[Amps] Non-inductive resistors

[Michael Tope]

Will,

Globar resistors are "bulk ceramic" types. That means that
the ceramic body isn't just a form for a thin film resistive trace
to be deposited on the form. To the contrary, in the case of
a globar type resistor, the ceramic material is the actual
resistor - see the following URL:

http://www.globar.com/ec/resistor.php.html

This configuration has very low inductance  compared to
a thin meandering trace. It also has very high peak power
capability. The techniques you describe do work, but I am
skeptical that they could achieve the same low levels of
inductance that are possible with a "bulk" or "composition"
resistive material configuration. Long thin traces have
considerable self inductance even if you don't have
reinforcing flux linkages between adjacent "turns".

When you say that something is "Non-Inductive" you have
to be careful to properly define what is meant be that
term. Something that would be considered "non-inductive"
in one application (say relative to a standard wirewound),
might be intolerabably inductive in another application
(compared to a composition resistor for instance). It all
depends whether your threshold for "inductance" is
1 microhenry or 10nanohenry.

73 de Mike, W4EF.......................................................


----- Original Message -----
From: "craxd" <craxd1 at ezwv.com>
To: <amps at contesting.com>
Sent: Sunday, September 26, 2004 7:54 AM
Subject: Re: [Amps] Non-inductive resistors


> Rich,
> According to what I read, those Globar and Ohmite resistors were wound
> with a similar process. I know I seen a cut away of one of those similar
> to Globar and it had similar windings under the ceramic cover.
> Especially the Slab Type resistors, which I have seen the insides of
> myself by dissecting a bad one. Globar says this about the Slab Type;
> "RF Dummy Load Circuits".
>
> According to the Radio Engineers Handbook, it shows nine possible ways
> (including Ayrton-Perry) to wind a resistor with "that minimize reactive
> effects". Under the section,"Non-reactive Wire-wound Resistors", it
> says,"To keep the inductance low, each turn should enclose the minimum
> possible area, and the wire should have as many ohms per foot of length
> as possible so that the length required to obtain the desired resistance
> will be small". Of course we all know this as we'd be winding an
> inductor, the more turns, the more inductance. Then it goes on, "In
> addition, it is desirable that adjacent turns carry current in opposite
> directions so that the residual inductance of an individual turn is
> neutralized by the effect of the adjacent turns. A low capacitive
> reactance associated with a resistor is obtained by arranging the
> winding in such a way that adjacent turns of wire have a low potential
> difference between them and as far apart as possible".
>
> The type I first mentioned about the loops, one behind the other, was
> actually known as the "Reversed Loop". The "Bifilar Series", is another
> which is almost similar. The Ayrton-Perry method was like I described in
> the last post.
>
>  >>-  "Also, there is no such thing as a conductor that does not have
> inductance."
>
> Well of course there's no conductor with no inductance, resistor or just
> plain wire. But there's methods to cancel what inductance that's
> possible, if not those Globar resistors wouldn't work, nor any others.
> Even though those tubular resistors look like a solid coating on the
> outside, there's a winding underneath that coating. The ones in the load
> I have here, the coating is thin enough you can actually see the outline
> of them. From the others I've seen and on the Internet of cut-away views
> from tubular ceramic, non-inductive resistors, they all had a winding of
> some form or another. The load here from Harris, had a small amount but
> was mainly due to using six resistors laid out with the wiring. It
> leveled out with approx. 30-35 pF of capacitance applied in parallel to
> the resistor bank. That means the inductance had to be really low.
> Looking at the Palstar load again, down in the lower left corner, you
> can see a doorknob capacitor. This in the inside view they give from the
> 5Kw load. The schematic for the 2.5 kw load shows a 33 pF cap there
> also. Both those loads (2.5 and 5 Kw) weren't there when I spoke to
> Palstar a few years back.
>
>  From all this, I still think a person can wind his own if he can find a
> suitable resistance wire or tubing.
>
> Will Matney
>
> R. Measures wrote:
>
> >
> > On Sep 25, 2004, at 8:41 PM, craxd wrote:
> >
> >> Since the discussion came up about dummyloads, and I remembered about
> >> how non-inductive resistors were wound, I couldn't think of the
> >> winding type name in the last post. The type of winding used is the
> >> "Ayrton-Perry" winding. It takes two lengths of nichrome wire which
> >> seriesd will equal 50 ohms or 25 ohms each. They are wound bifilar on
> >> a form. On one end, they are connected together and the other end has
> >> the two leads. This throws the two windings 180 degrees out of phase
> >> and cancels the inductance. This would be the same as hooking up two
> >> transformer windings out of phase and killing the inductance.
> >
> >
> > Will  --  This technique is okay for LF terminations, but not MF and HF.
> > -  Also, there is no such thing as a conductor that does not have
> > inductance.
> >
> >> This is exactly what is needed in the case of a non-inductive
> >> resistor. The windings would need to be spaced so they didn't contact
> >> each other along the length. The Alloy 87 I read about with 7/8 ohms
> >> per foot would be somewhat ideal for this. Something like two lengths
> >> 28-1/2 feet would equal 25 ohms each. It don't take much to wind up
> >> 28-1/2 feet of wire around a form. Every 3.8 turns around a 1"
> >> diameter form is 1 foot. Thats only 108-1/2 turns which don't take up
> >> too much space. I wouldn't see any problem experimenting with this is
> >> that alloy wire is affordable and large enough to do whats wanted.
> >>
> >> Will Matney
> >
> >
> > Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
> >
> >
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> >
> >
>
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