Hall Effect Current Sensor Magnelab

8/8/2019 Hall Effect Current Sensor Magnelab

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Pavani Korada [email protected]

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Hall Effect Current SensorsMagnelab

I. Introduction

A Hall Effect Current Sensor is a current transformer, which utilizes the Hall effect. This effect was first observedby Edwin Hall in 1879[1]. He conducted experiments ongold foils wherein he monitored the current flowing fromthe top to the bottom of a thin rectangular strip. He foundthat, in the presence of a magnetic field perpendicular tothe strip, the electrons were deflected to one side of thegold foil. This caused an excess electrical charge build up, which gave rise to a voltage difference across the right andleft side of the foil. This electric field (voltage difference), which is perpendicular to both the magnetic field and thecurrent flow, is called the Hall Voltage. In the absence of amagnetic field, or in the presence of a magnetic fieldparallel to the strip, there was no voltage differencebetween the right and left side of the strip.

Fig 1: The Hall Effect

II. Theory of Hall Effect

The Hall effect can be explained by the Lorentz forceprinciple. When a charge moves in a directionperpendicular to an applied magnetic field, it experiences aforce defined by the Lorentz Law. The direction of thisforce is perpendicular to the direction of propagation of the charge and that of the external magnetic field.

For an n – type semiconductor as shown in Fig. 2.a, the

majority of the charge carriers are electrons. The currentflows across the semiconductor in the positive x direction, with the external magnetic field in the positive z direction. The electrons under the influence of the Lorentz forcedrift away from the current line towards the negative y-axis, which results in the build up of excess charge. This givesrise to an internal electric field, which opposes the Lorentzforce. The electric field is measured as the Hall Voltage. The force on holes/positive charges as shown in Fig 2.b istowards the same side because of their opposite velocitiesand positive charges.

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(b)

Fig 2: Hall effect in a thin semiconductor bar with(a) negative and (b) positive charges

III. Operation of Hall Current Sensor

A Hall Current Sensor utilizes the principle of the Halleffect to detect the current levels. These sensors monitorthe gauss level created by a flow of current; they do not

measure the actual current flow. The current being measured (through the primary conductor) is passedthrough a flux-collecting core (which is generally a slottedtoroid) that concentrates the magnetic field on the Hallelement. The Hall element is a piece of semiconductormaterial, which produces the Hall voltage proportional tothe current flow. The Hall voltage is a low level signal, sogenerally a low noise high gain amplifier is used to regulatethe output of the Hall element. The Hall Element along with the evaluation and regulation circuitry is generally fabricated into a single IC (Integrated Circuit).



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Hall IC

The output of the sensor is linearly proportional to thecurrent in the primary conductor. Fig. 3 shows an exampleof the output waveform of these sensors. These sensors areratiometric; where the output voltage of the sensor is half that of the supply voltage, when the current in the primary conductor is zero. This voltage is called the Quiescent voltage (Vq). When the current flows in the positive

direction then the output voltage is greater than Vq, whenthe current flow reverses, the output voltage is leass than Vq. Saturation occurs when the current exceeds the rating of the sensor.

Fig 3 Linear Current Sensor Ideal Output

Different Configurations of Linear Hall Effect Sensors

1. Open loop Hall Effect Sensor

The configuration of an Open loop sensor is shown in Fig.4. Here the current (Ip) carrying primary conductor passesthrough the core, and the Hall IC is situated in the air gap

of the core. The current through the primary conductorcreates a magnetic flux in the core, which is sensed by theHall IC to produce an output voltage (Vo) proportional tothe input current Ip.

Fig 4: Open loop Hall Effect Sensor

2. Closed loop Hall Effect Sensor

Fig. 5 shows the configuration of a closed loop sensor.Here, the output of the Hall IC is amplified and driventhrough a coil wound around the core. This secondary current, Is, creates a secondary magnetic field in the core. The magnetic flux from the secondary coil is exactly opposite to that generated by the primary conductor andthis results in the cancellation of the magnetic flux in the

core. The current through the secondary coil is driventhrough a resistor to measure a voltage that is proportionalto the input current Ip.

Fig 5: Closed loop Hall Effect Current Sensor

IV. Design Example

An open loop Hall Effect Sensor is designed with a currentrating of ± 90 A. It is assumed that 1 Amp of current

generates approximately 6 Gauss of flux density. Based onthis assumption, the flux operating extremes aredetermined. Accordingly, a Hall IC that can operate withinthe desired flux density range is selected. The saturationlevels of the core and the gauss sensitivity of the Hall IClimit the peak current that can be detected.

The selected Hall IC has a quiescent voltage, Voq = 2.5 V which is typically half the supply voltage. It has a sensitivity of 4mV/G. The range of the output voltage for theselected Hall IC is 0.25 V to 4.75 V. Based on thesensitivity rating and the output voltage we can safely estimate that the IC can sense a flux of ±500 G. Theselection of the core is based on the size of the primary conductor or the dimension requirements if any.

The maximum effective permeability of the core with theair gap is calculated.

µ e = BLm/0.4π NI where,

Lm = mean length of the core, N = number of turnsI = current flowing in the primary

conductorB = flux generated due to the current flow

Based on the effective permeability the minimum length of

the air gap is calculated as,Lg = Lm [(1/µ e ) -(1/µ i )] 0.3937

where,Lg = length of air gap in inches

µ e = Effective permeability

µ i = Initial permeability of the core

The air gap should be large enough to house the Hall IC. If the gap is not large enough then a larger core is selectedand the minimum gap length is calculated again.

Input Current

Quiesecnt Voltage

Output Voltage

Ipeak -Ipeak

Vo

Gapped Toroid

Input Current

Vo

Is

I



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V. Results

Fig 6: Characteristic Graph of Design Example An Open loop Hall Effect Current Sensor is designed asper the considerations in the design example. Fig. 6 plots

the linear relationship between the sensor output and theinput current.

The current sensor designed here can sense AC or DCcurrent. The supply voltage of 5.09 V produces a quiescent voltage of 2.50 V. A positive current flow results in anincrease in the output voltage, while a current flowing inthe negative direction causes a decrease in the output voltage. The sensor is linear for an input current range of ±90A. Increasing the input current further causes the coreto saturate.

A single wire passing through the center of the coreprovides a sensitivity of 25mV/A with a peak current of

90A. The sensitivity increases as the number of turns of the wire around the core increases. Passing the wire 2 timesprovides a sensitivity of 50 mV/A with a peak current of 45 A. Passing the wire 5 times will provide a sensitivity of 125mV/A and a peak current of 18 A.

VI. References

[1] Hall, Edwin, "On a New Action of the Magnet on Electric Currents”, American Journal of Mathematics, Vol. 2,1879, p. 287-292

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Hall Effect Current Sensor Magnelab

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