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I want to create a solid state relay kind of circuit. The plan is to allow signals to pass through when active and not when inactive just like a standard relay. The issue is that I want to do this with a couple of signals, not just one. My idea is to connect N-channel MOSFETs like this that are driven with an VOM1271 optocoupler/MOSFET driver to allow current in both directions. This is one of the standard applications for the VOM1271 so I expect it to work with one MOSFET pair but what I am not so sure about is how the circuit behaves when there are two pairs (for two independent signals). Will they be interfering with with each other since they share the source pins in the middle or will they work fine. As an example, if I want to control both the power and ground pins of some device, will they short?

Circuit diagram

Will I be able to send two different signals on the different pairs? Like a serial TX through 1_A <--> 1_B and RX through 2_A <--> 2_B?

I could always go with one VOM1271 for each bidirectional pair but they are quite expensive relative to other chips so I would like to avoid that if possible

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  • \$\begingroup\$ Your MOSFETs are connected via the sources so everything (both "relays") will short together once they turn on. I see you're using low voltage FETs. If you don't really need the galvanic isolation from the VOM photocoupler, there are cheaper solutions. \$\endgroup\$
    – bobflux
    Apr 8 at 21:17

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Will I be able to send two different signals on the different pairs? Like a serial TX through 1_A <--> 1_B and RX through 2_A <--> 2_B?

Absolutely not without massive corruption.

If circuit_A does not have exactly the same applied drain voltages as circuit_B then, it ends in a mess. Imagine both sets of MOSFET are active (very low resistance) then, the common source connection is connected to V_A and to V_B hence, V_A has to equal V_B or you get a massive cross current flow.

It works when the MOSFETs are deactivated but, that doesn't give you much to be honest.

As an example, if I want to control both the power and ground pins of some device, will they short?

They sure will.

I could always go with one VOM1271 for each bidirectional pair but they are quite expensive relative to other chips so I would like to avoid that if possible

How deep is your pocket?

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    \$\begingroup\$ I was afraid so, thanks for the explanation \$\endgroup\$
    – ErikM
    Apr 9 at 4:12

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