Current Sensor Measurement and Challenges

By Carolyn Mathas

Contributed By Electronic Products

A current sensor is a device that detects electrical current (AC or DC) flow and generates a signal proportional to it. Measuring both low and high current with sensors to detect an overcurrent or undercurrent condition makes these devices ideal for providing status information or protection for electrified equipment, for example, in hybrid vehicles and fuel cells where it is critical to measure current and power with high accuracy and precision. Other automotive uses of current sensor ICs include HEV inverters and electronic power steering (EPS) systems.

In order to maintain accuracy over the temperature range inherent in automotive applications, from -40° to 125°C, many challenges exist, among them, drift. Also, at high currents and temperatures, failure rates are more apt to occur as a sensor may not be able to withstand higher current surges.

The measuring of high current is particularly problematic. Resistors used in many circuits often cannot handle the high voltage. As a result, measuring at this level requires a special-purpose current transformer or transducer to reduce the magnitude of the current in the circuit to a manageable level.

The two types of current transformers involved are split core and solid core. Split-core current (open) transformers can be placed near existing wires without circuit disruption. As such, while they are often expensive, they can reduce overall system cost. In comparison, solid-core current (closed) transformers require the circuit to be rewired to pass though the core, but they can deliver extremely precise and accurate current measurement compared with the open transformer variety.

Another major challenge of using current sensors is that the outputs of multiple sensors employed throughout a system are often very different. Consequently, if the wrong sensor is connected, or if it is connected improperly, equipment damage or a safety hazard to personnel can result.

Engineers can select from three main current measurement technologies. These are shunts with and without galvanic isolation, and open-loop and closed-loop Hall-Effect current sensors.

Shunts are self-heating at high current and when the shunt resistance is high, providing good signal amplitude, there are several variations of resistance with current that result. The advisory in this case is that materials must be selected carefully and temperature gradients minimized, or thermoelectric voltages will distort low current readings.

Open-loop sensors have problems at high temperatures or with temperature shift. Given that their performance can change with temperature, they can exhibit drift of offset and gain. They cannot, however, provide the gain stability of closed-loop sensors. Destructive overload capability favors open-loop current sensors. The rating represents the rating of the conductor passing through a device.

In comparison, closed-loop sensors absorb significant quiescent current when the primary current is high, causing self-heating. They handle short-duration high-current transients well. High currents often damage shunts as they act like a fuse. Thermal transients also alter their calibration.

The CR5200 Series DC Current Transducers by CR Magnetics (Figure 1) provide a DC signal proportional to a DC sensed current. Designed for direct current only, the 2 to 10 A range use an advanced Magnetic Modulator technology, while device ranges of 20 A and above use Hall-Effect technology.

CR Magnetics C5210, CR5211, and CR5220

Figure 1: The C5210, CR5211, and CR5220 are single-element, 0.79 in.-window, 2 to 300 ADC input range transducers.

The series is closed loop for accuracy, and features 35 mm DIN rail- or panel-mount, noncontact DC current sensing. Applications include power supply management, mobile applications, DC motor drives, and battery chargers and systems. The transducer is available in a split-core design.

The Allegro ACS710 (Figure 2) is a 120 kHz bandwidth, high-voltage isolation current sensor with integrated overcurrent detection. It provides an economical and precise means for current sensing in industrial, commercial, and communications system applications.

Allegro ACS710

Figure 2: The current-conduction path of the ACS710 is electrically isolated from the low-voltage sensor inputs and outputs allowing the sensors to be used in applications requiring electrical isolation, without the use of optoisolators or other costly isolation techniques.

The ACS710 consists of a precision linear Hall-sensor IC with a copper conduction path located near the surface of the silicon die. Applied current flows through the conduction path, and the analog output voltage from the Hall sensor linearly tracks the magnetic field generated by the applied current.

The voltage on the Overcurrent Input (VOC pin) allows customers to define an overcurrent fault threshold for the device. When the current flowing through the copper conduction path (between the IP+ and IP– pins) exceeds the threshold, the open-drain Overcurrent Fault pin will transition to a logic low state.

According to Allegro, high-level immunity to current conductor dV/dt and stray electric fields, offered by proprietary integrated shield technology, results in low ripple on the output and low offset drift in high-side, high-voltage applications.

Other features include:
  • A small-footprint package suitable for space-constrained applications.
  • 1 mΩ primary conductor resistance for low power loss.
  • High isolation voltage, suitable for line-powered applications.
  • User-adjustable Overcurrent Fault level.
  • The Overcurrent Fault signal typically responds to an overcurrent condition in < 2 μs.
  • An integrated shield is said to virtually eliminate capacitive coupling from current conductor to die due to high dV/dt voltage transients.
  • A filter-pin capacitor improves resolution in low-bandwidth applications.
  • The part offers 3 to 5.5 V single-supply operation.
  • It has factory-trimmed sensitivity and quiescent output voltage.
The LEM HY 5 to 25-P Current Transducers (Figure 3) are used for electronic current measurements (DC, AC, pulsed, mixed) with a galvanic isolation between the primary circuit (high power) and the electronic circuit. Features include Hall-Effect measuring, 2,500 V isolation voltage, low-power consumption, and an extended measuring range (3 x IPN).

HY Series Hall-Effect transducer from LEM USA

Figure 3: The HY Series Hall-Effect transducer from LEM USA.

The HY Series Hall-Effect transducer is designed for PCB mounting. The package is consistent for a range of currents from 5 A through 50 A. It is a bipolar device with galvanic isolation between the primary and secondary circuits. It has a voltage output and each current-range device has a measuring range of three times the nominal current.

Applications include switched-mode power supplies, general-purpose inverters, AC variable-speed drivers, uninterruptible power supplies, and battery-supplied applications.

LEM USA’s HMS xx-P Series Open-Loop Surface-Mount Current Transducers (Figure 4) are designed for low-current applications. They feature lower drift, improved accuracy over the temperature range, high reliability due to ASIC technology, improved manufacturing process efficiences, and they are said to meet critical industry standards. Applications include solar system, wind turbine, marine energy, motor control, small UPS, converter, power supplies for welding, laser cutting, among others, and such appliances as sewing machines, vacuum cleaners, white goods, and air conditioning.

LEM HMS series

Figure 4: The LEM HMS series is designed for low-current applications (Source: Datasheet).

Features include Hall-Effect measuring principle, 4,300 V isolation test voltage, extremely-low (12 mm) profile, single power supply (+5 V), and fixed offset and gain.

The advantages of the series include a small space-saving design, only one design for a wide primary current range, and high immunity to external interference.

Current sensing is an essential function in end uses ranging from safety-critical automotive and industrial applications to handheld devices, where determining power and efficiency are vital. Precision current monitoring allows designers to obtain critical information (such as motor torque, based on motor current) or diagnostic information. When even small changes in current can be detected it can lead to avoiding costly repairs and mechanical problems before they occur. This article has examined several types of current-sensing technologies and looked at some example devices. For more infromaiton on the products discussed here, use the links provided to access product information pages on the Digi-Key website.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Carolyn Mathas

Carolyn Mathas has worn editor/writer hats at such publications as EDN, EE Times Designlines, Light Reading, Lightwave and Electronic Products for more than 20 years. She also delivers custom content and marketing services to a variety of companies.

About this publisher

Electronic Products

Electronic Products magazine and serves engineers and engineering managers responsible for designing electronic equipment and systems.