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I am confused again. I just watched a recent YouTube video today where the host claims a flyback diode is not only unnecessary, but a bad idea for treadmill DC motors. He "demonstrates" it by quickly disconnecting and shorting the two motor wires causing the motor to break hard with heavy sparking at the brushes.

The video is a response to a commenter who suggested adding a flyback diode to the circuit to protect the PWM-based MC-2100 controller board. Many commenters, including the host, complained the MC-2100 is of poor quality because the MOSFETs often burn out when the motor stalls during use or when the direction is reversed. I find it odd since treadmills are fairly reliable and handle quite heavy loads.

I was under the impression that a flyback diode is essential for protecting circuits from inductive spikes when the motor's coil's magnetic field collapses. The reported damage to the controller suggests a diode wasn't used. Am I correct or is he correct? Why?

ASIDE: If he's wrong, it would be helpful if someone who's a bit more knowledgeable than I ]politely corrects him with a video comment. Wrong information online shouldn't go unchallenged.

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  • \$\begingroup\$ 50% of the stuff on the internet is suspect and what do you propose we do about that? Regarding the video (which I won't watch), you have to draw a distinction between back emf suppression (necessary to protect electronics) and, motor free-wheeling voltage generation (quite often can be left unchecked because the voltage generated is not greater than the voltage originally applied to drive the motor). \$\endgroup\$
    – Andy aka
    Commented Jan 20, 2023 at 10:21
  • \$\begingroup\$ break hard or brake hard? \$\endgroup\$
    – greybeard
    Commented Jan 20, 2023 at 10:23
  • \$\begingroup\$ There is flyback diode, and there is flyback diodes. With a full-bridge driver, braking would be less hard than max. acceleration. With a single active switch and the flyback diode connected directly across the brush holders, he may have a point. \$\endgroup\$
    – greybeard
    Commented Jan 20, 2023 at 10:31
  • \$\begingroup\$ ....In other words, when a brushed DC motor has the power removed and, it continues to spin, the voltage produced (generated) is exactly the same polarity as the original voltage applied hence, a diode will not have any effect. \$\endgroup\$
    – Andy aka
    Commented Jan 20, 2023 at 10:36
  • \$\begingroup\$ (FWIW, one reverse engineered schematic of said board shows a diode straight over the motor terminals.) \$\endgroup\$
    – greybeard
    Commented Jan 20, 2023 at 11:13

1 Answer 1

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The video is a response to a commenter who suggested adding a flyback diode to the circuit to protect the PWM-based MC-2100 controller board.

That's not necessary because the MC-2100 controller already has a flyback diode in it. Any controller that needs a diode should have one built in. If you don't see one in the circuit it may be using the body diode inside one of the MOSFETs.

He "demonstrates" it by quickly disconnecting and shorting the two motor wires causing the motor to break hard with heavy sparking at the brushes.

That will happen if the diode is connected the wrong way around. In normal operation it should be reverse biased and have no effect. It only conducts during the moment of turn off when the inductance of the windings cause a negative voltage spike as it tries to keep the current going.

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  • \$\begingroup\$ What could be causing the faulty controllers? \$\endgroup\$
    – user148298
    Commented Jan 20, 2023 at 17:23
  • \$\begingroup\$ From what I read, the diodes are usually placed as close to the motor as possible. I've even seen them added across the switch contacts. \$\endgroup\$
    – user148298
    Commented Jan 20, 2023 at 17:34
  • \$\begingroup\$ I recently had to replace the motor brushes in my 17 year old Weslo Cadence S8 treadmill. All the motor control electronics was on a single PCB (no diodes on the motor or wiring). Here's a typical modern 'universal' treadmill motor controller. You can see the dual Schottky diode to the right of the MOSFET. You can tell it is a dual diode because the two outer pins (anodes) are connected together and go to motor -. aliexpress.com/item/4000752080625.html \$\endgroup\$ Commented Jan 20, 2023 at 21:48
  • \$\begingroup\$ I suppose the FETs might burn out when the motor is stalled if the current limit doesn't protect it. The incline height motor is usually controlled by relays, not PWM. You must not put a diode across this motor because it would short it out in one direction. \$\endgroup\$ Commented Jan 20, 2023 at 22:00

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