I recently attempted to reverse-engineer a TRIAC dimmer switch that I found in a lamp. The circuit was very simple, and I'll provide a schematic below.

enter image description here

From what I could tell by reading the part numbers, the circuit used the following parts:

  • 250 kΩ linear potentiometer
  • 75 nF film capacitor with a 600 V rating
  • STMicroelectronics DB3 DIAC
  • STMicroelectronics BTA12-600C TRIAC

I recreated the circuit and swapped a few parts for similar but more accessible ones.

  • 250 kΩ linear potentiometer
  • 84 nF film capacitor with a 600 V rating
  • STMicroelectronics DB3TG DIAC
  • STMicroelectronics BTA20-600CWRG TRIAC

I did a PSpice simulation using SPICE models for the DB3TG DIAC and BTA20-600CWRG TRIAC that I found on the STMicroelectronics website, and the circuit behaved as expected in the simulation.

I was confident enough to order the parts and test the circuit on a breadboard. However, the bulb remained on through almost the entire potentiometer range. The bulb started dimming when the potentiometer was < 10 kΩ, and the dimming was sharp and barely noticeable. I verified that the potentiometer was linear and that I was using an incandescent bulb.

Is there anything fundamentally wrong with this circuit or my part substitutions? Is there an issue with implementing this circuit on the breadboard, like parasitic capacitance? I'm surprised this circuit didn't work because it was copied from an existing board with minor modifications and tested in simulation.

  • 1
    \$\begingroup\$ You have your answer below. There should be no need for a 600V capacitor if the circuit is wired correctly since the diac will break over at around 35V and the MT1-gate junctions have very low drop. So 100V should be more than adequate. \$\endgroup\$ Commented Jan 16, 2023 at 19:43

1 Answer 1


You have the TRIAC in backwards. Swap the MT1 and MT2 terminals so that both gate and MT terminals are on the same side of the TRIAC as the capacitor.


simulate this circuit – Schematic created using CircuitLab

Figure 1.

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  • \$\begingroup\$ Yep, that solved the problem! Thank you! I wrongly assumed that the MT1 and MT2 terminals were interchangeable because it's an AC device, and I got unlucky. Do you know the fundamental reason for TRIAC directionality? I imagine this gets into semiconductor physics. \$\endgroup\$ Commented Jan 17, 2023 at 4:43
  • 1
    \$\begingroup\$ See if electricaltechnology.org/2021/08/…. helps. \$\endgroup\$
    – Transistor
    Commented Jan 17, 2023 at 8:29

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