Can I add a step-down transformer to use a 220v DC power supply on 120v?

Hi,

the question is unclear. Draw a sketch.

But generally transformers are not suitable for DC.

Klaus

Generally, as long as its done on the mains AC side and the step-down transformer is sufficiently rated, you should be OK. Things to be careful of are the power rating and AC mains frequency. Most 120V countries use 60Hz mains and most 220V countries use 50Hz, there are small differences in the transformer design to cater for the frequency difference. If your power supply has a power rating on it (Watts or VA), use a transformer with at least that rating and preferably add at least an extra 10% to allow for inefficiency in the transformer itself. Using a step-down transformer with even higher ratings is not a problem.

Brian.

> Can I use step-down transformer to use a 220v (AC to )DC power supply on 120 (Vac)?​

No. You need to step-up the voltage.
AC to DC supplies rectify and charge up low impedance capacitors quickly to store energy and this may exceed the current rating and saturate the transformer on start-up and thus blow a fuse or burnout unless rated for the surge current of the supply.

If you have a 1:1 isolation transformer you can connect the output of this in series with the power to the input to get 2 x the input voltage

so about 220Vac - which appears to be what you are seeking.