I decided to try another experiment. I removed the motor from the 9505 controller and replaced it with a 1000 ohm resistor. I still get a spike whenever the voltage switches, and it has the classical RC circuit decay. The initial value is about 60 counts (6.2 mA per count) and it decays to a steady state value of 4 counts (24 mA), which is the expected current with the resistor. I can’t explain why the second peak is positive instead of negative.
Current waveform, 25% duty cycle, 1000 ohm resistor
Next I removed the resistor and tried it with nothing connected as a load. The curve looked almost identical to the previous curve, but it goes to a steady state value of zero, which is no surprise.
Current waveform, 25% duty cycle, open circuit
My final test for the day was to use a 150 ohm resistor to get a larger steady state current. In this case, I noticed the peak still starts at 60 counts but decays to the expected steady state value. It does weird things when it switches back to zero volts that I can’t explain.
Current waveform, 25% duty cycle, 150 ohm resistor
It appears for resistive loads the current always starts at 60 counts and decays to the expected steady state value. I do wonder what would happen if the steady state value was greater than 60 counts. From the decay curves, I figured out that the time constant for the decay is about 1000 ns or 1 us. I also noticed a third small spike just before 2000 counts. It looks to me like the circuit disconnects from one source about 30 counts before connecting to the other source.
I noticed in the 9505 manual that the minimum inductance is 500 uH. I assume my motor’s inductance is much lower than this. My current plan is to buy a 500 uH inductor and put it in series with the motor. It will smooth out the current to a triangular waveform that can be measured predictably. The only negative effect I foresee is that it will also filter changes in voltage and delay the response of the motor. The delay may not be large enough to worry about, though. I might also get a smaller inductor, about 250 uH, to try out as an intermediate step – less smoothing but less delay.