A **shunt** (aka a **current shunt resistor** or an **ammeter shunt**) is a high precision **resistor** which can be used to measure the current flowing through a circuit. Using **Ohm’s Law** we know that the voltage dropped across a resistor divided by the resistance of that resistor is equal to the current, therefore if we measure the voltage across a **shunt resistor** in a circuit, we can easily calculate the current.

For example, pictured above is a typical **100 Amp Shunt Resistor**. This can be used to measure currents of up to 100 Amps – although to prevent overheating it should really only be used to measure continuous currents of no more than 60-70 Amps.

* *If a shunt resistor overheats it can permanently change the resistance of the shunt.*

This shunt is calibrated such that the **voltage drop** across it is 100mV when the current flowing through it is 100 Amps. Therefore we can calculate the resistance of this shunt to be voltage divided by current = 0.1 / 100 = 0.001 Ohms (typically to within ± 0.25% accuracy). Therefore if a voltage drop of 28mV is measured (using a standard **multimeter** or 0-100mV range voltmeter), we know that the current flowing is 0.028/0.001 = 28 Amps.

(The power *wasted* by the shunt resistor is given by multiplying the voltage by the current = 0.028 * 28 = 0.78 Watts in this example.)

To save making this calculation manually each time it is possible to re-label a 0-100mV moving coil voltmeter so it instead reads 0-100 Amps. This would be achieved simply by sticking the word “Amps” over “mV” on the face of the meter which would now be an **ammeter** rather than a **voltmeter**.

### Using a Shunt Resistor in a Renewable Energy System

It is very important to know how much current is flowing in and out of the **battery bank** in a renewable energy system. When charging the current flowing into the batteries should never be more than 10% of the battery capacity – e.g. a 100Ah battery should not be charged with more than a 10 Amp current or it may be damaged and/or overheat.

It is also very useful to know how much current is being generated by a wind turbine or solar panel, because that information helps you to calculate how much **power** is being generated. For example a 12 Volt 15 Watt **PV Solar Panel** may produce a voltage of 18 Volts when it is very cloudy and 21 Volts when it is very sunny.

When it is cloudy you may measure a current of just 0.1 Amps and when it is sunny a current of 0.8 Amps. Power is equal to voltage multiplied by current, so the solar panel is generating just 2 Watts when it is cloudy and almost 17 Watts when it is sunny.

### Make Your Own Shunt Resistors

It is possible to **Make a Basic Shunt Resistor** very cheaply with some copper wire. Click here to find out more.