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 Post subject: How to tell if a resistor is non inductive?
PostPosted: Nov Sat 25, 2017 7:09 am 
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I need some non inductive power resistors, but I have seen some reputable businesses selling some that are wire wound. How does one do tests on them to see if that is true, what set up would one use?

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 Post subject: Re: How to tell if a resistor is non inductive?
PostPosted: Nov Sat 25, 2017 8:52 am 
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There are non-inductive wire wound resistors:

https://www.vishay.com/docs/31801/mra.pdf
https://en.wikipedia.org/wiki/Ayrton-Perry_winding

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 Post subject: Re: How to tell if a resistor is non inductive?
PostPosted: Nov Sat 25, 2017 1:39 pm 
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The simplistic answer is that you need to measure the inductance.....but you'd need an instrument that goes low enough.
In turn, "low enough" is defined by your application.

Buying parts with the right specs could be the easiest approach.

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 Post subject: Re: How to tell if a resistor is non inductive?
PostPosted: Nov Sat 25, 2017 3:29 pm 
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If I had them in-house I would measure them at the frequency of interest, or at a number of frequencies for a broadband application, with my General Radio Type 1606-A Radio Frequency Bridge. Other approaches not requiring a bridge might be feasible depending upon the resistances and frequencies involved.

Do you have the manufacturers, stock numbers, and any other markings for the resistors in question? Some of us have a variety of catalogs and might be able to look them up.

Bear in mind that non-inductive wirewound power resistors are not common and are significantly more expensive than ordinary wirewounds, so they are most likely ordinary wirewounds. If the supplier does not specify them as non-inductive they almost certainly are not.

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 Post subject: Re: How to tell if a resistor is non inductive?
PostPosted: Nov Sun 26, 2017 12:54 am 
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There are some high power film resistors which consist of a ceramic substrate which has been dipped or coated with a resistive material. They are extremely low inductance. Anything that is wirewound is inductive--the only question is, how much? There are ways, like the Ayrton-Perry winding, of greatly reducing the inductance of wirewound resistors and other devices, but they do not totally eliminate all parasitic inductance and capacitance.

You can measure the impedance of a component at RF frequencies using a signal generator, an oscilloscope or RF voltmeter, and a precision non-inductive resistor (doesn't need to be a power type). The resistor is placed in series with the component under test, and the signal generator is set to produce a voltage that is convenient to measure at the frequency or frequencies of interest across the two components. The generator output voltage minus the voltage across the precision resistor tells you the voltage across the unknown resistor, and the voltage across the precision resistor divided by its resistance tells you the current passing through both. So now you have the impedance of the unknown resistor. Rearranging Z = SQRT (R^2 + XL^2), (impedance = square root of resistance squared + inductive reactance squared) one can plug in the impedance just measured (Z), the DC resistance of the unknown resistor (R), and solve for XL, the inductive reactance of the resistor. The inductance itself can then be worked out from XL = 2pi*F*L.

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 Post subject: Re: How to tell if a resistor is non inductive?
PostPosted: Nov Sun 26, 2017 1:39 am 
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Non-inductive doesn't mean they will be non-reactive at all frequencies. There are also stray capacitances and unavoided parasitic inductances in real-world components. A network analyzer or an RX bridge similar to a Boonton 250 would be useful in the RF range.

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 Post subject: Re: How to tell if a resistor is non inductive?
PostPosted: Nov Sun 26, 2017 3:16 pm 
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To follow up on Chris' and Peter's responses, the most common use of non-inductive resistors is my field is in dummy loads for broadcast transmitters. I carry a Bird Termaline 8201 500 watt dummy for field work on low power AM transmitters, FM exciters and IPAs (Intermediate Power Amplifiers), and STLs (Studio-Transmitter Links).

The stray reactances described by Peter can be minimized for particular applications. My Bird is rated for a VSWR of less than 1.1 to 1 from DC to 1 GHz, and less than 1.2 to 1 from 1 GHz to 2.5 GHz. It uses the type of non-inductive power resistor described by Chris. The standard Bird method for reducing those strays is described in the 8201 manual:

"The Models 8201 and 8203 Termaline Load Resistor consist essentially of a carbon film-on-ceramic resistor immersed in a dielectric coolant. The resistor, individually selected for its accuracy, is enclosed in a special exponentially tapered housing. This provides a linear reduction in surge impedance directly proportional to the distance along the resistor. When surrounded by the dielectric coolant, the characteristic impedance is therefore 50 ohms at the front (connector end), 25 ohms at the mid-point to compensate for the resistance already passed over, and zero ohms at the rear where the resistor joins the housing, forming the return conductor of the coaxial circuit. This produces a uniform and practically reflection-less line termination over the stated frequencies for the load resistor."

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
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 Post subject: Re: How to tell if a resistor is non inductive?
PostPosted: Nov Sun 26, 2017 7:19 pm 
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Thanks for all of the replies. I found some 20 k resistors on Surplussales, but they are 25 w and I only need 5 w. I might have to get them, but they are only $10 each.

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 Post subject: Re: How to tell if a resistor is non inductive?
PostPosted: Nov Sun 26, 2017 10:04 pm 
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Mouser has 20K 5W metal oxide for 65 cents, metal film for 37 cents. Even if they're spiral cut, the inductive reactance will be negligible compared to 20K. If we were talking about smaller values and hundreds of megahertz, the lead length alone would be significant.

Mouser also has flat ceramic thick film 20K 10W for $3, probably overkill.


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 Post subject: Re: How to tell if a resistor is non inductive?
PostPosted: Nov Sun 26, 2017 11:07 pm 
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Okay, I can go that route. Thanks!

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 Post subject: Re: How to tell if a resistor is non inductive?
PostPosted: Nov Mon 27, 2017 1:47 am 
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What is the application for the resistors you need?

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 Post subject: Re: How to tell if a resistor is non inductive?
PostPosted: Nov Mon 27, 2017 6:43 am 
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radiotechnician wrote:
What is the application for the resistors you need?


I wanted to build this ''Tesla" coil that I discussed in another thread.
viewtopic.php?f=12&t=331404

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 Post subject: Re: How to tell if a resistor is non inductive?
PostPosted: Nov Tue 28, 2017 4:24 am 
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There are some wire wound resistors that use a winding that doubles back upon itself, so that the inductance of the resistance wire cancels itself out. They are generally used for audio or low frequency RF applications, where the inherent stray capacitance isn't a concern. Telsa coils run in the low LF region?

I've seen resistance wire dummy loads used for AM transmitters, but I assume they had used special engineering techniques to keep the load purely resistive.

Pete

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 Post subject: Re: How to tell if a resistor is non inductive?
PostPosted: Nov Tue 28, 2017 6:08 am 
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I would assume that if a reputable seller says they are non inductive, they can be used about anywhere a carbon comp resistor would be used. Tesla coils don't seem to require any special type of resistor. The resistors just need to handle the power requirements, and be of minimal inductance.

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