need simple circuit to control speed of grinder

Thread Starter

Rusty Rifle

Joined Dec 18, 2019
hi all
i found this circuit on YouTube
it should handle 2000 W
but when i connected my 900 W grinder
the POT has burned
do you have better circuit diagram ?294218239_534921988407836_2089208242378261045_n.jpg
I've never built or studied this type of circuit and so am not an expert, but here are some thoughts. In your sketch the pot looks like a little trimmer pot. If so, try a larger rotary or slide switch pot that can handle more power. The circuit doesn't look like the pot gets much current, but it's worth a try. (The maximum current will be when the pot is at 0% rotation.)
The capacitor is marked with 275 V rating. At 220 Vrms line, the PEAK voltage is 311 V (220 x 1.414). Could the capacitor be breaking down? You should replace it with a higher voltage capacitor, at least 350V.
This circuit is for dimming lights. Incandescent lamps are resistive. You are controlling a grinder, which likely has significant inductance. Inductances create large voltage spikes when they are switched off. Maybe this is causing the problem. Have you tested it using a 900 W resistive load like a heater, broiler, or 900 W of incandescent lamps?
If it works with resistive loads but not the grinder, you could try putting small capacitances across the grinder (rated at least 350V). If that doesn't solve the problem, you probably need to find a circuit designed for inductive loads like motors.


Joined Aug 7, 2020
The only capacitors I have ever seen marked "275V" are Class-X types, and they are rated at 275V AC. The voltage across the capacitor is limited by the breakover voltage of the diac, so even a 50V capacitor would not fail in this circuit.
The circuit is also used for speed control of universal motors. Most bench grinders have universal motors. Really big ones may have induction motors - you are unlikely to be able to speed-control induction motors successfully with this circuit - you need a variable frequency drive.
Inductances do create voltage spikes when they are switched off, but when controlled by a triac, they don't switch off until there is no current flowing, at which point there is no energy left to create a spike.
A snubber across the triac would be a good idea - 100nF Class-X in series with 100Ω 2W - but it will work without it especially if you have triac with the W suffix.


Joined Jul 18, 2013
Induction motor phase control, is usually limited to smaller motors, e.g. fans and small/light load applications. ~ 1/4hp max.