In this post I am going to talk about the types of technologies used in some battery monitoring devices. We can see what advantages we get using Hall effect sensors against Shunt (you can get a quick picture in this infographic).
##Types of sensors for current measurement
The most common types of sensors for current measuring are:
Resistive: Shunt. A resistor causes a voltage drop proportional to the current through the shunt resistor.
Inductive: Current Transformers. The wire to be measured passes through a magnetic core having a secondary winding which provides a tension proportional to the current flowing in the wire.
Magnetic (Hall effect): The sensor measures the magnetic field of a nucleus, generated by the current flowing through the wire to be measured, that coil the core.
Rogowski coil: measure the changes in magnetic field around a wire through which a current pass to produce a voltage signal that is proportional to the derivative of the current (di / dt) 3.
Now, I will explain with detail two of the most commonly used in photovoltaic power plants operating sensors, that you can see highlighted above.
##Resistive current shunt sensor
Resistance “shunt” provide an accurate and direct measurement of the flow, but do not offer any galvanic isolation. The shunt current is the least expensive solution available today for current measurement.
A shunt consists of a precision resistor ohmic value which passes a current i(t ), allowing a current proportional to the voltage and frequency falls between the terminals of the shunt resistor.
The low resistance shunt current offers good accuracy at low cost as well as a simple current measurement. When we talk about high-precision current measurement, we consider the parasitic inductance of the shunt.The inductance is typical in the order of only a few nano henry. This affects the magnitude of the impedances of the shunt at relatively high frequencies. Its effect is remarkable to reduce the power factor.
Because the current is essentially a shunt resistive element, the heat generated in the resistor is proportional to the current passing through it. This heating problem makes the use of shunt not a good option for high currents.
They are very low-value resistors to minimize power dissipation, low inductance value and a small reasonable tolerance to maintain an overall accuracy in the circuit.
##Current sensor for magnetic field ( Hall Effect )
The measuring principle of the current with a magneto-resistive sensor is direct. A current “ i “, that flows through a wire, generates a “H” around the same magnetic field that is directly proportional to the current. Measuring the intensity of this magnetic field with a magnetoresistive sensor, it can exactly determine the current.
When a driver carries a current, it produces a magnetic field, and a voltage that is perpendicular to both the current and the field is generated.This principle is known as the Hall effect.
The Hall element is basically a magnetic sensor. This requires signal conditioning to make the output available to be used for most applications.
The sensitivity of the magneto- resistive sensors can be easily adjusted, using different configurations, a single sensor can be optimized for a specific measurement application stream. The factors affecting accuracy are the mechanical tolerances ( such as the distance between the sensor and the conductor carrying the primary current ), the temperature deviation and the electronic sensitivity of the conditioning stage.
There are two main types of Hall effect sensors: open- loop and close- loop. The second provides better accuracy but at a higher cost, and most of Hall effect sensors that are in energy meters uses open design to achieve lower costs ring. These sensors have excellent frequency response and are able to measure very high currents.
To keep the resulting field to zero, mistakes associated with flow compensation or voltage levels for temperature, flow sensitivity and saturation of the magnetic core will also be efficiently canceled. The current Hall effect sensor closed loop also provides the fastest response time. The closed loop configuration has its limits by amount of current that can be measured from the device that handles only a finite amount of current compensation.
##Current sensors characteristics
The main characteristics that a current sensor should have for an energy measurement system design are:
Comparison chart of both measurement methods, Shunt versus Hall effect:
Linearity of measuring range
High current measuring capacity
Saturation of high current problems
Output variation respecting to temperature
Saturation and hysteresis
In conclusion we can say that by contrary the only type of sensor not recommended for measuring the current would be the Shunt resistor, due to its high power consumption and heat dissipation.
The Hall effect sensor represents a good choice because it has several advantages, which are: