Inductive resistors

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** Hi all,

the self inductance of WW resistors has been a hot topic here  -  as makers
rarely specify how much inductance their products have nor publish impedance
curves.

For most WW resistors over a few hundred ohms, inductance issues rarely
arise -  but for ones under 100 ohms and in the larger wattage ratings, it
can be wise to check.

One resistor is this category is sold by WES Components under the code  "
100W8 "  and is a 100 watt ( free air rated ) tubular ceramic type, wound
with a 2.3 mm flat strip conductor.  The wind diameter is 28 mm and the
length from beginning to end of the wind is 110mm. There are 31 turns of
strip.

The WES catalogue claims they are " non -inductive"  but this is complete
nonsense.

They make useful dummy loads for audio amplifiers and if submerged in water
will happily dissipate up to 1000 watts - being a hollow tube helps a lot
with water cooling.

Using an on-line calculator ( see below) gives an estimated inductance of
6.0 uH.

http://www.daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml

But as there are significant ( 1.3 mm ) gaps between each turn on the
resistor, the inductance value ought to be less than the calculated one.
However, the opposite proved to be the case as the resistors show an
inductance of  9.4 uH  (with a 1uF cap series resonance occurred at 52 kHz).

The reason is rather obvious, when you do one more simple test on the
resistor.

Anyone know ?


....  Phil



Re: Inductive resistors

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Pure guess - magnetic former? Incidentally, if that is the case it would
make the situation rather messy at high frequencies, where the magnetic
material became lossy.



Re: Inductive resistors

"Bruce Varley"
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**  Nice, white ceramic tube  -  as I said above.




....  Phil



Re: Inductive resistors
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My guess: whatever the wire is made of has a higher permeability
coefficient? Tried a magnet?

Tony


Re: Inductive resistors

"TonyS"
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** Good guess.

The strip is highly magnetic, probably a varierty of lower grade stainless
steel that is magnetic.

If you imagine a copper wire coated in steel -  the idea that it increases
the inductance is obvious.



...  Phil






Re: Inductive resistors
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Strange claim, as even a straight piece of wire is inductive (though
not very).


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At a guess, that it uses a flat strip rather than round wire. This
probably gives a greater surface area of metal around the diameter of
the ceramic "former" than round wire , and more of it would be
slightly closer to the centre (as it is flat)  ?

 assuming that this would even make a noticeable difference.

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Re: Inductive resistors
On Sun, 26 Jun 2011 01:35:18 -0700 (PDT), kreed


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That should be easy to test. Just unwind the 31 turns and stretch them
out in a straight line. Then repeat the measurement. I'd be extremely
surprised if that was the reason, though.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Re: Inductive resistors

"kreed"

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Strange claim, as even a straight piece of wire is inductive (though
not very).

** Nothing strange at all.

There are several ways to wind wire on a tubular former that cancel out all
inductance.

http://en.wikipedia.org/wiki/Resistor#Wirewound


....   Phil







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