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?.
Best Answer
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.
No.
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.