Maker Pro
Maker Pro

Thyristor/triac gate resistor size

punkieys17

Feb 7, 2016
43
Joined
Feb 7, 2016
Messages
43
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
 

Attachments

  • triggering-triac-from-a-moving-object.jpg
    triggering-triac-from-a-moving-object.jpg
    21.7 KB · Views: 4

ramussons

Jun 10, 2014
448
Joined
Jun 10, 2014
Messages
448
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.
 
Top