Difference between a DC SSR (solid state relay) and AC SSR

Question

I am trying to understand the difference between a DC and AC solid state relay. For electromechanical relays, I notice that they are often rated for some VAC load and a smaller DC load. Solid state relays, however, seem to be rated only for VAC or DC.

What would happen if one accidentally applied a DC load to a VAC solid state relay and vice versa?

Can one convert between each type with the addition of simple components (e.g. diodes, optocouplers, etc.)?

If I have a completely unmarked relay, how can I determine if it is for VAC or DC loads, preferably using a multimeter or similar device?.

Answer 1

What would happen if one accidentally applied a DC load to a VAC solid state relay and vice versa?

An SSR (solid state relay) that uses a triac as the switching element, when fed from a DC supply to a load that takes DC current, will never turn-off once activated. On an AC supply, by virtue of the supply alternating, it passes through zero every few milliseconds and, commutates the triac into an off state when it is no longer being activated by the control signal.

A DC SSR might not be capable of dealing with the reverse voltage situation of AC because it might only use a single MOSFET as the switch. What would happen then is that when the supply reverses, the MOSFET will act like a conducting diode hence, it would be inappropriate to use it in many situations.

Can one convert between each type with the addition of simple components (e.g. diodes, optocouplers, etc.)?

No.

If I have a completely unmarked relay, how can I determine if it is for VAC or DC loads, preferably using a multimeter or similar device?.

The same with any device that has no provenance; it's pretty useless and unreliable in all but the most desperate of situations. Don't get into that desperate situation is my advice. If you need an SSR, buy one that has provenance and a data sheet and one that is sourced via a reliable distributor from a recognized original manufacturing facility having decent quality standards.

What to check for when buying an electronic component or module.

Answer 2

An AC SSR normally uses a triac. When you release the gate voltage, the triac will continue conducting until the voltage across the triac (and the current through it) go to zero.

A DC SSR will use a MOSFET, and this will not provide the latch function of the triac. Furthermore, the MOSTET will only block current in one direction - in the other direction the body diode will conduct, so the SSR will conduct half the time even with no gate drive.

"What would happen if one accidentally applied a DC load to a VAC solid state relay and vice versa?"

If you use an AC supply (not load) with a DC SSR, you'll get power to the load half the time, with the other half of the time being either on or off depending on gate drive.

If you use a DC supply with an AC SSR, you're hosed. Once you've turned the load on, removing the gate drive will not turn off the load - and it will remain on until you turn off the DC supply (or remove the load from the SSR).

"Can one convert between each type with the addition of simple components (e.g. diodes, optocouplers, etc.)?"

Sort of. Sometimes.

If you put AC through a full-wave bridge and apply that to a DC SSR, that will work.

There is no easy workaround for a DC voltage and an AC SSR.