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