With mechanical relays, they can be inserted anywhere in a circuit.
Can SSRs be used anywhere in a circuit or are there certain gotchas that exist ?
Consider this simple example where SW* is a SPST SSR with a on resistance of 3mOhm. Can I expect the the resistors to be shorted (excluding the 3mOhm ) when the SSR fires ?
simulate this circuit – Schematic created using CircuitLab
Yes. In the circuit you have drawn you want to use SSR's instead of mechanical relays as you will be hot switching them (i.e. current or voltage is present when you open or close the connection) and this will decrease the lifetime of mechanical relays. SSR's do not have this effect.
However, mechanical relays have an advantage that when they are open, they are OPEN! You get great isolation when they are open. There is no leakage and low capacitance. SSR's have a tradeoff of on resistance vs capacitance. The product of that R x C is a constant for most SSR's product families. Thus if you want low on resistance, you also get higher off capacitance.
one other advantage of SSR's is they open and close much faster than most mechanical relays. An advantage for mechanical relays is their on resistance is usually much lower than SSR's. So it all depends on what you are trying to optimize for...
In your circuit with your particular SSR, it sounds like you will be OK.
However, this is not true in the general case. Your SSR apparently uses a FET as the switch. Other SSRs use SCRs and other devices. A SSR with SCR switch would not work in your case because of the voltage drop across it. That type of SSR is specified for a maximum voltage drop at some current, not on resistance.
Devices that have different characteristics will be optimal for different situations. In your case you are working with low voltages, so a low Rdson is more important that other issues. If you were switching the AC line to a motor, for example, the SCR type might be better, especially since they turn off at the current zero crossings.
Mechanical relays have their advantages too, like very low on resistance.
Pick the relay type according to what matters in your application. There is no one size fits all answer, which is also why various different relay types exist.
