Current Sensors Information
Current sensors measure AC and/or DC current levels. The sensors in this selection guide measure current and provide some sort of output that corresponds to the current being measured. The most important distinction to make when selecting a current sensor is whether AC and/or DC current needs to be measured. Another important specification to consider is if the sensor needs to be in-line with the circuit or if it works by being clamped around the wire to be measured.
Current Sensor Technology Options
The technology used by the current sensor is important because different sensors can have different characteristics for a variety of applications. Most sensors work because a current-carrying wire produces a magnetic field. Current sensing resistors are used when current is directly measured in the circut.
Current Sensing Technologies. Image Credit: nktechnolgoies.com
Hall Effect - Hall effect sensors consists of a core, Hall effect device, and signal conditioning circuitry. The sensor works when the current conductor passes through a magnetically permeable core that concentrates the conductor's magnetic field. The Hall effect device, which is mounted within the core, is at a right angle to the concentrated magnetic field and a constant current (in one plane) excites the Hall device. The energized Hall device is then exposed to a magnetic field from the core and it produces a potential difference that can be measured and amplified into process level signals such as 4-20mA or a contact closure.
Inductive- Inductive sensors use a coil through which a current carrying wire passes. This causes power to flow in the coil that is proportional to the current. This happens because of the magnetic field produced by flowing current. Inductive sensors are used for AC currents. The sensor has a wire-wound core and a signal conditioner. As the current conductor passes through the core, it becomes magnified by the conductor's magnetic field. Since AC current is constantly changing potential from negative to positive (general rate of 50 to 60 Hz) and therefore creating an expanding and collapsing magnetic field, a current is induced in the windings. The secondary current is converted to a voltage and conditioned to output process; signals such as 4-20mA or contact closures.
Magnetoresistive- The magnetoresistive effect is the property of certain materials to change the value of its resistance as a function of a magnetic field applied to it. If magnetic flux is not applied, the current flows straight through the plate. If magnetic flux is applied, a Lorentz force proportional to the magnetic flux density will deflect the current path. As the current path is deflected, the current flows through the plate for a longer distance, causing the resistance to be increased.
Current Sensor Specifications
Current sensors receive current inputs and provide outputs:
Current Sensor Circuit. Video Credit: Thundertronics / CC BY-SA 4.0
Performance specifications describe how current sensor will interact and operate in the desired environment.
- Measuring range is the maximum current the sensor is capable of measuring.
- Input voltage is the voltage required to operate the device.
- Frequency range describes the range of values of the input frequency that the sensor can operate.
- Response time is the interval between the application of an input excitation and the appearance of the corresponding output signal.
- Isolation voltage describes the maximum voltage that the sensor can handle to protect the devices connected to it. Voltages above this specification can damage the sensor and render the measurements inaccurate.
- Accuracy measures the closeness of the measured value to the true value. Current sensors should be calibrated on a regular basis to ensure accurate measurements.
- Operating temperature describes the temperature range the sensor is designed to operate in. Exposure to temperatures outside of this range can damage the sensor.
The type of output used by a current sensor depends on the desired application and operating environment.
Image Credit: allegromicro.com
- Analog voltage signals - Sensor output is an analog voltage signal.
- Analog current levels - The sensor outputs analog current signals. It may be the standard 4-20mA used for signal transmissions.
- Switches and audible signals - The sensor can also be connected to a switch or an audible alarm which indicates current level via a noise or process change.
- Modulated frequency outputs - The current sensor can output any of the frequency outputs listed below.
- Sine waves
- Pulse trains
- Amplitude modulation (AM)
- Frequency modulation (FM)
- Pulse width modulation (PWM)
- Self-powered current sensors do not require external power.
- DC powered devices require DC voltage.
- AC powered devices require AC voltage.
Mounting and Configuration
There are several mounting or installation options for current sensors.
- Panel mount installation is designed to be panel-mounted into a wall or modular unit.
- SMD is a circuit mounted device assembled from surface mount components and technology. SMD current sensors attach directly to the circuit board, without the need to fit wire leads through holes in the board.
- Clamp or bolt on sensors clamp or bolt onto the surface. These are best used for rough or extreme applications to prevent the sensor from falling off.
- Through hole technology (THT) mounts components on a printed circuit board by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board.
- Surface mount technology (SMT) adds current sensors to the printed circuit board (PCB) by soldering component leads or terminals to the top surface of the board.
- Handheld products are portable and do not require special mounting hardware.
Sensor types can be closed loop -- meaning they have to completely surround the current carrying wire -- or open loop where they don't need to make a complete circle.
Open loop vs. Closed loop current sensor. Image Credit: powerguru.org
- Open loop sensors are typically used in battery-powered applications. They require less power than closed loop sensors and are better able to withstand overloads.
- Closed loop current sensors provide faster response times and better linearity than open loop sensors. They also produce less noise, making them a good choice for power supply switching.
Both open loop current sensors and closed loop current sensors are used in hazardous environments.
- Isolated tip means that the active, measuring tip of the sensor is electrically isolated.
- High voltage current sensors are designed for high voltage applications.
- Intrinsically safe equipment is defined as "equipment and wiring which is incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration." This is achieved by limiting the amount of power available to the electrical equipment in the hazardous area to a level below that which will ignite the gases.
Image Credit: allegromicro.com
There are a variety of applications for current sensors. For example, some devices are used for power metering, control system diagnosis, current supply measurement, and the control of complex loads from electric motors. Others are used for charge integration and condition monitoring of rechargeable batteries. Specialized current sensors, such as those used in commercial appliances, often incorporate safety cut-off features and surge trips. High voltage current sensors are also available. PCB-mounted current sensors combine the benefits of a small footprint with the advantages of signal processing that can be achieved by using an existing microprocessor.
Current sensors that are identified as intrinsically safe (IS) equipment are low-power devices that meet design, installation, and maintenance standards established by the Instrumentation, Systems and Automation (ISA) society. Other current sensors include:
CSA Mark - Products that bear a CSA Mark have been tested by the Canadian Standards Association (CSA) and meet applicable standards for safety and/or performance. These standards are written and administered by organizations such as the American National Standards Institute (ANSI), Underwriters Laboratories (UL), and CSA International. CSA Marks may appear alone or with indicators. CSA Marks that appear alone are used with products certified primarily for the Canadian market, to the applicable Canadian standards. CSA Marks that appear with the indicators "C" and "US" or "NRTL/C" are used with products certified for both the U.S. and Canadian markets, to the applicable U.S. and Canadian standards.
RoHS - Restriction of Hazardous Substances (RoHS) is a European Union (EU) directive that requires all manufacturers of electronic and electrical equipment sold in Europe to demonstrate that their products contain only minimal levels of the following hazardous substances: lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyl and polybrominated diphenyl ether. RoHS became effective on July 1, 2006.
Underwriters Laboratories - Underwriters Laboratories (UL) is a non-profit organization that tests components, systems, and materials according to its published standards for safety. Products that receive UL approval bear a UL Mark.
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