# Using low voltage switch for high voltage application

I want to know if I can use the following switch to basically disconnect the 80V AC source.

The current through the switch should be 8mA. However, this is only for a few milliseconds.

I know this switch is rated for 16V @ 0.1A.

De to the time being short, how would I calculate if the switch will handle 80V AC at 8mA for 50 milliseconds?

simulate this circuit – Schematic created using CircuitLab

• According to your schematic, the pushbutton will get the full current from the relay coil when you push it. That's almost guaranteed to be more than 8mA.
– JRE
Commented Jun 6, 2023 at 16:01

You have most of the right parts in the wrong order. If you want the switch to disconnect the load using the relay, then you would have to wire something like this up:

simulate this circuit – Schematic created using CircuitLab

It's missing a few components: a snubber or TVS to deal with the relay coil inductive pulse during switching which, as @Christobol Polychronopolis points out, will not be kind to your switch; and I would throw in some kind of fusing depending on the strength of the source. SW1 must be rated for at least 80VAC (120V or 250V switches are common and cheap, but 8mA is a very small load, less current than the relay coil takes and probably even less current than most SSRs, so there's really no need for a separate relay unless you're married to that switch, in which case you'd have to add in some way to drop the input voltage to where the switch could handle it and size the relay coil to match.

8mA is also low enough current that minimum switching current would probably come into play. Most electromechanical switching elements (switches, relays, etc) have a minimum current rating to prevent the buildup of and/or clean off oxides that may form on the contacts. For low-current loads, special measures such as mercury wetting or gold alloy contacts must be implemented to ensure reliable performance.

It will not survive long. The voltage rating is an indication of the gap between the contacts. At 80VAC, it may arc before you press it, or it may continue to conduct through the plasma when released. Both will wear down the switch, and eventually it will increase in resistance or fuse closed.

As pointed out by JRE, you're counting the 8mA coming through the 10K load, but not the current through the coil. That will probably be higher, and in its current configuration with no flyback diode, it will certainly result in voltage spikes even higher than 80V. These will be even more destructive than the 8mA arc.

In this schematic, it's unclear why you're using the relay at all, since the power to the 10K load would be the same if you eliminated the relay and attached it directly to your 80V. Unless you're really using the NC contact, in which case the 10K load never gets power, except for briefly between the time the switch closes and the relay opens.

This switch, however, still will not work. :-( You're better off finding a high-voltage device with a switching threshold that your switch can handle, and use the switch to turn the HV device on and off.

You could use a hipot or insulation tester to determine the voltage at which the contacts arc or start drawing current. The specs show dielectric breakdown of 250 VAC but that does not mean the contacts can withstand that. You will also need to add a snubber across the relay coil sufficient to keep flyback transients as low as possible. And also make sure the relay draws well under the rated 0.1A. Please provide a datasheet or part number for the relay.

It would probably be best to use the NO contact of the relay to latch it on and disconnect the load, so that the switch will only draw current for a short time. Something like this:

simulate this circuit – Schematic created using CircuitLab