Solid state switch vs. MOSFET

  • Thread starter Jdo300
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  • #1
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Main Question or Discussion Point

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
 

Answers and Replies

  • #2
Averagesupernova
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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.
 
  • #3
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but two back to back can.
 
  • #4
Averagesupernova
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but two back to back can.
Reread my post. This will not accomplish isolation.
 
  • #5
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what about a triac with a photodiode on the gate.
 
  • #6
Averagesupernova
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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 alot 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, lets 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.
 
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  • #7
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Thanks for the clarification on that guys.
 

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