Current sensor

Below you can find relevant Current Sensor information - the links will lead you to our Danisense Current Sensor range.

An Introduction to Current Sensors

A current sensor enables measurement. Once a designer has a measurement, it can be used to monitor, control and protect systems. Current sensors are increasingly common in motor speed control, overload protection, fault detection, battery chargers, power supplies and programmable current sources. This article will look at the common types of current sensors and their advantages. At Danisense we most often refer to our product as a Current Sensor or a Current Transducer

6 Danisense Current Sensors in a test setup

What is a Current Sensor?

A current sensor is a device that measures how much current is flowing through a conductor. Implementations range from non-isolated direct measurement techniques to sensing the magnetic fields.

Current sensor architectures

Shunt Resistor Current sensor

This simple current sensor employs direct sensing. Using Ohms law (V=IR), the value of the shunt is known, the voltage across it measured with an amplifier, so the current is determined. This works with both AC and DC currents, but is a non-isolated solution.

A shunt resistor current sensor is not suitable for high voltage or high power applications, due to heat and power dissipating in the shunt. Typically, they are used with less than 20A and 48 V. The shunt resistor should be high tolerance, low temperature co-efficient and low resistance to improve accuracy and reduce losses. The physical implementation often employs several resistors in parallel to reduce the losses, resulting in a high component count. Shunt resistor current sensors require more board space, but are low profile solutions.

Hall-effect current sensors

The Hall-effect states, when a conductor (e.g. a metal plate) carrying a current is put into a magnetic field, a voltage is generated perpendicular to the flow of current. Without the magnetic field, the current will flow in a straight line, but with a magnetic field is applied, the path of the current will move to one side, creating a voltage differential. The voltage is directly proportional to the current, so it can be calculated.

A Hall-effect current sensor uses a Hall element. When the Hall element is powered and placed in a magnetic field perpendicular to the surface, it generates a voltage proportional to the magnetic field strength.

Ampere’s law states when a current flows in a conductor a magnetic field is generated. The current sensor has a magnetic core, which is used to concentrate the magnetic field generated by the current, into an air gap. The Hall element is placed into the airgap, and the resultant output voltage is directly proportional to the original current. Hall-effect current sensors offer good accuracy, compact size, fast response time, low insertion loss, low cost and work with both AC and DC.

Open Loop versus Closed Loop current sensors

The main concern with the open loop current sensor is linearity and temperature offset drift, both of which are addressed by the closed loop configuration.

In a closed loop current sensor, a second Hall-effect current sensor feeds the current back into a second magnetic coil, placed in the opposite direction. This compensates for non-linearities and temperature drift, provides a faster response and is relatively immune to electrical noise.

Both open and closed loop Hall-effect current sensors have an air gap, which is sensitive to external magnetic fields, making them unsuitable for applications where large EMF are present, such as MRIs, or large power supplies

Magento Resistor

A Magneto-resistor is a component whose resistance changes proportionally to the magnetic field. When a Magneto-resistor is placed in the air-gap, the primary current creates a magnetic field, which changes the resistance of the material, resulting in a current measurement. This approach has a lower offset temperature, is typically more accurate than a Hall-effect current sensor, but the air gap means it is still sensitive to EMC.

Fluxgate current sensor

A fluxgate is a device with a highly permeable core, whose current saturates quickly. When a fluxgate is driven with a square wave, the current’s profile becomes a series of positive and negative saturation and de-saturation cycles. Placing the fluxgate into the airgap of a core, where the magnetic field is concentrated, creates a shift in the zero-crossing point of these cycles. Using signal processing, the shift is turned into a current measurement.  Fluxgate current sensors have good offset and drift characteristics. There are several variations designed to increase performance.

Closed-loop

Similar to the Hall-effect current sensor, a closed loop architecture can be created. The current output is re-injected into a secondary element, which creates a magnetic field in the opposite direction. With this method, the magnetic field experienced by the fluxgate is always zero, eliminating offset and linearity issues.

Single Core and Double Core

The single core current sensor uses a coil without an airgap as the fluxgate element. By removing the airgap, the design avoids sensitivity to EMF, extending the useful applications to include power supplies and MRIs. It has high resolution, but, as the saturation occurs very quickly, the bandwidth is limited to a few Hertz. A double core current sensor increases the bandwidth by adding a winding core, which also increases the price.

Balanced Core

A balanced core current sensor uses two matched fluxgate elements placed in opposition. This renders the design immune to external factors, such as temperature and EMC, as it has an inherent compensation between the two elements. This approach achieves very high accuracy, even in challenging environments.

 

Fluxgates current sensors are more expensive than Hall-effect current sensors, the cores also need height, which can be a design constraint, however they offer the highest accuracy of the designs discussed. These last three designs have the advantage of no airgap, and so are suited to high EMF situations.

AC only current sensors

Rogowski Coil

A Rogowski coil is single layer winding on non-magnetic core, often referred to as an air core. The coil is placed around the wire carrying the current needing to be measured. As the polarity of the AC current changes, the magnetic field generated expands and collapses. The changing magnetic field created induces a current in the windings of the Rogowski coil, proportional to the primary current, which is processed to produce a measurement.

A major advantage of an air core, is the lack of magnetic saturation, creating a very linear output even for high currents. The Rogowski coil is used mostly for high currents and signals with high-frequencies harmonics, as its bandwidth extends into the MHz. The core can also be flexible, and easily wrapped around the test cable, making them suitable for difficult aftermarket installations, or live wires.

Current Transformers Sensor

A current transformer sensor is a Rogowski coil with a magnetic core that concentrates the magnetic flux inside the coil. This creates a direct relationship between the coil current and the primary current. They do not require a power supply, have very low power dissipation, and so can be used on higher signal levels. Due to their size and cost, they are mostly used on high power systems.

Considerations when selecting a current sensor

Each current sensor topology has its own advantages. A designer needs to consider the limitations present in the application, and which aspects are the most critical.

  • AC and DC, or just AC
  • High voltage, current or power
  • Board space, height, component count, cost
  • Response time, accuracy, bandwidth, speed
  • Environmental factors such as noise or EMF

 

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