Michael,
The Globar may be of a different design. However, theres some made with
those types of windings I'm speaking of in ceramic tubular resistors.
This is the way the resistors are in this old Harris load. They must
have low inductance because without any capacitance, the SWR was low
then, about 1.5:1-1.8: 1 or so. Six of these by the size and comparisons
to several manufacturers, are 250-275 watts each. The guy I got them
from mentioned adding capacitance in parallel then, but he had used the
doorknob cap(s) on a different project and I didn't get any. When it was
tested, it didn't show much of a change, maybe 1.5:1 after one minute or
maybe more from about 1.2:1 when cold (This after adding some
capacitance 30-35 pF). They got warm and was a little too hot to touch,
but cooled quickly with the fans. I wouldn't think they would hold up
though over maybe 4-5 minutes without damaging one or all. My guess is
around 2-3 minutes max for 1650 watts total rating and 5000 going in. If
they were oil or water cooled, then the rating would be way up there.
Back before this type of resistor (Globar), they made them the way I'm
speaking of. I double checked the Radio Engineers Handbook (1943), and
each was rated for RF duty, except for I think one, which the
capacitance might be too high on. To me, if they're non-inductive for RF
service, they should work, and really I can see why. They might have
more inductance than the Globar, but they will not be no where near as
high as a standard wirewound. If these didn't work for RF, I don't see
them listing the design in the Handbook and saying they all will work in
RF frequencies. I figured Rich was speaking of them maybe working in
audio frequencies, but the Handbook states clearly different. Plus, this
old Harris load works perfect to me and all six resistors are wound
resistors.
The "meandering trace" is a re-design of the "fish line" style included
in the book. Back then it was made by winding a wire coil around a silk
cord. Then, this was wound around a ceramic coil form. The ones today
have zig-zagging traces about the circumference. These very types I seen
listed for use in dummy loads for RF service and labeled non-inductive.
In the resistors I've seen there are two terms, non-inductive and
limited inductance. The limited inductance is the higher inductance of
the two. Another route was using two windings of opposing wound coils on
each end of the form. This would be the same as hooking two resistors in
series, with the same number of turns, but wound in opposite directions.
It would be the same using two RF coils in series, one having a right
hand twist, and the other a left. I experimented with this years ago,
(20?) and can't remember now how it worked.
I figure if it worked back in the good ol days, it would have to work
now. The only way to find out would be do some testing with some
resistance wire and a form. Apply the RF and see what you get. The SWR
will either be sky high, look like a pure resistive load, or somewhere
in between. My personal opinion is, if they're done correctly, they'll
work. Might not be perfect, but still work.
Will Matney
Michael Tope wrote:
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@ezwv.com>
To: <amps@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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