Mike McCarthy, W1NR wrote:
>Ohmite OX and OY series ceramic composition resistors can be used as
>replacements for carbons. They are non inductive unlike film and
>wirewound resistors. Available at Mouser, Digi-key, etc.
>
>See http://www.ohmite.com/cgi-bin/showpage.cgi?product=ox_oy_series
>
>Mike, W1NR
Where do Ohmite specify the inductance of these resistors, or show how
they are made?
We're straying away from the original topic, where inductance doesn't
matter at all, but many metal film resistors actually have quite a low
inductance - low enough for RF work up to 150MHz in some applications.
Tubular film resistors are made from a cylinder of metal or carbon film,
coated onto a ceramic body. To get the required resistance value, a
narrow spiral of material is cut away to lengthen the current path from
end to end. Some resistors use as little as 1.5 turns, leaving almost a
complete cylinder with a very narrow strip cut away. This gives a very
small inductance, almost as low as you'd get from a solid cylinder, and
very hard to measure (I tried).
To make the next higher resistance value, the manufacturer uses slightly
more of a spiral, so the inductance increases with the resistance; but
when they reach about 10 turns they switch to a higher-resistivity
material (and/or a thinner film) and start over again. This means the
next higher resistance value drops back to having a very low inductance.
Don't assume that inductance will make the resistor unusable for RF.
It's only a small-diameter spiral of a few turns, and if you measure the
inductance or even calculate it (the normal formula works quite well)
you'll find that even resistors with 7-8 turns are OK for most RF
applications. For example, 10x 470-ohm 2W metal film resistors in
parallel will make a 47-ohm load that has an acceptably low SWR up to at
least 30MHz. I've used about six 1k 2W MF in parallel for grid swamping
at 144MHz, and a small tweak in the grid tuning cap was all it needed to
cancel the inductive reactance.
However, there is a gotcha. The changeover points from highest to lowest
numbers of turns differ between manufacturers, and this can matter in
critical applications. Elecraft found this when they changed
manufacturers for the base swamping resistors in the K2 power amplifier.
A design that was previously OK became unstable with the new resistors,
which had exactly the same resistance but a different number of spiral
turns. So if low inductance really is important, it pays to check by
scraping off the paint.
--
73 from Ian GM3SEK
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