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Reducing DC motor speed w/min components

Discussion in 'Electronic Design' started by DaveC, Mar 10, 2007.

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  1. DaveC

    DaveC Guest

    My new cordless screwdriver runs on 7.2vdc Li-Ion batteries (pm brush motor).
    It's not variable but runs at one speed (2000rpm) which is nice when you need
    it, but for many tasks it's way too fast.

    I like everything about this tool except the speed. I'd like to add a second
    speed, half or maybe slightly slower than half.

    Obviously space is at a premium in cordless tools like this, so minimum
    component count for this modification is paramount.

    I know I can just hang a large resistor off of the battery and a large switch
    for routing current through it, but I was hoping for a solid-state solution.
    Is there any advantage to using a bipolar or MOSFET or such over the
    bog-simple resistor method? (ie, it requires a smaller switch; a transistor
    might be easier to mount externally than a resistor to cool it, etc.).


    Those who think a PIC can be adapted to this requirement, please be prepared
    to show your work. (c:

  2. A resistor will result in reduced torque as you apply load, as it will
    waste more power just when you need it. Something which drops a more
    constant voltage would be better, such as a zener diode, or just 4 or so
    regular diodes in series, and on a heat sink. Another way would be to
    use just one of the two Li-Ion cells, which will give half voltage.
    I'm not sure what effect it would have on the charging if one was
    significantly more discharged than the other.
  3. Lionel

    Lionel Guest


    (1) Solder all the I/O pins together & attach a wire. (2) Solder Vcc &
    Vdd together & attach a wire. (3) Wire as many such chips in series
    with the battery as required to get the motor speed to where you want
    it. ;^)
  4. Jamie

    Jamie Guest

    use a 555 timer, a logic level high current Fet and pulse width drive
    the motor, this is commonly done in a lot of variable speed drills.
    the electronics is built in the trigger.
    50% duty cycle gives you half speed.
    the idea of using PWM is that it's either on or off, analog control
    creates heat build up which is energy wasted and less regulation.
    You can get fancy enough with the controller and monitor the voltage
    output between the on pulses to use that has a regulator. The Motor will
    regenerate back voltage and is a good way to test actual speed. THey
    call this Armature feed back in most cases.
  5. DaveC

    DaveC Guest

    use a 555 timer, a logic level high current Fet and pulse width drive
    What makes a FET "logic level"? Is 2N7224 (a.k.a. IRFM150) a logic level
    device? I've got a few of them...


    Vds is 10v, so does this rule it out for use with 7.2v?
    Yeah, I was wondering about heat. Sounds like pwm is the way to go. Now, how
    to fit it all in...
    Don't want to get too fancy... just slow it down. Accurate speed regulation
    isn't too important.

  6. Jamie

    Jamie Guest

    Logic level Fets come in different ranges, but you should get one that
    turns on at about 3 volts. the logic fets normally turn on fully at that
    the Source of the Fet should be on the common side of the supply and
    assuming that you are using - volts for the common side, you use an N
    channel type.
    the Drain of the fet will connect to one of the armature leads while
    the other motor lead connects to the + side of the battery.
    use a diode across the armature leads to remove kick back high voltage.
    the cathode would connect to the + side of the armature.
  7. Salmon Egg

    Salmon Egg Guest

    It takes two lithium cells to get 7.2V. Use a single cell and add the means
    to have the single cell connect properly. That will give you half speed.

    -- Fermez le Bush--about two years to go.
  8. DaveC

    DaveC Guest

    It takes two lithium cells to get 7.2V. Use a single cell and add the means
    A really simple solution. But the battery pack is modular (encapsulated) and
    would require modification of the pack. Also, would result in imbalance of
    charge in the cells. And from what I understand about Li-Ion chemistry. they
    don't balance very happily during charge.

    I think a 555 PWM solution is in my future.

  9. DaveC

    DaveC Guest

    Jamie sez:
    This is now my plan. The only question remaining is re. reversing direction.

    How is motor reversing accomplished in commercial cordless drills? I see 2
    wires connecting the battery to the speed circuit/trigger, and 2 going to the
    motor. How can I do it simply?


  10. Lionel

    Lionel Guest

    The simplest possible way is to add a DPDT switch immediately before
    the motor, wired to reverse the polarity.
  11. ISTM that the right solution here is a plug-on
    reduction gearbox. Doing it mechanically would
    also do the required increase in torque. If there
    is not one on the market already then someone is
    missing a commercial opportunity.

    Say a 2.5 or 3:1 ratio, input/output shafts inline,
    make both shafts as hex sockets so that it can be
    used to increase or decrease speed. Supply a hex
    bar to couple the screwdriver/drill and gearbox
    hex sockets.
  12. Palindrome

    Palindrome Guest

    A two pole c/o series/parallel (slide) switch... and the modification is
    usually pretty trivial. The slide switch can be very small and won't
    require anything like the space that an electronic unit (including its
    own switch/control) will need.
  13. John Gilmer

    John Gilmer Guest

    Using only one of the two cells can cause problems as you end up with one
    cell completely discharged in series with a partly charged cell.

    You don't need a zenner diode. You just need one or two or three "regular"
    diodes. Each will drop from .5 to .9 volts depending upon diode size and
  14. DaveC

    DaveC Guest

    A two pole c/o series/parallel (slide) switch... and the modification is
    Thanks, Sue.

    Modifying the battery does look like the obvious route. As does the
    series/parallel switch (what's "c/o"?). I'll have to find one to handle max
    current, of course, in such a small package...

    The main objection I have to applying this mod is that it means I'll have to
    rely on my memory to place the switch in series before placing the pack in
    the charger. Charging two 3.6v lithium-ion cells in a 7.2v charger is Not A
    Good Thing (sm). I'll have to balance the pro's and con's. Unless you have an
    idea to help protect the cells & charger from that possibility... (c:

    FYI, it's this tool:
  15. jasen

    jasen Guest

    2000 rpm 7.2V cordless screwdriver? I don't beleive it.

  16. DaveC

    DaveC Guest

  17. That's an impact driver and I suspect that it needs the
    rpm to get the inertia to ride through the impacts. Its
    the impacts that torque the screw, not the direct motor
    speed/torque. Reducing the speed may not have the effect
    you desire.

    Notice the next driver down, which is not an impact, and
    which does the nearly the same torque by means of a larger
    step down in the gearbox.
  18. DaveC

    DaveC Guest

    Thanks for your comments, Tony.
    I want to double the use of this tool by providing lower speeds. Yes, the
    impact is useful for driving/undoing tight screws, but it's useless (way too
    fast) when, for example, installing small screws in covers on residential
    light switches or receptacles.

    By providing low-torque low speeds, I make this tool useful for small screws,
    too. Yes, I could buy another tool to cover that purpose, but for a few
    bucks, (and having a bit of fun in process of design and implementation) I
    can do this.

    (And I already own it...)

  19. DaveC

    DaveC Guest

    use a 555 timer, a logic level high current Fet and pulse width drive
    But this means that another switch must be put between the battery and the
    PWM circuit to turn it off. This spoils my "plug n play" solution. I was
    hoping to simply insert the circuit between the battery and the existing
    reversing switch but now must either add an on-off switch or...?

    I thought over the ideas suggested by some re. a rotary switch and dropping
    diodes. But I can't find a small rotary switch to handle the current (3A
    constant, 5A peak on start) -- there's precious little room for a HD rotary
    in there...

    I like the MOSFET w/speed pot idea best, but
    1) it gives only 90 percent max, so I thought to use a switch on the back of
    the pot to short across the MOSFET to give 100 percent
    2) the circuit needs a switch on the battery supply conductor to turn it off.
    Would be nice to use the switch on the pot, but I'd like to use that for max

    More ideas?

  20. DaveC

    DaveC Guest

    You don't need a zenner diode. You just need one or two or three "regular"
    I've thought about this method (without zeners). If each diode drops ~0.7v,
    to give 4 speeds (full speed plus 3 speeds obtained with diodes) with 7.2v
    battery would mean 3 diodes / speed, so:

    low speed 1 = 3 diodes (2.1v)
    low speed 2 = 3 more diodes (4.2v)
    medium speed = 3 more diodes (6.5v)

    That's 9 diodes in a pretty small space.

    Doesn't using a 1.8v zener (or one close to this v) for each 3 diodes perform
    the same purpose (with the added advantage of more effectively giving me 3
    evenly-spaced speeds)?

    Configure as a "diode ladder" and pick off voltages to switch to the motor? I
    presume I'll have to use a high-current zeners.

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