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# how to properly connect an oscilloscope to diagnose stepper motor?

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#### james56

##### New Member
I have a bipolar (4 wire) stepper motor that isn't working right.

It runs on 24VDC. How do I analyze the voltage waveforms going into the stepper motor? I know it has 2 coils. The possibilities should be:
coil 1: wire 1=+24V, wire 2=GND
coil 1: wire 1=GND, wire 2=+24V
coil 2: wire 3=+24V, wire 4=GND
coil 2: wire 3=GND, wire 4=+24V

I want to see which coil has current flowing through it at different times.

My scope has 2 probes, ChA and ChB. Each probe has a spring loaded tip and an alligator clip.

Do I hook up the the GND black alligator clips to the negative terminal of the battery (battery ground) then hookup the spring loaded tip to the base of one of the coils (wire 1). Then for the second probe, should I hook up the black alligator clip to battery ground and the probe to (wire 3)? Does this mean I need 2 scopes (4 total channels) to see what's going on in the stepper motor?

Or can I hookup the black end of the probe to one end of the coil (wire 1) and the springy tip to the other end (wire 2)? Then hookup the second probe to (wire 3) and (wire 4) ?

I tried the second one and a big spark took place so I took it off. Am I doing the write thing?

Are you sure it's a stepper motor?

A stepper motor requires a certain order of polarity change in both coils in order to run.

If you want to measure current through a coil you use a series resistor low enough to not drain too much power, but high enough to cause a voltage drop to measure..

Shunt (electrical) - Wikipedia, the free encyclopedia

Yes I'm sure it's a stepper. I know the polarity changes in the coil.

That's why I want to figure out how to look at the voltages with the scope to figure out the voltages at each of the coils and the direction.

I just need to figure how to hook it up...

Thanks!

Do I need to measure the current through the coil? I really just want to verify the circuit is firing properly.

Can't I just measure the voltages going into the motor coils somehow?

Hmm I've been thinking about it and the motor coil itself has a resistance of 15ohms or so.

Can I just hook up my scope across the coil (wire 1=black alligator) and (wire 2=scope clip) and read out the voltage?

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