Make a Shunt Resistor
Make A Shunt Resistor
Make your own rough and ready shunt resistor
electric circuit | education
A
shunt resistor (pictured above) is a high-precision resistor of very low resistance used by an
ammeter to measure the current flowing through an electric circuit.
Shunts are available in a range of sizes from under one Amp to many hundreds and thousands of Amps, but they can be very expensive.
Fortunately it is possible to make your own
shunt resistor from nothing more than a length of copper wire. While it will only be accurate to within 10%, and its accuracy will fall with changes in temperature much more so than a
real shunt resistor - making your own shunts is very cheap and easy.
Example DIY Shunt Resistor
In this example we will look at making a 0-10 Amp shunt resistor with a voltage drop of 10mV at 10 Amps. Therefore using a standard
multimeter or 0-10 mV
voltmeter
it will be easy to read the numerical value of the current (and ignore the units displayed) - i.e. if the voltmeter reads 6 mV we know this corresponds to 6 Amps of current, 2.4 mV corresponds to 2.4 Amps and so on.
Therefore, using
Ohm's Law it can be seen that since the voltage drop of 10mV divided by the current of 10 Amps give the required resistance of the
shunt = 0.001 Ohms, we need to make a 0.001 Ohm resistor.
Resistance of Copper Wire
Copper wire has a
resistance which depends upon its length and its diameter. The thicker the wire, the lower its resistance; the longer the wire, the greater its resistance. For example 24 AWG copper wire (see our
wire size converter for more information about
AWGs) has a resistance of 25.67 Ohms per 1000 feet of length at 25 degrees celcius.
Click here to view a comprehensive table of
copper wire resistance values.
Using Copper Wire to make a 0.001 Ohm Shunt Resistor
10 AWG copper wire (2.59mm diameter) for example has a resistance of 0.9989 Ohms per 1,000 feet. Therefore our 0.001 Ohm resisitor would require 1000 * 0.001/0.9989 = 1.001 feet of 10 AWG copper wire.
12 AWG copper wire (2.05mm diameter) has a resistance of 1.5883 Ohms per 1,000 feet and so 1000 * 0.001/1.5883 = 0.63 feet of 12 AWG wire would be required for the 0.001 Ohm resistor.
| Wire Length (feet) = |
1000 * Target Resistance (Ohms)
|
| Ohms per 1,000 feet |
|
Basic Shunt Resistor
In the simplest case copper wire of the calculated length is simply soldered in series into the circuit to be measured and the postitive and negative leads of the
voltmeter connected to those joints. Unfortunately this would not be a very accurate ammeter because the resistance of the soldered joints can affect the results.
Calibrated Shunt Resistor
The
shunt resistor will be much more accurate if it is calibrated by comparing the results it gives to those given by a precise
ammeter. An
ammeter should be connected into the circuit so the true value of the current flowing through it can be measured and this reading compared to that given by the
voltmeter and
copper wire shunt combination.
A length of copper wire a good 25% longer than calculated above should be joined in series with the circuit to be measured as shown in the above schematic. One of the leads of the
voltmeter should then be soldered near to one end of the copper wire. The remaining voltmeter lead can then be moved along the length of copper wire until the voltmeter reading is the same as that of the ammeter. The voltmeter lead can then be soldered into place leaving a calibrated relatively accurate
homemade shunt resistor.
Thick wire has lower resistance than thin wire therefore a more accurate
shunt can be made with thick wire, but it will be more expensive. To measure large currents thick wire
must be used as thin wire will overheat.
Article Last Modified: 12:52, 28th Feb 2007Comment on this Article
If you have any comments on this article, please email them to
neil@reuk.co.uk.
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