i am looking at some Polypropylene capacitors and am wondering which voltage i should get(using in a 12v circuit).
My main question is: Do capacitors change capacitance vs voltage?
Meaning if i got a 47 uf 400v capacitor and applied 12v DC to it, would it hold more than a 47 uf 500v capacitor?

 

No. Capacitance is capacitance and totally independent of working voltage.

 

If you using in the filter after rectifier, then when the voltage is high, and the cap will be low, and the voltage is low, the cap will be high.

The rating voltage of cap is fixed, it won't change when you using in the low power circuit.

In the low power circuits, I like to use two times or more rating voltage, when the power is increasing then the rating voltage will reduce, because the price of caps, when the rating voltage more higher then the price will getting higher.

 

The size of a capacitor is important.

Small 100V poly caps are for very small currents only. Not more than 50mA.

Large big 400V caps can handle 3 Amps or something like this.