Controlling Speed of Power Tools using SCR

By reading this article, you will be able to build a variable speed setup for,
Angle Grinder
Wood Working Routers
Air Blowers
Chop Saws
Circular Saws/ Skill Saws
For Mini Precision Tools with AC Motors

This setup also can be used for the below purposes, Dimming circuit for Filament Bulbs
Dimming circuit for GLS Bulbs
Can control Kitchen Mixer

This circuit is basically a voltage regulating unit, which works on SCR (Silicon Controlled Rectifier). Currently, in the market, there are two types. One is rated at 2000 W and the other at 4000 W. You can make a decision depending on your power requirement.

Note: This circuit will not work with ‘Power Tools’ which have the following features. Speed Pre-Selection, Soft-Start, Restart-Protection. If this circuit is bypassed, it will work.


Work on a table. Not on the floor. If a person gets in contact with 220 V, the electricity makes the person weaker. The brain tries to break contact due to reflex action. If the person is lying on the floor, he won’t be able to gather up the energy to escape the situation. Hence work safely.

The SCR unit produces heat. Provide ventilation to avoid damage to the circuit.

Which SCR to buy?

Like said, SCRs are available in different power ratings. Lowest at 2000 w and up to 4000 w or more in future. Previously these circuits had a higher cost of production. Today, technology has advanced and these SCRs are easily available for the end consumer.

Purchasing an SCR solely depends on the power consumed by the device to be used with. As an example, an angle grinder with a power rating of 600 watts will consume more or less 600 w in usage. But the initial current will be high that we need to take care of. Hence it is always better to consider twice as much it consumes during working.

SCR Speed Control For Angle Grinder and other Power Tools with Universal Motor

A 2000 watt SCR will run smoothly with a 1000 W load without much heating. I personally use a 2000W SCR to control the speed of my 5 inch 1200 watt Angle Grinder. I had built this circuit in 2018. Still running fine. No issues whatsoever.

Wiring the SCR

In this project, it’s recommended to provide earthing to the SCR body is made of metal (Metal enclosure carries a very small leakage about 20-30 volts if measured against earth) While preparing enclosures, make sure the terminals don’t short with each other due to the enclosure. Make connections according to the below diagram.

SCR Speed Control For Angle Grinder and other Power Tools with Universal Motor Wiring Diagram Setup

Tip: No need to connect the neutral to the SCR. The neutral terminal on the SCR is commonly connected. Electronic circuits like SCRs, DC Voltage Boosters, Buck Converters do not necessarily need the Negative input.

Configuring The SCR for Speed control

You need to set the minimum voltage output in the SCR using the screw on the W103 Potentiometer (Blue Colored) which is already mounted on the circuit.
(A easy to understand video link has been provided at the bottom)
Before you do this, you need to connect the circuit to the mains, the output to be measured with a Multimeter, and the device to be used with the SCR should be connected. You can use a GLS Bulb too. If you connect an Angle Grinder, remove the disc, flange, and nut. Remember not to place the circuit on a metal table while configuring it as this will short the circuit and burn it. 

Note: The minimum voltage output depends on the AC voltage input. Assume, the day you set the SCR if the input voltage was 250 volts, and you set the output to 50 volts, then you will always receive 50 volts as output unless and until the input varies.

Now, at some point of the day, if the input voltage drops to 230 volts, ( Yes! Voltage will vary. It’s not in your control) then the output voltage of the SCR drops too. Don’t decide that the SCR has gone kaput. 

Tip: If no device is connected to the SCR output, you will receive full AC output itself. 
Watch this simple video demo. Please ignore the background noise.

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6 thoughts on “Controlling Speed of Power Tools using SCR”

  1. Thanks for the video, I just got a 2000watt scr from Amazon, the same model shown in this video. It works fine on my stanley 550 watt drill, but there is no need for it as I can simply control speed by selecting speedknob or pressure on the trigger.I bought a CUMI marble cutter cg125 hoping that with this scr I would be able to cut wood as well, but unfortunately there is no speed difference at all, the drill goes from zero to full speed but this cutter stays at full speed . Can you check this at your end with some other marble cutter and confirm whether this works ?

  2. Technically it should work for any Universal Motor. This circuit works on all my power tools including marble cutter too which I use on wood. Looks like you have not set the potentiometer to minimum volt output. While the cutter connected, adjust the POT for minimum RPM. I think its already in its max setting

  3. Thanks for your advice, it was my fault, I had tested earlier with a stanley drill and set the potentiometer for that particular drill, it was after you commented that i had the idea to reset the pot and start once again so it works now the control window is small just a small turn takes from zero to full, but thats fine for me.

  4. Hi,

    Pl confirm why the same will not work with soft start or restart protection grinders.

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