I've seen quite a few .22uF capacitors in power supplies, especially in things like printers, there generally are about two caps each. These are on 120 V AC, 60 Hz; they're rated for 250 volts.
I'm wondering what their purpose is. Several things lead me to think that they're definitely not being used for mains decoupling. Small size, fairly high reactance ~12057 Ω, etc.
Can someone shed some light on the use of these sorts of caps in power supplies?
EMI suppression. These things usually have switching power supplies. These caps are supposed to bypass the high frequency switching harmonics to keep the line cord from radiating them into the radio spectrum. Ask me how I know? As a Ham, I'm tired of listening to my neighbor's cheap Chinese crap on 40 meters.
I have seen anywhere from .022µF to .68µF for AC line filtering. Most cheap smps have small value rated caps, however a power filter/surge protecter could have up to .68µF.