The present invention relates to electricity power meters and in particular to power meters which can be connected to the circuitry of an individual electrical appliance .

Power recording units are, of course, known, as is a meter which shows the costing of the power consumption. Such a meter is shown in U.K. Patent 2 041 588 in which the power consumption is read as a 0 succession of discrete samples of a parameter of the mains supply, which are then fed via an analogue to digital convertor to a microprocessor operating in accordance with a stored programme. From which a signal is generated to display the power consumption and another 5 signal to display the accumulated cost of the power consumed.

However such a device suffers from several disadvantages, the main disadvantages being the discrete sampling, and the consequential use of the 0 microprocessor. The present invention seeks to overcome these disadvantages.

In one broad form the invention comprises a power meter adapted to continuously measure electrical power consumed by a load, which utilises input information on 5 cost/kWh to calculate and display the total energy consumption and total cost accumulated during a time interval.

In another form the invention comprises a power meter adapted to continuously measure electrical power 0 consumed by a load and which continuously convert the measured electrical power consumed by the load into a pulse train signal with frequency proportional to the rate of power consumed, and which calculates and displays the energy consumption and the cost of the energy 5 consumed.

An embodiment of the present invention overcomes the beforementioned problems of the prior art by providing a power meter having a power sensor/transducer, which converts continuously electrical power into a pulse

frequency signal, and utilizes programmable, digital frequency dividers to provide signals to display the power consumed and the costs of the power consumed.

Electrical power meters according to a preferred embodiment of the present invention comprise:

1. Means for continuously sensing the instantaneous power taken by the load and producing an analog output signal which continuously represents the power taken by the load; 2. Means for producing a pulse-train signal whose frequency continuously represents the instantaneous power taken by the load, thus being less sensitive to electrical noise interference than a sampling system; 3. A system of touch-sensor, push button or switch programmable digital frequency dividers, rate multipliers, counters and timer to calculate the energy consumption and cost over a time interval initiated by the use and with a price/k h information input by the user. 4. The use of decimal rate multipliers to convert k h into cost information.

5. Features 3 and 4 above allow the use of electonic circuits selected and designed in view of implementation as dedicated VLSI chip(s) , thus ensuring a lower manufacturing cost and a higher reliability of the instrument than those using a sampling method for calculating power and energy.. β. Touch-sensors or push-Duttons or switches and digital displays to input information and display results. These features result "in advantages of this embodiment of the present invention over known instruments or apparatus intended for similar applications: a) Higher accuracy and reliability, due to lower sensitivity to interferenced of VFC based systems; b) Ease of production and lower manufacturing costs in large production runs due to ease of implementation in latest ASIC - VLSI

microelectronic technology.

(ASIC = Application Specific Integrated Circuits VLSI = Very Large Scale Integrated Circuits) Other embodiments of the instrument of the present invention might differ in: the sensor type used to continuously measure the power taken by the load and produce an analog signal reflecting this power; the transducer type used to convert the analog signal representing the power into a pulse-frequency; the detailed structure of the programmable digital dividers, rate multipliers, counters, timers and displays; - being a small, portable, self-contained instrument, inserted by the user between the appliance and the electrical load, or being a panel instrument, installed in an appliance, or - being an instrument intended for switchboard installation, used for total energy metering in an installation; being a single-phase or a three-phase energy/cost meter. The features and advantages of the present invention will now be described with reference to an embodiment for the present invention in which:

Fig. 1 illustrates schematically a block diagram of the analog circuitry of the present invention; Fig. 2 illustrates schematically a block diagram of the digital circuitry of the present invention;

Fig. 3 illustrates a circuit diagram detailing one implementation of the block diagram shown in Fig. 1; and Fig. 4 illustrates a circuit diagram detailing one implementation of the block diagram shown in Fig. 2. The block diagram of the analog circuitry which performs power sensing is shown in Fig. 1. The active and neutral conductors from the mains supply form the input signal 1 of the power meter. A load (not shown)

may be connected to the active, neutEral and earth conductors of a power point 2, for example, a standard 10 amp power point. The earth conductor of the input signal 1 is connected to the earth conductor of the power point

5 2. The load may be an appliance, or a household circuitry, which is to be measured by the power meter and display the amount of power being consumed or its accumulated cost. The instantaneous input signal 1 is sensed with a voltage divider (in other embodiments it

10 might be sensed with a transformer) to produce a voltage signal V, which is proportional to the instantaneous input signal 1. For example, V. = 5 volt corresponding to an input signal 1 of 240 volts. The instantaneous current drawn by the load connected to the power point 2, 15 is sensed by a series resistance R connected between the neutral terminal of the power point 2 and the neutral terminal of the input signal 1. The voltage V„ measured across ^' the series resistance R is thus proportional to the instantaneous current drawn by the load. For 20 example, V- = 1 volt corresponding to the instantaneous current drawn by the load. (Again, in other embodiments, it might be sensed by a current transformer.) The voltage and current signals V. and V- are then applied to operational amplifiers 3 and 4 to give impedance

25 buffering such that changes in load resistance do not affect the proper circuit operation of the power meter. The signals V • and V are then applied to an Analog

Multiplier 5, which produces a voltage output signal V- which is proportional to power consumed by the load. The

30 signal V- is then applied to a Voltage-to-Frequency

Converter 6 which produces a sinusoidal signal which has a frequency which is dependant on the magnitude of the signal V^. In an alternative embodiment, the analog multiplier and the Voltage-to-Frequency Converter may be 35. combined in a single functiona-. Integrated Circuit. An , optocoupler 7 may then be used to interconnect the analog

circuitry with the digital circuitry--(shown in Fig. 2) whilst providing electrical isolation between the two. An optocoupler typically consists of a light emitting diode which emitts electromagnetic radiation at the frequency of the sinusoidal signal, and a photo-sensitive transistor to receive this electromagnetic radiation and convert it to a pulse train for processing in the digital circuitry shown in Fig. 2.

In Fig. 2, the digital pulse train produced by the circuitry of Fig. 1 is further processed to provide a display of the energy consumed (in kilowatt hours), the power (in kilowatts), the time elapsed, or the actual cost (in dollars) of the power consumed by the load. Each of the functions may be selected on the keyboard 8. The rate, that is, the cost per kilowatt hour, is entered using the numerals on the keyboard. Any rate from 0.1 cents to 99 cents per kilowatt hour may be entered. If an error is made during entry, the clear button 'C may be pressed and a new rate is selected. The rate selected may be automatically displayed, and used to multiply the frequency of the incoming pulse train. Once the rate is selected, the circuitry uses the value, which is keyed in, to calculate the various functions which may be displayed. The power, cost or time elapsed may be displayed by pressing the appropriate keys on the keyboard 8. The power (in kilowatts) is displayed by pressing 'P ¹ , the cost (in dollars) is displayed by pressing '$', and the time (in days, hours, minutes) may be displayed by pressing 'T' . The signals for displaying the power, cost, or time are generated by the 'Digit Latch and Decimal Point Control Module' 9. This module is a state machine which selectes the appropriate division, depending on the rate selected by the keyboard. The appropriate division is carried out by the 'Digital Frequency Divider' 10 and the 'Rate Multiplier' 11. In the Digital Frequency Divider 10, the frequency of the incoming pulse train is divided by either 1, 10 or 100 depending on the rate selected. The rate multiplier 11 coverts the incoming pulses to a

- 6 - pulse train of frequency proportional- to_ the selected rate (or cost per kilowatt hour) .

The remainder of the circuitry operates on the pulse train to convert it into the desired display. This circuitry consists of two 'And gates' 12 and a '4 Decade Counter/latch' 13 both of which are controlled by the 'Gate and Counter/Latch Reset Controller' 14. The 'Gate and Counter/Latch Reset Controller* 14 also causes the reset/start and stop sequences for measuring the cost of power consumed. The '4 Decade Counter Latch' 13 accumulates the costs or energy. When the Power function 'P' is selected, the counter is automatically reset every n seconds, displaying the average power. Also provided is a timer 15 which keeps track of time from minutes to 99 days. The display selector 16 then tranmits to the display 17 the appropriate signal depending on the selected keyboard function. The display consists of four 7-segment L.E.D. units and also, 4 L.E.D.s to indicate the selected display function. Alternatively a variable display can be provided to show any of the required parameters or individual displays can be provided for each of the functions.

In Fig. 3 is shown a circuit diagram illustrating one implementation of the analog circuitry as shown in the block diagram of Fig. 1.

Fig. 4 shows a circuit diagram illustrating one implementation of the digital circuitry as shown in Fig. 2. It has a keyboard, a display, a rate multiplier and a digital .frequency divider shown as 8, 17, 11 and 10 respectively in Fig. 2. It is distinguished by a single

Integrated Circuit _s labelled as 8748 on Fig. 4 which performs the functions of: firstly, accepting the rate from the keyboard and setting the rate into the rate miltiplier (shown as 11 in Fig. 2) ; secondly, accumulating the pulses representing the cost or power

(shown as 7 in Fig. 2) ; thirdly, accumulating the pulses representing elapsed time (shown as 15 in Fig. 2) ; and finally, controlling the display to show the cost or power (shown- as 14 in Fig. 2) . In this implementation,

by using VLSI custom integrated circuits., economy in parts is achieved.

It will be recognised by persons skilled in the art that numerous variations and modifications may be made to the invention as described without departing from the overall spirit and scope of the invention as broadly described herein.