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SPDT Solid-state Relay

  1. Sep 9, 2008 #1
    Hello All,

    I have been trying to think of a way to make a Single-pole double-throw solid-state relay circuit out of some MOSFETs that I have laying around. I determied that I wanted to this SSR to be bidirectional so that it will behave as much like a mechanical relay as possible. I first started with a SPST version using two Mosfets with their source pins tied together and used as a ground reference for the driver IC, which has it's own floating power supply (battery) referenced to the same point independent of the load circuit. I then took it a step further and added another pair of MOSFETs and a driver so that the setup could be driven as the compliment of the first pair. I drew a circuit diagram showing my idea but I'm wondering if it is ok that both drivers are referenced to the same floating ground. Could someone take a look and let me know if this is a good way to do this?

    Jason O

    Attached Files:

  2. jcsd
  3. Sep 9, 2008 #2


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    Science Advisor
    Homework Helper

    From what I can see the two MOSFETs connected to B are shorted out of the circuit by the common ground.

    Just a guess, but you may want to look into H-Bridge circuits.

    IIRC they do make a variant of a triac that can be turned off by control signal.
  4. Sep 9, 2008 #3
    Yeah I thought there was something funny there. Good catch. The triac idea sounds interesting but i really want to stick with the use of Fets because of the fast switching times. As for the H-Bridge design, thats an interesting approach as well but then I'm not sure how to make it bidirectional, and I'm not sure that the general setup woud be good for a bidirectional switch type of setup.

    Are there any circuits out there that use FETs in an bidirectional SPDT configuration?
  5. Sep 9, 2008 #4


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    Staff: Mentor

    You would make such a switch out of an analog switch topology, with a mix of n-channel and p-channel FETs. Here is one example of a SPDT analog switch from Maxim:


    Unfortunately, the datasheet doesn't show you the FET topology....

    This one has a better internal schematic (although drawn kind of funny):


    And this is the basic analog switch schematic that I was looking for (but ignore the "amp" at the bottom):

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