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I have a tin can bipolar stepper motor. It has two stators A & B.

When I disconnect the motor and measure the differential voltage across A, I get this:

enter image description here

It's actually strange as we are using a h-bridge configuration, so I would have expected a negative voltage as we are pulsing High, Low, nevertheless I am not too worried about this.

The issue that is causing major confusion to me is the next trace:

enter image description here

The sequence is correct but rather than go to zero, the voltage is only dropping to approximately half that of the high voltage.

The motor is actually stepping correctly and in the right direction.

Is the low voltage being high linked to the direction of movement? I have been looking for an oscilloscope trace of a h-bridge online in order to compare what I have seen but found nothing . Does anyone know if this is expected behaviour?

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  • \$\begingroup\$ Have you wired the scope correctly? Do you know that scope has the GND potential tied to the ground. Probably using two probes on each phase line will reveal the exact voltage. \$\endgroup\$ – Marko Buršič Jul 4 '16 at 10:23
  • \$\begingroup\$ @MarkoBuršič The scope has actually been shipped for calibration and won't be back until Friday. I did not know the ground was tied to ground, maybe that's the issue. I tried again with an iso-tech and changed the coupling from DC to AC, (bit rusty). That is showing negative values now but the circuit is failing, (arggh), so will have to debug. I'll post again if I find out the issue. Thanks for feedback Marko. \$\endgroup\$ – SeanJ Jul 4 '16 at 11:51
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RDSON in the transistor and other resistive artefacts result in a voltage offset.

This is illustrated below:

enter image description here

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