I've been searching forums for a while trying to figure out exactly how to do this. I want the same functionality that a mechanical relay does.
When the "coils" are de-energized, I want the 'heel' and 'back' to be connected.
When the "coils" are energized, I want the 'heel' and 'front' to be connected.
It's easy to use an N-channel FET and the gate be in place of where the coils would be. So, when voltage is applied to the gate, it allows current through the heel and front.
But can I use a P-channel for when the coils are de-energized and I want the heel and back contacts to be connected?
Any help will be appreciated.
Update
This is what I was picturing. If voltage is applied to the gates, I want the heel to be connected to the front. If there is an absence of voltage, I want the heel to be connected to the back.
Would this work? Where do I need to put additional components and why. Feel free to alter my values. The 1k resistor is to replace the resistance of the coils of the current mechanical relays in place.
Best Answer
You want "form C" capability. Without a lot more information on what limitations you're willing to accept, it is difficult to answer this, but I'll take a crack at it.
Since you are willing to accept an N-channel MOSFET with source grounded as your Normally Open contact, and the Common is therefore grounded, I'll assume you can accept the Normally closed also with the Common grounded. Keep in mind that both loads have to be from some positive voltage relative to ground). The below schematic requires a supply Vg for the N.C. MOSFET gate.
simulate this circuit – Schematic created using CircuitLab
Another way is to simply buy an opto-isolated MOS relay that uses a depletion-MODE MOSFET for the N.C. contact. Using a depletion-mode MOSFET is about the only way to get a closed contact with no power applied.