Shunt Resistors

Shunt resistance
High electrical power, apart from the use of high voltage, can only be realized with high electrical currents. However, high currents cannot be measured easily with commercially available amperemeters, since these usually stop at a few amperes. Therefore, a way must be found to divert the majority of the heat-generating current away from the meter rather than letting it flow through it. The easiest way to do this is to use a parallel circuit, where the parameters of the branch are known very precisely. Shunt means "derivation, branch" and is also used as an expression for parallel connection. Today, products specialized for current measurement are used here, the shunt resistors. They make it possible to measure very high currents with impressive precision. They have a very low electrical resistance (in the milli-ohm range) and, in modern products, are equipped with separate terminals for current and voltage. If you want to measure a high current, you build these components directly into the main circuit and use the voltage taps to measure the voltage drop using a voltmeter. Since the resistance of the shunt is known, the current - in the case of direct current - can be easily calculated using Ohm's law.

Resistor element, terminal block and terminals
The resistor material itself used in the shunts is a special copper-nickel-manganese alloy known as manganin. Manganin is excellent for these measurement applications because it has a very low and predictable thermal resistance coefficient and remains very stable over time. The terminal block is usually made of brass or copper where the high current wires are connected to the shunt and where the measurement terminals are located. The shunt has one position for heavy current carrying conductors and two smaller terminals for attaching the voltage sensing wires. The voltage drop is measured at the sensor terminals and this allows the current across the shunt to be determined.

Rated current and output voltage
Rated current is the current through the shunt specified for the product that results in the specified voltage drop, or millivolt output value. In other words, this means that the rated current flows through the shunt exactly when the output voltage assumes the value specified on the data sheet. This voltage serves as the reference voltage when selecting the component, because it allows the load (the decreasing power) on the element to be calculated directly (for DC current: power = current x voltage). The value is also indicated on the element itself and at the same time corresponds to the maximum power for which the component is specified.

Operating current and power reduction
Under normal operating conditions, we recommend that customers operate the shunt at no more than 2/3 of its rated current. On our data sheets, this value is specified as the operating current. This safety factor ensures a long service life.

Tolerances
The shunts have a standard tolerance of 0.25% for the voltage output and are calibrated at room temperature. Many of our models are also available with tolerances as low as 0.1%.

Rated power
The shunt dissipates the power generated by the voltage drop across the resistive element by generating heat. This power dissipation is subject to Ohm's law and results in a very low current consumption of the components, since the resistance is in the milli-ohm range. Nevertheless, it must be ensured that the heat generated can be dissipated. A 10-A shunt with 100 mV output voltage is operated at the maximum permissible power.

Power P = rated current I x voltage drop U = 10 A x 100 mV = 1 Watt

Operating temperature
Shunts operate most accurately at temperatures between 30 °C and 60 °C. Care must be taken to ensure that the heat generated by the power input can be dissipated. Therefore, please observe the installation instructions. Shunts can of course also be operated at lower temperatures, but this will reduce the accuracy. In this case it is recommended to calibrate the component separately for this deviating range. In principle, the temperature of the resistance elements should be limited to 125 °C for normal operation. If the temperature exceeds 145 °C, permanent damage must be expected. Please ensure suitable heat dissipation.

Temperature coefficient
The Temperature Coefficient of Resistance (TCR) indicates how the resistance value changes as a result of temperature changes, including self-heating. Our TCR of the resistive element material manganin is usually expressed in ppm/°C (parts per million per degree Celsius) and has a typical value of 20 ppm/°C.