Eks loaned me a Tek AM503 Current Probe Amplifier, one of those gorgeous instruments that Just Works: a clamp-on DC to 50 MHz Hall Effect current meter. Because it’s electrically isolated from all the hideous electrical hash that surrounds any stepper motor driver circuit, it doesn’t see much of the garbage that pollutes any current sensor depending on a series resistance and a differential amplifier.
Which lets you take pix like this:
From top to bottom:
- Generated 1/rev sync pulse
- Winding current at 200 mA/div (1 A peak current)
- Step drive pulses at about 3 rev/s
The initial ramp occupying the first third of each step comes from the motor’s L/R time constant coupled with the 9 V supply I was using. Back of the envelope: 2 mH / 2 Ω = 1 ms. With 8 V (9 V less MOSFET drops &c) applied, the initial slope = 8 V / 2 mH = 2500 A/s, so in 75 ms it rises 187 mA: close enough.
The small ripples show the A4988 chopping the current to maintain the proper value for each microstep.
Looks just like the pretty pictures in the datasheet, doesn’t it?
The Smell of Molten Projects in the Morning 
Have you ever played with these guys?
http://www.allegromicro.com/en/Products/Part_Numbers/0712/index.asp
I like it, but wow you’d have to pay close attention to the maximum power dissipation limits: the datasheet makes no bones about the necessity for adequate heatsinking through the package pins.
Running 30 A through four pins of an SOIC package requires more courage than I’ve got, that’s for sure!
Might have to try for some samples of the 5A version… thanks for the tip.
Sparkfun offers a handy breakout for this chip:
http://www.sparkfun.com/products/8883
Cool on the SparkFun board. Too bad they did not put a SPI ADC on there while they were at it. Really though, Allegro should make a SPI version of that part with a built-in ADC and reference. That would be NICE !
The fact that they rolled the response off at 34 Hz (!) says that it has real noise trouble; I’m not sure anybody who buys that board will be delighted with the results. Heck, you can’t even measure 60 Hz power line current!
In fact, the full 80 kHz doesn’t go very far with stepper current. At 1600 steps/rev and assuming you want enough bandwidth for the 5th harmonic, that’s 10 rev/sec = 600 rpm. Fast for a Thing-O-Matic, not so much elsewhere.
Worse, current chopper frequencies are right up around 80 kHz, so it’s useless for that.
The AM503 I’m using has a bandwidth limit switch that cuts it down to a mere 5 MHz. Now that is a Hall effect current probe!