Wirewound resistor

pweber · 13th September 2010, 02:23 PM

Hi,

Just a quick question from an amateur. I bought some non-inductive wire-wound resistors, value 25Kohm, 12W. I would suspect a little bit of induction, but when I measure inductance with a LCR meter across these resistors, I read a value of 60mH.

Is 60mH a normal value of a 'non-inductive' 25K OHM 12W resistor??? Or am I doing a wrong measurement (1kHz sample with a Tecpel LCR-612, +/-1% precision, not a very reliable machine, but enough quality for this issue), I hook the resistor up like I would do with an inductor. Maybe the resistance (25k) is to high for a reliable measurement?

I wonder what you think.

Frank Berry · 13th September 2010, 03:24 PM

A 25K resistor has a lot of turns of wire. A non-inductive resistor isn't actually non-inductive. They are made with two turns of wire. One is wound clockwise and the other is wound counter-clockwise. Each one with 50K of resistance.
In a perfect world, the inductance would be cancelled. In most cases, there is some residual inductance because the balance between the two resistance windings isn't perfect.
Have you measured the inductance on more than one resistor? Do you get similar results? If so, it's probably normal for that particular resistor value.

bob91343 · 13th September 2010, 05:41 PM

Is that millihenries or microhenries? Sixty millihenries is a lot of inductance. But the test leads contribute some, maybe a microhenry. Short the resistor and measure, then remove the short.

It's probably bifilar wound. They take the wire for the resistor, double it up, and wind it on the core. Thus the leads end up on one end.

Can you measure the inductance at some other frequency? Bifilar resistors may have less inductance but they have a lot of capacitance, so maybe you are measuring near the resonant frequency.

Things are not as simple as one might think.

Elvee · 14th September 2010, 08:39 AM

Quote:

Originally Posted by pweber

Hi,

Just a quick question from an amateur. I bought some non-inductive wire-wound resistors, value 25Kohm, 12W. I would suspect a little bit of induction, but when I measure inductance with a LCR meter across these resistors, I read a value of 60mH.

Is 60mH a normal value of a 'non-inductive' 25K OHM 12W resistor??? Or am I doing a wrong measurement (1kHz sample with a Tecpel LCR-612, +/-1% precision, not a very reliable machine, but enough quality for this issue), I hook the resistor up like I would do with an inductor. Maybe the resistance (25k) is to high for a reliable measurement?

I wonder what you think.

60mH at 1Khz is only ~360ohm @1Khz, which is much lower than 25K.
It's simply caused by the insufficient vectorial discrimination of the instrument (some of the real component of the impedance leaks into the imaginary part).
You would need a pretty good impedance-meter to perform such a measurement at 1KHz. Low cost instruments are able to reject the minor parameters only if their magnitude is significantly below that of the major parameter.
You could make the measurement at 100KHz, this should make the job easier for your tester.

pweber · 14th September 2010, 12:06 PM

Thank you all for responding.

All resistors give the same reading, and are really is 60mH (not uH). And shorting the leads give met almost zero. I can only switch to down to 120Hz sampling, but the reading is unstable then (never seen that before).

That, and your replies, all lead me to the conclusion that I am not capable of measuring accurate inductance of complex circuits with this machine, but only part directly attached to the leads. It would be interesting to see what values a capable LCR meter returns though.

Thanks all for your help, never suspected that a wire-wound resistor would challenge my machine so much.

Enzo · 15th September 2010, 02:06 AM

Might be interesting to test the test setup with something not wound at all, like a carbon comp resistor or maybe the element of a pot. Select one of similar 25k value. See how much inductance you measure there. That might give an idea how much contribution to the reading the test arrangement made.

bob91343 · 15th September 2010, 03:41 AM

I am amazed that a resistor labeled non inductive would have so much inductance. I mean, that is a substantial fraction of a Henry! Noninductive, indeed...

Try measuring some resistors that aren't so labeled.

13th September 2010, 02:23 PM	#1
pweber diyAudio Member Join Date: Jun 2006	Wirewound resistor Hi, Just a quick question from an amateur. I bought some non-inductive wire-wound resistors, value 25Kohm, 12W. I would suspect a little bit of induction, but when I measure inductance with a LCR meter across these resistors, I read a value of 60mH. Is 60mH a normal value of a 'non-inductive' 25K OHM 12W resistor??? Or am I doing a wrong measurement (1kHz sample with a Tecpel LCR-612, +/-1% precision, not a very reliable machine, but enough quality for this issue), I hook the resistor up like I would do with an inductor. Maybe the resistance (25k) is to high for a reliable measurement? I wonder what you think.

13th September 2010, 03:24 PM	#2
Frank Berry diyAudio Member Join Date: Sep 2003 Location: Midland, Michigan	A 25K resistor has a lot of turns of wire. A non-inductive resistor isn't actually non-inductive. They are made with two turns of wire. One is wound clockwise and the other is wound counter-clockwise. Each one with 50K of resistance. In a perfect world, the inductance would be cancelled. In most cases, there is some residual inductance because the balance between the two resistance windings isn't perfect. Have you measured the inductance on more than one resistor? Do you get similar results? If so, it's probably normal for that particular resistor value. __________________ Frank

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13th September 2010, 05:41 PM	#3
bob91343 diyAudio Member Join Date: Mar 2010	Is that millihenries or microhenries? Sixty millihenries is a lot of inductance. But the test leads contribute some, maybe a microhenry. Short the resistor and measure, then remove the short. It's probably bifilar wound. They take the wire for the resistor, double it up, and wind it on the core. Thus the leads end up on one end. Can you measure the inductance at some other frequency? Bifilar resistors may have less inductance but they have a lot of capacitance, so maybe you are measuring near the resonant frequency. Things are not as simple as one might think.

14th September 2010, 12:06 PM	#5
pweber diyAudio Member Join Date: Jun 2006	Thank you all for responding. All resistors give the same reading, and are really is 60mH (not uH). And shorting the leads give met almost zero. I can only switch to down to 120Hz sampling, but the reading is unstable then (never seen that before). That, and your replies, all lead me to the conclusion that I am not capable of measuring accurate inductance of complex circuits with this machine, but only part directly attached to the leads. It would be interesting to see what values a capable LCR meter returns though. Thanks all for your help, never suspected that a wire-wound resistor would challenge my machine so much.

15th September 2010, 02:06 AM	#6
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	Might be interesting to test the test setup with something not wound at all, like a carbon comp resistor or maybe the element of a pot. Select one of similar 25k value. See how much inductance you measure there. That might give an idea how much contribution to the reading the test arrangement made.

15th September 2010, 03:41 AM	#7
bob91343 diyAudio Member Join Date: Mar 2010	I am amazed that a resistor labeled non inductive would have so much inductance. I mean, that is a substantial fraction of a Henry! Noninductive, indeed... Try measuring some resistors that aren't so labeled.

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