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I am trying to protect a MOSFET from motor flyback with two Schottky diodes. There is a DPDT reversal switch to reverse the direction of the motor. Is the included circuit accurate?

MOSFET: 55 V, 49 A IRLZ44N
Diodes: 1N5819
Motor: +12 V @ 3-6 A
DPDT: ON-OFF-ON paddle rated 10 A

enter image description here

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    \$\begingroup\$ Diodes should be serial wired. They short always motor as designed. \$\endgroup\$
    – Antonio51
    Commented Nov 23, 2022 at 1:09
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    \$\begingroup\$ The circuit is as accurate as you’ve drawn it. Will it work? No as you have back to back diodes that will clamp the motor voltage to 0.3V or so. Use one diode from the mosfet drain to 12V or a bidirectional transzorb across the motor or a varistor across the motor. My choice would be a varistor. \$\endgroup\$
    – Kartman
    Commented Nov 23, 2022 at 1:11
  • \$\begingroup\$ Your circuit will go bang due to the back to back diodes. \$\endgroup\$
    – Autistic
    Commented Nov 23, 2022 at 1:48

2 Answers 2

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That circuit will destroy itself the first time it is energized because the diodes are shorting the motor.

You can do this:

schematic

simulate this circuit – Schematic created using CircuitLab

The switch will spark if it's opened under load, but plugging the motor has problems inherently and can damage things. Alternatively you can add diodes from each side of the motor to GND and +12. This assumes the 12V supply has a reasonably large capacitor on the output or is a battery, since some of the energy is pumped into the supply.

schematic

simulate this circuit

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    \$\begingroup\$ The ideal solution might be a full bridge, which provides better PWM control. \$\endgroup\$
    – PStechPaul
    Commented Nov 23, 2022 at 3:20
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As mentioned in the comments, the reverse paralleled diodes will not do as you intend. If you are applying pwm to the gate of the FET, tranzorbs or varisters will just get hot. They might be effective when the direction is reversed by operating the switch while there is still current in the inductor.

Placing a Schottky diode as shown in the circuit diagram below will allow the motor's current circle back to the motor. The fly-back voltage (inductive kick) is caused by the current having nowhere to go. The method shown allows the current to follow a low resistance path from one terminal of the motor to the other.

The Schottky diode also prevents periodic tripping of the protection diodes.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ An RC snubber may work better than the varistor or bidirectional TVS device. But it will dissipate some power in its resistor because of the PWM. \$\endgroup\$
    – PStechPaul
    Commented Nov 23, 2022 at 2:51
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    \$\begingroup\$ @PStechPaul: Agreed. If the direction reversal always occurs when the inductor current is zero, then the transient protection is not required. \$\endgroup\$
    – RussellH
    Commented Nov 23, 2022 at 2:58
  • \$\begingroup\$ RussellH, What TVS would you recommend and is the schottky diode 1n5819 okay? \$\endgroup\$
    – B0SC0
    Commented Nov 24, 2022 at 14:29
  • \$\begingroup\$ RussellH, What TVS would you recommend and is the schottky diode 1n5819 okay? The circuit is powered with a +12v@20A battery. This will be one of three driver boards for three motors that will be wired with fifty feet of 16ga wire to the motor . PWM with a LM555 timer. Motors are +12v@3A. \$\endgroup\$
    – B0SC0
    Commented Nov 24, 2022 at 14:40
  • \$\begingroup\$ Would this be an appropiate size TVS? P6KE18CA...TVS DIODE 15VWM 32.5VC DO15.... 32.5V Clamp 123A (8/20µs) Ipp Tvs Diode Through Hole DO-15 (DO-204AC \$\endgroup\$
    – B0SC0
    Commented Nov 25, 2022 at 14:53

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