Re: [Amps] Non-inductive resistors

[craxd <craxd1@ezwv.com>]

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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