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I'm relatively new to using motors and electronics, and wanted to know if this was possible. I am trying to understand the motor circuit of a PCB I am analyzing. The PCB doesn't use a FET H-Bridge but rather two SPDT relays. I realized however that with this configuration you cannot use PWM to control speed. It may still be functional, however, because the application only uses a high speed setting (12V) and a low speed setting (5V) which could come from either the 12V battery or the 5V MCU they use to power the motor. However, the motor draws more current under higher resistance, which decreases voltage, showing constant power. From that I concluded that PWM isn't possible, though I am unsure if that is a sound assumption. There is also on the PCB two identical N channel mosfets that are close by, which my instincts tell me are for the motors as well.

My questions are as follows:

  • Is it possible the relays and mosfets follow the schematic I've made below? Does it make sense?
  • If it does make sense, how would you configure the flyback diodes to protect the circuit?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Put the MOSFET on the bottom, "after" RLY2. Then you only need one. Oh, you didn't design this PCB? It is quite strange if the MOSFETs are like you have drawn. The logical design is to use one MOSFET at the bottom. Perhaps the MOSFETs are actually to control the relay coils. But, this is also an odd arrangement of relays because it is very easy to create a short circuit. \$\endgroup\$
    – user253751
    Feb 24, 2023 at 23:21

1 Answer 1

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If you're using relays to form an H-bridge, the more common configuration would look like this:

schematic

simulate this circuit – Schematic created using CircuitLab

With this configuration it's impossible to short your power supply through the relays, whereas with your version you're just looking for trouble.

If you want to PWM the motor too, then you only need one MOSFET:

schematic

simulate this circuit

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  • \$\begingroup\$ could you explain why switching the relay configuration is better, and does this mean flyback diodes aren't needed? Also, why is it better to put the mosfet on the negative side? (I just learned about high side vs low side mosfets, isn't high side better?) \$\endgroup\$
    – 1W0n_29
    Feb 25, 2023 at 4:23
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    \$\begingroup\$ I did explain why it's better - in the 2nd paragraph. If you want to add flyback diodes then you can add them in the same configuration as in a transistor H-bridge. Low-side N-channel MOSFETs are far easier to drive than high-side. \$\endgroup\$
    – brhans
    Feb 25, 2023 at 4:30

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