I am trying to design a circuit that protects 127V appliances if plugged in a 220V socket (in Brazil 127 and 220V sockets have the same shape and it's easy to plug a 127V appliances in a 220V socket by mistake).

The circuit should be self-resetting without intervention so a standard crowbar is not ideal because of the fuse which must be replaced. The appliance is fairly high power (1200W) so I couldn't find a resettable fuse that can handle the voltage and current. I came up with the following circuit, which is supposed to turn on the TRIAC when the supply voltage is 127V and turn it off when the voltage is 220V. R2 represents the protected appliance: Overvoltage protection with a TRIAC

My idea is that when 127V is applied, there will be around 3.35V at the output of the LM7805 and the transistor remains "off". Then the gate of the TRIAC is connected to the output of the 7805, which I hope would cause it to trigger. If, however, 220V is applied, the 7805 will output its full 5 volts. This will turn on the transistor, which in turn will pull the gate of the TRIAC to 0V and thus will prevent it from triggering. However, when I try to simulate it in Multisim, the TRIAC remains off no matter what the input voltage is. I think the transistor switches on and off properly though.

I must be missing something significant, but I can't figure out what. I hope someone can point out where my mistake is, or if this circuit makes any sense at all. I am open to all suggestions, including for completely different designs.

Thank you all in advance.


2 Answers 2


However, when I try to simulate it in Multisim, the TRIAC remains off no matter what the input voltage is. I think the transistor switches on and off properly though.

The signal that attempts to activate the triac (Q1 emitter) is referenced to a floating supply and isolated from the triac (in all other respects) by the transformer.

enter image description here

This means it can never push/pull current through the triac's gate.

I'm not ruling out other problems too but, once you have this corrected your sim should indicate other potential issues.

  • \$\begingroup\$ Thank you for your input. I thought there was something wrong with the way I'm trying to drive the TRIAC. So basically I should get rid of the transformer? \$\endgroup\$
    – tungsram
    Feb 5, 2019 at 13:07
  • 1
    \$\begingroup\$ I think you have bigger problems than just replacing the transformer. For instance, shorting out the live/neutral with a triac doesn't sound a great idea to me unless the 50 ohm is meant to be your load? Try it in your sim and see. \$\endgroup\$
    – Andy aka
    Feb 5, 2019 at 13:10
  • \$\begingroup\$ Yes, the 50 ohm is supposed to be the load. \$\endgroup\$
    – tungsram
    Feb 5, 2019 at 13:15
  • \$\begingroup\$ How about a normal wide range power supply? \$\endgroup\$
    – winny
    Feb 5, 2019 at 14:36
  • \$\begingroup\$ There are space constraints. It should be a small PCB that ideally can be integrated into the appliance. \$\endgroup\$
    – tungsram
    Feb 5, 2019 at 14:49

It might be easier & better to use a relay with a 120VAC coil. Energize the coil only if the input voltage does not exceed some threshold. If the turn-on transient triggers your triac, you may damage the connected device before you even get started.

  • \$\begingroup\$ I made it with a normally closed relay and the coil is energized if the input voltage exceeds the threshold. However, I am still looking for ways to do it with a semiconductor instead of a relay. \$\endgroup\$
    – tungsram
    Feb 6, 2019 at 14:49
  • \$\begingroup\$ I would suggest a normally open relay, otherwise you will briefly get overvoltage on your output, which could damage something. \$\endgroup\$ Feb 6, 2019 at 15:59

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