Solid state switch or MOSFET: Which is the better relay replacement?

[Jdo300]

Hi, I've been hearing people talk about using solid state switches/relays as replacements for relays on boards and I was wondering if someone could explain to me the difference between a solid state relay and something like a MOSFET or transistor. Why not just use one of those?

Thanks,
Jason O

 

[Averagesupernova]

A true solid state relay is able to pass AC and offers very good isolation between the control signal and the actual switch. A single MOSFET or BJT cannot offer this.

 

[light_bulb]

but two back to back can.

 

[Averagesupernova]

but two back to back can.

Reread my post. This will not accomplish isolation.

 

[light_bulb]

what about a triac with a photodiode on the gate.

 

[Averagesupernova]

Yeah I suppose, I've never looked into it. The original argument was:

Hi, I've been hearing people talk about using solid state switches/relays as replacements for relays on boards and I was wondering if someone could explain to me the difference between a solid state relay and something like a MOSFET or transistor. Why not just use one of those?

So I'm going to stick with that. A triac with a photodiode on the gate sounds a lot like a solid state relay which the OP was questioning the replacement of with a single transistor. A triac is basically 2 SCRs back to back. An SCR is basically 2 transistors arranged in a fashion so they latch on until current is interupted. To provide the isolation needed it takes an opto-coupler which is usually 2 devices, but usually the output is 2 transistors in darlington configuration. So, let's count up the devices. One triac consists of 4 transistors, 2 more transisors for the photo darlington pair, and an LED. That's 7 individual components. Quite a stretch from the OPs idea of a single part.

 

[Jdo300]

Thanks for the clarification on that guys.