This is a bit of a vague question as I'm not 100% sure what I'm asking so I apologise in advance, I'm just hoping to better understand something I noticed the other day.

In my endless tinkering I recently dismantled a cordless power drill to cannibalise the DV motor, as well as the variable speed control trigger assembly to be used on another project which requires a variable speed motor with good torque. The battery power source for the drill is rated at 18V (measured closer to 19V with a voltmeter) and this works perfectly. However, I would like to wire it up directly to a mains plug so that I can run the motor for long periods of time (on the order of a few hours). I happened to have a mains power supply laying around with a measured output of ~21V (rated 24V). I assumed this was probably "close enough" and hooked up the circuit and what I found surprised me.

Under normal circumstances, if the motor/speed controller are connected to the 19V battery supply that it came with, and I gradually depress the speed controller (which I assume is just a potentiometer?), the speed of the motor gradually increases expected as expected. However, when I connect it up to the 21V supply (also DC) and gradually depress the speed controller just as before, the motor doesn't turn at all until the button is almost completely depressed, then suddenly the motor whirls into life at its maximum speed! So when the source voltage is only 2 volts higher, the potentiometer(?) starts behaving more like an on/off switch.

I was not expecting this. Does anyone have some explanation as to why this would be the case? I was expecting little to no significant difference in behaviour, maybe a slight speed increase due to the higher voltage, but for the speed control to just stop working is strange to me.

To be clear, the speed controller still works fine, it's not broken. If I hook it back up to the 19V supply, everything works as normal.


1 Answer 1


A likely possibility is that your AC powered power supply just cannot supply the peak current needed to start your DC motor via its controller. It's called stall current and is usually much higher than full load current.

When any power supply cannot supply enough current, its output voltage drops significantly and now you are in the realm of a thousand possibilities. Check that the power supply can deliver full-load AND starting current for the motor.

Also, running it at 21 volts rather than 18 or 19 volts could be enough, in the long run, the harm the controller and/or motor. You cannot assume that because something is seen to run at 19 volts you can put 21 volts on it with impunity.

Hey it works on 21 volts, now lets try 23 volts etc...

  • \$\begingroup\$ Ah yes! You might be on to something there. In my haste, I grabbed any old power supply with a similar voltage but I forgot to account for the amperage. I believe the DC power supply I used was meant to be used to charge some kind of battery, so perhaps it cannot output enough current to start the motor. I will experiment further, thanks for the insight. \$\endgroup\$ Jun 12, 2017 at 8:59

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