Stepper controller triggers over-current protection on bench supply?

Support Motor controllers/drivers and motors
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I’m working on a project that takes a 12VDC power supply – will be a car battery in the field (literally: agriculture). After giving up 0.2V or so to a diode to protect against reverse connection, this is split three ways, with buck regulators providing 5.0V (logic level for this project) and 3.3V (to power a laser diode) and the unregulated ~11.8V going over to the VMOT connection on Pololu’s A4988 carrier.

While working out the details and testing on my desk, I’m using a cheap-but-not-awful DC Regulated Power Supply (TekPower TP3005T; rated for up to 30V, 5A). I found that whenever I increased the current sense potentiometer to read more than about 0.270V, the power supply’s over-current protection kicks in and drop the voltage to around 6V supply.

This concerns me because I’m used to that happening only when I accidentally touch the wrong thing and short something out :blush: I think it’s overreacting with some sort of unpleasant interaction between the two current limiting circuits, but thought I should ask…

  • Is this known / expected behavior?
  • Do I need to do more to isolate VMOT from VCC? (I do have the 100uF electrolytic across VMOT; and the buck regulators have 100uF across their inputs and 220uF across their outputs)

When I run it from a battery, I can easily set the current limiting voltage pot on the A4988 up to ~0.42V and I do read the desired maximum full-step ~0.75A current measured in-series. (I received older boards with 50mOhm sense resistors; the coils are rated 2A and I did put a heat-sink on the chip). The total circuit draw is only 0.30A so that really shouldn’t be troubling the bench power supply’s over-current protection.

So, I think everything is probably okay, but with a B.A. in music and a M.S. in computer science, I would appreciate either reassurance or warnings from anyone more knowledgeable!

I did see this in the manual for the power supply:

If the load is too small, or very close to short circuit, please turn of [sic] the OCP function, because the OCP will detect a large current that is over the set OCP current, it will shut off the power supply automatically, the power supply will not be able to work at all. Or you can set the Constant Current Value smaller than the OCP current value, the power supply will work properly.

Does the current limiting in the A4988 cause a situation “very close to short circuit?” It does seem to be okay if I disable the OCP on the supply.

Thanks for your thoughts!

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Hello.

It is unclear why the over-current protection on your power supply is triggering. Devices like the A4988 can draw large amounts of power in short bursts, but it doesn’t seem like it should be drawing 5A or more from a 12V supply. Setting a current limit (the “constant current value” your manual refers to) seems like a reasonable solution. Do you have access to an oscilloscope with a current probe you could use to check for current spikes?

-Nathan

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Thanks for that thought and suggestion. While I do have an old hand-me-down 15MHz oscilloscope (from my thesis advisor who I think salvaged it while he was a grad student!) and a small handheld digital scope, neither has a current probe. I had to google that, actually, and as they cost ~USD300-800, I doubt it’s something I could easily wire up! I do have a clamp for a line-level ammeter, but I figured a better bet to get an idea of this would be to set a multimeter to min/max mode and try to measure something that way. The Fluke 112 I used claims “100ms to 80% of signal” response time; don’t know whether that would be fast enough to capture the “short bursts” you mentioned, but it did find something.

The bench supply seemed less panicky today and I couldn’t get it to trigger OCP until I had the A4988 putting 1.4A into a coil in full-step mode which is way above where I want to run it and possibly too near the 2A maximum limit, though I had stuck on a heat sink and I didn’t ask it to do that for more than 30s. At that load, the max current detected from the 12V supply was a bit over 3A. I switched to a set of 8xAA batteries and got the same values; definitely not linear behavior compared to running it at half that current. Maybe spike actually is over 5A, or maybe the circuit is responding to the velocity of the sudden current draw and being proactive.

When I set the coil current back down to 0.75A, the maximum supply current detected is about 0.400A (average around 0.330A) which seems entirely manageable.

Since you didn’t mention any known issues with using bucking voltage regulators along side the stepper controller – that was my main unknown as it’s my first project using them – I guess I won’t worry about it.

Thanks again, Nathan.