[volt-nuts] Anyone know how to make stable inductors?

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Fri Aug 21 08:24:48 EDT 2015


On 20 August 2015 at 02:38, Todd Micallef <tmicallef at gmail.com> wrote:

> Here is a DIY guide to making some lab standards. It is detailed with some
> component values.
>
>
> http://nopr.niscair.res.in/bitstream/123456789/4848/1/JSIR%2065%286%29%20510-513.pdf
>

OK, I tried this.


I was hoping to use a box I had with the connectors on already, but that
shorted all 4 shields, so was no use. I cobbled together 2 x 1000 Ohm
resistors, and a 100 pF cap. I had components mounted on leads, and leads
about 600 mm long, and the return point about the same length, so I could
not expect much accuracy, but I was not going to waste much time on this.

Between 1 kHz and 100 kHz, I was getting values around the 75-100 uH
inductance. I had a self-resonance at a few hundred kHz, so obviously it
went wrong there. Also below a few hundred Hz, the inductance started going
negative too. Given how poor my setup was, I am not going to give the
actual values I measured. I am inclined to believe their basic method works
(of sorts).

But having looked at it in more detail, the paper is *exceeding* poor, so I
am *firmly* in agreement with Brooke Clarke that it is pseudoscience.

The first paragraph says that inductors wound on a coil have unwanted
parasitic components, such as coil resistance and distributed capacitance.
OK, we all know that is true, but using the component values they give in
table 1, make for an inductor with an exceeding high series resistance

* 10 uH is 200 Ohms
* 100 uH is 632 Ohms
* 1000 uH is 2000 Ohms

I bought some 33 mH inductors a week or two ago and they had a series
resistance of under 200 Ohms, so I think 200 Ohms for 10 uH is rather
excessive!

I think most engineers, if presented with a component in a black box with
two terminals and asked to guess what it is, they would assume it a wire
wound resistor rather than an inductor.

Also on the first page, the authors mention it uses the 4 terminal pair
(4TP) system, and describes the advantages of the 4TP system. They
conveniently fail to mention their inductors (resistors?) rely on this.

So overall, a totally useless paper IMHO, perhaps befitting of the April
Fools joke I first thought of, but certainly not a credible scientific
paper.

Dave


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