I'm currently in the process of developing a protected 12v- 250mA (current limited) DC supply. The problem is I need to design the supply so that if 240vac (RMS) is connected across its output terminals (ie the 12v250ma output) the power supply circuit is protected.
I'm not talking about surge or transients here either - i do mean 240vac across the outputs for extended periods of time (ie minutes/hours).
After the 240v is removed the supply should go back to normal operation.
I was thinking perhaps a solution could be to use something like a trisil in parallel across the outputs, with a series polyswitch to the load. That way the trisil will take the inital hit, after which the polyswitch will heat up and disconnect the circuit.
Anyone have any tips / solutions. I have attached my simplified serial tx circuit along with the idea i just put foward.
One problem i can see with the polyswitches is that the ones rated at 240vac seem to be few an far between. At the general parts outlets, RS and Farnell, their polyswitch range seem to go up only to 60vac or so.
Look foward to some replies.