How to Design a dimmer that don't depend on the load's resistance

Thread Starter

Bassem Faris

Joined Jun 22, 2022
Hi all
I have an angle grinder which have Maximum power of 950 watt
And my country provide 220 volt as voltage source
But it have one speed and i want to make a dimmer to control its speed.

I used this circuit but it provides at least 130 VAC
It make the machine rotate so fast
How can i make a dimmer to provide zero volt at least?

Note : i used big heat sink IMG20220623004544_BURST000_COVER.jpg


Joined May 3, 2013
Controlling a motor by voltage control is not a preferred option. At lower voltages, the motor may not have enough torque and it will stall.
A PWM driver will be a much better option if it is a DC motor.
A VFD drive will be fine for an AC motor.


Joined Jul 18, 2013
Triac controllers for Universal motors are plentiful and cheap, you do not really have to build one.
But why do you need speed control on an angle grinder?
Look up Onsemi/Fairchild AN3003 & AN3006
Also a fancy one
Also you may find it an advantage to place a suitable bridge rectifier before the motor.
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Joined Nov 6, 2012
There are many "Tool-Hacks" You can find on the Internet that quite often
use super-cheap and plentiful Angle-Grinders for a Power-Source.
But what many people don't realize is that, as a general-rule, they are either OFF or ON,
with lots of quirky-behavior and complaining anywhere in-between those 2 states of operation.

For an application that requires slow, smooth, controlled, application of Power,
they can be rather radical and uncontrollable, ( and very noisy ).

Smooth-Control, while retaining adequate Torque to actually do some work, requires RPM-Feedback.
Without RPM-Feedback, if You slow down the Motor, then apply a Load, it will just stall-out.


Joined Jul 18, 2013
The feature of the series field/Universal motor is that it has the advantage of high torque down to low RPM, one reason why they were/are used as automotive starters.
The reason is that as the motor-load/current increases, this causes a correspondingly higher field current, hence higher torque.
In some instances, removing the load altogether can cause the motor to destruct due to over-speeding. They should always have some kind of residual load.
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