For circuits using TTL ICs the supply voltage is a great concern and a slight increase in supply from the rated 5V may damage the IC. Using fuses alone does not solve the problem because a fuse may take several milliseconds to blow off and that’s enough time for the IC to get damaged.
In this circuit a crowbar scheme is used in which a triac short circuits the power supply and burns the fuse. The burning time of the fuse is not a concern because the power supply is already shorted by the triac and the output voltage will be zero. When the output voltage exceeds 5.6 volts the zener diode D2 conducts and switches ON the triac T1.Now T1 acts as a closed switch, shorting the circuit. The output voltage drops to zero and fuse gets burned off. Since the switching of triac takes place within few micro seconds there will be no damage to the TTL ICs or any other such voltage sensitive components in the load circuit.

Circuit diagram with Parts list.



  • Assemble the circuit on a general purpose PCB.
  • If 1A Bridge is not available, make one using four 1N4007 diodes.
  • The trip voltage can be varied by varying the values of D2 and R2.
  • All capacitors must be rated at least 25V.
  • The transformer T1 can be a 230 V AC primary, 12v secondary, 2A step-down transformer.


  1. hi
    my doubt is,using 7805IC which is a regulated voltage so output will be constant.I could not understand how voltage will exceed pls give a solution

  2. pls say why are we using these values for the components which are used in the circuit

  3. can you please help me to have an schematic diagram for a 4.5 volts power supply .. please ..

  4. hi frnds,plz plz can any1 mail me the procedure n the ckt diagram for the design of 5V regulated dc power supply

  5. Chaitanya

    Anyone Please mail me the circuit of variable voltage source that are to be used in lab to give a supply of 0-30v dc from ac