Shunt (electrical engineering)
As a shunt (pronounced ʃʌnt), also known as shunt resistance or shunt resistance indicated, an electrically conductive originally device referred to, which at a part of a circuit connected in parallel is, an electric current of this part derive.
If a current intensity is to be measured that is greater than the maximum directly measurable current intensity in the adjacent picture, a shunt resistor is connected in parallel to the ammeter in order to expand its measuring range. It is designed in such a way that at the desired full- scale value, the portion flows through the measuring device and the (mostly larger) remainder flows through the shunt.
The term shunt has evolved and also describes a current measuring resistor , that is a low-resistance electrical measuring resistor , often equipped with separate current and voltage terminals. This is inserted directly into the live line. With the voltage measuring device connected in parallel to this type of shunt , only a mostly negligible current is diverted.
Application for current measurement
principle
In the case of high currents, a guide value greater than 10 A, the current measurement is usually carried out by measuring the electrical voltage on a shunt with a voltage measuring device. These shunts are often designed for a voltage of 60 mV or 150 mV at the nominal value of the current strength. The current strength in the shunt is calculated according to Ohm's law . Example: If the voltage drop is 55.0 mV across a shunt of 0.500 mΩ, the current is 110 A.
The shunt is built into the line of the current to be measured. The small voltage drop across this resistor is measured. The contact resistances in the current terminals are often greater than the measuring resistance, but quantitatively unknown. There is a significant drop in voltage of unpredictable magnitude across the terminals; therefore the measuring voltage at the shunt is often tapped via two additional voltage terminals (also called Kelvin terminals or Kelvin contacts) located between the current terminals in a four-wire connection.
Electronic circuits with operational amplifiers can generate a current to ground or a voltage to ground from the voltage, which is usually “floating” at an increased potential.
Construction
Shunts are mechanically robust for measuring high (> 100 A) currents, for example made of metal strips or rods with strong screw contacts for the load circuit (consumer circuit) and two smaller connections for the measuring device. Large shunts made of parallel metal rods can be screwed directly between busbars. Materials for the resistors are, for example, Manganin , Isotan , Isabellin , which are characterized by a low temperature coefficient of the specific electrical resistance and a low thermal voltage against copper. Sufficient heat dissipation is required.
Furthermore, small shunts are manufactured for soldering into printed circuits ; these too often have Kelvin contacts. Current measurement with a shunt has decreased somewhat with the advent of current sensors (these also offer potential isolation ), but shunts are an inexpensive and precise method of current measurement and are used in power electronics assemblies for current monitoring and control in addition to measuring devices .
Low inductance designs
If a current intensity with short rise times or high frequencies is to be measured, special designs with low parasitic inductance must be used. Axial or even worse axially wound resistors cannot then be used. More suitable are bifilar wound resistors or special designs, such as the coaxial shunt, which consists of two tubes plugged into one another through which the current flows in opposite directions. Other low-inductance designs are the Möbius resistor or wave-shaped resistor foils.
Other uses
Further examples with components connected in parallel for the task of deriving an electrical current from a circuit part:
- A capacitor can divert unwanted high-frequency signals to ground .
- A Zener diode together with a series resistor stabilizes the voltage across a load by diverting the current coming from the series resistor, which the load could only absorb with an increase in voltage.
- A varistor is suitable for protection against overvoltage . In normal operation, its resistance is very high, while in the event of an overvoltage the resistance is reduced by powers of ten almost without delay and discharges charge.
- A freewheeling diode is used to protect against voltage peaks when switching off an inductive DC voltage load, for example a relay coil , by diverting the current forced by the inductance from the switch and leading it into a closed circuit.
- A shunt winding is a field winding in a shunt machine that is connected in parallel to the armature circuit or part of it.
However, these components are not referred to as shunt in common parlance - even in these applications.
Web links
literature
- Adolf J. Schwab: electrical energy systems . 2nd Edition. Springer, 2009, ISBN 978-3-540-92226-1 .
Individual evidence
- ↑ IEC 60050, see DKE German Commission for Electrical, Electronic and Information Technologies in DIN and VDE: International Electrical Engineering Dictionary IEV. Entry 151-13-32.
- ↑ IEC 60050 - IEV [1] entry 313-09-04.
- ↑ a b DIN EN 60617–4: 1996 Graphic symbols for circuit diagrams - Part 4: Circuit symbols for passive components , August 1997, entry 04-01-10
- ↑ Rupert Patzelt, Herbert Schweinzer (Ed.): Electrical measurement technology. Springer, 2nd ed., P. 305
- ↑ ROHM Semiconductor: Shunt resistor (sic!) [2]
- ↑ Measuring amplifier, data sheet with circuit diagram [3]
- ↑ Measuring amplifier, data sheet with circuit diagram [4]
- ↑ SMD current measuring resistor, data sheet with picture [5]
- ↑ Klaus Schon: High-voltage measurement technology: Basics - measuring devices - measuring methods. Springer Vieweg, 2016, p. 177
- ↑ Adolf J. Schwab: High-voltage measurement technology: measuring devices and measuring methods. Springer, 2nd ed. 1981, p. 156
- ↑ IEC 60050 - IEV [6] entry 411-37-10.