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I am working on a PCB that is powered by a 12 V DC source, in the circuit I have a 12 V DC motor (drawing about 4-6 A continuous), as well as other digital CMOS elements powered by a 5v step down voltage regulator (12 V -> 5 V). I need to create an ON-OFF (non momentary) mechanical switching mechanism which switches the circuit on and off.

The thing is, that looking at SPST switches and other types of high current mechanical switches, they are all quiet expensive for what they do, so I came up with a simple solution that uses a single 30 V, 50 mA ON-OFF pushbutton and a suitable N-channel MOSFET, where instead of connecting all of the source terminals of the components in the circuit to ground, I connect them to the drain of the MOSFET, and the gate of the MOSFET is connected to the 12 V supply through the pushbutton.

Here is the schematic of what I came up with (question below): enter image description here

I know this would work with something like powering a single LED, but would it be ok to do in a mixed voltage circuit, CMOS components and dc motor driver circuit? (assume everything works ok when the switch is removed and every LOAD source is tied directly to ground).

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    \$\begingroup\$ "(non momentary)" For future reference, this is usually called latching. \$\endgroup\$
    – winny
    May 28, 2021 at 11:43
  • \$\begingroup\$ Vgs rules the on/off of a MOSFET. Since you are switching your supply 12V how to you generate the additional voltage required to enhance it. It will partially turn on in your circuit and light LEDS but your motor will cause it to get very hot. Gates are capacitive, it is best to switch then on and off fast and keep the MOSFET out of its linear range. When they are in the linear range they are dissipating power which causes them to warm up. \$\endgroup\$
    – Gil
    May 28, 2021 at 14:45

2 Answers 2

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but would it be ok to do in a mixed voltage circuit, CMOS components and dc motor driver circuit?

Simply, yes.

But I would use a high-side switch instead, to keep the circuit grounded when it's off.

schematic

simulate this circuit – Schematic created using CircuitLab

  • The configuration above allows you to switch the circuit on and off with a logic-hi signal. If you don't want that and if you want to simplify the circuit, you can remove M2, RG, and RGS2, then place the switch between M1's gate and ground.
  • A safer solution is to use back-to-back arranged MOSFETs. But the one above works as well.
  • A PMOS with low RDS-on and enough continuous ID rating is needed.

A side note: When using an NMOS as a switch, keep the pull-down resistor (between gate and source) low. Placing a 1M resistor is nearly nothing different than leaving the gate open. Since an NMOS is a high-input-impedance device, it can still accidentally turn on (e.g. when you touch the SPST) if a high resistor is used. A 10k-ish resistor can be used instead. Plus, it's always a good practice to use a series resistor for the gate, rather than a direct connection from the switch.

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A few traps:

  • inductive loads can burn your mosfet if you turn it off fast enough and there's no kickback suppression diode
  • capacitive load can take a current surge when starting and melt the mosfet
  • 1MOhm and long wires catch noises and may turn the system ON unwantedly
  • 12V Vgs may be too much to the mosfet and it gets easily worse if there's electromechanical loads

I recommend RC filtered Vgs to slow down (see NOTE1) possible capacitor chargings, smaller than 1MOhm resistor to reduce sensitivity to noises (RC filter helps this, too), voltage limitation for Vgs and some protective measures against too high voltages caused by loads.

NOTE1: Needs some calculations to be sure max allowed current is not exceeded, but also not too much energy is dissipated during the transition.

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