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I am new to electronics. I am watching this video.

At 4:03, you can see a diode that connects the two pins of the motor:

enter image description here

What role does the diode play?

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  • \$\begingroup\$ Search "flyback diode". It only works if the drive current through the motor is expected to be in one direction since it just shorts the motor in the other direction. An H-bridge where drive currents can flow through the motor both ways must be handled differently). \$\endgroup\$
    – DKNguyen
    Jul 20, 2021 at 4:18
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    \$\begingroup\$ This is a flyback diode used to protect the MOSFET from destruction by the energy of the motor when it is turned off, and is discussed in this post (they use the symbol for a coil as the motor, but the principle and physics are essentially the same). If you need additional clarification, feel free to edit your post. \$\endgroup\$
    – nanofarad
    Jul 20, 2021 at 4:18
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    \$\begingroup\$ Does this answer your question electronics.stackexchange.com/questions/95140/… \$\endgroup\$
    – Lars Hankeln
    Jul 20, 2021 at 4:20

1 Answer 1

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This is called a Flyback Diode.

Motors tend to have a large inductance.

Once current gets going in an inductor it doesn't want to stop. If you try to stop it anyway by cutting off the current path the voltage will spike to a very high level and probably damage something (the mosfet in this case).

The diode gives the current somewhere to go. It routes it back to the other side of the inductor and allows the current to flow in a loop until it dies down due resistive losses.

The diode is sometimes also called a Freewheeling Diode for this reason.

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    \$\begingroup\$ can we flip the diode? \$\endgroup\$
    – Dat
    Jul 20, 2021 at 4:51
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    \$\begingroup\$ No, flipping the diode would cause a short whenever the MOSFET is on. \$\endgroup\$
    – Unimportant
    Jul 20, 2021 at 5:52
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    \$\begingroup\$ It would be an error which leads to wrong circuit that will damage components if you did flip it. \$\endgroup\$
    – Justme
    Jul 20, 2021 at 5:59
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    \$\begingroup\$ @Dat You could flip the diode, but then you'd also have to flip the voltage and the MOSFET type. This would make the motor go in the other direction. (A better way of thinking about that: Flip the motor without flipping the diode or anything else in the circuit. Then the motor also goes in the other direction). \$\endgroup\$
    – orithena
    Jul 20, 2021 at 13:57
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    \$\begingroup\$ This needs more in the "why" area. Suggest something like "This diode's prime purpose is to extend the life of the motor and delay costly replacements and repairs. Stopping the motor with no diode results in high electric currents hammering in the circuit. If the high-currents are allowed, then contactor surfaces like brushes and points and switches will eventually have chunks blown out of them, or suffer from arcing, or become heat-welded shut." \$\endgroup\$
    – Criggie
    Jul 21, 2021 at 4:34

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