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small dc motors

Discussion in 'Electronic Basics' started by [email protected], Jan 18, 2006.

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  1. Guest

    Hi, does anyone know how I change the start/stop points on small dc
    motors? I have a few that I need to control at low revs. When I run
    them through a pot, the voltage gets to a point then they just go on at
    almost full speed. Any ideas?

    Thanks, Raul
  2. Lord Garth

    Lord Garth Guest

    Assuming you want to control the motor speed, there are several links
    to simple pulse width modulators...
  3. ehsjr

    ehsjr Guest

    Here's a very simple PWM circuit to control your motors
    at lower RPM. Be aware that these small motors usually
    have some minimum speed that you can't expect to go below.
    For example, don't expect to get 1 RPM out of a motor
    rated for 12000 RPM.

    +12 --+----+-----------+--------------+------+
    | | | | |
    | | | --- |
    [1K] | ------------ /_\ [Motor]
    | +-----|4 8 | | |
    P | | +------+
    50K O<---+-----|7 | |
    T | | 555 | c/
    | \ / | 3|---[150R]---| TIP31
    [1K] --- +-|6 | e\
    | | | | | |
    | +---+-|2 | |
    .1 [C1] | | |
    | +-|5 1 | |
    | | ------------ |
    | .01 [C2] | |
    | | | |
    Gnd --+--------+-------+---------------------+

    Both diodes are 1N4001. Depending on the motor,
    you may need to put the TIP31 on a heat sink. On
    a heatsink, it's good for up to 3 amps.

  4. Guest

    Thanks guys, do you think they will work with lower voltages like 1.5v
    or 3v? The motors are small.

    Ed- is there another schematic of your circuit? I have a hard time
    reading mono-spaced font schematics. I'm not that good yet. I'm sorry.

  5. Jasen Betts

    Jasen Betts Guest

    I've had some success with this circuit

    +V --+--. _.--+
    | \ /| |
    | \ / |
    | ~~~T~~~ |
    | | |
    | | |
    | v | _
    ~ |

    I discovered it before I understood it (by connecting parts more or
    less at random) I used a 2n3055 transistor (because it was the largest
    I had) and put the circuit on a cable with a 2.5mm plug and used it to
    slow a regular tape recorder to about half speed ...

  6. Guest

    Last year a guy named Robert gave me this schematic which I have been
    using for a year now with no problems.
    (, when I try to use it on
    some motors the transistors get hot and burn up. I have been using it
    on a 6v motor out of a cd player with no problems. Can I change this in
    some way so it will work?

  7. Lord Garth

    Lord Garth Guest

    Use a fixed width font like courier to view his post...

    If you use OE, click view, text size, fixed.
  8. Geir_HK

    Geir_HK Guest

    My guess is that the opamp is very fast, and the transistors is very
    slow, resulting one transistor start conducting before the other
    transistor cease.

    It'll be even worse if that opamp is a type of "voltage comparator".
    Then it may switches rapidly between low and hi state (making
    transistors hot as I described above) in order to keep the average
    voltage about equal to center-tap on the pot.
  9. ehsjr

    ehsjr Guest

    I'll send a jpg to your E-mail.
  10. ehsjr

    ehsjr Guest

    Those transistors are too small for driving some of
    your motors. Use power transistors like the TIP31C
    and TIP32A. Put each transistor on a heatsink.

  11. Guest

    Thanks Ed, I got it.

    Here's my last question then I'll go away until I've built it.

    Can I make the circut reversable? So I can control the motor in both

    Thanks, Raul
  12. Ross Mac

    Ross Mac Guest

    If you use a PWM controller the applied DC control voltage, usually +-10vdc
    or less will control the direction and speed of the motor. And on top of
    that it will stop on a dime and if you add an encoder or tachometer feedback
    attached to the motor or device you are driving you will get servo lock at a
    perfect zero volts. Add a computer with a DAC card to read the encoder
    counts and you can close the loop and have full positioning control.....Ross
  13. ehsjr

    ehsjr Guest

    I assume your question applies to the schematic I
    sent to your e-mail - but an answer that will work
    with any DC motor circuit is to use a DPDT switch
    or relay to reverse the wires that connect to the
    motor. The diode across the motor is replaced by
    a resistor-capacitor snubber.

    I'll send a jpg to your e-mail.

    One caveat - there are DC motor assemblies with
    gearing that prevent reverasal.

  14. Bob Monsen

    Bob Monsen Guest

    Look at the output transistors again. It is a voltage follower.

    Bob Monsen

    "a scientific man ought to have no wishes, no affections, -- a mere
    heart of stone"
    -- Charles Darwin
  15. JeffM

    JeffM Guest

  16. Guest

    To anyone who might know. Ed mentioned switching the transistors to
    larger ones, "Those transistors are too small for driving some of
    your motors. Use power transistors like the TIP31C
    and TIP32A. Put each transistor on a heatsink."

    I'm using KN3904(NPN) and KN3906 (PNP), the data sheets on TIP31C and
    TIP32A don't say which is PNP or NPN. Does it matter?

  17. That's funny, the datasheet I looked at clearly says on the first page.
    ;-) TIP31 is NPN, TIP32 is PNP.
  18. But then you should use a motor that costs more than $1 retail.
  19. Ross Mac

    Ross Mac Guest

    Agreed there Homer!....Ross
  20. Guest

    Anthony, you're right. I did'nt see it was listed at the top. Thanks,
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