Re: EMI Filter Freq

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Re parallel filters...
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> I would expect it to reduce the filtering, by roughly
> 50%.  (6db).

Hmm.. not entirely sure about that... (although in practice, mileage may
vary).. But,  if half the current goes through each filter, then half the
"noise" also goes through each filter, and is attenuated by the requisite
amount..., then summed back together... So you get something like:

1 filter:
Signal Out = Signal In * .001                (for 30 dB)

2 filters

Signal Out = 2 * ( (SignalIn/2) * .001) = Signal In * .001

However, since it is non coherent combining, etc... I'm not sure it would
work like this....


>
> > this will make the .1uf caps = .2uf and the 450uH inductors
> > = 900 uH.....??? maybe.
>
> It's iffy to combine component values this way, when
> each is part of a separate circuit.  And i expect the
> inductors would be 'in parallel' (roughly) so the net
> inductance would be _halved_: 225 uH.

Say, though that the filter is basically a PI circuit C-L-C... hooking two
in parallel will give you 2C - L/2 - 2C.   The "resonant" or cutoff
frequencies should stay the same (since it is proportional to LC and 2C*L/2
= LC...)  Parasitics might (will!) bite you probably...

I suspect that most line filters also depend to a certain amount on the
driving and load impedances... (part of those test conditions).



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