# Calculation of supply current in stepping motor driven using chopping driver

I'm using STK 672-330 to drive a unipolar 2 phase stepping motor. The situation can be seen roughly in the circuit diagram below. I set Vref so that the peak phase current becomes 1.2 A. When I measured the current in the phase A (blue arrow), i confirmed that it correctly peaked at 1.2A . But then I measured the current from 24V (red arrow) it looks more like DC 400 mA ish.

I did a few research and I found out that according to this source (around last page) supply current is calculated using this formula:

Isupply = IM · ( VM ⁄ Vsupply )

where IM is motor winding phase current, VM is motor winding voltage.

In my situation: Vsupply= 24V, IM = 1.2A, VM = 3.6V (winding resistance is around 3 Ohms) so it gives me Isupply = 180 mA for each phase. Which gives a total 0f 360 mA. This result more or less agrees with my measurement.

But why does this equation holds? I find it strange we need to divide with Vsupply. If the current flowing in the phase peaks at 1.2A, why there is no such peak appear in the 24 V line? Where does the current come from and go to?

I have a hunch this has something to do with the PWM chopping by the FETs inside the driver IC. But I can't find satisfactory explanation for the continuity of current.