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how to bypass dremel tool internal variable speed control?

Discussion in 'Electronic Basics' started by [email protected], Sep 10, 2007.

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  1. Guest

    I'm on my second electronic variable speed control inside my Dremel
    model 395 tool. This one just crapped out with the same temperamental
    symptoms as the last one. I need to use the tool tomorrow night, and
    would like to bypass the internal variable speed circuitry to simplify
    it; maybe buy an external control later.

    There aren't any wiring diagrams I could find on the Dremel site. I'm
    thinking if I had one for the model 275 tool (single speed) and the
    395 (electronic variable speed), I might be able to make the 395 into
    a 275 by just jumpering some wires. I'll probaqbly need to at least
    retain the variable speed assembly, since the brushes fit into it.

    Can anyone help?
  2. Jamie

    Jamie Guest

    Obvious, it's junk! Buy a new model dremel.
    or better yet, by one of those no name brands
    like I did, looks like it's made in the same place
    but cost much less!
    $29.00 for a complete set of cutters, stones with
    variable speed unit.
    I think it was "All Trade" or "TradeAll" or something like that.

  3. Hi

    Just take it apart, I'm pretty sure it's just a "potentiometer"
    (variable resistor) so it would only have 3 leads, one from the external
    wire to the pot., one from the pot to the motor, and one from the other
    external wire to the other lead of the motor.

    Cut things off, plug the external leads directly to the motor.

    You can make yourself something that would do the same with a variable
    light controller (those you have to control your house lights) Get one
    of these, one wall plug unit and one extension cord.

    PWR IN (1) to VARIABLE (1)
    VARIABLE (2) to PWR OUT (1)
    PWR IN (2) to PWR OUT (2)

    it would be an awesome idea to tape everything together to cover the
    connections once you're done so you don't get shocked everytime you want
    to slow it down or speed it up :)

    Have fun

    (PS I also have one of the new Dremel units and it smokes too ... Uh.)
  4. lj_robins

    lj_robins Guest

    I'm not positive, but I would think if you just soldered a wire around
    the speed control it would run at full speed, I could be wrong though.

  5. Guest

    Just take it apart, I'm pretty sure it's just a "potentiometer"
    Unfortuantely it doesn't seem that simple. The speed control has what
    looks like a some kind of semiconductor/IC (3 leads), a diode, and
    maybe a fixed resistor in addition to the slide pot. I don't know if
    they're doing pulse width modulation or what.
  6. Trevor Jones

    Trevor Jones Guest

    If you cannot figure that out by looking, you probably should not be
    dicking around with it.

    The last fried speed control in a chinese one I bought, took a buck's
    worth of a new triac to fix.

    Trevor Jones

  7. I doubt they have put a stepper motor in there, they wouldn't sell for
    20$ each! -- You still should only have two leads coming out of the
    motor. Confirm?
  8. Btw you said there was a IC ... how many pins does it has, can u give
    out the ID (in case it's a bridge or something so your motor would be DC
    (Uhm?!) and wouldnt be a great idea to plug it right into a wall outlet!
  9. James Sweet

    James Sweet Guest

    Of course it's not gonna be a pot, it would have to be far too big and burn
    up a lot of power. Instead they use what is essentially a light dimmer. The
    semiconductor you see is a triac, the diode is a diac to trigger it, if you
    just jumper together the right two pins on the triac, the motor will be
    forced on.
  10. Ninja

    Ninja Guest

    It sounds like you're describing a light dimmer circuit. See for an example of a typical
    circuit. If your circuit is like that one, running a jumper wire across TR1
    will get you going at full speed. I presume you can figure out which two of
    the three thyrister leads. And you do know your safety rules for working
    with mains powered equipment, right?
  11. the speed control is a triac based phase control, the same as a light
    dimmer - typically it has two wires, just short the two wires together and
    the dremel will run full speed all the time.

    by the way, typical failure is just noisy pot, try cleaning carbon track
  12. Mine had a smd triac BT134W which was faulty.

    I replaced it (easy!!) with a new one, and
    the dremel has worked years after that. The
    triac costs about 1 usd.. The parts are on a
    white ceramic circuitboard.

    Kristian Ukkonen.
  13. Arfa Daily

    Arfa Daily Guest

    Don' it just hurt to the core, James ... ? !!! ;~)

  14. B Fuhrmann

    B Fuhrmann Guest

    hillpc wrote ...
    This is one of those cases where "If you don't know already, you probably
    shouldn't be doing the job".

    The wiring should be simple enough to do it by inspection. If it isn't, you
    really need the schematic and the ability to understand it.

    Unlike the other poster, I really doubt that the motor speed control is just
    a pot.
  15. Guest

    Thanks, folks. This discussion is exactly the type of info I
  16. Jamie

    Jamie Guest

    That's a simple phase control SCR circuit.
    the diode is a DIAC.. etc..
    if it's not firing, I would check the pot and

  17. Every Dremel tool I've had apart used a universal motor, and the
    speed control was a simple dimmer circuit. This one might be PWM, and
    run the motor on DC.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  18. Jamie

    Jamie Guest

    Most of the cordless drills these days use PWM power FeT drivers.

    I modified a cordless drill with a mini PIC and Bridge to
    perform regulated torque control, auto reverse and then forward
    again until maximum torque was no longer peaking. Did this so that
    the drill would have a TAP mode in it. I stuck a mini pot on the back
    side of the handle to set the torque level.

    if his dremel is also cordless, It may also be using it a PWM?
    who knows. how ever, with the part count, I'm guessing he's using a
    corded unit with a phase control.

  19. None of those I've seen used an IC in the speed control. Also, he
    didn't mention a filter capacitor, so id may be a simple dimmer
    circuit. It's hard to tell from such a vague description. Part numbers
    would have been a big help.

    BTW, have you seen the small DC powered clone at Harbor Freight? It
    runs on 12 VDC, and comes with an AC adapter. I was thinking about
    using one (or more, with different sized drills) with a homebrew CNC
    machine to drill PC boards.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  20. Jamie

    Jamie Guest

    Hmm, No, I haven't checked into Harbor Freight in some time how ever, I
    think you'll find that a lot of named brand tools we know are now being
    made by the same people that make the no-name brands from China.
    For example, I have a rotary tool that in all respects is a dremal.
    bu t the name isn't of course.
    As far as drills with PWM, the Craftsman 1/2 drive chuck cordless uses
    PWM driver board which is mounted as part of the trigger. the speed pot
    slider is on the board. It employs an IC chip with a logic level Power Fet.

    We have some electric real movers that are still being modified by the
    manufacturer because they can't seem to get one to last any longer than
    2 months in our shop. First they had drive problems where it wouldn't
    start half the time. This was an elaborate board with a micro driving
    what looked like a Mosfet H-bridge.

    Any ways, we sent them back, the next set that came our way, they
    modified with the speed control in the handle of the unit. All they
    did was employ a speed control trigger slide switch from some existing
    cordless drill system.
    Those were very simply units, a single Power Fet with a 555 timer
    driving it. Not sure if it was variable freq pulsed or PWM? Anyways,
    those have a switch in the slide that initially connected the + batt
    lead to the Vcc and Drain of the Powerfet. the Minimum speed was too
    much on initial start. Those would burned them self's up in the switch.!
    oh well, so much for engineering.
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