Control relay from 2 circuits

Question

I have a relay that is controlling 220VAC input to a PSU. That PSU is controlling the relay coil at 5VDC so that when the PSU powers off the input is cut as well. Problem is how do I start it in the first place. I can wire up a small 4.5V battery with a button on the relay coil that when I press will allow power to the PSU and then the PSU will power the relay and everything will continue working as planned. I'm worried about combining 2 circuits like that, what I can do is I can put diodes on both the 5V from the PSU and the little battery circuit to not allow current from the other circuit to flow through but I'm wondering would it be a problem if I didn't? I'm not quite sure what will happen but I don't want to find out the hard way.

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. A latching mains power-on circuit.

The circuit of Figure 1 might do the trick.

• A push to make switch energises the PSU.
• A contact of the relay latches the power on.
• A push to break switch unlatches the circuit to power down. Note that this may have to be held for several seconds if the power supply is slow to shut down - particularly when lightly loaded.