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Old 7th Dec 2017, 11:05 pm   #21
Refugee
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Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,549
Default Re: Mini-drill; Speed Controller Problem

Just get any DC motor and connect a low value resistor in series with it and a low value capacitor across it.
On a scope you will see something like an unregulated DC signal.
Pick off the AC with a coupling capacitor into a high gain amplifier so that it clips to a square wave.
If you can get a PWM chip to track it to match a variable oscillator you should be able to get a wide range of loading at a constant speed that is adjusted by varying the oscillator.
If you can find a drum motor board off an old VCR you might be able to reverse engineer it should work. It will work like a DC servo.
Another solution would be an F to V converter and a POT on regulated DC driving a PWM chip.
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