# Thyristor/triac gate resistor size

#### punkieys17

Feb 7, 2016
45
So I have a 230v a.c. supply switched pump that uses the exact circuit below. After a major failure the resistor was fried. I am puzzled by my calculations as the resistor in the circuit was no where near the wattage. I take the 230v a.c. and tb at gives me a peak of say 330v. The gate current is shown as between 5 and 50ma. Taking 30ma as the target this gives me 330v / .03=11,000 ohms. However the wattage using v2/r gives 330×330/11000=9.9W. The resistor was more like a quarter watt in size. What am I missing please and how is it calculated

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• triggering-triac-from-a-moving-object.jpg
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#### ramussons

Jun 10, 2014
467
You are missing the fact that once the Triac fires, the voltage across the resistor will be nearly 0 volts!
So the wattage of the resistor would be 0 x 0 /11000 = 0.

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