GB2130449A - Time switches - Google Patents

Abstract

A time switch for operation in synchronism with a low tariff period defined by a time-controlled multi-tariff electricity meter is synchronised with the meter by a crystal-controlled oscillator 72 driving a stepper motor 76, which in turn drives a 24 hour timing disc via a gear train, and is provided with a backup battery 70 to maintain its operation in the event of a power cut. The timing disc carries two adjustable targets which operate a changeover switch 48 controlling the load. A "boost" switch 26 is provided, to enable the load to be switched on for hourly periods outside the off-peak period, by turning on a thyristor T1 which energises a relay 58 to close a switch 56. The changeover switch 48 directs the main supply to either of two terminals 52, 60 which may be connected to high and low power water heating elements for use during off-peak and peak periods respectively. <IMAGE>

Description

SPECIFICATION Time switches This invention relates to time switches, and is more particularly but not exclusively concerned with a time switch adapted for use with a two-tariff electricity supply to control a load, such as a water heater, such that the load primarily uses off-peak electricity supplied at the cheaper of the two tariffs.

Atypical two-tariff electricity supply is that currently available in the United Kingdom underthe name 'Economy 7'. According to this tariff, off-peak electricity, that is electricity generated typically between midnight and 7.00 a.m., is supplied at a significantly lower price than electricity generated outside this period.

Electricity supplied on the Economy 7 tariff is metered by a two-tariff electricity meter, which contains a timing device, typically based on a synchronous motor directly driven by and directly deriving its timing from the 50Hz supply, for switching the meter between its two tariffs. To ensure that the tariff switching times of this meter remain synchronised with the predetermined off-peak period, even in the event of a power cut, the meter is typically provided with a spring reserve mechanism capable of taking over the function of its timing device for up to 48 hours.

In order to take advantage of the Economy 7 tariff for loads such as water heaters, the load is controlled by its own, separate, time switch, which has a principal 'ON' period intended to be synchronised with the predetermined off-peak period. The time switch also typically has an override (or 'booster') switch for switching on the load outside the principal 'ON' period, normally for up to an hour at a time, although it will be appreciated that electricity supplied outside the predetermined off-peak period is charged at a significantly higher price.

To realise the full benefit of the Economy 7 tariff in relation to a time-switch-controlled water heater or similar load, it is essential that the time switch should remain synchronised with the switching times of the two-tariff electricity meter. However, currently available time switches for use with this tariff suffer from the disadvantage that they lose this synchronism in the event of a power cut, since they are based on a synchronous motor of the type mentioned above, but do not have a spring reserve mechanism. Further, to provide them with a spring reserve mechanism would make them rather expensive. The result is that, after a power cut, the consumer must remember to reset the time switch controlling his water heater or like load to the correct time, otherwise a significant proportion of the electricity used for that load will be charged at the higher price.

The currently available time switches for use with the Economy 7 tariff suffer from the further disadvantage that the timing of their principal 'ON' period is defined by a cam, and is therefore fixed. If, therefore, the supplier wishes to change the timing of the off-peak period, a different cam would need to be fitted in the time switch.

It is an object of the present invention to provide a time switch suitable for use with a two-tariff electrical ity supply, in which the abovementioned disadvantages of the currently available time switches are alleviated.

According to the invention, there is provided a time switch of the kind having a principal ON period intended to be sychronised with a low tariff period defined by a multi-tariff electricity meter, the time switch comprising: an AC input for connection to an AC power supply which is metered by said multi-tariff electricity meter; changeover switching means having first and second states in which said AC input is coupled to first and second AC outputs respectively; DC-operated timing means including a stepper motor arranged to be driven by timing pulses, said stepper motor being arranged to set said changeover er switching means to its first state for at least one time period intended to be wholly within the low tariff period defined by said meter;; normally-open switching means connected to control the supply of the power to said second AC output, said normally-open switching means being manually-settable to its closed state for up to a predetermined maximum time; and a DC power supply circuit connected to said AC input and arranged to generate a DC power supply for said timing means from the AC supply at said AC input, said DC power supply circuit including a standby battery arranged to maintain operation of at least said timing means for a given minimum period in the event of interruption of the AC supply at said AC input;; said time switch further comprising switch driving means coupled between the stepper motor and the changeover switching means, and a housing containing at least the stepper motor, the switch driving means and the changeover switching means, wherein the switch driving means includes a timing disc driven by the motor and having switch-operating targets secured to its periphery at circumferentially spaced positions which define the respective times at which said changeover switching means is switched from its first state to its second state and back again by said targets, and wherein the positions of the targets are adjustable, upon opening said housing, in order to permit the switching times defined thereby to be changed.

The invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a somewhat schematic front view of a time switch in accordance with the invention; Figure 2 is a simplified block circuit diagram of the electrical and electronic circuitry of the time switch of Figure 1; and Figure 3 is a schematic view of part of the inside of the time switch of Figure 1.

The time switch shown in Figure 1 is indicated generally at 10, and is intended for use with a two-tariff domestic electricity supply metered by a two-tariff meter (not shown): typically, this meter defines a cheap or off-peak tariff period during the night, e.g. from midnight to 7.00 a.m.

The time switch 10 comprises a rectangular plastics housing 12 having a stepped front face 14 whose upper (and much larger) portion 14a is covered by a removable transparent cover 16, and whose lower (and much smaller) portion 14b is substantially flush with this cover.

Projecting through a suitable orifice in the portion 14a of the front face 14 of the housing 12, so as to stand slightiy proud of this face, is a circulartiming dial 18, which is driven as will be described hereinafterto rotate clockwise at one complete revolution everytwenty4our hours. The timing dial 18 has a slightly knurled rim 20, by which it can be rotated clockwise by hand, in order to align a radial pointer (or clock hand) 22 marked on it with the correct time on a twenty-four hour clock scale 23 provided on the portion 14a ofthefrontface l4ofthe housing 12 around the dial.

Also set into the portion 1 4a of the front face 14 of the housing 12, beside the timing dial 18, is a two-position, two-pole, switch 24, whose function will be described hereinafter with reference to Figure 2.

In the lower portion 14b of the front face 14 of the housing 12, and thus not covered by the cover 16, are a push-button 'boost' switch 26, a red indicator lamp 28 associated with the boost switch 26 and an orange indicator lamp 30. The switch 26 and the indicator lamps 28 and 30 are horizontally aligned, and their functions will also be described with reference to Figure 2.

The electrical and electronic circuitry of the time switch 10 is shown in Figure 2, and is disposed inside the housing 12 of Figure 1. This circuitry comprises two AC input terminals 40 and 42, which are adapted for connection to the live and neutral wires respectively of a normal domestic AC power distribution circuit, e.g. 240 volts A.C. at 50Hz, or 110 volts AC at 50 or 60Hz. The live AC input terminal 40 is connected via one set of contacts 44 of the aforementioned switch 24 (i.e. one pole of this switch) to the movable (or changeover) contact 46 of a changeover switch 48, one of whose output contacts 50 is connected to a principal live AC output terminal 52 of the time switch 10.The other output contact 54 of the changeover switch 48 is connected, via a set of normally-open contacts 56 of a relay 58, to a secondary live AC output terminal 60 of the time switch 10. The neutral AC input terminal 42 is connected via another set of contacts 61 of the switch 24 (i.e. the other pole of this switch) to a neutral AC output terminal 62 of the time switch 10.

The indicator lamp 30 is connected between the output terminal 52 and 62, while the indicator lamp 28 is connected between the output terminals 60 and 62.

The AC input terminals 40 and 42 are also connected to respective AC inputs of a DC power supply circuit 63, the former terminal being connected via a fuse FS1.

The DC power supply circuit 63 can take any convenient form, but typically comprises a diode rectifying bridge 64 connected between the AC inputs. Between the positive and zero DC outputs 65, 66 of the bridge 64 is connected a network comprising a pair of current-limiting resistors R1, R2 and a diode Dl, which are connected in series with each other and with a circuit consisting of a smoothing capacitor C1 in parallel with the series combination of a further resistor R3 and a zener diode ZD1. For operation with a 240 volts AC supply, both resistors R1, R2 are used, but for a 110 volts AC supply, one of them is shorted out. The DC output of the circuit 62 appears across the zener diode ZD1, whose cathode and anode are connected to positive and zero DC power supply terminals 67 and 68 respectively of the circuit 63.

A forward biassed diode D2, a resistor R4 and a 1.2 volt, 100 mAH, nickel-cadmium battery 70 are series connected between the terminals 67 and 68, the battery also being connected between the DC power supply inputs of a crystal-controlled-oscillator circuit 72. It will be appreciated that the battery 70 is rechargeable. Thus, when AC power is present at the input terminals 40,42 of the time switch 10, the circuit 62 generates the DC power supply required for the oscillator circuit 72, and simultaneously keeps the battery 70 fully charged. In the event of a power cut affecting the supply of AC power to the terminals 40,42, the fully charged battery 70 will continue to keep the oscillator circuit 72 in operation for up to 48 hours.

The oscillator circuit 72 is controlled by a 32768 Hz quartz crystal 74, and includes frequency dividing circuits to produce output pulses at a frequency of 1/2Hz. These output pulses are applied via a capacitor C2 to a miniature stepper motor 76, e.g. of the kind made by H. Hechinger of Schwenningen, West Germany, which drives the timing dial 18 of Figure 1 via a suitable reduction gear train (not shown). This same gear train also drives a timing disc 80, which is shown in Figure 3.

The timing disc 80 of Figure 3 is coaxially secured to the same shaft as the timing dial 18, but is disposed inside the housing 12, behind its front surface 14: Figure 3 is actually a view of the back of the disc 80. Fixed to the rim of the disc 80, on the rear face thereof and at positions which typically correspond to the aforementioned times of midnight and 7.00 a.m. but which are adjustable, are two targets 82,84. These targets cooperate with a cruciform rotary cam-driving member 86, whose integral cam 88 in turn operates the changeover switch 48 of Figure 2 so that it takes up its illustrated, 'off-peak', position at the time determined by the target 82 and its non-illustrated, 'peak', position at the time determined by the target 84.

The positive DC power supply terminal 67 (Figure 2) is connected to the movable contact 50 of the push-button 'boost' switch 26, whose output contacts 91, 92 are connected to the set and reset inputs of a D-type bistable circuit 94. The 0 output of this bistable circuit is connected to the clock input of another D-type bistable circuit 96, whose 0 output is connected to the gate of a thyristor T1. The Q output of the bistable circuit 96 is coupled back to its D input, and is also connected to the reset input of a counter 100.

The counter 100 has a count input 101 connected to receive the Hz output pulses produced by the oscillator circuit 72, and an overflow output 102 at which it produces an output pulse when it reaches a count of 1800. The overflow output 102 of the counter 100 is connected to the reset input of the bistable circuit 96.

The anode of the thyristor T1 is connected to the positive DC output 65 of the bridge 64 via the coil 104 of the relay 58 and a current-limiting resistor R5, while its cathode is connected to zero DC power supply terminal 68 (and thus to the zero DC output 66 of the bridge 64). A diode D3 is provided in parallel with the coil 104, to limit the back EMF it can generate.

The time switch 10 is typically intended for controlling a twin-element water heater (not shown), such that the main or principal heating element of the water heater uses only off-peak electricity, i.e.

electricity supplied at the cheap, off-peak, tariff, as defined by the aforementioned meter. To this end, the AC input terminals 40,42 are connected as described earlier to the AC distribution circuit of the aforementioned two-tariff electricity supply. The principal heating element of the water heater is then connected between the AC output terminals 52 and 62, while the lower power, or secondary, heating element is connected between the AC outputtermin- als 60 and 62. Finally, the timing dial 18 is set to indicate the correct time (typically GMT in the UK) on the scale 23.

Once the time switch 10 is installed and set to the correct time, the stepper motor 76 drives the timing disc 80 so that the targets 82, 84 operate the changeover switch 48 into its illustrated 'off-peak' position at midnight every night and back into its non-illustrated 'peak' position at 7.00 a.m. each morning, as described earlier.

Assuming the switch 24 is in the ON position, then while the switch 48 is in its 'off-peak' position, the principal heating element of the water heater is energised: to provide the user with a visual indication of this, the orange indicator lamp 30 is also energised.

If the user does not want the principal heating element to be energised for any reason, he or she simply sets the switch 24to the OFF position (as illustrated in Figure 2). It will be noted that this does not stop the operation of the oscillator circuit 72, nor the resultant driving of the timing dial 18 and timing disc 80 by the motor 76 in response to the output pulses from the circuit 72.

If the user wishes to use the water heater outside the off-peak period, then he or she simply presses the push-button 'boost' switch 26. This causes the bistable circuit 94 to produce at its Q output a positive-going output pulse which sets the bistable circuit 96 (since this latter bistable circuit is arranged to be initially in its reset state). The setting of the bistable circuit 96 produces a logic level 1 signal (a positive voltage of about 4 to 5 volts) at its Q output, which renders the thyristor T1 conductive, thus energising the relay 58 and so closing its normallyopen contacts 56. The secondary heating element of the water heater is therefore energised.The user is again provided with a visual indication of this, since the red indicator lamp 28 is also energised, red being chosen for this lamp to indicate that peak (and therefore relatively expensive) electricity is being consumed.

The setting of the bistable circuit 96 also produces a logic level 0 signal (a near-zero voltage) at its 0 output. This not only readies the bistable circuit 96 to be reset by the next pulse from the bistable circuit 94 (if not already reset as described below), but also permits the counter 100 to start counting upwardly from the zero count to which it had been set and held by the logic level 1 signal previously present at its reset input.

When the counter 100 reaches its full house count of 1800, i.e. after one hour, it produces at its output 102 an overflow pulse which resets the bistable circuit 96. This in turn renders the thyristor T1 non-conductive, thus de-energising the relay 58 and so de-energising the secondary heating element of the water heater. Additionally, the counter 100 is reset to, and held at, a count of zero.

If, after pressing the push-button 'boost' switch 26, the user wishes to turn off the secondary heating element before one hour has elapsed, the switch 26 can simply be pressed again, thus causing the bistable circuit 94 to reset the bistable circuit 96, with the consequences described in the preceding paragraph.

It will be appreciated that pressing the pushbutton 'boost' switch 26 when the switch 48 is in its off-peak position has no effect.

If the water heater has only a single heating element, then the output terminals 52 and 60 can simply be shorted together by means of a shorting link 110 provided for that purpose.

The use of DC-operated timing means, constituted by the crystal-controlled oscillator circuit 72, together with the back-up battery 70, ensures that the switching times of the switch 48 remain substantially wholly within the off-peak period defined by the associated two-tariff meter, even in the event of power-cuts. Once the timing dial 18 has been set to the correct time, therefore, it is extremely unlikely that the user will ever need to reset it. Additionally, the switch 24 will probably be operated relatively infrequently, e,g. when the user is away from home for one or more nights. It is for this reason that the timing dial 18 and the switch 24 are protected by the transparent cover 16. Conversely, since the pushbutton 'boost' switch 26 is likely to be used more often, it is positioned for convenience outside the cover 16.

As already indicated, although the positions of the targets 82 are typically set to produce operation of the switch 48 at midnight and 7.00 a.m., these positions, and therefore the switching times, are adjustable. Thus if the electricity supplier wishes to change the off-peak perod defined by the two-tariff meter, it is a relatively easy matter for the same person who adjusts the meter also to adjust the time switch 10.Alternatively, the supplier may wish to have different off-peak periods in different parts of the total area supplied, or where many such time switches are to be used in a single building, e.g. an apartment building, the supplier may prefer to arrange that the switching times of the time switches are slightly staggered, to avoid abrupt changes in the total power consumption in the building: in either of these cases, it is relatively easy for the installer to adjust the time switch 10. However, since it is necessary to remove the front face 14 of the time switch 10 in order to effect this adjustment, the user will not be able to change the switching times, and thus lose synchronism with the meter, by accident.

Several modifications can be made to the described embodiment of the invention. In particular, the motor 76 and the switch 48 can be replaced by an all-electronic timing circuit similar to the circuit 100 arranged to operate a changeover relay similar to the relay 58. In this case, the timing dial 18 is replaced by a suitable electronically-operable time display, with provision for setting the timing circuit to display the correct time.

Claims (10)

1. Atime switch of the kind having a principal ON period intended to be synchronised with a low tariff period defined by a multi-tariff electricity meter, the time switch comprising: an AC input for connection to an AC power supply which is metered by said multi-tariff electricity meter; changeover switching means having first and second states in which said AC input is coupled to first and second AC outputs respectively; DC-operated timing means including a stepper motor arranged to be driven by timing pulses, said stepper motor being arranged to set said changeover switching means to its first state for at least one time period intended to be wholly within the low tariff period defined by said meter;; normally-open switching means connected to control the supply of the power to said second AC output, said normally-open switching means being manually-settable to its closed state for up to a predetermined maximum time; and a DC power supply circuit connected to said AC input and arranged to generate a DC power supply for said timing means from the AC supply at said AC input, said DC power supply circuit including a standy battery arranged to maintain operation of at least said timing means for a given minimum period in the event of interruption of the AC supply at said AC input;; said time switch further comprising switch driving means coupled between the stepper motor and the changeover switching means, and a housing containing at least the stepper motor, the switch driving means and the changeover switching means, wherein the switch driving means includes a timing disc driven by the motor and having switch-operating targets secured to its periphery at circumferentially spaced positions which define the respective times at which said changeover switching means is switched from its first state to its second state and back again by said targets, and wherein the positions of the targets are adjustable, upon opening said housing, in order to permit the switching times defined thereby to be changed.

2. A time switch as claimed in Claim 1,wherein the DC-operated timing means comprises a crystalcontrolled oscillator circuit arranged to produce said timing pulses at a substantially constant frequency.

3. A time switch as claimed in Claim 1 or Claim 2, wherein the normally-open switching means is connected in series between said changeover switching means and said second AC output.

4. A time switch as claimed in any preceding claim, wherein the timing disc is connected to a timing dial which is disposed at least partly outside said housing so as to be visible from the front of the time switch and whose angular position can be manually set from the front of the time switch.

5. A time switch as claimed in Claim 4, wherein the portion of the front of the time switch containing the timing dial is covered by a transparent cover, and wherein the normally-open switching means is manually operable by means of a switching device accessible from the portion of the front of the time switch not covered by the cover.

6. A time switch as claimed in any preceding claim, wherein the normally-open switching means comprises a relay, and further comprising an electronictiming circuit for defining said predetermined maximum time.

7. Atime switch as claimed in Claim 6, wherein the timing circuit comprises a counter connected to receive and count said timing pulses.

8. Atime switch as claimed in Claim 6 or Claim 7 when dependentfrom Claim 5, further comprising a two-state circuit arranged to switch back and forth between its two states in response to successive operations of the switching device, and wherein the relay is responsive to one state of the two-state circuit to enter its closed state, and the two-state circuit is connected to be set to its other state by the timing circuit when the timing circuit reaches a predetermined count which defines said predetermined maximum time.

9. A time switch as claimed in any preceding claim, further comprising a connecting linkfor selectively connecting together the first and second AC outputs.

10. A time switch as claimed in any preceding claim, further comprising first and second indicator lamps connected to be respectively energised when said first and second AC outputs respectively are energised.