Current measurement?

While I'm waiting for the parts to arrive, I do some read-up on what to do once they get here. Right now I'm trying to wrap my head around current-adjusting.

Right, so what the wiki says and what most of the forum posts I find says is that I should use the formula Current=VREF(8*Rs), and that the Rs value for the Polulu is 0.05Ω, but if it's a Stepstick it's 0.02Ω, and... Well, with all the ebay finds these days it can be a tad bit bothersome figuring out what you've got, since some of them only says "A4988 stepper driver".

Anyways, couldn't I read the current value directly from the motor with the multimeter on connector 2B, like this? Also, since the pin layout is consistant, that should work independent of what driver one's using, right? Or have I overseen something here?

Except it's a chopped AC current so you might get wierd readings. Also if you accidentally break connection while the motors are powered it can fry the driver. Probably easier to check the resistance of the current sense resistor with the multimeter when powered off. Either that or email your seller.

Oh, I never thought about that. Of course that's easier (and safer).

And yeah, I could probably ask the seller, but I want to know things by heart rather than having to rely on others =) Sooner or later I'll find myself in some situation where I don't have a seller to ask ;-)

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loidor
Right, so what the wiki says and what most of the forum posts I find says is that I should use the formula Current=VREF(8*Rs), and that the Rs value for the Polulu is 0.05Ω, but if it's a Stepstick it's 0.02Ω,

Stepsticks are four times the resistance of genuine Polulus, so .2Ω, not .02Ω. I = VREF / (8 * RS), or rearranged, the VREF you're looking for for the current rating of your steppers is VREF = I * 8 * RS.

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and... Well, with all the ebay finds these days it can be a tad bit bothersome figuring out what you've got, since some of them only says "A4988 stepper driver".

So measure the sense resistor that's on your driver. Orient the board so that the main IC (the square one with 28 pins coming off the 4 sides) has the dotted corner in the top left corner. Along the top look at the 2nd pin from either end. Follow the trace to what is usually the two largest resistors on the board. Those are the sense resistors. Measure either of them to find the resistance. Tolerances will vary, so it may not be exactly .05 or .2 but they should be close enough that it should be one of those two values. If you can't figure it out, post a clear picture of the top of the driver and we can tell you which resistor you need to measure.

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cdru
Stepsticks are four times the resistance of genuine Polulus, so .2Ω, not .02Ω. I = VREF / (8 * RS), or rearranged, the VREF you're looking for for the current rating of your steppers is VREF = I * 8 * RS.

Of course it is. Occasional brain fart or something.

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So measure the sense resistor that's on your driver. Orient the board so that the main IC (the square one with 28 pins coming off the 4 sides) has the dotted corner in the top left corner. Along the top look at the 2nd pin from either end. Follow the trace to what is usually the two largest resistors on the board. Those are the sense resistors. Measure either of them to find the resistance. Tolerances will vary, so it may not be exactly .05 or .2 but they should be close enough that it should be one of those two values. If you can't figure it out, post a clear picture of the top of the driver and we can tell you which resistor you need to measure.

Thanks! I'll have a look once it arrives