Three phase auto changer circuit


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.

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