even have to get a special FET.

    - motor
    PWM ^
    generator |
    .--------. |
    | | |-+
    | | |
    | |------------->|-+
    | | |
    '--------' |
    (created by AACircuit v1.28.6 beta 04/19/05


    Tim Wescott
    Wescott Design Services

    Posting from Google? See

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at
  3. Palindrome

    Palindrome Guest

    How about a length of resistance wire and a crocodile clip slider? When
    you find the right point, chop the spare resistance wire off and replace
    the crocodile clip with a permanent connection. Or a loudspeaker
    volumer control rheostat? Or add rectifier diodes in series, one by one,
    until the desired speed is obtained. Or a few 1.2v christmas tree light
    bulbs, adding more until the speed is good - they can even be "power
    indicators". Or some quantum tunneling tape or pills and stick weights
    on it until the speed is right?

    It really doesn't need an active device acting as a variable resistor
    when a real variable resistor will do the job nicely.
  4. John E.

    John E. Guest

    Palindrome sez:
    Thanks for your reply, Sue.

    If I could just put a pot in series and adjust that, I'd try it. But it looks
    like whatever controls it will have to dissipate ~5w. A bit much for a cheap
    pot. I could just buy a bunch of hi-W resistors (or -- as you brightly
    suggest -- diodes) and mix and match to see what speeds I could cobble up.

    But I was hoping for a bit more elegant solution which will allow the artist
    the option of choosing the proper speed without me needing to change the
    resistor (or diode) so she may see "how that looks".

    Once a speed is settled on, it will probably remain fixed for good, but
    getting there -- I'd like to have a bit of adjustment available, so I don't
    have to be around during the trial.

    Thanks again,
  5. Palindrome

    Palindrome Guest

    12w 1 p rotary switch + a handfull of diodes/resistors?

    You can get 5W* pots, no problem - as I suggested, have a look at
    speaker volume controls. * and higher..
  6. Guest

    Use Tim Wescott's ckt.
  7. Phil Allison

    Phil Allison Guest

    "John E."

    ** Use an LM 317 voltage regulator IC after the wart.

    Allows the voltage to be smoothly varied from 1.2 volts up.

    Works just as good as PWM methods - but with more heat loss.

    ........ Phil
  8. PeterD

    PeterD Guest

    One (power) transistor, one small pot. Think 'adjustable voltage
    regulator' the first power supply every experimenter builds.
  9. jasen

    jasen Guest

    here's one I have used in the past, only 2 parts

    +V --+---. _.------ > to motor
    | \ /|
    | \ /
    | ------- large NPN
    | | (or mid size with heatsink)
    | |
    | V
    1K lin |
    gnd ---------------+---------- gnd

    not particularly well regulated
    I used a surplus 2n3055 transistor and a 1K pot and was able to slow down
    the motor in a tape deck and record stuff at a lower speed....

    if you need more precision you may consider using a variable voltage
    regulator or a 555 based pwm.

  10. Marra

    Marra Guest

    I would use a PIC with a PWM output driving the motor through a
    transistor pair.
  11. Rich Grise

    Rich Grise Guest

    You can get high-power rheostats, but they're spendy (~US$30.00 new at
    Maybe shop around your local surplus dealers.

    Good Luck!
  12. Rich Grise

    Rich Grise Guest

    There's a new convention around here. Henceforth, any person posting "Use
    a PIC" without providing your code, circuit, and development kit is
    to be automagically deemed a "Troll".

  13. Ken Smith

    Ken Smith Guest

    You may be pushing the limits on a LM317 depending on the wall wart you
    use. Fortunately they protect themselves quite well.

    The LT1083CP is a lot easier to get the heat out of and doesn't need as
    much head room. The parts count to use it is very low. You have to have
    10uF at least on the output.
  14. Ken Smith

    Ken Smith Guest

    No, you only need one transistor. You can use a LM555 to level shift up
    to drive the gate of a MOSFET. The voltage is much too low to get an NE-2
    into the circuit too.
  15. Sjouke Burry

    Sjouke Burry Guest

    Put an emittor follower behind the potentiometer.
  16. bw

    bw Guest

    You want one part called a rheostat. Assuming the motor near full load
    at 500 mA then the resistance will be E/I = 10 ohms. Filament control
    rheostats were often zero to 25 ohms at around 5 watts. Odds are you
    will end up using a 10 ohm resistor. If thats not enough, try 20 ohms
    or so.
  17. Jamie

    Jamie Guest

    That's all well and fine how ever, a must smaller and more
    ergonomic approach would be to use a 555 driving a Hex Fet which
    can all mount on a back of a panel mount pot.
    More than likely the Fet will not need a heat sink.
  18. John E.

    John E. Guest

    Jamie sez:
    Looks interesting.

    Wiring details, please?

  19. Jim Thompson

    Jim Thompson Guest

    The fundamentals of constant speed motor control are shown at...

    The only caveat is that the external RM may not track the internal RM
    over temperature.

    Some day, when I'm in the mood, I will ponder that problem ;-)

    ...Jim Thompson
  20. Jamie

    Jamie Guest

    I don't have any ASCII translation program here to post
    prints in the NG.
    It's a very simply circuit using a 555 timer varying the
    TH (threshold) charging time via a POT to vary the time
    width of the timer.
    Output can directly drive a Hex fet gate. The source
    would be on the common side (-) and the Drain on one motor
    lead while the other motor lead is connected to your + supply.
    Also, make sure you have a protection diode across the motor
    leads for reverse discharge from the coils of the motor to protect
    the FET.
    Look at Digikey or mouser for Hex fets or Logic Fets in the
    N channel family.
    you want one that can turn on at aprox 50% of your Vcc.
    Below links points to a print that uses a non logic level
    fet and some bias resistors.

    THe 555 timer will handle up to 200 ma output, so if you're
    driving a light load, you may not even need a transistor.
    also, you could use a NPN transistor in Emitter flower mode
    to drive the motor..
    it's your pick.
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