proposed zero voltage crossing switch

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denison

Joined Oct 13, 2018
328
A triac is pretty slow - it takes about 1us to switch on. During that time your 360Ω resistor will have full mains voltage across it, and will dissipate 150Watts, but 150 W for 1us, 100 times a second is an average of 15mW, and that's the worst case scenario if you were triggering it at mains peak. If you are triggering it at zero crossing, it will not switch on until Vmains/360Ω exceeds the trigger current (35mA). The worst case dissipation is then 440mW, which will last for approximately 175us - average power dissipation about 4mW.
If your triac MT2 terminal becomes disconnected from the load and the opto-triac remains connected, the opto-triac will blow up the resistor, so make sure you track the pcb so that it can't happen.
Also note that the 360Ω resistor must withstand the full mains voltage, so make sure you use a component rated for at least 375V (and that excludes most surface mount resistors smaller than 2512).
To use a 1206 resistor (most of which are rated at 200V), put two 180Ω in series.
Don't quite get your calculations but the key thing is the short time power is applied across the resistor. Don't know the ratings of resistors. I have a 390 ohm 1/2 watt metal film resistor from Jaycar. Can't find the specs for these. Do you think it would be ok?
 

Ian0

Joined Aug 7, 2020
9,667
Don't quite get your calculations but the key thing is the short time power is applied across the resistor. Don't know the ratings of resistors. I have a 390 ohm 1/2 watt metal film resistor from Jaycar. Can't find the specs for these. Do you think it would be ok?
They vary - you'll have to check the voltage specification.
 
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