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Inductive resistors

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Phil Allison

Jan 1, 1970
0
** 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
 
P

Phil Allison

Jan 1, 1970
0
"Bruce Varley"
"Phil Allison"
Pure guess - magnetic former?


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




..... Phil
 
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Franc Zabkar

Jan 1, 1970
0
Strange claim, as even a straight piece of wire is inductive (though
not very).

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
 
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Phil Allison

Jan 1, 1970
0
"TonyS"
My guess: whatever the wire is made of has a higher permeability
coefficient? Tried a magnet?

** 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
 
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