Electronic – the difference between a solid state relay and a transistor


Along with the difference between a solid state relay and a transistor, I'm also interested to know if there are transistors that act a lot like the typical relay modules you can find on ebay (where you only need a 5V signal from the microcontroller to toggle the state of the transistor…) A lot of transistor datasheets I've seen so far have wildly different saturation specifications (where the collector/emitter value is highly influenced by how much voltage is applied to the base), but this saturation idea doesn't really apply to a electromechanical relay…


Doing a search on Digikey's website for solid state relay gave me a list of components that all have input voltages close to 1.25 V… Am I correct in believing the solid state relay is a lot like what I just described?

Best Answer

One essential feature of a relay, solid state or not, is that the input and output are isolated. In practise this means optical isolation in the SSR (solid state relay) case. In contrast, ye olde phashioned mechanical klunkety-klunk relays are magnetically isolated. One could conceivably make a solid state relay using magnetic isolation in various forms too, but optical isolation makes more sense for the requirements.

So solid state relays are more than just a tranistor, triac, or whatever is used to perform the actual switching. They have an isolated input that then ultimately controls the solid state switch. In practice, this usually means at least a LED and phototransitor in addition to the switching element. That is all packaged together and called a Solid State Relay.