Purpose of 0.22uF caps in power supply?

[crapalanche]

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?

 

[MikeMl]

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.

 

[Nigel Goodwin]

They are for interference suppression, both TO and FROM the unit.

 

[Speakerguy]

This page will answer all your questions.
ABC's of Safety (Interference Suppression) Capacitors for Tube Radios

The caps you are seeing are probably 220nF X2 type safety caps. Very common value for that application. If you've ever seen a filtered power entry module like this, they contain all the X and Y safety caps as well as a common mode choke.
https://media.digikey.com/photos/Schurter Photos/5110.0343.1.JPG?cshift_ck=null&client_id=5042

If you look at the data sheet for that part, you can see the attenuation in dB for the high frequency EMI crap that it blocks:
https://www.electro-tech-online.com/custompdfs/2010/09/typ_5110.pdf
(scroll to third page)

 

[ben7]

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.