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Solid state switch vs. MOSFET

  1. Mar 27, 2007 #1
    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?

    Jason O
  2. jcsd
  3. Mar 27, 2007 #2
    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.
  4. Mar 27, 2007 #3
    but two back to back can.
  5. Mar 27, 2007 #4
    Reread my post. This will not accomplish isolation.
  6. Mar 27, 2007 #5
    what about a triac with a photodiode on the gate.
  7. Mar 27, 2007 #6
    Yeah I suppose, I've never looked into it. The original argument was:

    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.
    Last edited: Mar 28, 2007
  8. Mar 28, 2007 #7
    Thanks for the clarification on that guys.
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