The circuit given here is designed by Mr.Seetharaman V.S ( from Chennai.

Seetharaman’s comments about the circuit: This circuit is a modification of  High & Low voltage cut-off with delay& alarm circuit  appeared in Circuits today, which I have tried and found to be quite reliable. You can adopt this circuit with small modification. Use a transformer with secondary 15 – 0 – 15 AC Volt at 500mA, for 18Volt relay operation. Normally any modern electrical / electronic equipment can operate with 230 Volt ± 15% AC supply. That is, it can stand normal voltage operating range of 195 to 265 Volts. It may misbehave beyond this voltage range. You can choose the practical voltage required for the low end high cut off to change over to other phase.

Circuit diagram.

three phase auto changer connection diagram

Connection diagram.

three phase auto changer circuit


  • Set VR3 for minimum voltage to switch on the relay (Say 195 Volt input).
  • Set VR1 to switch off the relay above a particular voltage (say 260Volt input).
  • You can use a 100μF 40 Volts good make in parallel with relay for chatter free operation.
  • Each Phase you have to use one module as above.
  • The relay interconnections are shown in the connection diagram above.
  • T1 can be a 15-015 V secondary, 230V primary, 500mA step down transformer.


  1. Ravindra Gaydhane

    Sir please give a wiring diagram for 3 phase electric connection in home
    Some time 1 phase out 3 is not coming then we can take it from other phase how can we wiring connections for change over switch Please give me connection diagram

  2. please give the 3phase 2phase autochange circuit
    i am villing to the circuit
    It is used Ic details
    How to work the circuit
    Findout on pic or microcontroller used?
    All ready determine the PIC12F508ic
    So the clarified this pic IC

  3. Looking for full battery cut of circuits diagram.

  4. habeebllahi tijani

    pls am not understand how to connect it together with dis 3phase pls explain for me thanks

  5. sir,how can i make use of the relay in this cricuit with a capacitor parallel is it going to step up the voltage to the requirement?

  6. Seetharaman

    Hi Rambabu you may kindly shift the connection for paralleling to other phase after the relay towards the load side. This should solve the problem.

  7. rambabu potluri

    sir in above ckt if r-ph and y-ph fails r-ph will not get power . if 2 c/o relay we can manage even one phase is avalible we can get power on 3 phases

  8. Sir In our school a 2.8kva lpg generator got, kindly give advice how to install the generator to our mains supply at very low cost Ours is a government vocational higher secondary school

  9. Seetharaman

    Hi Segzy this is a relay to run single phase from another phase, equipment when the phase to which it is connected fails. when the relay is on the load of the phase will be connected to the in-comer. when in-coming fails it will change over to next phase. VR1 & VR2 are for voltage, low and high permissible limit set for that phase.
    for the relay operation pl go through the earlier project

  10. Pls sir i ned a bter xplanatn on d rlay intrconectns.hw ‘ll i use it 4 3phas.what’s statmnt i hav 2use one modul.R3 vr1,R5 VR2?thanks