This invention relates to current sensor assemblies arranged for connection in series with an electrical circuit.

There are many applications where it is desirable to be able to monitor current in electrical equipment and systems. T nowledge of current and power consumption can enable power supply to be managed in the most effective way and can enable the detection of faults. Although current sensors are available, they are generally expensive and bulky.

It is an object ofthe present invention to provide an improved current sensor assembly.

According to the present invention there is provided a current sensor assembly ofthe above-specified kind, characterised in that the assembly includes a first conductive member adapted to contact a first conductor in the circuit, a second conductive member adapted to contact a second conductor in the circuit, a layer of conductive material connected at one end to the first conductive member and at its other end to the second conductive member such that current flow in the circuit between the first and second conductors is via the layer of conductive material, that the conductive layer is arranged such that the temperature rise at different parts ofthe layer caused by current flow through the layer is different, that the assembly includes a plurality of temperature sensors located to sense the temperature ofthe layer at the different parts, and that the assembly provides an output representative of current from the outputs ofthe temperature sensors.

The conductive layer may have different cross-sectional resistances at different parts so that the resistance heating ofthe parts is different. The conductive layer may have wide regions at opposite ends and a narrower region between the ends. The conductive layer may

be in thermal contact with materials of different thermal conductivity at different points along its length such that heat dissipation is different at the different points. One of the materials of different thermal conductivity may be diamond.

The sensor assembly is preferably of a planar form, the first and second conductive members being located on opposite surfaces ofthe assembly. The sensor assembly may have an aperture extending through the first and second conductive members and adapted to receive a terminal post projecting from the first conductor, the second conductive member being electrically insulated from the terminal post by an electrically-insulative collar in the aperture. The sensor assembly may include three temperature sensors, the layer of conductive material being arranged such that the temperatures t, and t ₂ at opposite ends is less than the temperature t ₃ at a point between its ends, the assembly including a circuit for calculating t ₃ - (t,+t ₂ )/2 and for deriving an indication of current accordingly.

A current sensor assembly according to the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is an exploded perspective view ofthe sensor assembly in a part of a circuit;

Figure 2 is a cross sectional side-elevation view ofthe sensor assembly;

Figure 3 is a plan view of a part of the sensor assembly; and

Figures 4 and 5 illustrate alternative ways of deriving outputs from the sensor assembly.

With reference first to Figure 1, the sensor assembly 1 is a tag of planar form and rectangular shape with a circular aperture 2 through its thickness, close to one end. The assembly 1 is secured to a first conductor 3 by means of a threaded terminal post 4 projecting

upwardly from an upper surface ofthe conductor, through the aperture 2. The first conductor 3 is a power line formed by a metal bar of rectangular section having a flat upper surface at least as wide as the sensor, so that the entire lower surface ofthe sensor assembly is in electrical contact with the conductor. The circuit is completed by a second conductor in the form of a flexible cable 5 and metal connector pad 6 at one end ofthe cable. The pad 6 has the same shape and size as the assembly 1 and has an aperture 7 in a location corresponding to the aperture 2 in the sensor assembly. The pad 6 and the sensor assembly 1 are clamped to the conductor bar 3 by means of a nut 8, a locking washer 9, a metal washer 10 and an insulating washer 1 1. The arrangement is such that current flows from bar conductor 3 to the cable 5 via the sensor assembly 1 or in the opposite direction.

With reference now also to Figures 2 and 3, the sensor assembly 1 comprises an upper and lower copper plate 20 and 21, which form the upper and lower surfaces ofthe assembly. The two plates 20 and 21 are separated from one another by a microengineered wafer assembly 30 and an electrically-insulative ceramic spacer 23. The spacer 23 is of aluminium oxide and is of rectangular shape with an aperture 24 and a collar 25 projecting upwardly around the aperture. The collar 25 fits within an aperture 26 in the upper plate 20, the aperture 24 in the spacer aligning with an aperture 27 in the lower plate 21. The aperture 27 in the lower plate 21 and the aperture 24 in the spacer 23 together form the aperture 2 in the sensor assembly 1, so that the terminal post 4 can make electrical contact with the edge ofthe aperture in the lower plate but is insulated from contact with the upper plate 20.

The wafer assembly 30 is located to the right ofthe spacer 23 and includes a lower silicon substrate 31 with a metallized lower surface 32 soldered to the upper surface ofthe lower plate 21. Two diamond interlayers 33 and 34 extend across the width ofthe upper surface ofthe substrate 31 at opposite ends, being separated from one another by about one third the length ofthe substrate. The diamond interlayers 33 and 34 are thermally conductive but electrically insulative. The space between the two interlayers 33 and 34 is occupied by a central pad 35 of a thermally-insulative material and by two side regions 36 and 37 contaimng processing electronics for the sensor assembly formed in the silicon substrate by

conventional integrated circuit techniques. A thin layer 40 of an electrically-conductive material, such as silver, is deposited on top ofthe diamond interlayers 33 and 34 and the central pad 35, the layer typically being about 12 microns thick. The layer 40 extends the entire length ofthe substrate 31 and varies in width along its length so that its cross-sectional resistance varies correspondingly. More particularly, the two ends 41 and 42 ofthe layer 40 extend substantially across the entire width ofthe substrate 31 whereas the central region 43 overlying the pad 35 occupies only about the central one third ofthe width ofthe substrate. The length of this central region 43 is about one third that ofthe layer 40 and is separated from the end regions 41 and 42 by respective tapering regions 44 and 45, so that the layer 40 has a bow-tie shape. The resistance between opposite ends ofthe layer 40 is typically about 0.003 ohms.

Three temperature sensors 46 to 48 contact the upper surface ofthe silver layer 40. The sensors 46 and 48 may be manufactured by surface machining a layer of a semiconductor deposited on the upper surface ofthe layer 40 to make it temperature sensitive. For example, the sensors may be band-gap devices. Alternatively, the temperature sensors could be formed in a part ofthe silicon wafer assembly 30 in contact with the underside ofthe silver layer 40, or they could be conventional discrete devices mounted in contact with the layer, such as similar to those sold by Analogue Devices under Part Number AD590. One sensor 46 is located in the wide region 41 at one end ofthe layer 40 towards one edge, another sensor 47 is located in the wide region 42 at the other end ofthe layer towards the opposite edge, and the third sensor 48 is located centrally in the narrow region 43 ofthe layer. The three sensors 46 to 48 are electrically connected to the two processing regions 36 and 37.

The wafer assembly 30 is completed by an upper silicon cap 50 having a downwardly- extending wall 51 around its periphery. The lower edge ofthe wall 51 is bonded to the upper surface ofthe substrate 31, so that the upper surface ofthe substrate is enclosed in an hermetically sealed cavity 52. The upper surface ofthe cap 50 is metallized and soldered to the underside ofthe upper plate 20.

Electrical connection to the layer 40 is made by etching a recess 53 through the left¬ hand end ofthe cap 50, so as to expose the upper surface of one end ofthe layer 40, and then filling the recess with solder 54 to form an electrical path between the upper plate 20 and the layer 40. A similar recess 55 is etched through the substrate 31 at its right-hand end to expose the lower surface ofthe other end ofthe layer 40, and then filling the recess with solder 56 to form a connection with the lower plate 21.

Current data can be supplied from the sensor assembly 1 via a separate wire 60, as shown in Figure 4, to a databus 61. Low voltage power for the processing circuitry 36 and 37 can be supplied by separate low voltage wires (not shown) or it can be derived from the bar conductor 3 and a chassis earth connection 62. Alternatively, where the power line 3 supplies an altemating current, power for the sensor assembly 1 can be derived by an inductive pick- off 63 formed from a flat silicon coil in the sensor, as shown in Figure 5. The problem with deriving power from the main power line itself is that, when no power flows between the conductor 3 and the cable 5, the sensor assembly 1 would be derived of power, so no output would be provided. This problem could be overcome by incorporating into the sensor assembly a rechargeable, thin film micro battery to provide a signal when line power is unavailable. Instead of supplying data via a separate line 60, it could be encoded and supplied along the power line itself, whether this be of a dc or ac kind, as shown in Figure 5.

When no current flows in the system, the temperatures t ₁ , t ₂ and t ₃ at the three temperature sensors 46 to 48 is approximately equal, so that the differential temperature: t ₃ -( tι+ 1 ₂ )/2, is approximately zero.

When current does flow in the system, this passes from the bar conductor 3 through the lower plate 21 and to the right-hand end ofthe silver layer 40 via solder 56 in the recess 55. Current then flows along the layer 40 to its left-hand end and to the upper plate 20 via solder 54 in the recess 53. For a current of 10 amps, the voltage drop across the sensor would be about 0.03 volts, with a power dissipation of about 0.25 watt. Current can then flow into the cable 5 via the pad 6. Current flow through the layer 40 causes its temperature to rise as a

result of I R losses in the conducting layer. The heating effect this produces, however, will not be even along the length ofthe layer. One reason for this is that the cross section resistance varies along the length ofthe layer. Also, the rate of heat dissipation is different at different points along the layer 40 because ofthe variation in surface area and the different thermal conductivities ofthe materials underneath the layer, the diamond interlayers 33 and 34 at the ends having a greater thermal conductivity than the insulating pad 35 under the central region 43 ofthe layer. As a result of these effects, the temperature t ₃ ofthe central, narrow portion 43 ofthe layer 40 will be greater than the temperatures i, and t ₂ at the wider ends 41 and 42 ofthe layer so the temperature differential t ₃ -( tι+ 1 ₂ )/2 will have a positive value that is approximately proportional to the square ofthe current at any instant. Typically, the power dissipation in the narrow, central part 43 ofthe layer 40 would be about 4.5 times that of each ofthe wider end regions 41 and 42. In fact, the relationship is more complex than this but the processing circuitry 36 and 37 compensates for this after calibration, such as by use of a simple look-up table, to provide an output signal representative of he equivalent current.

The diamond interlayers 33 and 34 serve other purposes than to increase the heat dissipation at the ends ofthe layer 40. They also serve to provide electrical insulation and help transmit medium term variations in ambient temperature to the interior of the sensor.

The low thermal mass ofthe sensing element 40 and 46 to 48 helps ensure a rapid response to current changes. By monitoring both ti and t ₂ , and using the average reading, the effect ofthe thermal path and temperature differences between the bar conductor 3 and the cable 5 is minimized. The system is more sensitive to high currents than to low currents because the temperature differential is proportional to the square of current; this is an advantage in power management systems. Sensor assemblies with different film resistances could be used for measuring different current ranges. The sensor assembly could be used to monitor other conditions. For example, a high terminal resistance might be indicated by conditions in which: t, >- t ₂ and t _] > t ₃ , or t ₂ > X and t ₂ ≥ t ₃ . Dangerous overheating might be indicated by the condition in which t ₂ >- >- tj or t| > t ₂ .

The low cost ofthe sensor assembly enables it to be used, even in domestic appliances, where it is desirable to be able to monitor current, such as for control or safety purposes. The sensor assembly is very compact, it is mechanically robust and is tolerant of very high vibration and shock. The assembly is inherently safe since, under short circuit load conditions, the film would fuse open circuit at a predictable overload current, with all the fusing debris being contained within the structure ofthe wafer. The sensor assembly is non¬ polarized and can be used accurately to monitor alternating currents of any waveform, as well as direct currents since the assembly is responsive to the heating effect ofthe current.