The diacs, because of their symmetrical bidirectional switching characteristics, are widely used as triggering devices in triac phase control circuits em­ployed for lamp dimmer, heat control, universal motor speed control etc.

Although a triac may be fired into the conducting state by a simple resistive triggering circuit, but triggering devices are typically placed in series with the gates of SCRs and triacs as they give reliable and fast triggering. Diac is the most popular triggering device for the triac. This is illustrated in the following applications.

triac lamp dimmer circuit
triac lamp dimmer circuit

1. Triac Lamp Dimmer Circuit.

The circuit for a triac controlled by an R-C phase-shift network and a diac is given in figure. This circuit is an example of a simple lamp dimmer. The triac conduction angle is adjusted by adjusting the potentiometer R. The longer the triac conducts, the brighter the lamp will be. The diac acts like an open-circuit until the voltage across the capacitor exceeds its breakover or switching voltage (and the triac’s required gate trigger voltage).

Diac Heat Control Circuit
Diac Heat Control Circuit

2. Heat Control Circuit.

A typical diac-triac circuit used for smooth control of ac power to a heater is shown in figure. The capacitor C1 in series with choke L across the triac slows-up the voltage rise across the device during off-state. The resistor R4 across the diac ensures smooth control at all positions of potentiometer R2. The triac conduction angle is adjusted by adjusting the potentiometer R2. The longer the triac conducts, the larger the output will be from the heater. Thus a smooth control of the heat output from the heater is obtained.



  1. ahmed samy nasser

    what is the value of the components in heat control circuit
    pls send me by mail

  2. danny boy

    i like this site and i want more information regarding semi conductors.

  3. Hi. What would be the value of the component if tha load is 1500wats.

  4. hi
    what value of capacitor should i use to execute this circiut…?

  5. as what i’ve know,they cannot..because,triac/diac are most oftenly used as triggering device..^^

  6. hellow
    please help me. i need to circuit that control speed of an electro motor with traic and diac. I have water cooler that consist water pump and fan electro motor. water pump has a constant speed but fan motor ( single phase – 1000 Wat – 5 ampere ) has variable speed that both control with single circuit. one bottom for pump and another for fan that control with potentiometer. please send to my Email ”“. very very thanks a lot of.

  7. Can DIAC/TRIAC be used as an voltage regulator like SCR?

  8. Seetharaman

    Hi Blingzy you cannot test a diac with multimeter will show open on both direction, since the test voltage of DMM are not greater than 4 volt. you require around 35 to 40 volts @ 5mA to check them.

  9. can you give some information ?
    especially the procedure .thanksx

  10. pls i need all the application of both traic and daic

    • triac/diac are most oftenly used together as triggering device.
      for example:
      lamp dimmer
      heat control and
      universalmotor speed control

  11. Is there posibility to add some component to use Light dimmer switch for small heater element, pls advice us, because ther is quite difficult to find specified heater dimmer in my area but still a certain circuit component can find.

    best regards,

  12. what is the value of the components in heat control circuit
    pls send me by mail
    t.y this is my mail hijeff_angasmo@y.c tnx

  13. mahetab alam

    i am working on a project titled AUTOMATIC ROOM POWER CONTROL,employing two ldr and IC’s,i want to know that it will work or not?

  14. seetharaman

    Hi Mishra Diac is nothing but 2nos 27-33volt zener diods connected back to back. (DIode used for AC application). If you want to check its continuty with a meter in resistance or in diode check range, it will show open only, due to its break down voltage is of 27 volt minimum on either directions) You can have the data on diac from

  15. Patha Sarathi Mishra

    hi…..i actually need some data for application of daic in industry. if u like to mail me some text i’ll b grateful.

  16. thanks for the circuit especially inthe area of heater. if i want to use this circuit to operate 1500W load what will be the value of components