Dick Cappels' project pages http://www.projects.cappels.org

How to Make Copper Wire Resistors
DIY low value resistors.

If you need a low value resistor quickly, or happen to need one with a
temperature coefficient of resistance of +0.393%/°C its easy enough to make.

The first time I had to make a resistor, I was in an apartment in Bangkok, well more than an hour's ride and half an hour's walk to the nearest electronics component outlet. Basically, it would have been a half day trip to go out and look for a 0.1 ohm resistor that I needed for the
EDFET amplifier breadboard. I needed a ballast resistor to go between two emitter followers. Something small fraction of an ohm would do, so I reached for a spool of magnet wire.

Above: The first time I needed to make a resistor.
The red axial leaded component is a 0.1 ohm copper wire
resistor.

I like to use resistors as the winding form for these hand-made resistors. If you just take a resistor and wind a bunch of wire on it, you have made something with resistance, but it will also be a pretty good inductor. Using the winding method illustrated below will result in a low inductance for the resistor. Making the wire into a twisted pair assures better coupling between the wires, thus assuring lower overall inductance.

Above: Low inductance winding method, illustrated and placed in the Public Domain by the Zureks.

The method applied in the example below can be extended to a wide range of resistance values. While you can get a given resistance with anywhere between a short thin wire and a long heavy wire, two things should be kept in mind: Long heavy wires will have a longer time constant, which may be a significant factor, depending up on the application, and long thin wires have lower fusing currents. Please check the wire tables for the wire you intend to use for fusing current.

For the purpose of illustration, the table below shows the resistances of some common copper wire sizes at room temperature (25°C).

AWG#                Area                 Ohms/meter
Square mm

22                  .320                 .0529

24                  .205                 .0891

25                  .162                 .105

28                  .081                 .213

30                  .051                 .338

32                  .032                 .537

36                  .0127                1.36

Notice that as the AWG (American Wire Gauge) size increases, the area of the wire decreases and the resistance also increases. Different wire diameters have different resistances per meter.

Below you can see how I made my low inductance, low power 0.1 ohm shunt current sensing shunt for a wattmeter that required an approximately 3300 ppm temperature coefficient to compensate for the temperature coefficient of the gain of a differential pair.

At 15 watts, the shunt will dissipate approximately 390 micro watts; I don't think self heating is going to be a large factor in accuracy. Having established that, let's get on with the design of the shunt.

AWG #30 copper wire has a resistance of approximately 103.7 milliohms per foot, or 3.40 milliohms per cm. This means that a 0.1 ohm resistor would require 34.0 cm of AWG #30 copper wire. I carefully measured 34 cm enameled magnet wire, leaving a few mm for soldering, most of which would be shorted to the wire leads.

I then scraped the enamel from a few mm at each end for soldering and tinned the leads.

After tinning, I folded the wire to make it a pair of conductors that is shorted at one end, (where the loop is formed) and then twisted the pair until there were approximately 2 twists per centimeter.

At that point, I soldered the two ends to the leads of a 1/2 watt resistor. The resistor was to become the bobbin upon which the wire is to be wound. The value happened to be 11.5K ohms, but anything resistor of 100 ohms or higher would have worked fine.

I wound the pair of wires around the resistor body, as evenly as I could. You can just see the loop where wire was folded in two, at the lower left of the winding.

Usually, I apply fingernail enamel to keep the wire safely held in place, but this time, I used heat shrink tubing, and I think it turned out well. Not only is the wire held in place, but it is also protected against accidental abrasion.

After assembly, I checked the inductance on my digital inductance meters, both of which have a resolution of 100 nanohenries (An Even Better LC Meter), and could not see any inductance registered. The inductance of a similar 1.0 Ohm current shunt was found to be a little more than 2 microhenries, so based on that, the inductance of this, 1/100 the amount of wire, must have been very small indeed.

A check with Digital Volt-Ohm-Meters with a resolution of 0.1 ohms confirmed that the resistance is in the area of 0.1 ohms, and performance in the circuit provides additional confirmation that the value is acceptable for use in the circuit.

Sad note: While preparing this web page on 24 June, 2011, I learned of the passing of Robert Pease in an automobile accident on after leaving a memorial for Jim Williams who had died just over a week prior. This makes June, 2011 a very dark month for the analog community.  I think the article in the link below summarizes the great losses.
http://realityandreason.blogspot.com/2011/06/jim-williams-and-bob-pease.html

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First posted in June, 2011. Updated July 2011.

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