# A question of a controlling PSU safely

Is there a type of relay which is AC powered (does not require a DC supply)? Or any solution to the following issue?

To simplify the question, inside an enclosure there is an SMPS and a module which drives 6A as follows:

I need a external button switch which controls a relay; and when the switch is closed, the line (L) should be off so the system power is off. I could use 12V to power such a relay but the problem is when the PSU is off the relay will be useless.

If I had a non-latching normally open relay and a sperate PSU I could achieve that. But in this case I don't want to use an extra PSU for relay. Is there any work around for this kind of situation?

• Could you simply put the switch in series with the line, or do you need a relay so that you can turn it off with electronics? May 2, 2022 at 12:24
• Google tells me there are relays with 230V AC coils (not sure how a relay coil could work on AC, but if not, you can always rectify it) May 2, 2022 at 12:25
• Safetywise the terminals at the outside of the enclosure should not be AC thats why dont want to put the switch in series with the line. The terminals will be exposed May 2, 2022 at 12:33
• use a switch that doesn't expose the terminals? May 2, 2022 at 12:34
• The L + N terminals shown in your diagram have to be AC. May 2, 2022 at 12:34

simulate this circuit – Schematic created using CircuitLab

Figure 1. RLY1 inverts the logic of SW1.

Relays are commonly available with 120 V and 230 V AC coils.

... when the switch is closed, the line (L) should be off so the system power is off.

I'm just checking that your wording is what you actually mean. In electrical speak a switch is closed when it is conducting. In Figure 1, when SW1 is closed RLY1 will be energised and U1 powered off. (That's what you have asked for.)

• The relay control inputs should be DC in my case(like max 12V) the control inputs will be exposed so it has to be a non-latching DC-AC relay. SW1 terminals in your diagram will be floating outside of the enclosure. I know sounds weird but it has to be like that and SW1 should be safe for person who touch its terminals May 2, 2022 at 12:38

Here's how.

The fully-insulated toggle switch is mounted in a cutout made in the enclosure.

The enclosure should also have a protective earth connection.

Only half kidding but you could use a garbage disposal air controlled switch. They come in a variety of attractive finishes. Photo from Lowes.