Snap Acting Relay Circuit

When a Relay driver circuit is used in Sensor based electronic circuits, false triggering may occurs due to unwanted relay clicking. The input signal level is not sharp in most sensor circuits and the transition stages are slow. So the relay driver transistor partially turns on/off . The relay coil will not become fully energized leading to vibration of the relay contacts and heating. A Snap Acting relay driver with Schmitt trigger action can eliminates this.

The diagram below is the relay driver circuit with Snap Acting for the clean switching of the relay. When the input signal at the base of T1 decreases, it turns off allowing T2 to conduct by giving base current through R3.When T2 conducts, its emitter current increases and this raises the voltage at the emitter of T1. This rapidly reduces the base current of T1 and it switches off rapidly. When T1 turns off rapidly, T2 turns on faster and relay energize. This snap action reduces the time gap between the working of T1 and T2 and prevents relay chattering.

Snap Action Relay Circuit

Note: The value of R2 must be higher than the resistance of the Relay coil to give more current through R5 when T2 turns on and less flow when T1 turns on.


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  • Mihai

    There’s something wrong with values of R3, R4.
    Q2 can’t saturate with only 56mV in its base. As as matter of fact you can get rid of these two resistors. This scheme (R3, R4-divider) is a legacy from germanium era.

    The value of BE(base-emitter) voltage for germanium transistors is comparable with Vce_sat (around 0.15V), that is why a resistive divider was necessary for cutting-off T2.

    With silicon transistors you can easily obtain Vce_sat around 100…200mV which is
    much less than Vbe(on) of approx 0.65V.

  • Ameer

    Dr Mohan kumar
    You provoid a great and useful information in electronics I realy appriciate you.
    Thanks and keep it up.

  • Sivanand.V

    The transistor T2 will never be switch on because of very low value of resistance R4 make it at least 4.7 KILO

  • Ed

    the time delay relay circuit you posted ,,, PAGE, does not fully open,,had to go to another circuit page to send this to you,,it opens but only too the bottom of the intro paragraph and no further,,no schem/circuit

    • D.Mohankumar

      Dear Ed
      The Time delay circuit, is posted recently. The page is opening in my pc.Try once again to see whether it is getting or not

  • D.Mohankumar

    The Relay break or close suddenly under tension.This prevents relay chattering.

  • D.Mohankumar

    The Word Snap is used to describe – To pull apart or break with, move swiftly and smartly, give way abruptly under pressure or tension, open, close, or fit together with a click, A sudden breaking, Made or done suddenly,

  • Boris

    What is snap action? Snap = break, click, crack or what?