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I have designed a AC power control using TRIAC, facing issues while driving an inductive load. In the above attached waveform, I have fired at 15% of half cycle (both positive and negative). The issue arises after that, if I fire at positive half cycle then we can see a spike generated in negative half cycle which is back-EMF generated due to inductive load same happening when fired at negative half cycle. Is there any method to remove this? A snubber circuit is also designed across TRIAC. I tried possible resistor and capacitor combination but it didn't solve the issue. TRIAC driving circuit is attached above. After higher firing angle like 50% the back-EMF starts suppressing the conducted part of the sine wave and hence a lot of distortion is happening in waveform. I am using a voltage based zero cross detection to detect sine wave. I am connecting inductive load across J5 and AC supply across J7.

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  • \$\begingroup\$ Please note that spaces need to be used after punctuation in English. \$\endgroup\$
    – JYelton
    Commented Aug 28, 2023 at 2:15
  • \$\begingroup\$ You mention an inductive load. The waveform shows a normal inductive load. What is your question? \$\endgroup\$ Commented Aug 28, 2023 at 2:41
  • \$\begingroup\$ My only question is how can I remove extra spike which comes when I am firing triac?if I connect an resistive load I am able to fully control the AC sinewave from 0 to 180 degrees.This case is not happening in case of inductive load.I want to supress or remove that extra energy from both cycles. \$\endgroup\$ Commented Aug 28, 2023 at 4:54
  • \$\begingroup\$ What is this inductive load? Because it looks like the power factor is basically 0. \$\endgroup\$
    – Bryan
    Commented Aug 28, 2023 at 4:54
  • 1
    \$\begingroup\$ So you want the first one to look like the second? Cut off this part? i.sstatic.net/eHwCN.png That part is the definition of an inductive load in your circuit; use a current probe to verify proper operation. You don't remove it, you account for it in your control design (say by advancing firing angle the appropriate amount). \$\endgroup\$ Commented Aug 28, 2023 at 8:53

1 Answer 1


It is a normal waveform for a TRIAC based dimmer with inductive load, should be no issue. The TRIAC will switch off when current is bellow holding current, with a resistive load this will happen also at zero voltage. In case inductive load, the inductance will hold the TRIAC switched on some time, depends on inductance, and the only way to switch off at zero voltage is by adding a circuit to force current to zero value. The snubber is used with inductive load to limit dV/dt when the TRIAC turns off (to avoid false firing), and also helps to establish some current (above latching current) when turning on.

If it is mandatory to switch off at zero voltage, you can do it with a MOSFET or IGBT based solution (there are many solid state devices with optocoupler input), but it is necessary to modify control signal, you need to hold it until zero voltage detection. Take in account that you will get a voltage spike when switching off current because inductance.

  • \$\begingroup\$ Or indeed, peak voltage detection if a flux-balanced drive is desired! (The ideal amount of shift can be determined from component L and R.) \$\endgroup\$ Commented Aug 28, 2023 at 8:56

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