A shunt resistor is a very low value, high power resistor that is connected in parallel with a low range meter, so it helps the meter increase its measurement capability significantly.

　　Many times, you may find it difficult to measure high currents with a modern multimeter. If you have ever considered purchasing an industrial shunt to solve this problem, you will realize how costly these shunts can be. While industrial shunts are very accurate, they usually cost more than the circuits they may monitor!

　　However, there is a simpler, cheaper option that can perform just as well in most cases: you can build your homemade shunt with a few cents worth of wire and a little technical expertise. It only takes a few minutes, and it can be fun!

　　What is a shunt?

　　A shunt is simply a very low value resistor (usually less than one ohm) that is used to assist in measuring current. As shown in Figure 1, the shunt resistor R..SH is connected in parallel with the meter to reduce its sensitivity by a set value.

Shunts do this by bypassing or "shunting" most of the current flowing through the meter. Thus, a shunt resistor allows you to convert any inexpensive conventional meter, such as a 0-1 milliamp meter, into a rugged 0 to

20 amp meters.

　　Choosing a Shunt Meter

　　Before attempting to make your own shunt, you must first purchase a suitable shunt meter. When choosing a meter, look for one that is in good working health and has a properly calibrated scale on the front.

　　For example, if you require a meter to measure 10 amps full scale, choose a meter with a scale range of 0 to 1.

　　If you want a full scale measurement of 30 amps, select one with a 0 to 3 scale.

　　Shunt Meter Resistance

　　To construct a shunt, you must know the meter's internal resistance.

　　Therefore, select a device that has its internal resistance marked. In most cases, this will be on the front or back of the meter using small characters around the terminals.

　　Assuming you already have a meter, but don't know what its internal resistance is, there is a quick way to find out. Pick up your digital multimeter (DMM)

and set it to its maximum resistance range. Connect the red (positive) lead of the digital multimeter to the positive terminal of the analog meter and the black (common) lead to the negative terminal of the analog meter.

　　By sending a small amount of current to the device under test, the digital multimeter can determine its internal coil resistance.

　　This type of measurement should not be attempted with an analog multimeter. These anaogue multimeters test resistors with higher currents, which may damage some of them.

　　Now, continue testing the lower DMM resistance range (remember, you start with the maximum value) until you see the analog meter's needle reach a full scale reading.

　　Make a note of the value on the digital multimeter and use a marker to write down the value on the back of the meter. Please be careful and perform this procedure carefully. You can easily damage the mA meter if you move it too fast and inadvertently fix it.

　　How to Build a Shunt Resistor

　　A small piece of copper wire is used to create the shunt. Since any wire will show some resistance, we can use this feature to build a shunt resistor.

　　To build a shunt, you must first calculate the amount of current that may pass through it. For example, if your meter is capable of measuring 20 amps full scale, the shunt wire should be able to safely carry that size of current.

　　Suppose you want to build a 20 amp shunt with a 0-1 milliamp residual analog shunt with a 0-1 step panel.

　　Select the most appropriate gauge wire from any copper wire diagram available on the internet. It is important to remember that the lower the wire gauge and the larger the diameter, the greater the current that can be safely handled.

　　250 per ampere

round mil is more than adequate for most amateur applications. Divide the circular mil of the selected wire (found in the copper wire chart) by the maximum current you decide to use through the wire to determine the circular mil per ampere of the shunt wire.

　　Circular mils per ampere = (circular mils of wire) / (current through the wire)

　　According to the copper wire diagram, the cross-sectional area of a 12 gauge wire is 6530 circular mils. We divide this by 20 amperes to get 326 circular mils/ampere, which should be sufficient.

Wires in the 12 gauge range are readily available and can be found at most hardware stores. Next, you can use the following formula to get the resistance of the shunt.

　　R.SH = RM/(n - 1)

　　where R.SH shows the shunt resistance, RM indicates the resistance of the analog meter, and n indicates the multiplication factor of the shunt.

　　In our setup, since we are using a 0-1 milliamp meter, 1 milliamp = 0.001 amps and n = 20 amps/0.001 amps, or 20,000.

　　Now, let's assume that the internal resistance of the analog meter is determined to be 81 ohms. Plugging this resistance value and n = 20,0000 into the above equation yields

　　R.SH= 81Ω/(20,000-1) = 0.00405Ω

　　This resistance looks small, doesn't it? A shunt with this resistance is suitable for passing a current of about 20 amps. For full scale deflection, this will allow 0.001 amps (1 mA)

flowing through the meter.

　　Now the length of the shunt must be calculated. Remember that the 12 gauge wire provides a resistance of 1.619 ohms/1000 feet as shown in the copper wire diagram. Therefore, the length of the shunt wire (LS)

can be calculated as follows.

　　LS= R.SH/(x Ω/1000 feet) = 0.00405/(1.619Ω/1000 feet) = 2.5 feet

　　Using a 0-1 mA meter with an internal resistance of 81 ohms to measure 20 amps full scale, the 12 gauge shunt should be 2 feet 6 inches long.

　　Now, the contact resistance of the meter may be a problem for shunts of this length. Considering the shunt resistance of 0.00405 ohms, even a solid solder connection may exhibit a high resistance.