# Thread: how see the current resistance sense voltaje on osciloscope?

1. ## how see the current resistance sense voltaje on osciloscope?

hi all,
could someone tell me how see the current in the sense resistor with an osciloscope? i'm trying but i dont know if it's well done. I put gnd on gnd board and connect the probe in the sense resistor, but i got a trace with sin and cos both coil, its so or it's a mistake? can i see one coil in the sense resistor or it's always the 2 coils to see in the sense resistor?

2. Are you using an old analog scope or a fancy new digital scope?

If old analog........

Make a simple differential circuit using an op-amp and 4 resister (2 inputs one output) You might need to attenuate your two inputs channels or you could "rail" your op-amp output. This circuit will subtract one channel from another. Connect an input channel to each side for your current sense resistor and connect the op-amp circuit output to one channel of your oscilliscope.

You could also use this same circuit to look at stepper motor coil current. If you made two of these circuits you could even look at both the SIN and COSIN coils of a single stepper motor.

(A simple differential op-amp circuit is something handy to have in your electronics tool-box)

If new digital scope..........

Connect two o-scope channel inputs to each side of the sense resistor. find the o-scopes mathmatical functions and subtract the lower potential channel from the higher potential channel.

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In both solutions, this will give you the voltage across the resistor (or stepper motor coil) Ohm's law says I= V / R. Divide the single trace on the o-scope by a resistance to get current.

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You could also use an o-scope current probe. However, not everyone has a current probe.

3. hi, thanks a lot for your reply, i will try with that.

4. ## current sense

when i connect the input oscilloscope channel in one current sense resistor it show the picture above. Is this correct or am i doing any mistake?

5. I guess that you are talking about a stepper motor but is it bipolar or unipolar? From the picture I think you have not synced your scope well. I get similar things all the time and it happens more if the signal is not so steady. Try to do it with the trigger/level pots.

I am not sure about stepper theory and whether you could see the reflected waveform from the other winding. As I know this will happen in unipolar motors.

Todor

6. it's bipolar driver. I dont think it's the 2 windings because if i connect the other input oscilloscope in the other current sense i get the same picture but 180º degree. I will try with trig input.