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R85 in the following circuit keeps getting burned out when the photo-triac is turned on. It may last for a few minutes before burning out. It is a smallish 1/10 W resistor, while R83 is in DIP package.

I am confused about what's causing R85 to exceed its power rating. AC_HOT is 220 VAC. The load is an inductive load (an incline motor, rated at 15W).

Is there anything in the following circuit that does not make sense? Any pointers are appreciated.

enter image description here

BT137-600E datasheet

PC1S3063NTZF datasheet

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  • \$\begingroup\$ Where is the load connected? \$\endgroup\$ – Andy aka May 14 '13 at 11:36
  • \$\begingroup\$ The load is across LUG10 and AC_NEUT. I have edited the post. Thanks. :-) \$\endgroup\$ – Brian Wang May 14 '13 at 11:54
  • \$\begingroup\$ If you include links to the datasheets for the components you're using, it makes it a lot easier on people who might be interested in answering your question. \$\endgroup\$ – Dave Tweed May 14 '13 at 11:57
  • \$\begingroup\$ How is the gate current supposed to be limited? Aren't you going to get 10ma * 220V = 0.2W through it? \$\endgroup\$ – pjc50 May 14 '13 at 12:11
  • \$\begingroup\$ @DaveTweed Thanks. I will include the links next time. :) \$\endgroup\$ – Brian Wang May 14 '13 at 12:18
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The datasheet for the triac says the average gate power is .5 watts. You need a resistor rated for at least that. Also, If we look at the Triac wikipedia it discusses that 3Q triacs are designed to operate heavily inductive loads. This would explain it functioning better. Normal TRIACs or 4Q triacs have problems due to high voltage and current angles when they turn off at zero crossing. They experience a voltage spike which can turn them back on again. Commutations (▲v/▲t) can be improved with the use of snubber networks. They are designed to provide at need to help balance the circuit. With the introduction of 3Q triacs this problem was avoided. If shopping for a device like this you can simply search for "Snubberless Triacs" which is what they are often called.

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There is nothing wrong with your schematic that I can see. The gate resistor should only dissipate a few milliwatts average, since it stops conducting once the triac turns on. (10mA for a couple microseconds every 1/2 cycle).

What do you mean "DIP" package for R83- I would expect that to burn up first if it had a similar rating.

I wonder if you have the triac wired incorrectly, or if one of the parts is defective.

Make sure the triac wiring is correct. The view of the package is from the top.

enter image description here

If the triac gate is shorted to the MT1 or the triac is damaged such that it never triggers, you will see something like what you have observed, however R82 will see a lot more power dissipation.

If you still need help, posting a photo of the circuit would not hurt.

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