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How to Make Copper
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
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
I like to use
resistors as the winding form for these hand-made resistors. If you
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
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).
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,
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
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
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
for soldering and
tinned the leads.
After tinning, I folded the wire to make it a
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
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
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.
Contents ©2011 Richard
Cappels All Rights
Reserved. Find updates
First posted in June,
2011. Updated July 2011.
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