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:
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.