GB1575111A - Current monitoring circuits including hall effect devices - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO CURRENT MONITORING CIRCUITS INCLUDING HALL EFFECT DEVICES (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company, of 190 Strand, London, W.C.2. England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to electrical circuits including Hall effect devices for responding to current changes in a monitored circuit.

According to the present invention there is provided an electrical circuit for responding to changes in the electrical condition of a monitored circuit, in which a current in the monitored circuit or a current derived therefrom is caused to flow in the winding(s) of the magnetic circuit of a Hall effect device, in which the sense current of the Hall element of the Hall effect device also flows in a resistive impedance in series with the element, in which the output of the Hall effect device or an amplified version thereof is applied to one input of a comparator, the voltage developed across the resistive impedance or an amplified version thereof being applied to the other input of the comparator, and in which the output from the comparator indicates whether the current applied to the Hall effect device's winding(s) has or not exceeded a threshold whose value depends on the characteristics of the Hall element and the circuit parameters associated therewith.

In the above arrangement, the current flowing in the Hall element is also the current in the resistive impedance, both of which are influenced by ambient temperature and other variations of the circuit. Since these determine the inputs to the comparator, the deleterious effects of variations of the ambient temperature, etc are at worst minimized.

Embodiments of the invention will now be described with reference to the drawings accompanying the Provisional Specification, in which.

Figure 1 is a circuit diagram, with inset graph, explanatory of the basic principles of the invention.

Figure 2 is a practical example of a circuit embodying the invention.

Figures 3 to 9 show in simplified form various applications of the circuit shown in Figure 2.

The invention provides a current sensor which senses the current, Ic, flowing in the coil of a conventional magnetic circuit which has an air gap in which a Hall Effect device is located. The Hall element is a flat plate-like element with a magnetic field derived from the electrical current to be monitored passing through it at 90" to its surface. A sense current flows across one dimension of the Hall element, and a potential difference is produced between two electrodes at right angles both to the sense current direction and the magnetic field direction. An electronic circuit associated with the Hall Effect device produces at its output terminal a logic level signal, VOUT, which changes state at a particular value of Ic (the current in the device's magnetic circuit winding or windings) defined as the current threshold ICT. so that: VOUTO for IC < ICT and VOuT~vssfor Ic > IcT In the present arrangements the operating threshold ICT of the device is, as far as possible, maintained substantially constant over variations of power supply voltage V55 and ambient temperature. This is achieved partly by the design of the Hall Effect element or slice which has low offset and a sensitivity factor independent of temperature variations, and partly by its use in conjunction with electronic circuits embodying this invention.

The principle on which the circuit is based will now be described with reference to Figure 1. The circuit includes a differential amplifier A, the closed loop gain of which is determined by the ratio R2 R1 and a comparator C. The current Ic to be monitored flows, as indicated, in the winding or windings of the Hall device's magnetic circuit, and the magnetic flux, the sense current Is and the output potential difference are mutually at right angles to each other. The comparator compares the amplified Hall voltage at the output of A with a voltage developed across a resistor RT in series with the Hall element, so that the current Is flowing through the element also flows through resistor RT.

The Hall output voltage VH, is given by: VH=IC.NC.IS.Sh+Vs(K1-K2) 1 where IC is the current in the coil, Nc is the number of turns on the coil, 15 is the current through the Hall element, 5h is the sensitivity of the Hall element, Vs is the voltage drop across the Hall element due to Is, and K, and K2 are geometric factors indicating the voltage distribution across the element's output terminals 1 and 2, and the difference between them accounting for the Hall element's offset voltage.

The voltage output VA of the amplifier also includes a term due to the amplifier offset voltage, VAc, SO that: R2 VA= (VH+VAO) 2 R1 Substituting for VH from equation 1 we get: R2 VA [IC.NC.Is.Sh+VS(K1K2)+VAc] 3 R The output of the comparator C changes state when: VA-IS RT1VCo=O where Vco is the comparator offset voltage.

Thus from equation 3 and 4 we can derive the current threshold I CT' at which the output voltage VOUT, changes state. Thus we have: R2 [ICT.NC.IS.Sh.+VS(K1-K2)#VAO]=IS.RT#VCO 5 R1 dividing equation (5) by Is and re-arranging we get:

Thus assuming that the error terms (in the horizontal bracket) are small compared to RT.R, R2 and the Hall element sensitivity 5h is constant with temperature, the current threshold ICT of the device is substantially independent of ambient temperature and supply voltage Vss.

Figure 2 is a practical application of the principles set out above, in which two amplifiers Al and A2 and two comparators Cl and C2 are provided, so that two outputs are available each responsive to different directions of current flow in the coil of the magnetic circuit which carries the current being monitored. There are, in this case, two resistors, RT1 and RT2, in series with the Hall element, and it can be shown that the positive operating current threshold at which the output OP, switches is determined uniquely by the value of RT1 and that the negative operating current threshold at which output OP2 switches is uniquely determined by the value of RT2. These resistors may be preset to cancel out the effect of residual errors in order to achieve the required operating current thresholds.

A current limiter CL is provided in series with the Hall element to limit the current in the element, and the resulting dissipation therein, so that the circuit can operate over a very wide range of power supply voltage, (e.g. 5-30V). If this is a constant current device, it also has the additional effect of making the error contributions due to the amplifier and comparator offset voltages constant (see equation 6), which makes the operating threshold of the device more stable. The output of each comparator drives an open-collector output transistor T, and T2 from comparators C, and C2 respectively.

These two output devices permit alternative output connections to suit various detector functions using the standard circuit to Figure 2. Normally, the strap S is in circuit, but can be removed so that comparator C, may be connected as an integrator. The manner in which these options are used for various detectors is illustrated in Figures 3 and 4, the terminal numbers on the block labelled Hall electronics corresponding to those indicated on Figure 2. Terminal 2 (not shown on Figure 2) is in the limiter CL. Figure 3 shows the two output pins 6 and 7 connected together in wired OR manner with a common pull up resistor R3, so that a nonpolarised detector is achieved. This is the equivalent function to a conventional electro-magnetic relay in which the contacts are operated by either direction of current flow in the relay.

Figure 4 shows how the two outputs may be independently connected to separate pull up resistors R4 and R5 so that either or both of the outputs may be used in a polarised function when one wishes to sense the direction of current flow in the coil.

Figure 5 shows how the two outputs may be used to set and reset respectively a latch in a telegraph relay equivalent to the signal available at the contact of a conventional telegraph relay such as a Carpenter relay. If the resistors RT1 and RT2 (Figure 2) are adjusted so that the positive and negative current thresholds are equal, the output of the latch is a distortionless repetition of a double current signal applied to the coil, as shown in Figure 10.

Figure 6 shows that with a resistor R connected between terminals 3 and 4, and a capacitor C connected between terminals 4 and 5, and using OP, (terminal 6) the device performs the ring trip detection function when the appropriate RC time constant is chosen. This circuit is an improvement on the circuit described in our Patent Specification No. 1,479,094 because its operation is independent of variations of power supply and ambient temperature, but it is used in the manner described in that Patent Specification.

Figures 7 and 8 show how the non-polarised wired OR output connection may be used in conjunction with a pair of balanced windings to form a standard telephonic loop signal detector and also a differential signal detector.

Figure 9 shows how the wired OR output connection may be given a time delay function with an additional capacitor C2 connected to ground. The connection is useful for a detector which has to detect the presence of a ringing signal on a telephone line. A capacitor is connected in series with the coil so as to prevent the detector coil indicating a false ring trip via its d.c. path to the exchange.

Explanatory wave forms are given in Figure 11.

The Hall effect element can be preferably made of silicon, in which case the element comprises an n-type epitaxial layer typically 0.3 cm-3 Qcm resistivity, within an area defined by a p-type isolation diffusion. The n-type layer, typically 5 Mm to 20 m thick, would be grown on a high resistivity p-type layer, where the resistivity is approximately 5x greater than the n-type layer. Standard emitter diffusion plus metallisation contacts from bipolar processing techniques can be used.

The characteristics of a Hall element of resistivity 0.5 Qcm, thickness, 7.5 ,um were measured over a temperature range to 1500 C, and a flat temperature response for output was obtained up to 1500C where intrinsic behaviour starts, using the circuit of Figure 2. In this temperature range, the resistance of the device increases by typically 1.9 times. The selection of such a Hall element design, together with the circuit design described, provides a circuit using a Hall effect device which has high temperature stability. The problem with previous Hall elements is that offset variations with temperature become large above 80"C. With the present design, where the side contacts are relatively large compared with the length of the element, it is found that the offset variations can be kept low, up to 1500C. Typical elements might be rectangular or cruciform, but with the length of the side contacts more than a tenth of the length of the side edge. This reduces the Hall output for the element, but lowers the offset value and improves temperature stability.

The circuit described may also be used with a Hall element and discrete components, hybrids, and silicon monolithic devices where the Hall element and circuitry are situated on the same silicon chip. The high temperature stability means that the device can be placed in a heated environment, e.g. a car engine, for control purposes, e.g. ignition, without expensive temperature compensating circuitry.

WHAT WE CLAIM IS: 1. An electrical circuit for responding to changes in the electrical condition of a monitored circuit, in which a current in the monitored circuit or a current derived therefrom is caused to flow in the winding(s) of the magnetic circuit of a Hall effect device, in which the sense current of the Hall element of the Hall effect device also flows in a resistive impedance in series with the element, in which the output of the Hall effect device or an amplified version thereof is applied to one input of a comparator, the voltage developed across the resistive impedance or an amplified version thereof being applied to the other input of the comparator, and in which the output from the comparator indicates whether the current applied to the Hall effect device's winding(s) has or has not exceeded a threshold whose value depends on the characteristics of the Hall element, and the circuit parameters associated therewith.

2. An electrical circuit as claimed in Claim 1, wherein the Hall element output voltage is connected to the two inputs of an operational amplifier via resistors, wherein the negative input of the amplifier is connected via a further resistor to the output of the amplifier, wherein the positive input of the amplifier is grounded via a further resistor, and wherein the comparator is in a further operational amplifier to whose positive input the output of the first amplifier is connected, the junction between the resistive impedance and the Hall element being directly connected to the negative input of the further operational amplifier.

3. An electrical circuit as claimed in Claim 1, and wherein there are two amplifier comparator combinations connected to the outputs from said Hall element and the resistive impedance is so arranged as to be responsive to different directions of current flow in the monitored circuit.

4. An electrical circuit as claimed in Claim 3, and wherein the resistive impedance is in two portions in series with the junction between the Hall element and the first of those portions connected to an input of one of the amplifiers while the junction between the two resistive impedances is connected to said two comparators.

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

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. The Hall effect element can be preferably made of silicon, in which case the element comprises an n-type epitaxial layer typically 0.3 cm-3 Qcm resistivity, within an area defined by a p-type isolation diffusion. The n-type layer, typically 5 Mm to 20 ssm thick, would be grown on a high resistivity p-type layer, where the resistivity is approximately 5x greater than the n-type layer. Standard emitter diffusion plus metallisation contacts from bipolar processing techniques can be used. The characteristics of a Hall element of resistivity 0.5 Qcm, thickness, 7.5 ,um were measured over a temperature range to 1500 C, and a flat temperature response for output was obtained up to 1500C where intrinsic behaviour starts, using the circuit of Figure 2. In this temperature range, the resistance of the device increases by typically 1.9 times. The selection of such a Hall element design, together with the circuit design described, provides a circuit using a Hall effect device which has high temperature stability. The problem with previous Hall elements is that offset variations with temperature become large above 80"C. With the present design, where the side contacts are relatively large compared with the length of the element, it is found that the offset variations can be kept low, up to 1500C. Typical elements might be rectangular or cruciform, but with the length of the side contacts more than a tenth of the length of the side edge. This reduces the Hall output for the element, but lowers the offset value and improves temperature stability. The circuit described may also be used with a Hall element and discrete components, hybrids, and silicon monolithic devices where the Hall element and circuitry are situated on the same silicon chip. The high temperature stability means that the device can be placed in a heated environment, e.g. a car engine, for control purposes, e.g. ignition, without expensive temperature compensating circuitry. WHAT WE CLAIM IS:

1. An electrical circuit for responding to changes in the electrical condition of a monitored circuit, in which a current in the monitored circuit or a current derived therefrom is caused to flow in the winding(s) of the magnetic circuit of a Hall effect device, in which the sense current of the Hall element of the Hall effect device also flows in a resistive impedance in series with the element, in which the output of the Hall effect device or an amplified version thereof is applied to one input of a comparator, the voltage developed across the resistive impedance or an amplified version thereof being applied to the other input of the comparator, and in which the output from the comparator indicates whether the current applied to the Hall effect device's winding(s) has or has not exceeded a threshold whose value depends on the characteristics of the Hall element, and the circuit parameters associated therewith.

2. An electrical circuit as claimed in Claim 1, wherein the Hall element output voltage is connected to the two inputs of an operational amplifier via resistors, wherein the negative input of the amplifier is connected via a further resistor to the output of the amplifier, wherein the positive input of the amplifier is grounded via a further resistor, and wherein the comparator is in a further operational amplifier to whose positive input the output of the first amplifier is connected, the junction between the resistive impedance and the Hall element being directly connected to the negative input of the further operational amplifier.

3. An electrical circuit as claimed in Claim 1, and wherein there are two amplifier comparator combinations connected to the outputs from said Hall element and the resistive impedance is so arranged as to be responsive to different directions of current flow in the monitored circuit.

4. An electrical circuit as claimed in Claim 3, and wherein the resistive impedance is in two portions in series with the junction between the Hall element and the first of those portions connected to an input of one of the amplifiers while the junction between the two resistive impedances is connected to said two comparators.

5. An electrical circuit for responding to changes in the electrical condition of

a monitored circuit substantially as described with reference to the drawings accompanying the provisional specification.