Is it possible to add a rheostat/pot to a cordless power drill?

Discussion in 'The Projects Forum' started by KDCNewYork, Dec 5, 2013.

  1. KDCNewYork

    Thread Starter New Member

    Dec 5, 2013

    I've been assigned a project that requires the use of an 18v cordless power drill, with the speed controller modified to that the drill can be made to be run at a selectable, continuous speed with minimal user intervention, and (relatively) non-invasive modifications to the drill's innards.

    My quick and dirty theory was that it may be possible to:
    1) Tape the trigger to the 'full speed' position
    2) Install a pot/rheostat on the positive side of the DC motor, therefore controlling the voltage at small increments.

    Am I even in the right ballpark?
    a) If yes, what would be a safe value for the pot for constant use on an 18v drill?
    b) If not, then would you be kind enough to recommend a solution?

    Thank you very much for your time.
  2. crutschow


    Mar 14, 2008
    A resistor in series with the motor would not work well. Such motors use PWM to control the speed which is much more efficient and gives better speed control than a series resistor.

    Better would be to substitute a regular pot for the trigger resistor. Measure the trigger resistor value and buy a pot of the same or close value. Then connect the pot in place of the trigger element. Likely you will need to connect the pot as a rheostat (wiper connected to one end of the pot). Then just adjust the pot to the desired speed and leave it there.
    KDCNewYork likes this.
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Juts taping the trigger meets the goal as it was presented..
    KDCNewYork likes this.
  4. MaxHeadRoom


    Jul 18, 2013
    The 'selectable' speed will will not be regulated without feedback of some kind.
    KDCNewYork likes this.
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
    Your best option is to tape the trigger as described and then remove the battery and connect the speed controller to the battery terminals. The controller should be a PWM type, not a rheostat or a potentiometer.

    I assume by using the phrase 'selectible', that you don't need the exact speed, but just need to vary the speed. And that you don't need constant speed at varying loads.

    If you need constant speed at varying loads, then you need the feedback as discussed above.
    KDCNewYork likes this.
  6. KDCNewYork

    Thread Starter New Member

    Dec 5, 2013
    Thank you, everyone for your time. I really appreciate it.

    The PWM solution sounds like the way to go, but with the project deadline looming, my rudimentary understanding, and (what seems to be) a lack of a turn-key solution available in NYC or on the internet, Perhaps I need a simpler solution.

    Maybe a regular electric/AC drill and a Variac?

    The speed of the drill does not have to be exact- I intend to spin a small driveshaft at low RPM that requires some torque to turn it. I need the speed to be variable because the director of the project is going to see the effect of different speeds on camera. The chuck also makes the drill an attractive option, since I can use it to attach directly to the shaft quickly and easily- not to mention it fits in the budget.

    All suggestions are greatly appreciated. Truly. You have no idea.
  7. KDCNewYork

    Thread Starter New Member

    Dec 5, 2013
    This seems to me the easiest, as I would be simply bypassing the trigger resistor with a compatible pot(as rheostat)
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
    I would put a pipe hanger around the drill handle the one shown below, and use a bolt through it which depresses trigger. the more you screw in the bolt, the faster it goes. This works; I've done it. super simple

    KDCNewYork, shortbus and Metalmann like this.
  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
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