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I am working with the following circuit, which is from the MOC3043M datasheet with a BT137 at the location of the unnamed triac:

Page 7, MOC3043 data sheet

This circuit works fine for me if I leave the snubber out. However, with the snubber in place, the BT137 is on all the time.

I would look forward to any suggestions as to why this is happening.

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    \$\begingroup\$ What exactly is your load when you are making the measurement? And are you using the exact values in the schematic? \$\endgroup\$ – Spehro Pefhany Nov 9 at 0:26
  • \$\begingroup\$ Load during testing is: no load or a 3w LED bulb. \$\endgroup\$ – Martin Rowe Nov 9 at 7:48
  • \$\begingroup\$ Two snubber configurations tried: 1. As shown 39R/0.01uF 2. Roxburgh RE1201 (120R/0.1uF) \$\endgroup\$ – Martin Rowe Nov 9 at 7:56
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I have placed a 0.47uF/450v cap across the 330R (cold side) resistor. Tested by repeatedly applying 240vac while heating (50 degrees C - the max temp my wife's hairdryer will go!) the triac. No miss-firing that I could detect.

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