Hall Effect Current Sensors

Three of the LEM technologies (open loop, closed loop and Eta) are based on the Hall effect, discovered in 1879 by American physicist Edwin Herbert Hall at Johns Hopkins University in Baltimore. They are used in both our current sensors and voltage sensors portfolios.

Hall Effect - Current Sensors Technologies

open loop current sensors

Open Loop Current Sensors (O/L)

Open loop current sensors use the simplest implementation of the Hall effect. They provide the smallest, lightest, and most cost effective current measurement solution while also having very low power consumption.

Features

  • Small package size
  • Extended measuring range
  • Reduced weight
  • Low power consumption
  • No insertion losses

Operation principle Open Loop O/L

The magnetic flux created by the primary current IP is concentrated in a magnetic circuit and measured in the air gap using a Hall device.

The output from the Hall device is then signal conditioned to provide an exact representation of the primary current at the output.

Open Loop Current Sensors Advantages and Limitations

Open loop sensors ensure DC, AC and complex current waveforms measurement while providing galvanic isolation. As we said, the benefits of O/L current sensors include low cost, small size, lightweight, and low power consumption. They are also especially advantageous when measuring high (> 300 A) currents. As with most magnetic based measurement techniques, insertion losses are very low. Primary current overloads can be easily handled although it may result in some magnetization of the core creating an offset shift, called remanence or magnetic offset.

When compared to other technologies, the limits of open loop sensors are moderate bandwidth and response time, a larger gain drift with temperature, and a limitation on the
current frequency product (power bandwidth).

In many applications the advantages outweigh the limitations and an open loop solution is advised.

Typical application of Open Loop Current Transducers 

Open loop current transducers are used in several applications as the key element of a regulation loop (current, torque, force, speed or position for example) or simply to drive a current display.

Typical applications include:

closed loop current sensors

CLOSED LOOP CURRENT SENSORS (C/L)

Compared to the open loop sensors, Hall effect closed loop sensors (also called Hall effect "compensated" or "zero flux" sensors) have a compensation circuit that dramatically improves performance.

Features

  • Wide frequency range
  • Good overall accuracy
  • Fast response time
  • Low temperature drift
  • Excellent linearity
  • No insertion losses

Operation principle Closed Loop C/L

The magnetic flux created by the primary current IP is balanced by a complementary flux produced by driving a current through the secondary windings.

A hall device and associated electronic circuit are used to generate the secondary (compensating) current that is an exact representation of the primary current.

Hall Effect - Voltage Sensors Technologies

closed loop voltage sensors

Closed Loop Voltage Sensors (C/L)

It is also possible to measure a primary voltage with galvanic isolation using the same LEM Hall Effect technology.

These voltage sensors are based on the more sensitive and accurate current measurement technologies, such as closed loop Hall effect designs.

The main difference from a current sensor is the addition of an internal primary winding with a large number of turns, allowing the sensor to create the necessary ampere-turns to measure the small primary current.

Features

  • Measurement of high voltages
  • Safety isolation
  • Good overall accuracy
  • Low temperature drift
  • Excellent linearity

Operation principle Closed Loop C/L

A very small current limited by a series resistor is taken from the voltage to be measured and is driven through the primary coil. The magnetic flux created by the primary current IP is balanced by a complementary flux produced by driving a current through the secondary windings.

A hall device and associated electronic circuit are used to generate the secondary (compensating) current that is an exact representation of the primary voltage. The primary resistor (R1) can be incorporated or not in the voltage sensor.

Closed loop current sensors advantages and limitations

Closed loop current sensors ensure DC, AC and complex current waveforms measurement while ensuring galvanic isolation. The main benefits of this technology include very good accuracy and linearity, low gain drift, wide bandwidth, and fast response time. Another advantage is the output current signal that is easily scalable and well suited to high noise environments. At the same time, closed loop sensors are available in voltage output configurations. Just like open loop current sensors and similar to most magnetic based measurement techniques, insertion losses are very low on closed loop current transducers.

On the other hand, closed loop technology is showing limits with its high current consumption from the secondary supply (which must provide the compensation as well as bias current), the larger dimensions (more noticeable on high current sensors), a more expensive construction compared with the simpler open loop designs and a limited output voltage due to the internal voltage drops across the output stage and secondary coil resistance.

Here again, the advantages of C/L current sensors often outweigh the limitations and the accuracy and response of this solution is desirable over other alternatives. The application requirements will help to determine the best solution.

Typical applications of closed loop current transducers

Closed loop transducers are the perfect fit for applications requiring high accuracy, wide bandwidth and fast response time. They are often used as the key element of a regulation loop for the control of current, torque, force, speed and/or position as well as for the protection of semiconductor devices.

Typical applications are the same as for open loop sensors. Although in this case, higher performance results can be expected:

  • Frequency inverters and 3-phase drives
  • Power factor correction converters
  • Electric welding equipment
  • Power supplies and Uninterruptible Power Supply (UPS)
  • EV motor control
  • Automotive battery management system (BMS)
  • Electric traction systems
  • Trackside circuit breaker and rectifier protections
  • Energy management systems

Closed loop voltage sensors advantages and limitations

Closed loop voltage sensors also allow for DC, AC and complex current waveforms measurement while ensuring galvanic isolation. The benefits and limitations are the same as for closed loop current sensors.

Therefore, C/L voltage sensors offer very good accuracy and linearity, low gain drift, wide bandwidth, and fast response time, while also providing an output current signal that is easily scalable and well suited to high noise environments. Here again, we benefit from very low insertion losses.

On the limitations side, we got high current consumption from the secondary supply, the larger dimensions, a more expensive construction compared with open loop sensors and a limited output voltage.

The benefits of closed loop voltage transducers often counterbalance the limitations. Its accuracy and response makes it a recommended solution over other choices.

Typical applications of voltage loop current transducers

Closed loop Hall effect voltage sensors are used in many applications to detect, monitor and regulate voltages. A typical application is the monitoring of input, output and DC filter voltages of frequency inverters, where accuracy and isolation are of primary importance.

Typical applications include: