ac rated caps

[Rosewood]

Can someone explain the difference between caps that are rated for ac like 450vac compared to a cap rated at 450vdc? You see 125vac in the primary side sometimes when older amps used a dc rated cap. Can the dc rated replace the ac rated?

[Wakarusa]

Apples and oranges. Capacitors are typically really designed for DC environments with the voltage rating specified with the tag WVDC for "Working Volts Direct Current". The AC rating of a capacitor is, so far as I've seen, always less than the DC rating (so a 600WVDC film cap would commonly be rated 250VAC). As I understand it, big AC voltages stress the cap with the constant large charge/discharge cycles.

[rooster]

The AC rating of a capacitor is, so far as I've seen, always less than the DC rating (so a 600WVDC film cap would commonly be rated 250VAC). As I understand it, big AC voltages stress the cap with the constant large charge/discharge cycles.

Well....

A 250VAC rating, assuming a sine wave, is about 700VDC peak-to-peak, which kinda lines up with the 600WVDC rating, since the 700VDC will only be reached for a small part of the duty cycle. Basic electronics.

rooster.

[Rosewood]

Thanks guys, is there a formula for replacing a cap with only the ac rating with a regular cap with the dc rating? It looks like a dc rating 3 times bigger than the ac rating would be a ball park??