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Given the circuit below:

enter image description here

(I simply plan to switch the light bulb on/off and I will later add an optotriac to the gate.)

As I understand, the circuit should work in this configuration, and the light bulb should glow. But it does not. When I connect the live, it flashes a little, and then nothing, I tried more times, it does the same.

The resistor is working, I measured after every try. The triac in off state shows low resistance between gate and MT1, all other pins are unconnected. If I replace the resistor with a jumper wire, the light bulb glows, but all the current flows through the wire from live to gate and from the gate to MT1. I also tried a snubber circuit between MT1 and MT2 (100n 100R) but nothing changed.

What could be the problem? Is my triac blown? Is this a working design at all?

(I used this as the base of my desing: http://www.instructables.com/id/Small-Triac-Switch/)

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  • \$\begingroup\$ Re your title .. it is 'triac' not TRIAC. Triac is not an acronym. \$\endgroup\$
    – Spehro Pefhany
    Oct 12, 2014 at 13:25

3 Answers 3

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Here's an invaluable reference which indicates that your load's on the wrong side of the TRIAC and your gate resistance is way too high.

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  • \$\begingroup\$ I tried swapping MT1 and MT2 but it didn't work at all, even with a jumper wire instead of the resistor. \$\endgroup\$
    – lszabi
    Oct 12, 2014 at 13:01
  • \$\begingroup\$ You can't just swap MT 1 and MT2 to move the load, you've got to connect the load between mains hot and MT2, the gate resistor from MT2 to the gate, and MT1 to mains neutral. But... the TRIAC might be toast. Test it by turning it on with a DC supply and load, and it should stay latched until you cut the power to the load. \$\endgroup\$
    – EM Fields
    Oct 12, 2014 at 13:30
  • \$\begingroup\$ @EMFields The only reason to put the Triac referenced to Neutral is to include "safe-ish" unisolated electronic control. To the basic operation of the Triac in a set-up like this or the original schematic, the only thing that can help a few tiny transients here or there is putting the triggering path through the Load first, so flipping the entire Triac drawing here would have gained all the needed advantage of the set-up you describe. As to the paths in this design, there is full agnosticity to phase and neutral, on account of AC. \$\endgroup\$
    – Asmyldof
    Oct 12, 2014 at 15:51
  • \$\begingroup\$ @Asmyldof: You're right; thanks for the reality check. :-) \$\endgroup\$
    – EM Fields
    Oct 12, 2014 at 19:01
  • \$\begingroup\$ @EMFields No worries :-) The original link is a good read for anyone starting out though. \$\endgroup\$
    – Asmyldof
    Oct 12, 2014 at 19:03
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The resistor value at the link you indicate is '470R' which is 470\$\Omega\$, not 47,000\$\Omega\$ (47K).

Your conclusion about where the current flows with 0\$\Omega\$ is also probably incorrect.

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  • \$\begingroup\$ I know the resistor value is mismatching, but according to the triacs datasheet the current would be enough. I tried with 470R 1/4W resistor, but it was blown right away. I don't think that supposed to happen. \$\endgroup\$
    – lszabi
    Oct 12, 2014 at 13:13
  • \$\begingroup\$ You're right, that is not supposed to happen. If the connections were correct, the triac is defective, however they typically fail 'on'. If you put the resistor in there and omit the MT2 connection, a blown resistor is to be expected since the triac can't take over and shunt the resistor. \$\endgroup\$
    – Spehro Pefhany
    Oct 12, 2014 at 13:21
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Turns out my triac was broken. I replaced it with another and now the circuit works properly.

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