Simple DC power delay circuit

jacky May 25, 2010 7 Comments

The circuit diagram shown here is of a simple DC power delay circuit that is based on an SCR. This circuit is a very handy one and can be employed in many applications. The working of this circuit is very simple. When the input power is applied the capacitor C2 charges through resistor R2 and when the voltage across the capacitor just exceeds the Zener diode D3’s breakdown voltage, it breaks down and the SCR H1 is triggered and the delayed power will be available at the delayed OUT terminal.
Circuit diagram.

dc power delay circuit

  • The circuit must be assembled on a good quality PCB.
  • The Zener diode must be rated half the input supply voltage.
  • The current capacity of the circuit depends on the SCR and here it is 4A.

  • Fred
    February 9, 2018

    I build this circuit and it works fine. I do have some questions:
    The role of R2 and C2 is clear. But what is the function of the other components?
    * R1/C1?
    * D2 ? I assume that this diode for the case the circuit that is powered is somehow inductive. In my case that is not the case. Can I take it out?
    * D1?

    Why is a thyristor used and not a plain transistor?

  • abhijeet kadam
    July 13, 2015

    extra information are required of project details

  • kevin
    December 10, 2014

    Why when you say this is very simple, that you dont explain what C1,R1 and D2 do?

  • KI4DKX
    January 20, 2013

    I presume that the zener is equal to the supply in. No comment was made about it.

  • Abel
    February 27, 2012

    Hi, to control the delay time?

  • Santhosh
    May 26, 2010

    How many seconds it can make delay
    please reply…

    • Bob
      July 26, 2012

      Resistor R2 sets the delay by controlling the current that C2 uses to charge. Most likely, increasing the value of R2 will increase the delay time.

      A potentiometer (variable resistor) could be useful for this.

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