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PWM DC motor circuit?

Discussion in 'Electronic Basics' started by Terry Pinnell, Jul 10, 2004.

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  1. Could someone kindly save me a bit of time in finding a suitable
    circuit for driving a DC motor with PWM please? My first hour of
    googling has found many threads, but so far no specific relevant

    The DC motor is from a cordless screwdriver, used originally with two
    NiCad C cells, and duly marked as '2.4V'. I ran it briefly at 5V and
    assume it would tolerate that, giving me higher torque potential. It's
    geared, and (unloaded with 2.4V applied) gives 250 rpm. That's too
    fast for my curtain control application so I want to be able to reduce
    it. Hence PWM.

    I'll probably use limit switches and a C/O relay for the
    forward/reverse/off control, at least while getting the challenging
    mechanicals sorted (as per separate thread). So I don't need to
    consider the complications of combining PWM with an H-Bridge.

    My first thoughts are to use a 5V supply (derived from a convenient
    fairly heavy duty 12V supply I have nearby) with a variable duty cycle
    555 (or maybe a 4001/4011 equivalent), directly driving a medium power
    MOSFET, with the motor between its drain and 5V. I'll breadboard that
    shortly, but meanwhile:
    - will that simple approach be reliable?
    - any snags or protective measures to watch out for?
    - is there an 'optimum' frequency range?
    - if I designed a *high* duty cycle from 555, which I recall makes it
    easier to achieve a wide d/c range, what is most efficient way to
    configure the output MOSFET stage?
    - I happen to have a 2N3055 (or similar) already neatly mounted on a
    h/sink, from a previous project; any major reason why I shouldn't use
    that instead of a MOSFET?

    As mentioned in the curtain thread, I'd like to build this asap, as
    the full-speed motor action is adding to my design and testing

  2. This is a really clever design that works very well indeed in
  3. Al Borowski

    Al Borowski Guest

    that, or a cheap single-chip microcontroller + MOSFET + capacitor/diode
    snubber? =)

  4. The main problem with using a 555 based design is going to be the
    1/3-2/3 range of available supply volts. That may not be a problem in
    your particular case but it's nice to have the full rail-rail o/put
    this quad op-amp gives you, IMHO.
  5. John Jardine

    John Jardine Guest

    My cooker extracter uses a pot', setting a 555 pwm, driving a TIP110
    darlington, directly feeding an ex Alfa-Romeo radiator fan. Frequency is
    arbitrary at a couple of kHz. No heatsinking, no protection or
    jiggery-pokery parts, other than a diode across the motor.
    There must be all of 50p's worth of bits and it's run fine for years.
  6. Thanks. I'm probably going to have to get a bit more complex in this
    case, due to a combination of the reversal requirement plus automatic
    stopping at each extreme. In considering that very simple '555 driving
    MOSFET' appoach, (or the epanorama site version), I'd vaguely thought
    I could implement these additional facilities with a relay and
    microswitches. But of course that's not possible; the motor will be
    permanently connected with one specific polarity, one end to Vcc and
    the other via the MOSFET (or NPN BJT) to ground.

    However, I might *still* build such a simple PWM circuit, purely to
    get the speed down while I experiment. At present, I barely get time
    to apply power before the cord flies off the pulley or fouls up in
    some way!

    But for my final circuit, it now looks as if I'll need an H-Bridge
    approach. That looks more easily done with two NPN and two PNP BJTs,
    but I'd welcome others' views please.

    How's the Alfa getting on without its fan?
  7. Alfas don't have fans anymore. They've all gone over to Ferrari.
  8. John Jardine

    John Jardine Guest

    I really missed that Alpha after they sent it to the
    great-scrapyard-in-the-sky. At the time it had been a useful source of 12V
    project motors and that nice multi-strand cabling that only seems to turn up
    in car wiring looms.

    For the project I wouldn't dream of using H bridges. Damned things are
    always messy, ugly and require effort to make 'em work reliably .
    Why the motor can't be reversed?. Surely just regard the Vcc and fet wires
    like what they basically are, i.e. A pos and neg supply for the motor. Run
    'em through the reversing relays and limit switches as normal. The wiring
    runs will add a bit of resistance and inductance but at (say) audio chopping
    frequencies, the motor won't remotely notice any difference.
  9. Bob Wilson

    Bob Wilson Guest

    Using a microcontroller to replace a simple quad-comparator-based PWM circuit
    (such as the one mentioned) would seem to be a little silly. Not only is the uC
    more expensive than the comparator, it also requires time spent on programming
    (and debugging). The comparator-based PWM is so simple, that it would be up and
    running before the code for a uP version wer even half finished.

  10. Another potentially interesting link I found on the epanorama site was

    Could someone give it a try and confirm that it's inaccessible please?

    Googling on some of the key words from the epanorama outline also so
    far unsuccessful.

    "Motor controller uses fleapower - A simple, permanent-magnet dc motor
    is an essential element in a variety of products, such as toys, servo
    mechanisms, valve actuators, robots, and automotive electronics. In
    many of these applications, the motor must rotate in a given direction
    until the mechanism reaches the end of travel, at which point the
    motor must automatically stop. This circuit implements The design is
    optimized for a supply voltage of 3 to 9V, making it well-suited to
    battery-powered applications."
  11. I read in that Terry Pinnell <[email protected]
    Remove '#dc' from the URL and instead scroll down the page you then get
    (about a kilometre!) and look under 'DC motors' and then sub-heading
  12. Al Borowski

    Al Borowski Guest

    Want to bet on that? :)

    I already have code for simple things like PWM - I'd use a cheap 8 pin
    PIC with ADC ($2 AUD in single quantites). The only parts you'd need
    would be an 8 pin uC, a pot for PWM adjustment, and the FET and snubber.

    I reakon, if I had the parts on hand, I could get it going in 10
    minutes on breadboard. No having to hunt down a quad comparitor or the
    needed resistor values - I just have a heap of cheap micro's for
    jellybeans when I do things like this.


  13. Steve Taylor

    Steve Taylor Guest

    ....and all the software written and debugged.....
  14. Al Borowski

    Al Borowski Guest

    /me points to where I wrote "I already have code for simple things like
    PWM" :)


  15. Thanks John (where've you been?), but that gives same result I got.
    IOW, I end up on page

    which says: "We are sorry, but the article you've requested cannot be
    found. You may try searching for the title in our Article Search
  16. I used to *add* stuff to my cars, rather than take bits away <g>.
    Started more than a couple of decades ago on my own cars, when even
    basics like windscreen wiper speed control were not always provided.
    Later, whenever I got a new company car, I'd have to strip out any
    gadgets I'd added. Mind you, as built-in accessories became
    increasingly generous, there was less need to add my own projects
    anyway. What car now doesn't have a burglar alarm, ice alert,
    sidelights-on warning, etc? In fact, it was only as recently as last
    week that I got around to adding my very first gadget to my '96 BMW
    328SE. It's one of the few things apparently not covered in all modern
    cars: what I call a 'Pedestrian Horn'. A footswitch just above my left
    foot (the car's auto) initiates a rapid and *relatively* quiet
    bip-bip-bip of the horn/siren, to warn little old ladies or horse
    riders etc just around the next bend, or that apparently oblivious
    driver or passenger emerging from a car ahead. Also has advantage that
    I don't need to take hands from steering wheel. The challenge was
    access to the horn wiring and getting it inside the car. (Just a low
    duty cycle astable driving the existing horn relay.)

    Back to curtain motor control. (Can't postpone the admission any
    longer!) I'm embarrassed to say that you're quite right, of course.
    Don't know what I was thinking. Clearly I should be able to use a
    simple single MOSFET or BJT circuit for the PWM speed control, and my
    relays/microswitches for F/Off/R control, as you say.

    Your post couldn't have been more timely, as I was going to get stuck
    in today and build the H-Bridge design I found yesterday in this
    month's edition of EPE mag. That's actually a neat design (NPN & PNP
    pairs), and incorporates all the facilities I want. But it would have
    been time-consuming to biild and test. Thanks for the heads up.
  17. cpemma

    cpemma Guest

    At lowish PWM frequencies (50-150Hz) a simple 555 + switcher transistor
    circuit can give about 5%-95% duty cycle at almost full supply volts, but a
    2-3 opamps will give 100% for little more in parts count or cost.

    Some more circuits at
  18. I read in that Terry Pinnell <[email protected]
    Very busy, and travelling abroad a lot.
    I think you may be picking up a bad proxy. Even the URL with the #dc in-
    page reference works for me. I've copied the text on DC motors and I can
    send it to you as a Word document if you want it.
  19. Thanks, yes please. Very odd why I can't get access to either of those
    yet you can. Must admit I don't really understand what 'picking up a
    bad proxy' means. My personal hygiene is pretty good ...
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