Current sensor

Current sensors are electrical devices with which the current can be measured in cables and busbars usually electrically isolated (non-contact ) from the electric currents induced by the magnetic flux density.

It is between sensors which can detect only AC, and those that can capture direct and alternating currents, distinguished.

• 3.1 Thermal Sensors
• 3.2 Faraday effect
• 3.3 The magnetoresistive effect without toroidal

• 4.1 Automation Technology
• 4.2 Vehicle Industry
• 4.3 Energy Technology
• 4.4 Power Electronics

AC sensors

Alternating current sensors are referred to as a current transformer in the strict sense. They usually consist of a toroidal core and are like a transformer running. The alternating magnetic field of a to be detected through which the alternating current induced in the conductor an alternating current detection coil, which is proportional to the measurement current through the reciprocal turns ratio. CTs represent current sources and are to avoid impermissibly high output voltages or core heating with a maximum permissible so-called burden ( terminator ) specified. Unused transformer are short circuited.

For small flows current transformers have a separate winding for the to be measured AC, for large currents - the areas go according to type up in the range of several kilo amperes - the current transformers on the solid conductor is moved (eg a bus bar ) so that only a single " Swirl " is given by the transducer through.

Other models:

• Designed for AC currents current clamps with large opening that can be " folded " over the current to be measured conductor.
• Ferrite toroidal core current transformers in switching power supplies, inverters or other higher-frequency applications
• Rogowski current transformer: coil without core to the conductor to be measured

By selecting the turns ratio of the coils involved a large measuring range can be covered without affecting the primary circuit appreciably. A current transformer according to this principle do not require external energy to operate.

DC sensors

DC sensors can also detect inherently alternating currents and transients. You need an auxiliary energy source ( supply voltage ).

Transducer with Hall probe

In principle, the current sensors, which are equipped with Hall sensors, sensitive to temperature and may optionally be temperature-compensated. Very high currents and high temporary inrush currents can magnetize the core material used, leading to measurement error and zero drift (offset error ) performs remanence.

Direct imaging current sensors

These sensors work with a Hall sensor. In this case, a slotted ring core of a possible linear, soft-magnetic ferromagnetic material is used, the current-carrying conductor which either surrounds or to which a primary winding with few coils is applied. The Hall effect sensor itself is housed (air gap) in the slot. The air gap also limits the magnetic flux density, the linearized relationship between the magnetic field and current and so allows measurements over a wide range. The measurement signal of the Hall sensor being proportional to the magnetic field and hence the current. At very low currents, the picture is not exactly linear, possibly may even make the Earth's magnetic field disturbing.

According to this principle usually work also current probes, which can detect AC and DC currents. These current probes consist of a hinged ring core.

Compensation current transformer with Hall probe

These sensors are similar in structure to direct imaging sensors. On the core surrounding the conductor, however, a coil is additionally applied, which is traversed by a compensation current, which is generated by an electronic circuit such that the magnetic fields of the measurement current and the compensation current ( counter-field ) of the sensor. A compensation winding to the terminals of the measuring resistor externally schleifter generates a voltage proportional to the current, which forms the output signal.

Compensation converters are more accurate than directly imaging Hall current sensors, among other things can thus be minimized with accurate measurements and low currents, the influence of the geomagnetic field. They have lower offset and linearity errors. But they are also more expensive and require more power supply.

Compensation current transformer with soft magnetic probe

These sensors also have a core made of ferromagnetic material, but it has no air gap, but an integrated soft-magnetic sensor. It controls as well as for the compensation current transformers with a Hall sensor on an electronic current through a compensation winding, so that the magnetic flux is zero. These current sensors have much smaller offset, hysteresis and temperature error as a Hall current sensors.

More current sensors

Thermal sensors

For high-frequency currents sometimes a wire is used, which is heated by the current to be measured. This heating is measured using a suitable device, such as a Hitzdrahtmesswerk or - with appropriate temperature coefficient of electrical resistance of the wire - a resistance measurement.

In motor circuit breakers and miniature circuit breakers bimetallic strips are used, either by itself flowed from current or wear a thick filament winding. Such current sensors have the following RMS current deflection and a certain thermal inertia and are therefore particularly suitable for overcurrent protection devices. They are also used in the secondary circuit of current transformer to turn off in case of excess current of the main circuit breaker.

Faraday effect

Here, the circuit in which the current to be measured, surrounded by a transparent optical medium, for instance with a glass fiber. The polarization direction of light passing through the medium linearly polarized light beam is rotated by the magnetic field surrounding the conductor due to the Faraday effect. Of the electric current in the conductor can be determined by measuring the optical rotation of this plane. Such fiber- optic current sensors, FOCS abbreviated from English- fiber optical current sensor, or MOCT of English magneto optical current transformer, work without magnetic influence of the conductor and also offer a potential separation even at very high voltages. Accordingly, they are used, inter alia, in the electric power industry in the field of high-voltage systems as an alternative to current transformers. Another application is to measure the very high currents in electrolysis technology.

The measuring range of FOCS goes up to 500 kA ( pulse currents) and can achieve an accuracy of 0.1%.

Magnetoresistive effect without toroidal

Compact power sensors that use the magnetoresistive effect and thereby manage without external toroidal core for magnetic field shaping.

The sensor is located in an integrated circuit. The current is conducted through the component, or it is placed over the conductor track to be measured on the circuit board, as shown in the adjacent figure. The measuring range is a few 100 mA up to several Amps.

Types of current sensors

Automation Technology

In automation technology, signals with 0 ... 10 V or 4 ... 20 mA are common to transmit various physical quantities analogous. For this signal level above current sensors and current transformers are not suitable, because they do not provide sufficiently high resistive loads or do not have the offset value of 4 mA. Therefore, current sensors with 0 ... 10 V or 4 ... 20 mA output are also provided for the automation technology. They have to an integrated signal processing, including scaling, often outputs the average in an AC RMS and often the case of direct current.

Vehicle industry

Current sensors which are used in motor vehicles, for example to To monitor battery voltage and electrical system, often have CAN and LIN - bus connections, which are implemented with microprocessors. Other types provide a PWM output.

Energy Technology

Current transformers and sensors for high voltages often have the shape of a post insulator. For low voltage (< 1000V ) it stuck on the power rail. Since in many cases they also have to withstand lightning strikes and the short circuit current, they are mechanically stable to withstand the magnetic forces can. They are often encapsulated or coated with synthetic resin. To display the current moving iron works are often used, the scale is adapted to the gear ratio. The secondary current is typically 5 amps. Moving iron works show the rms value.

Power Electronics

Current sensors for PCB solder are typical here, with the Messstromwindung is part or not. There are both such as a finished component as well as user-specific structures of ferrite toroidal cores for use in switching power supplies, inverters.

Manufacturer

Manufacturers of current sensors include:

• BFi OPTiLAS GmbH, Germany
• LEM, Switzerland
• Honeywell, United States
• Vacuum melt, Germany
• Wang Enterprises, China
• Asea Brown Boveri ( ABB), Switzerland
• Allegro Microsystems, USA