Re: RF filtering resistors

Tesla List wrote:

> Original Poster: dferguso-at-ebmail.gdeb-dot-com
> i'm planning on using an air core choke / resistor setup as part of my RF
> filtering circuit, but when i went to purchase
> the resistor ( 500 ohm 50 watt) i noticed that it was coil wound like an
> inductive coil ( it had 100 to 200 turns or so , pretty much closely wound,
> on about a 5/8 inch diameter ceramic tube about 4 inches long. can i
> calculate the inductance of this and
> use this value as part of the filtering capability, or is the standard to
> "ignore" this intrinsic value of the resistor
> and use a separate choke to provide the necessary inductance? - thanks for
> any advice, doug

     If you want to include inductance as part of your filter (and I think
it's a good idea) I would recommend that you use a purpose built inductor
because the voltage across a filter inductor can be very large (on my coil
an LC filter, 11Kv drive I record over 50kv across the filter inductors).
    The 2 common geometry's for this are to have a long single layer helix, or
a flat pancake inductor. In the case of the pancake it is vital to bring one
wire out at the center of the coil, if you try to run it up the side of the
winding, you will get flash over.
    The inductors I use are wound from something like 400 turns of 24swg
enameled wire wound in a disk shape 10mm thick and about 90mm across, with the
whole winding dipped in epoxy.

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