optional galvanic isolation, n-channel mosfet?

Hello, I need help with some type of way to connect 2 isolated ground planes in certain situations...

I have 2 circuits on the same board that need to be isolated by default, but have an option to not be isolated and share 5v power in other applications.

The simple solution would be to connect them when needed with 2 sets of jumpers, one for ground and one for 5v. However, I would like an LED to indicate that grounding isolation is DISABLED.

Current needs: The circuit A will need to share 5v DC of 50-350ma of current with circuit B, if isolation is disabled.

My first thought is to use a jumper to connect the 5v rail of circuit A, which will feed 3 things:

  • an optoisolator or similar to connect the 2 ground planes together
  • a resistor and LED to indicate that the circuit A is sharing 5v power with circuit B, AND ground isolation is DISABLED
  • a diode leading to circuit B to share the +5v

My problem is I don't think an optoisolator is the best choice, and can't handle the current. I was thinking about an N-channel mosfet, but I don't know if I can retain safe galvanic isolation when the switch is off.

I am looking for a part that is less than 0.25 USD, if possible.

Any suggestions would be great, thanks!
Mike

Hi, it may be worth just using a DPDT Relay, the LED and its resistor connected across its coil.
Because you will not be sure of the direction of current flow through the gnd to gnd connection and the 5V to 5V connection, you cannot use an opto with a transistor output.
Also the gnd connections will need to be as close a possible to zero ohms to keep all your gnd references.

Tom....... :slight_smile: