GB2107070A

GB2107070A - Hall effect switch

Info

Publication number: GB2107070A
Application number: GB08125482A
Authority: GB
Inventors: Dennis William Revell; Anthony William Sweet
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-08-20
Filing date: 1981-08-20
Publication date: 1983-04-20
Also published as: GB2107070B

Abstract

In a Hall effect switching circuit, the Hall element (HS) is connected across the DC supply in series with a resistor (R1) the value of which is and small compared with the resistance of the Hall element. This resistor sets the threshold of the Hall element independently of its current and enables a fixed voltage drive to be used to balance offset variations more effectively. The Hall voltage is amplified by an operational amplifier (A1) the output of which is applied to the + input of an operational amplifier (A3) acting as a comparator to determine the overall threshold of the circuit. The voltage for the - input of the comparator (A3) comes from operational amplifier (A2) whose output is adjustable by a variable resistor (R7) in a resistive bridge (R4-R5-R6-R7). <IMAGE>

Description

SPECIFICATION Hall effect switch This invention relates to Hall effect switching circuits, especially (but not solely) for use in automatic telecommunication exchanges.

A Hall effect switch uses a small flat element of a semiconductor material, which is placed in a magnetic field which is substantially normal to the surface of the material and which represents a condition to be monitored. A control direct current flows between two terminals on opposite edges at right angles to the magnetic field, with the result that a potential difference is set up at right angles both to the control current and to the magnetic field. Two further connections extend from two other edges of the element to the associated circuitry. This potential difference, plus any current flow caused thereby is proportional both to the value of the magnetic field and the value of the control current.As used in a telephone exchange the magnetic field can be varied in accordance with, for instance, the current in a subscriber's line, so that the device can be used for loop detection, ring trip detection, etc.

A feature of such a device is the existence of a so-called offset, i.e. in the zero flux condition of the device a potential difference other than zero is present at the output of the element. This is usually a nuisance, and one of the objects of the invention is to obviate the effects thereof.

According to the present invention, there is provided a Hall effect switching circuit, which includes a Hall effect element connected in series with a resistance whose value is small compared with the resistance of the Hall element, connections from the Hall element to an amplifier which amplifies the voltage generated in the Hall element and sets the switching threshold, a comparator to one input of which the output of said amplifier is applied, and a compensating voltage connected to the other input of said comparator, which compensating voltage determines the Hall output voltage at which the switching circuit responds, wherein said compensating voltage is adjustable to balance the effects of amplfier and Hall element offsets.

An embodiment of the invention will now be described with reference to the accompanying drawing.

The Hall element is connected across the supply in series with a resistor R1 the function of which is to fix the operating threshold of the element, as will be seen below. Its output connections extend via resistors R2 and R3 to the inputs of an operational amplifier Al , whose output goes to the positive input of an operational amplifier A3, which acts as a comparator to fix the threshold at which the amplifier A3 responds.

The other input to A3 comes from another operational amplifier A2.

Since the output of a Hall effect element is proportional to the slice current, it is conventionai practice to drive it with a fixed current. However, the offset of the element is proportional to the applied slice voltage. Further, due to the strong variations of Hall element resistance R, with temperature, the offset of the element and common mode voltage cause strong variations of the element's threshold with temperature. The use of a series resistor RT, whose value is small compared with the resistance Rs to set threshold independently of slice current enables a fixed voltage drive to be used to balance offset variations more effectively.

In the circuit shown in the drawing, the amplifier Af amplifies the output from the Hall element HS, and feeds it to the positive input of the comparator A, with the resistor RT setting the switching threshold independently of Hall element current Vz/(Rs+RT), where Vz is the voltage fixed by a zener diode ZD. This is because the output from Al rises by

while the Hall voltage is proportioned to

so the threshold flux density,

where k is the product of Hall element sensitivity and the gain of the amplifier Al.

The voltage at which the amplifier A3 responds is fixed by the output to its negative input from the operational amplifier A2. The input potential dividers R4--R5R5 and R6-R7 connected across the DC supply. The resistor R7 is variable, so as to provide a voltage of either polarity equal to the net offset voltage, which includes the effect of resistor network unbalances. It can therefore be used to balance Hall element offset, common mode errors and operational amplifier offsets simultaneously without introducing threshold errors due to variations of the Hall element resistance RS.

The output of the comparator A3 drives a conventional transistor output stage T, which can be used as a relay drive or logic drive, as required.

Claims

1. A Hall effect switching circuit, which includes a Hall effect element connected in series with a resistance whose value is small compared with the resistance of the Hall element, connections from the Hall element to an amplifier which both amplify the voltage generated in the Hall element and set the switching threshold: a comparator to one input of which the output of said amplifier is applied, and a compensating voltage connected to the other input of said

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Hall effect switch This invention relates to Hall effect switching circuits, especially (but not solely) for use in automatic telecommunication exchanges. A Hall effect switch uses a small flat element of a semiconductor material, which is placed in a magnetic field which is substantially normal to the surface of the material and which represents a condition to be monitored. A control direct current flows between two terminals on opposite edges at right angles to the magnetic field, with the result that a potential difference is set up at right angles both to the control current and to the magnetic field. Two further connections extend from two other edges of the element to the associated circuitry. This potential difference, plus any current flow caused thereby is proportional both to the value of the magnetic field and the value of the control current.As used in a telephone exchange the magnetic field can be varied in accordance with, for instance, the current in a subscriber's line, so that the device can be used for loop detection, ring trip detection, etc. A feature of such a device is the existence of a so-called offset, i.e. in the zero flux condition of the device a potential difference other than zero is present at the output of the element. This is usually a nuisance, and one of the objects of the invention is to obviate the effects thereof. According to the present invention, there is provided a Hall effect switching circuit, which includes a Hall effect element connected in series with a resistance whose value is small compared with the resistance of the Hall element, connections from the Hall element to an amplifier which amplifies the voltage generated in the Hall element and sets the switching threshold, a comparator to one input of which the output of said amplifier is applied, and a compensating voltage connected to the other input of said comparator, which compensating voltage determines the Hall output voltage at which the switching circuit responds, wherein said compensating voltage is adjustable to balance the effects of amplfier and Hall element offsets. An embodiment of the invention will now be described with reference to the accompanying drawing. The Hall element is connected across the supply in series with a resistor R1 the function of which is to fix the operating threshold of the element, as will be seen below. Its output connections extend via resistors R2 and R3 to the inputs of an operational amplifier Al , whose output goes to the positive input of an operational amplifier A3, which acts as a comparator to fix the threshold at which the amplifier A3 responds. The other input to A3 comes from another operational amplifier A2. Since the output of a Hall effect element is proportional to the slice current, it is conventionai practice to drive it with a fixed current. However, the offset of the element is proportional to the applied slice voltage. Further, due to the strong variations of Hall element resistance R, with temperature, the offset of the element and common mode voltage cause strong variations of the element's threshold with temperature. The use of a series resistor RT, whose value is small compared with the resistance Rs to set threshold independently of slice current enables a fixed voltage drive to be used to balance offset variations more effectively. In the circuit shown in the drawing, the amplifier Af amplifies the output from the Hall element HS, and feeds it to the positive input of the comparator A, with the resistor RT setting the switching threshold independently of Hall element current Vz/(Rs+RT), where Vz is the voltage fixed by a zener diode ZD. This is because the output from Al rises by while the Hall voltage is proportioned to so the threshold flux density, where k is the product of Hall element sensitivity and the gain of the amplifier Al. The voltage at which the amplifier A3 responds is fixed by the output to its negative input from the operational amplifier A2. The input potential dividers R4--R5R5 and R6-R7 connected across the DC supply. The resistor R7 is variable, so as to provide a voltage of either polarity equal to the net offset voltage, which includes the effect of resistor network unbalances. It can therefore be used to balance Hall element offset, common mode errors and operational amplifier offsets simultaneously without introducing threshold errors due to variations of the Hall element resistance RS. The output of the comparator A3 drives a conventional transistor output stage T, which can be used as a relay drive or logic drive, as required. Claims

1. A Hall effect switching circuit, which includes a Hall effect element connected in series with a resistance whose value is small compared with the resistance of the Hall element, connections from the Hall element to an amplifier which both amplify the voltage generated in the Hall element and set the switching threshold: a comparator to one input of which the output of said amplifier is applied, and a compensating voltage connected to the other input of said comparator, which compensating voltage determines the voltage at which the switching circuit responds, wherein said compensating voltage is adjustable to balance the effects of parameters including Hall element offset.

2. A circuit as claimed in claim 1, in which said compensating voltage is derived from the output of a further amplifier, in which the further amplifier is an operational amplifier having the voltages of its two inputs fixed by respective potential dividers connected across the DC supply for the circuit, and in which one of the resistors in said potential divider is variable to provide said adjustment, positive and negative, of the compensating voltage.

3. A Hall effect switching circuit substantially as described with reference to the accompanying drawing.

New claims or amendments to claims filed on 20 May 1982 New or amended claims:

4. A Hall effect switching circuit, which includes a Hall effect element connected in series with a resistance whose value is small compared with the resistance of the Hall element, such that a threshold for operation is set irrespective of the current in the Hall element, connections from the Hall element to an amplifier which both amplifies the voltage generated in the Hall element and sets the switching threshold of the circuit, a comparator to one input of which the output of said amplifier is applied, a compensating voltage being connected to the other input of the comparator, said compensating voltage determining the voltage at which the circuit responds, and a further amplifier from the output of which said compensating voltage is derived, in which the further amplifier is an operational amplifier having the voltages on its two inputs fixed by the respective potential dividers connected across the DC supply for the circuit, and in which one of the resistors of said potential dividers is variable to provide an adjustment of the value of the compensating voltage, and hence the voltage at which the circuit responds.