[Amps] Non-inductive resistors

Sun Sep 26 22:49:56 EDT 2004

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 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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>>
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