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enter image description here

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The fault' pin (18) was checked. It seems to not be triggering, but it's been years since I used an oscilloscope. I'm not confident, and it may be triggered low:

If the die temperature exceeds safe limits, all FETs in the H-bridge will be disabled and the nFAULT pin will be driven low.

enter image description here

When I connect a 12 V wall outlet power supply to the driver, no ratchetting occurs. The specs say the driver can handle up to 45 V though. To confirm it is not the fault' pin (18), I will try to figure out how to set the scope to trigger on a falling edge. Besides the overcurrent protection kicking in, what could cause the ratchetting behavior?

Edit: Generic stepper motor specs

Used an off the shelf CNC driver board. It has 100 uF capacitors on each driver:

Can I use 250μF capacitor instead of 100μF?

There may have been LC voltage spikes. So that may be the issue. But I will order a new PCB and populate it with 100 uF caps to truly find out.

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  • \$\begingroup\$ Tie a resistor, such as 1k, from nFault to Vdd 2.5-5VDC. Now you can 'scope it and see if it actually is pulled low. \$\endgroup\$
    – rdtsc
    Commented Jul 13, 2021 at 19:45
  • \$\begingroup\$ Two questions: What is the motor current and a link to the specs would help. Second are the grounds of both power supplies connected? Update: Can you post links to the parts. Also be sure you have enough torque. \$\endgroup\$
    – Gil
    Commented Jul 13, 2021 at 19:47
  • \$\begingroup\$ @Gil I believe they are Nema 17 and Nema 11. All grounds are connected. Question edited with specs. \$\endgroup\$
    – Adamelli
    Commented Jul 13, 2021 at 19:50
  • \$\begingroup\$ What does 'ratchetting' mean in this context? I think you are just trying to step the motor but your description and title are inconsistent / confusing as to what you are asking. \$\endgroup\$
    – gcr
    Commented Jul 13, 2021 at 22:38
  • \$\begingroup\$ @gcr It's like a car wheel moving except it has teeth. It's like gear were to replace a car wheel. The result is unwanted "ratcheting" and not a smooth movement. Also, it starts off smooth, but then eventually starts ratcheting. \$\endgroup\$
    – adamaero
    Commented Jul 14, 2021 at 13:02

1 Answer 1

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I used 330 uF capacitors. That appeared to fix the ratcheting. I never said that my previous answer did not answer the question.

Warning: This carrier board uses low-ESR ceramic capacitors, which makes it susceptible to destructive LC voltage spikes, especially when using power leads longer than a few inches. Under the right conditions, these spikes can exceed the 45 V maximum voltage rating for the DRV8825 and permanently damage the board, even when the motor supply voltage is as low as 12 V. One way to protect the driver from such spikes is to put a large (at least 47 µF) electrolytic capacitor across motor power (VMOT) and ground somewhere close to the board.

When I first started this project, there was also no ratcheting. So this may be the "solution." It's my best guess right now, and appears to be behaving normally.

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  • \$\begingroup\$ Adamelli - Hi, "I never said that my previous answer did not answer the question." You certainly seemed to say that, which is why your previous answer was deleted. Quotes from comments there: From another user to you: "Is this supposed to be an answer to your question? [...] Are you trying to say "I used an off the shelf CNC driver board and it solved the problem."?" Response from you: "no....................." So it certainly seemed like you confirmed your previous "answer" was not an answer, hence I moved it into the question. If you want to explain more, I can start a chat room. \$\endgroup\$
    – SamGibson ♦
    Commented Jul 15, 2021 at 15:25
  • \$\begingroup\$ Since you wrote the question, the only time you would normally write an answer, is when you have solved the problem yourself. Please can you clarify: (a) Have you solved the problem (no further help needed)? In which case, please "accept" your answer in about 3 hours from now (i.e. 48 hours after you asked the question) to close the whole topic, or (b) Do you still want help? In which case this isn't (yet) an answer, and should be an update in the question. Which applies here, (a) or (b)? Thanks. \$\endgroup\$
    – SamGibson ♦
    Commented Jul 15, 2021 at 15:27

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