EMI Filter question

[underwaterchippie]

Hello Gents,

This machine had previously an EMI filter before the Power Supply (12V, 37.5A) pictured below.

How do I work out the Filter Amperage?
I’m looking at the following 10A filter below:

FN9264-10-06 | Schaffner, FN9264 10A 250 V ac 50 → 400Hz, Flange Mount EMI Filter, Tab, Single Phase | RS

uk.rs-online.com

Much appreciated

 

[rjenkinsgb]

12V at 37A is 444 Watts.
That's about 4A at 115V or 2A at 230V.

Add an allowance for inefficiencies plus a bit of headroom.

I'd personally aim for 5A minimum for 230V supply, or 8 - 10A for 115V supply.
I do like to over specify things a bit, "brick outhouse" grade design, though.

3A and 6A should actually be OK if you want to economise.

 

[AnalogKid]

The filter you have selected is a pretty basic design, with only one common-mode choke. For a higher performance filter, the tradeoffs are size and cost. Overall, I prefer ones with the IEC connector built-in. Most people don't catch that inside a chassis, the wiring between the power connector and the filter input is an antenna that can both radiate and receive EMI.

Separate from that, here is a broad generalization: For any given filter's physical size and shape, the lower the current rating, the better the filtering performance.

ak

 

[Tony Stewart]

Line filters are used as bi-directional low pass filters (LPF) They are rated for AC Line current not DC output current shown on the product label.

Ingress may be lightning-induced transients which are attenuated in voltage and thus stretched in time (microseconds) and uses the line inductance to raise its impedance.

Egress is the noise voltage going out, created by the SMPS switched currents which may interfere with other electronics nearby. The noise is attenuated by redirecting the current above the grid frequency from inductors into X and Y rated grid safety plastic-film capacitors.

The 120/240V differential currents pass thru with a differential Y capacitor to suppress pulses, while the common mode noise is diverted to PE ground with smaller X Caps that are chosen to limit current to safe levels, in case of a ground fault and a human touches between metal case and ground.

The ratio of impedances from Zo=sqrt(L/C) The Schaffner, FN9264 10A 250 has the following specs.

Capacitance	2.2 nF, 100nF
Height	35mm
Inductance	210 μH

The best line filters use a CM choke for CM noise and 2 separate inductors for differential mode (DM) noise.