As a follow up to my previous question.

I want to develop a small circuit, which can detect, if a stepper motor is driven.

After checking all options/provided as answers, I decided, that probably the best and most reliable way for my case, is to connect a either voltage/current detection circuit to the stepper motor wires. At first, I thought to use an isolated hall current sense sensor, such as these ones from Allegro. However these are quite expensive (and also alternative from other makers I checked).

So I came up with the following circuit to convert the +24V to -24V votlage on the stepper motor wires to a voltage detectable by an MCU digital IO pin. The circuit is intended to work as follows:

  • Bridge rectifier to get only +24V Voltage
  • Resistor + Zener to get the voltage to the acceptable level of my MCU (digital input).
  • Small capacitance to smooth

With the digital input then I should be able to detect, if the stepper motor is pulsed by the driver. But I have some question regarding grounding + protection (as I need this to work reliable for some years).

  • Will this work (in an LT-Spice simulation it does)?
  • Is it OK, to just connect the GND of this circuit to the rest of my board GND at the shown position?
  • Do I need to add additional protection (clamping diodes, TVS etc.) to make it reliable?

enter image description here


1 Answer 1


That will work if that winding goes from driven, to undriven, back to driven again, sufficiently slowly that the capacitor after the rectifier can discharge enough to be noticed.

Note that some stepper motor drivers drive both windings all the time, for maximum holding torque. Your sensed winding would then flip from +ve to -ve very quickly, and you may not notice the change after your capacitor.

Perhaps a more reliable, and simpler, method would be an opto-coupler, which turns on when the winding is energised in one direction only. This would need the motor to advance a few steps for the change to be noticed in the worst case.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thanks for your answer and I will give the opto solution definitely a try. But I am still confused on the topic of grounding. Let's say in either your of my solution I want to add a TVS diode, to what I would connect anode? \$\endgroup\$ Feb 28, 2022 at 2:36
  • \$\begingroup\$ @KarlKarlsom Why would you want to add a TVS diode? Once you can answer that question, you'll know where to put it. You don't need one in my solution. However, that ground on your circuit may compromise the entire operation with currents to the ground of the H-bridge driver, not salvagable by a TVS diode, maybe requiring some isolation like an opto, or separating the H-bridge and MCU supplies. \$\endgroup\$
    – Neil_UK
    Feb 28, 2022 at 5:33

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