I'm constantly reading this fact that a Triac acts as a diode and two transistor. Can someone explain it.enter image description here

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    \$\begingroup\$ Please edit your question with a link to a page that says that. \$\endgroup\$
    – TimWescott
    Commented Sep 28, 2019 at 15:47
  • \$\begingroup\$ I think you may be confused by the fact that, while an SCR is most accurately modelled as two transistors (as @analogsystemsrf shows in their answer), it's commonly said to "act like a diode" when it's on, because an on-state SCR has an I-V curve very much like that of a diode. I'm not sure how best to write this up into an answer, but perhaps other users here could. \$\endgroup\$
    – Hearth
    Commented Sep 28, 2019 at 16:23

2 Answers 2


Triacs are nothing like two transistors and a diode. Read the Wikipedia article or (better, I think) the GE SCR manual ca. year 197x which will explain things better than anyone here is likely to spend the time doing.

There are four possible quadrants of operation. The commonly shown two-SCR "equivalent" (which would be like four transistors) is also very misleading (it fails to explain how Q2 and Q4 work at all (reversed gate polarity from MT2 polarity), even if you correct the SCR to a complementary type).

Look at the layer structure and read the explanations to understand. Some things (like the transistor as two diodes) cannot be even roughly understood by breaking them down into smaller parts (from the GE manual above-linked):

enter image description here

  • \$\begingroup\$ I understand your point. But can we say Triac( not SCR) acts as a diode And two transistor? \$\endgroup\$ Commented Sep 28, 2019 at 16:45
  • \$\begingroup\$ You can say it, but it is rather far from the truth unless you're looking at operation in only Q1 or Q3. And the diode is not required. \$\endgroup\$ Commented Sep 28, 2019 at 16:47
  • \$\begingroup\$ Well, thank u.... I posted the original question I saw on electrical4u website \$\endgroup\$ Commented Sep 28, 2019 at 16:50
  • \$\begingroup\$ Ignore the question, it's wrong. You might get the answer right if you read whatever text had the statement in the first place, but that doesn't make it correct. The SCR is a closer answer, but the diode is not required. A programmable UJT (not UJT) is also similar (maybe that's what they meant, because there might be a need for the diode if real transistors were used). \$\endgroup\$ Commented Sep 29, 2019 at 17:46

The view of an SCR, from what I read (and built, and tested) as a kid, is this


simulate this circuit – Schematic created using CircuitLab

Each side of the model provides voltage gain, and to "turn on" the SCR requires the GainSide1 * GainSide2 be > +1. The gain of either side is gm*Rload, where gm is the transconductance and Rload is 1Kohm for this model. Each side has (-) gain, the product being positive.

We now compute the condition to have |gain] >= 1.

Gain being gm*Rload, we find gm = 1/Rload = 1/1000 = 0.001 amps/volt.

A bipolar has gm of 1 [amp/volt] at 26 milliAmps thru the emitter.

A bipolar as gm of 1/26 [or 0.039 amp/volt] at 1milliAmps thru the emitter.

A bipolar has gm of 0.001 [amp/volt] at 26 microAmps thru the emitter.

However, to achieve 26uA thru the emitter, you'll need about 0.5 volts or 0.6 volts across the emitter-base, which requires 500uA or 600uA current into the Trigger node, because the 1Kohm shunts most of the Trigger current.

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    \$\begingroup\$ Great explanation. But I guess this is about an SCR, but can a TRIAC be said to act as a diode and two transistor. \$\endgroup\$ Commented Sep 28, 2019 at 16:46

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