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My controller has a sinking output (aka it supplies ground).

I am able to use an N-channel MOSFET to switch the load as if the MOSFET circuit worked like a normally-closed relay, see diagram below:

Sinking output controlling MOSFET to work as a NC relay

Is it possible to use a N-channel MOSFET to switch the load like a normally-open relay? I have played around with both depletion- and enhancement-type N-channel MOSFETs and neither seem to work.

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    \$\begingroup\$ Can your controller handle +24V on its open drain? \$\endgroup\$
    – Matt S
    Commented Jan 27, 2023 at 21:16
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    \$\begingroup\$ Yes it can, 5-36VDC. It's a BLDC motor controller with some basic IO. \$\endgroup\$
    – jlaufer
    Commented Jan 27, 2023 at 21:19
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    \$\begingroup\$ You should always (whenever possible) provide a link to the datasheet of specific components such as infineon.com/dgdl/…. The ON/OFF labels of your circuits are reversed (although they may indicate your intention and not actual performance). And you probably should have a free-wheeling commutation diode across the inductor (relay coil). \$\endgroup\$
    – PStechPaul
    Commented Jan 27, 2023 at 23:38
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    \$\begingroup\$ @PStechPaul That’s not actually the part I’m using in my project but a part with a PSpice model that works for the illustration. A datasheet would draw attention away from the point of the question I believe. Also if you look at the diagram you see there’s a label for what output I’m talking about. When the switch is closed that indicates the output is on. And yes obviously there will be a fly back diode but why include details when it had nothing to do with the question? \$\endgroup\$
    – jlaufer
    Commented Jan 28, 2023 at 5:44

2 Answers 2

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The "normally-closed" dual of a "normally-open" enhancement-mode MOSFET is, as you noted, a depletion-mode MOSFET. But depletion-mode MOSFETs are pretty uncommon (I've never needed one) and I don't know of any that can run at a continuous 16 A.

Though depletion-mode MOSFETs do have their uses, enhancement mode does fine for load switching. Usually it's as simple as inverting the logic in firmware. Output 0 for on and 1 for off.

If this isn't possible for you, your real problem is that enhancement-mode MOSFET logic is inherently inverting. (Specifically, in this configuration it's a common-source amplifier, which is an inverting amplifier.) Turning on an nMOS with a logic HIGH pulls the output LOW. Turning on a pMOS with a logic LOW pulls the output HIGH.

So if you can't change the firmware logic, just add another inverting stage, as such:

schematic

simulate this circuit – Schematic created using CircuitLab

Something along those lines should do the trick. Closing the switch pulls M1's gate low, which lets R1 pull M2 high. Opening the switch lets R3 pull M1's gate high, which shorts M2's gate to ground.

Make sure you choose transistors that can withstand a \$V_{GS}\$ of more than 24 V1, and your load transistor will also need to withstand a \$V_{\text{DS,off}}\$ of more than 48 V. You won't need a beefy load transistor on the first stage, just pick something that can withstand the voltages you need. I'd also recommend a flyback diode on any inductive load being switched by a MOSFET, otherwise you'll see damaging voltage spikes.


1 The IRF1010EZ you specified in your design can only handle a \$V_{GS}\$ of ±20 V, which is typical in my experience. You will have to limit the gate-source voltage, e.g. with an extra resistor or zener diode.

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schematic

simulate this circuit – Schematic created using CircuitLab

If you can use a PMOS, this should work.

Relay coil on/off Switch on/off

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  • \$\begingroup\$ Please explain reasons for downvote. I realize that the OP specified NMOS, but no reason was given for that restriction. \$\endgroup\$
    – PStechPaul
    Commented Feb 13, 2023 at 19:49

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