On Jun 27, 3:26 pm, "Dave" wrote:
Can someone tell me what a feedthrough capacitor actually does? One source
says it passes DC or low frequency signal through a panel, and another says
it passes any signal through said panel. Are they nothing more than a
feed-through device with a small amount of capacitive reactance? My
Encyclopedia of Electronics leaves me hanging on this one...

Thanks for any help,

Dave

A good feedthrough capacitor is arranged so that the capacitor part
has very low parasitic inductance and resistance between the center
fed-through conductor and the case. The case may be intended to be
soldered to a shield can, or may have threads so it can be mounted
using a nut (or perhaps into a tapped hole in the shield can). They
are used to get relatively low frequency signals into and out of
shielded areas, most commonly. You can get them in various
capacitance values. 1000pF is common, for example. Overall circuit
design will determine what frequency range can be passed. For
example, 1000pF driven by a 1 ohm source impedance will have a -3dB
corner frequency of about 160MHz, but driven from a source impedance
of 1000 ohms, it will be a 160kHz corner frequency. It's pretty
common to use them in conjunction with a series inductance or ferrite
bead on one or both sides, very close to the feedthrough, so that RF
isn't picked up by the lead and fed through because the impedance is
too low--the inductance or bead guarantees a reasonably high impedance
at RF frequencies. You can also get feedthroughs that have not only
shunt capacitance, but also series inductance or built-in ferrite.
(Note that the ferrite provides some inductance at moderate
frequencies, but at higher frequencies--up to a point--the series
impedance becomes mainly resistive. That helps damp resonances. See
curves of impedance versus frequency for parts found on the TDK
website, for example.)

Hope that helps...

Cheers,
Tom