I have a 120vac electrical circuit that is connected to a relay and to a on-off switch. When I put the switch to off position, sometimes the 120vac circuit goes in reverse and still feeds power to the relay. What type of diode should I use to prevent reverse voltage?
Welcome to the forum.
The polarity reversal 60 times per second of a 120 VAC line can not bypass a mechanical switch. This is because a mechanical switch when properly wired completely breaks the circuit, just like removing the wire.
AC continuously reverses direction so a diode can not block AC. It can only ever block one half of the AC creating pulsed DC.
There must be some kind of wiring or circuit design error if the switch does not completely kill the AC power.
If you can post your circuit diagram I suspect somebody will quickly spot the problem.