Ian,
Ohmite does not say anywhere in their catalog or spec sheets about
inductance. I asked their engineering department and was told that they are
non-inductive.
Mike, W1NR
----- Original Message -----
From: "Ian White GM3SEK" <gm3sek@ifwtech.co.uk>
To: <amps@contesting.com>
Sent: Tuesday, March 28, 2006 3:00 PM
Subject: Re: [Amps] Need some 220K 2 watt carbon resistors
> 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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