Sep 012014

TRIAC. Power control on alternating current (AC)

Triac is a semiconductor device that belongs to the family of thyristor control devices. It is essentially two SCR connected in parallel but on the opposite direction and they share the same gate. (See the diagram).

Triac is only used with alternating current and like the SCR, it is triggered by the gate. Triac works with alternating current, so there will be one part of the wave that is positive and one negative. The positive part of the wave passes through the triac as long as there is a triggering signal on the gate, so the current will flow from the top to the bottom (The current passes through the SCR going downwards).

Triac Symbol

In the same way the negative part of the wave passes through the triac as long as the triggering signal is applies to the gate, so the current will flow from the bottom to the top going upwards. In both cases the triggering signal is obtained from the same pin (the gate).

It is possible to control the moment the trigger happens (The moment the signal is applied to the gate) and the time each SCR will be ON. (Remember that a SCR is ON only when it has been triggered (activated) and there is a minimum positive voltage between anode 1 and anode 2 for each SCR.

Then, if we can control the time that each SCR is on, it is possible to control the current that is delivered to a load and therefore the power it requires. For example: A very common application is the incandescent lamp dimmer (a phase control circuit).

Incandescent lamp dimmer - TRIAC application


  • Vin: the applied voltage to the circuit (AC)
  • L: lamp
  • P: potentiometer
  • C: Capacitor
  • R: Resistor
  • T: Triac
  • A1: anode # 1 of Triac
  • A2: anode # 2 of Triac
  • G: Gate of the Triac

TRIAC waveform

Triac controls the flow of the alternating current that passes through the lamp (load), moving continuously between the “on” state (when the current flows through the triac) and the “cut off” state (when the current does not run through the triac)

If we move the potentiometer knob, we can change the charging time of the capacitor making the phase difference between the power supply voltage and the voltage applied to the gate to increase or decrease.

Note: the phase difference between two signals or waves is the angle (time difference) between the two signals when they cross the x axis (the time axis).

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  2 Responses to “TRIAC or Triode for Alternating Current”

  1. Can we reduce power consumption by dimming the lamp with this circuit ??

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