adding a capacitor to a switch mode PS's output

Thread Starter


Joined Mar 11, 2010
Hi - Here's a quote from a leading manufacturer of stepper motor (chopper) drives.

The drive works best with unregulated power supplies though regulated linear and switching power supplies may also be used. What matters is the power supply must have a large output capacitor and an unregulated supply intrinsically has one... ...A 2000 to 10000uF capacitor should do.
I always use unregulated supplies for driving steppers and have never had a problem but I know others choose to use switching supplies.

I recently advised someone who was using a switching supply and who was experiencing much lower than expected (low speed) torque that they might consider following the above advice to see if it improved things.

The response was to the effect of "never add a capacitor to the output of a switching supply - it will seriously screw with the supply's internal electronics".

So now I'm confused. Tried searching for advice/examples/papers on supply design etc. but to no avail for this particular question. Can anyone shed any light on the pros and cons please?



Joined Jul 9, 2011
I'm not sure about the torque issue.

The output capacitor of a SMPS, however, is part of the feedback loop and needs to be taken into consideration.

I would not say, NEVER add a capacitor to an SMPS output. Even with different values it can still work.

The SMPS may start to oscillate at different load levels/input voltages, though.


Joined Oct 26, 2011
Another issue here is "fold-back current limiting" that some power supplies have--this could be causing the power supply to trip as it attempts to charge the capacitor on power-up.

Thread Starter


Joined Mar 11, 2010
Thanks both for your replies. I should make clear the system in question does NOT currently have a capacitor. It was when I suggested adding one that the issue came up.