Switching AC with Triacs and Zero Crossing Triac Output Optocouplers

One of the simplest ways to switch AC without making too much noise …
↓↓↓ Complete description, time index and links below ↓↓↓

Warning: This video involves AC mains, which can cause serious injury or death!

There comes the time when you need to switch mains AC. Of course you want the rest of your circuit to be isolation from mains AC. And there are many ways to do that, from relays over fancy MOSFET circuits to off the shelf solid state switches.

An inexpensive, simple solution is to pair a Triac with a suitable optocoupler. That is if you can live with a some losses. As always, the devil is in the details. Who would have thought you can spend twelve minutes calculating two resistors

Intro …
00:00 Intro – a real world example and the obligatory warning
01:03 Requirements – isolation, zero cross switching and resistive loads only
03:04 Alternatives – there are many, an optocoupler with a Triac is one of the simplest
Design …
04:02 Circuit – optocoupler, Triac, two additional resistors and you’re done
08:51 Parameters – drive voltage, max AC peak voltage and max AC RMS current
09:38 Gate resistor – limiting current through the optocoupler and the Triac gate
16:25 LED resistor – trigger the optocoupler without damaging the microcontroller
21:40 Heatsink – keeping the Triac cool enough
Test …
24:35 Setup – there’s a lot on the bench
26:58 Basic function – it works just fine
28:29 Zero crossing – it switches at the right time
Caveats …
29:06 Zero crossing – is not what is says, it’s just an approximation
30:55 Current spikes –when driving a capacitive load, even with zero crossing
31:19 Switching power – even worse spikes up to maximum AC peak voltage
33:14 Heatsink – it’s not just the thermal resistance of the aluminum profile
Wrap-up …
34:27 Wrap-up – that’s all folks

Optocoupler, photocoupler, optoisolator, opto-isolator, optical isolator,

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