GB2265015A - Electricity meters - Google Patents

Abstract

In an electricity meter wherein a relay (22) is operated (10) to supply current to a load (not shown) during a permitted period, the holding current flowing in the relay coil subsequent to operation is arranged to be less than the current required for operating the relay. For this purpose a transistor (30) has a base which is supplied (30, 29) with current through alternative resistors (32, 34), one having a lower resistance for relay operation and one having a higher resistance for holding the relay in the operated condition. Alternatively, a relay (12) with a bistable magnetic circuit may be set to an operating state (40, 18, 14) and the operating current then switched off, a permanent magnet in the relay holding the relay in the ON condition. <IMAGE>

Description

Title: Improvements in and relating to Electricity Measuring Meters Field of invention This invention concerns meters used to measure the quantity of electricity, usually measured in Kilowatthours (KwH) consumed by a load such as a domestic or business installation.

Background to the invention Electricity can be charged at different rates according to different times of the day or night it is used. In one arrangement a circuit breaker typically a heavy duty relay is operated by a timer to enable electricity to flow to a particular load only during the permitted period or periods during each 24 hours. The act of closing the circuit breaker (relay) requires a relatively high current but once operated, the circuit breaker usually requires only a relatively small holding current to keep its contacts closed.

It is an object of the present invention to provide an improved control circuit for such a relay-circuit breaker, which takes account of these differing current requirements.

Summary of the invention According to the broadest aspect of the present invention a relay control circuit comprises current limiting means which is effective to limit the current flowing in the relay coil when the relay has been operated to close a set of contacts associated therewith.

By limiting the current flowing once the relay has operated to just the level required to maintain sufficient attraction between magnet and armature to keep the contacts closed, so the power required whilst the relay is kept operated (normally during permitted periods) is kept to the minimum.

Typically the relay coil is supplied with current via a transistor, the collector circuit of which can deliver sufficient current to close the relay and the current limiting feature is achieved by supplying the base of the transistor from two different current sources, one for closure having low impedence and the other (to merely hold the relay closed) having a higher impedence.

Where the transistor itself is controlled by a microprocessor, and the latter is arranged to provide a low voltage (typically 5 volt) supply to a base current feed resistor, the microprocessor is arranged to supply the low voltage to first one and then the other of two resistors each of which serves the base of the transistor, one of the resistors having a higher resistance than the other so as to limit the base current when the second resistor is used.

Where the microprocessor has two similar outputs each of which can independently be set high or low, the two different resistors connected to the base of the transistor may be connected separately to the two said outputs and in order to produce sufficient collector current to operate the relay circuit breaker, both outputs are forced high but after a short time the output connected to the lower resistance base feed is forced low leaving only the higher resistance path supplying current to the transistor base.

In an alternative arrangement only a single output from the microprocessor is needed in combination with a timing circuit which may be incorporated in or external to the microprocessor, such that when the single output of the microprocessor is forced high, both systems are supplied with current and the timing circuit operates to break the link to the lower value resistor after a predetermined period of time to thereby reduce the base current of the transistor after the relay has operated.

Where a timing circuit is employed, the microprocessor is arranged to provide a reset signal to ensure that a timing circuit is retriggered should it be necessary, when the microprocessor output is forced low so that when the same output is again subsequently forced high, the timing circuit is ready to operate in the predetermined manner as previously described.

The invention will now be described by way of example with reference to the accompanying drawing which contains part of the block circuit diagram of a solid state electricity power measuring meter which contains a microprocessor for controlling both measurement and operation of the meter.

With reference to the drawing, a microprocessor 10 controls the base current to a main contactor 12 via transistors 14 and 16 to set and reset the main contactor.

To this end the base current of each of the two transistors is supplied from junctions 39 and 40 via 4.7Kohm resistors 18 and 20. when output 40 of the microprocessor 10 goes high, base current is available for transistor 14 causing the set input of the contactor 12 to trigger.

If output 39 goes high, base current is available for transistor 16 thereby causing the reset input of contactor 12 to go high.

By using a bistable magnetic circuit, it is only necessary for the transistors 14 and 16 to operate momentarily for a sufficient length of time to generate a pulse of sufficient magnitude to cause the magnetic circuit to change state thereby opening or closing the contacts of the contactor.

The advantage of such a relay is that the quiescent condition is maintained by means of a permanent magnet or other mechanism which does not require the supply of electrical power so that junctions 39 and 40 need only be momentarily forced high to generate appropriate pulses for operating the contactor 12. For most of the time the contactor 12 draws no current.

Unfortunately such contactors are expensive and where the meter is to provide a switching facility for timed operation of selected loads (a so-called white meter facility) a more conventional relay is employed having normally open contacts which are closed when the relay is energised.

Such a relay is shown at 22. The relay coil 24 is shown connected in parallel with a back EMF diode 26 between an unregulated power line 28 and the collector of an NPN transistor 30. Base current for the transistor is obtained from terminals 29 and 30 of microprocessor 10 via two resistors 32 and 34 respectively. Resistor 32 has a resistance of 4.7 KOhms and resistor 34 a resistance of 10 KOhms.

Transistor 30 is connected to the opposite terminal of the unregulated power supply via 9 low resistor 36 typically of 150 Ohms and a suitable transistor is type BC 237B.

If junction 30 is forced high, a relatively high base current is available to transistor 30 causing a large current to flow in the winding 24. Resistor 32 is selected so as to ensure that base current attributable to it is sufficient to close the relay contacts.

On the other hand if terminal 29 only is forced high, the base current available to transistor 30 is not sufficient to cause the transistor to close the relay from an open condition but is sufficient to maintain the relay in its closed condition if it has already been switched into that condition.

In accordance with the invention therefore the microprocessor is programmed so that when the relay 22 is to be closed, to supply power to the selected load during a permitted period of time, at least junction 30 is forced high (and typically both junctions 29 and 30 are forced high) so that there is plenty of base current to ensure reliable closure of the relay. However the programming is arranged so that shortly after junction 30 has been forced high, it is again forced low leaving only junction 29 high. The current maintained by the 5 volt level on junction 29 is sufficient to keep the relay closed for the remainder of the permitted period.

At the end of the permitted period, junction 29 is also forced low causing transistor 30 to turn off and disable the relay coil 24, contacts 22 open and power is removed from the selected load.

A preferred microprocessor is type TMS 370C.

The remainder of the meter circuit is shown in the drawing but is not referred to in this application since it is not relevant to the operation of the power saving feature associated with the relay 22.

In addition to the microprocessor based power measuring circuit, a simple power supply is also shown for producing 5 volts DC for driving the processor and related semiconductor devices.

Although the circuit diagram shown in the drawing refers to a card operated prepayment meter, the feature of the invention may be applied to any form of electricity power measuring meter and is not restricted to a card operated prepayment meter.

Claims (9)

Claims

1. An electricity meter wherein a relay is operated by a timer to allow electricity to flow to a load, wherein a relay control circuit comprises current limiting means which is effective to limit the holding current flowing in the relay coil after the relay has been operated to close a set of contacts associated therewith.

2. A meter according to claim 1, wherein the relay is operated by a timer to control supply of electricity to a particular load during a permitted period.

3. A meter according to claim 1 or claim 2, wherein the current is limited to a value just sufficient to maintain attraction between magnet and armature which keeps the holding contacts closed, so the power required whilst the relay is kept operated is kept to the minimum.

4. A meter according to any of claims 1 to 3, wherein the relay coil is supplied with current via a transistor, the collector circuit of which can deliver sufficient current to close the relay, and holding current limitation is achieved by supplying the base of the transistor from two different current sources, one for closure having lower impedance and the other, for holding the relay closed, having a higher impedance.

5. A meter according to claim 4, wherein the transistor is controlled by a microprocessor which is arranged to provide a low voltage (typically 5 volt) supply to a base current feed resistor, the microprocessor supplying the low voltage to first one and then the other of two resistors each of which is connected to the base of the transistor, one of the resistors having a higher resistance than the other so as to limit the base current when the second resistor is used.

6. A meter according to claim 5, wherein the microprocessor has two similar outputs each of which can independently be set high or low, the two different resistors connected to the base of the transistor being connected separately to the two said outputs and, in order to produce sufficient collector current to operate the relay circuit breaker, both outputs are forced high but after relay operation the output connected to the lower resistance base feed is forced low, leaving only the higher resistance path supplying current to the transistor base.

7. A meter according to claim 5, wherein a single output from the microprocessor acts in combination with a timing circuit which is incorporated in or is external to the microprocessor, such that when the single output of the microprocessor is forced high both resistors are supplied with current, and the timing circuit operates to break the link to the lower value resistor after a predetermined period of time thereby to reduce the base current of the transistor after the relay has operated.

8. A meter according to claim 7, wherein the microprocessor is arranged to provide a reset signal to ensure that the timing circuit is retriggered when the microprocessor output is forced low, so that when the same output is again subsequently forced high the timing circuit is ready to operate.

9. An electricity meter as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.