# What Sort of Wire is used in Wire Wound Potentiometers?

#### JasonAshworth

Joined Jan 1, 2018
1
Hello,

I'm hoping to build a giant (500mm) slider potentiometer for an electronic musical instrument.

I'm going through some scenarios, and it seems like for a 100k ohm resistor, I'll need a wire with something like 8k ohms per foot. Does that seem right? I can't find any such material... Here's where I got that number:

I plan to wrap the wire around a 6mm diameter non-conductive rod.

Let's say the wire is .8mm diameter— that's 625 wraps around the rod (500mm/.8mm), and just under 19mm for the circumference of the rod (6mmx3.14), so 625 turns x 19mm per turn = 11.875m of wire to wrap along the length of the rod. If I want there to be 100k ohms across that length, I'll need 100k ohms / 11.875m = 8,421 ohms per meter... right? (I'll admit math was never my strong suit.)

What sort of wire is used in professional wire-wound potentiometers? It seems nichrome wire doesn't get anywhere close to that sort of resistance. Are you aware of any sort of wire material with an abnormally high electrical resistivity? Somebody mentioned somewhere that spark plug wire is carbon at the core, and if stripped it might have a high resistance— does anybody know if this is true (I'm wondering this more for a separate project/idea)?

Any help or input is greatly appreciated.

Thanks,
Jason

#### ebp

Joined Feb 8, 2018
2,332
0.8 mm diameter would put you in the realm of a high-power wire-wound potentiometer. I think some of my 300 watt power pots are wound with wire of about that size. For what you require you would need wire of very much smaller diameter. It will be difficult to handle, fairly fragile and probably hard to obtain. And you would need a zillion turns. There are other resistance wires made, but they will be even harder to obtain in small quantity. MWS Wire Industries makes several types. I don't know what their minimum order is these days. There should be good data available on their website.

46 AWG nichrome wire will have about a tenth of the resistance you are looking for. It is about 0.04 mm in diameter.

You might be better off with the heavier wire and using a low excitation voltage, then amplifying the voltage from the slider. There would be considerable electrical noise when the slider was moved, so you would need to low-pass filter it to get something usable. This of course assumes that you can use a voltage rather than a resistance to accomplish what you want. Voltage to current conversion is easy. Voltage to resistance conversion isn't.

#### Sensacell

Joined Jun 19, 2012
3,396
If the goal is a 500 mm linear slide input device, a capactitive sensor would be much simpler to build.

Then it's just pieces of circuit board, or metal plates.
The processing circuitry is more complex, but the sensor itself is simple.

#### DNA Robotics

Joined Jun 13, 2014
646
In Forrest M. Mims III. Engineers Mini Notebook on Sensor Projects

On pages 28 & 29 he made a strain gauge with a wide graphite pencil line. You could make a long potentiometer with a sliding contact that way. It may last a while. It may not.

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#### recklessrog

Joined May 23, 2013
985
As DNARobotics has said, you could make a long slider potentiometer with graphite. A very long time ago, I had to do an emergency repair to a master fader on a mixing desk. I mixed some graphite filings from an HB pencil into some conductive paint that consisted of silver particles suspended in a varnish. I only made a short piece ( about 5mm) on the end of the track and the repaired piece had a resistance of about 2K. It was still working 5 years later and we never could obtain an exact replacement of the original slider.
What sort of current are you proposing to pass through it?

#### DNA Robotics

Joined Jun 13, 2014
646
I made a dense pencil line 1/8 " wide X 10" long on paper. It measured 800K ohms at the ends and varied about 80K per inch. Not all pencils are graphite. Graphite is also used as a lubricant so it might last a long time.
You could use a spring loaded carbon brush pack from an alternator or motor as a sliding contact. It will be easy to mount. Alternators have 2 brushes, side by side.

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#### crutschow

Joined Mar 14, 2008
33,965
What exactly is the purpose of this pot?
That will help of us make good suggestions to solve your total problem, rather than suggestions for your perceived solution to that problem.

For example what's the basis for "it seems like for a 100k ohm resistor" requirement?

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#### MisterBill2

Joined Jan 23, 2018
17,728
Crutschow is totally correct. I have examined failed multiturn pots and they have very fine resistance wire wrapped around what looks like a fiberglass strand and then coiled into a housing. Unwinding one of those may work for you except that they never come in values that high. 10K is the very maximum that I have seen. So I suggest a carbon rod supported on a non-conductive base. The rod would come from a scientific supply house.
The carbon conductors in ignition cables are cloth type material and would wear away very rapidly.
AND, if the resistance is used in an electronic circuit, you will have serious noise pickup problems with a high impedance like that in an unshielded condition.
You could use a nichrome wire and redesign the circuit to work at a much lower impedance, perhaps ten ohms, maybe even 100 ohms.