Inductive load connected to Triac distorting waveform

Thread Starter


Joined Apr 13, 2017
I am using a self designed home automation system(5 loads + 1 fan) where the fan speed is being controlled in five different steps(0 - off,1,2,3,4,5 - full)by zero cross detection & triac firing control method.So far,the circuit is running fine over a year.
Today,i bought my Micsig differential probe(DP10013) & thought to attach it across the Fan to see the chopped waveform.So I did & found that the chopped waveform is being distorted.
The main principle of the circuit is when a zero cross is detected,the MCU will delay some time(for say,5ms for speed 1 , 3.5ms for speed - 4 etc) & turn on the triac by a simple H-L pulse of 100uS.The Triac will turn off automatically during the next zero cross.For speed 5(full speed),the MOC3021 as well as the triac will always stay on.
After the Triac turned on(No problem here),when the waveform gradually reaches zero,then a heavy spike is occuring.I am using BTB16-600BW snubberless Triac with MOC3021 Optocoupler with snubber.
My question is Why this is happening & is it somehow going to create problem in the future either for the Fan or the circuit or is it normal?
I am attaching the circuit & different waveforms,Please take a look.
Another thing I would like to ask that should I replace the MOC3021 with snubberless FOD420?Will that create any difference?






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Thread Starter


Joined Apr 13, 2017
I have two similar setups.In the 2nd circuit,i replaced the MOC3021 with a snubberless FOD420.Also removed the snubber circuit(followed the circuit given in the FOD420 datasheet).
In the 2nd setup,the Fan size is much smaller.Here the spikes are also less in amplitude as compared to the first one.This is mainly happening because of the inductive load back EMF/something like that(I think so) because whenever I removed the fan,the waveform became clean instantly.Take a look below.

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Joined Jul 1, 2009
Pay attention to where you place your probes. If you have one probe inside the parallel circuit formed by the fan (an inductor), and the snubber diode), then you're going to see the inductive kick-back- but your triac won't see it, because of the snubber. If you cannot put a probe between the snubber and the triac (in case the snubber is built in), then you can't see what the triac is actually seeing, you just have to understand what you're looking at- and I think you proved it by removing the fan and seeing the wave-form clean up. No kickback.