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# EMI filter Question?

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##### Full Member level 5
Do a Linear power supply needs an EMI filter?
I know EMI filter suppress noise spikes but is it from the supply towards the AC outlet or from AC outlet to the device or both ways ways?
In linear power supply i often see a capacitor connected to mains and a resistor in parallel, is that a snubber circuit?

I was once having a s spike on my AC outlet because of lousy fan dimmer so just to check i installed an EMI filter of 3Amps on the mains and spike was gone. the Emi filter was of this kind https://www.tavi.co.uk/ps2pages/ohland/emi.jpg

A linear power supply normally only needs an EMI filter if there are EMI sources nearby that may disrupt the operation of the supply or its load. An EMI filter is bidirectional and will suppress noise and spikes in either direction (although the attenuation factor may be different in opposite directions, depending upon the filter design and the impedances it sees).

The resistor and capacitor connected to the mains is likely a simple EMI filter. "Snubber" circuits are typically associated with the suppression of spikes from inductive circuits.

Points: 2

### KonstanU

Points: 2
How did you calculate the values of capacitors and inductor?

Do a Linear power supply needs an EMI filter?
Of course not ! EMI filter will be used when your regulator or your PSU is producing some noises or it's current has HF components . thus an EMI filter can eliminate them ( trap them ) and then they can not go through the main power line or preceding stages . but EMI filter will let the main voltage go through your input as well .
In linear power supply i often see a capacitor connected to mains and a resistor in parallel, is that a snubber circuit?
Are you sure that , that is capacitor ? most of the times a varistor will be used in series with a low value resistor to eliminate fast variations which are applying into the input .
But in parallel and ... ! cn you show me one of the schematics that you've saw such an arrangement in that ?
I was once having a s spike on my AC outlet because of lousy fan dimmer so just to check
Most of the times dimmers ( standard dimmers ) have some filters to prevent such problems . ( internally , of course ! )

Best Wishes
Goldsmith

A linear power supply normally only needs an EMI filter if there are EMI sources nearby that may disrupt the operation of the supply or its load
Hi crutschow
I afraid but i'm disagree with this statement . EMI filter must be designed for a certain value of BW to suppress it only ! all of the transmitters and dimmers and switching devices are not working with the same frequencies .
every standard system won't let any unwanted harmonic go through the main power line that create any bad effect on other devices .

The resistor and capacitor connected to the mains is likely a simple EMI filter
Sorry , but a capacitor and a resistor in parallel can not cover the concept of EMI filter . because if the current has HF components thus it can not do anything ! EMI filters are dealing with differential transformers ( inductors in fact ! )

Best Regards
Goldsmith

KonstanU

Points: 2