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Re: Question about Line Filters
A typical line filter is, in fact a "pi" section low pass (inductor in
series, cap from each end to ground, and is symmetric, as far as loss goes.
The significant difference is going to be where the "ground" is connected.
Some filters have the output "ground" (i.e. the green wire) also filtered
(albeit in a different way), so that the "ground" doesn't conduct the noise
out. The only true ground for the filter is the case.
By the way, in the vast majority of cases, the line filter is NOT there to
keep crud from the line getting into the box, but the converse,
particularly so you can meet Part 15 FCC specs. If you have a switching
power supply, it generates all sorts of RF hash, which you don't want
propagating back into the line.
So, I'd actually install that unknown filter in the "correct" direction.
A little research with the filter manufacturer will generally give you all
the info you need on what the internal configuration.
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Question about Line Filters
> Date: Saturday, April 24, 1999 12:31 PM
>
> Original Poster: Brent L Caldwell <stretchmonster-at-juno-dot-com>
>
>
>
> Dave and Alex:
> Hey, about those filters, how are they constructed? Do they really have
> a polarity? What I mean is: does the filter REALLY operate differently
> in one direction that the other?
> If it is a high pass filter, it seems to me like they wouldn't because
> all the high pass filters I've ever built didn't have a polarity, you
> just ran current through them and the output on the other side was
> dependent on what the input frequency was. You could flip the filter 180
> and connect the source to the output and the load to the input and it
> would do exactly the same thing.
> But then again, I have never torn apart those ones you buy at the store
> that are in metal boxes, so I wouldn't know.
>
> Brent
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