IMPORTANT UPDATE (Update 6), scroll down to see.

I have been trying to make an AC (air conditioner) soft starter for my home AC unit.

I have built the following circuit and connected it to my AC unit compressor. As soon as the AC starts the TRIAC fails and becomes a short between T1 and T2 pins. I have tested the circuit with smaller loads like a small 1 HP air compressor and it worked fine.


enter image description here



Any ideas on why the TRIAC keeps failing?

Update 1:

I forgot to mention that the AC unit has a big capacitor on it but I couldn't read it's value.

Update 2:

AC unit specifications: Phase 1, Volts 220-240V, FREQ 50Hz, Max Input 15.8A

Update 3:

Start-up waveform:

The sequence starts like this and the on time keeps getting longer until it gets to the end which is always on.

Green is the Zero cross signal from the optocoupler. Yellow is the TRIAC_PIN

enter image description here

Later on:

enter image description here

Update 4:

The 'Soft start' effect on the scope with an air compressor: The differential probe is on a 1 ohm shunt.

Without soft starter: enter image description here

With the soft starter:

Current rises slowly over time. enter image description here

Update 5:

Here's the current (yellow) and voltage (green) waveform on the compressor:

enter image description here

Update 6:

Okay, so I decided to scope the AC unit without the starter and results were quite different, have a look the following:

The current shunt is 0.1 ohm.

AC unit startup: enter image description here

Normal Operation: enter image description here

So, apparently the startup current is almost around 100A (70A RMS)! with 150 - 160 phase angle between current and voltage. Under normal operation it consumes 23A (16A RMS) but the current wiggles! have a look at this video.

Update 8:

I sacrificed another TRIAC to check it whit scope and see what is destroying it but, I couldn't find anything.

Current = yellow x 10, voltage = green x 10 enter image description here

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    – SamGibson
    Jun 18 '21 at 17:55
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    – SamGibson
    Jun 18 '21 at 17:55

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