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.


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.

enter image description here

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.

  • 1
    \$\begingroup\$ I assume by heal/front/back you mean the NC/NO contacts. NO is normally open and NC is normally closed. When the relay is not powered the common pin is in contact with NC, when the relay is activated NC disconnects and NO connects. What kind of load are you talking about? voltage/current? \$\endgroup\$
    – alexan_e
    Jan 22, 2014 at 0:27
  • \$\begingroup\$ 13V would be used to activate the relay. Current is less than 50mA \$\endgroup\$
    – brett s
    Jan 22, 2014 at 16:23

1 Answer 1


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.

  • \$\begingroup\$ JFETs are like depletion-mode MOSFETs, except that they are readily available, and not usually able to handle high currents. \$\endgroup\$
    – Phil Frost
    Jan 22, 2014 at 3:15
  • \$\begingroup\$ @PhilFrost Depletion mode MOSFETs are readily available, in N-channel, high voltage and reasonable current, if needed. If you need a P-channel, a JFET is probably the only game in town, unless there are some RF MOSFETs I've forgotten about. \$\endgroup\$ Jan 22, 2014 at 3:23
  • \$\begingroup\$ Ok thanks, The current that would be going through each relay's contacts would be at most 5mA. I'll post a schematic of how I'm picturing it. \$\endgroup\$
    – brett s
    Jan 22, 2014 at 16:25

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