Today let’s talk about a real basic component: Relays. Simply speaking relays are electrically operated switches, that’s it. They are primarily of two types: ElectroMechanical Relays(EMR) and Solid State Relays(SSR).
EMRs are the classic relays that work using an electromagnetic coil to physically move contacts. When you apply current to the coil, it creates a magnetic field that pulls the contacts together, completing the circuit. The contacts physically move, which can create a clicking sound and involves mechanical wear over time, so it’s rated only for a finite number of cycles. Since there is a physical switch, isolation is pretty high between input and output when it is OFF. Another problem is that the switching speed is pretty slow in 10’s of ms.
SSRs on the other hand use semiconductor components like thyristors, triacs, or MOSFETs to switch the circuit. No moving parts. It contains optocouplers(LED-based) to isolate the input and output sides. Much faster and durable than EMRs as there is no movement and are ideal for reasonably fast switching. Silent too. A thing to remember with SSRs is the fact that it can get heated with a larger current flow due to its ON resistance. EMRs also do suffer from some contact resistance, but they are usually built bulkily to wick away the heat.
Now how does an SSR differ from a power MOSFET? In a way, there isn’t much difference except in SSRs your input section has an opto coupler and the switching element can be a thyristor, triacs or transistors. Think of it as a fancy wrapper on Power MOSFETs. Do note that switching freq of Power MOSFETs will be faster than on SSRs as SSRs LED isolation is what is driving the control element. It will take time to switch.
SSRs and Power MOSFETs rule most use cases. I really can’t think of a case where EMRs might win out unless it’s a very large current/voltage scenario. Does anyone know any use cases where they are still useful? Would love to hear about it.
