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 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!