I connected the following AC chopper circuit to control the brightness of a lamp and it is working as expected.

Expected behavior: I observe that when the pulse is 0, the light is completely off. When I connect the pulse to 1, it is glows with full brightness.

Resistive load to AC chopper

When I connected an AC universal motor in parallel with light bulb as shown below, the circuit was damaged.

AC chopper for Universal motor

Explanation of bad functionality: When I saw the circuit is damaged, I moved back to the first circuit with just light bulb. I observe that when the pulse is 0, the light is glowing with low brightness. When I connect the pulse to 1, it is glowing with full brightness.

After the damage, I observed that the gate source voltage is behaving as expected but the MOSFET is not switching off completely.

Please suggest what corrections I can make. I am using a bulb in parallel with the motor to provide freewheeling.

The optocoupler I am using is given below.

Optocoupler chip

Edit: More info:

When I connected the motor, I kept the MOSFET in off-state. As soon as I connected the plug, the motor started rotating. It didn't take any on/off cycles to damage the circuit.

Oscilloscope results: When I checked the wave forms, I observed the following.

The optocoupler output when input it zero (0V).

Optocoupler output when input it 0V

The optocoupler output when input it one (5V).

Optocoupler output when input it 5V

The Drain-Source voltage in off-state

Drain-Source voltage in off-state

The Drain-Source voltage in on-state The Drain-Source voltage in on-state

New understanding: The waveform of gate source voltage should be full wave rectifier, but it looks like a half wave rectifier. So, I suspect the diodes of the bridge rectifier are not working.

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    \$\begingroup\$ It could be the motor draws too much current; it might be there is a large inductive "kick' causing high-voltage spikes across the motor... more info needed. \$\endgroup\$ Oct 21, 2021 at 2:15
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    \$\begingroup\$ What's the turn-on and turn-off time of that MOSFET via that optocoupler? \$\endgroup\$
    – winny
    Oct 21, 2021 at 9:58
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    \$\begingroup\$ Didn’t answer my question. Hook up your oscilloscope and measure Vgs. \$\endgroup\$
    – winny
    Oct 21, 2021 at 14:02
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    \$\begingroup\$ @winny, You are right. Observing with oscilloscope gave a better idea of the problem. I cannot post images in the comments, I am updating information in the question. \$\endgroup\$
    – SKGadi
    Oct 22, 2021 at 13:48
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    \$\begingroup\$ Whoa! Are you sharing ground on both sides of your rectifier? \$\endgroup\$
    – winny
    Oct 22, 2021 at 14:08

2 Answers 2


I think you should place a RC or RCD snubber parallel to transistor DS. Al least look the avalanche rating of the MOSFET, yours has only few tens of mJ energy, also the rated current is questionable - only 2A.


When you switch the MOSFET off, the current has to flow further since the inductor current is always continuous function of time, so it can't drop to zero immediately. For this reason, the Vds voltage rises at very high level until the MOSFET enters in avalanche mode. The entire inductor energy \$\dfrac{L\cdot i^2}{2}\$ is dumped in the MOSFET. If this energy exceeds the max. allowable, then the transistor will got damaged.

Better yet to not use the avalanche mode, rather use a RCD snubber.

Is this snubber needed for MOSFET?

enter image description here

When the MOSFET goes off, the inductive energy is stored into a capacitor, which is emptied upon next turn on - through a limit resistor.

  • \$\begingroup\$ I will update the circuit. However, I want to point the fact that the circuit damaged as soon as I plugged it in. I didn't even switch on the MOSFET. It was in off-state before I plugged in. \$\endgroup\$
    – SKGadi
    Oct 21, 2021 at 13:29

I finally found the answer. Based on the comments and suggestions from the community, I used oscilloscope and tested step by step and found that one of the diodes is failing in the bridge circuit. I replaced the bridge circuit with a new one and it is working as expected.

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    \$\begingroup\$ It is quite likely that a snubber is necessary to drive a motor, as others have suggested. Replacing the rectifier may help temporarily, but without the snubber, you may have a new failure. \$\endgroup\$ Nov 3, 2021 at 15:43

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