Can you please elaborate what you mean by a strong current persisting when unplugged?
By your usage, it seems you may be referencing the supply voltage keeping the processor of the DCC1120 awake for a few seconds when power is unplugged?
Your easiest solution is probably to just monitor the VoltageInput channel from your DCC1120, which would provide a measure of supply voltage until the remaining power runs out and the channel disconnects. Similarly, you can monitor the voltage across the supply terminals of the DCC1120 yourself if you would prefer.
Thanks for the quick reply. To elaborate, the system remains capable of powering the motor for several seconds after the main 24V power supply has been disconnected.
My hypothesis is that the capacitors on the SAF module and/or the DCC1120 controller retain a significant charge. The controller's logic remains powered via USB, allowing it to accept new commands and discharge this stored energy into the motor.
Reproduction Steps
The system is fully powered (24V supply connected to the SAF, USB connected to the PC).
I run a C application to operate the motor, then command it to stop.
I physically unplug the 24V power supply from the SAF module. The USB connection remains active.
I restart the C application and send a command to activate the motor.
Result: The motor runs for a couple of seconds, even under load, with no power supply connected.
Next Steps & Goal
Your suggestion to monitor the VoltageInput channel on the DCC1120 is excellent, as it should confirm my hypothesis. I will try that next week.
My ultimate goal is to address the safety implications of this behavior. A system that can still move after the main power is cut presents a potential hazard. I need to find a reliable method to detect this "latently energized" state to ensure the equipment is truly safe to service.
I have reproduced a similar setup, and under normal conditions the DCC1120 with an SAF2020 only stays connected for about one second before running out of power, with nothing else drawing power. We also verified the calculations that the capacitance on these boards is not sufficient to power any significant load on their own for any significant length of time.
When you say you command your motor to stop, do you actually wait for it to stop? It is possible the regenerated power from a motor slowing down could power the electronics while the motor is spinning down.
If you are still concerned about the motor receiving commands for whatever reason, you can also recommend engaging the E-Stop for the duration of the service, which guarantees the motor can not be powered as long as E-Stop is engaged.
Yes. And, indeed, the motor runs for about 1 second (after power unplug).
> you can also recommend engaging the E-Stop
The concern is more about the residual current within the boards while the motor is not activated (activation does indeed discharge capacitors ; the motor activation post-unplug was a proof of concept about the residual current), than the motor itself running.
For for the sake of peace of mind, like some PC chargers have, we'll add a LED at the right place on the board(s) to witness any residual current.
Yes unfortunately very little can be done about that, most electronics will suffer from staying powered by capacitors after being unplugged. LED indication is good if it is a concern.
If power consumption isn't a huge concern you could also consider adding a bleeder resistor to dissipate the power faster.
