I'm having trouble replacing a SPDT relay with a MOSFET circuit.

I have an SPDT relay that serves as an emergency brake of a small motor when the power goes off, shorting both motor inputs.

actual schema

It's slow and I want to change with some MOSFET. I've read about depletion-mode MOSFETs, but they are hard to find for min. 20-30 A current.

I need this relay because when the power goes off, my 12 V motor goes on for some seconds.

  • \$\begingroup\$ Have you considered simply using an off-the-shelf solid-state relay? That would make things easiest for you as it would be a simple replacement of the device (possibly with one resistor added, depending on input type). A slightly more natural method to someone experienced would be to use one FET and one diode, but then you need to drive the FET properly (with a voltage higher than "FROM_DC-CONTROLLER" if it's the more common N-channel variety). \$\endgroup\$
    – Hearth
    Nov 27, 2022 at 19:14
  • 1
    \$\begingroup\$ What do you mean by "slow"? The milliseconds it takes the relay to actuate or the actual braking time? A MOSFET will be similar, maybe not be quite as effective, as a relay contact in braking. \$\endgroup\$ Nov 27, 2022 at 19:18
  • \$\begingroup\$ You are doing what is called dynamic breaking. That does not work with all DC motors. What type of motor is it and post a link to its technical information. \$\endgroup\$
    – Gil
    Nov 27, 2022 at 19:20
  • \$\begingroup\$ Please indicate NC and NO contacts and please indicate the polarity of the DC controller output (as shown in your question). \$\endgroup\$
    – Andy aka
    Nov 27, 2022 at 19:54

2 Answers 2


If you need bipolar control of the motor and braking, then it's worth considering a H-bridge: -

enter image description here

Image from Discrete H-bridge Circuit For Enhanced Vibration Motor Control. So, if you want the motor to rotate one way you'd activate the top-left and bottom-right MOSFETs. If you want it to rotate in the other direction you'd activate top-right and bottom-left MOSFETs.

If you want make it to brake the motor you can activate both lower MOSFETs or both upper MOSFETs. If you don't need full direction control then you only need two MOSFETs: -

enter image description here

Lower MOSFET acts like a brake. This is called half-bridge motor control and, for either circuit, you can get chips that incorporate all the power MOSFETs plus logic inputs for direction and breaking and free-run.


You would need two MOSFETs and a circuit that would apply voltage to the gate of the MOSFET across the motor. But you could accomplish almost the same thing by using a commutating diode across the motor, which will conduct current when the drive voltage is removed. The braking action will work even better if you add a resistor in series with the diode (or NC contact of the SPDT relay).


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