GB2362048A

GB2362048A - An in-line lighting delay switch using a normally-closed relay

Info

Publication number: GB2362048A
Application number: GB0105297A
Authority: GB
Inventors: William Cook; George Heftman; Manjit Jutla
Original assignee: TIMEGUARD Ltd
Current assignee: TIMEGUARD Ltd
Priority date: 2000-03-04
Filing date: 2001-03-05
Publication date: 2001-11-07
Anticipated expiration: 2021-03-05
Also published as: GB0005184D0; GB0105297D0; GB2362048B

Abstract

A time-delay switch 1 includes a relay 8 which is energised to open its contacts 9, from a bridge-rectifier 12 that, being connected in series with a capacitor 13 across the switch-terminals 4 and 5, draws low current through the load (filament lamp 3) during the OFF switch-state. The main-current path through the switch 1 between the terminals 4 and 5 is via the contacts 9 in series with a push-button switch 10 which when operated by-passes the capacitor 13 transitorily to increase current flow through the rectifier 12. This increases the charge of a storage capacitor 23 and turns ON a transistor 27 so as to turn OFF a FET 16 in series with the relay-coil 11. The relay 8 is thereby de-energised so that the contacts 9 close completing the path between terminals 4 and 5 (switch 1 turned ON) for powering the load (lamp 3). The transistor 27 remains ON to hold the FET 16 OFF for a period determined by discharge of the capacitor 23, after which the FET 16 turns ON to re-energise the relay-coil 11 and bring about opening of the contacts 9, turning the switch 1 OFF. The circuit may be applied to stairs and landings.

Description

2362048 Time-Delay Switches This invention relates to time-delay switches.

5 The invention is particularly, though not exclusively, concerned with time-delay switches for use in mainspowered lighting circuits where the switch is connected in the live power line in series with the lamp-load.

Switches of this kind are commonly used on staircases and landings for controlling power supply to one or more lamps, the switch being operated manually to initiate power connection through the switch for a predetermined limited period. In this context, the time-delay switch replaces the conventional ON-OFF switch of the lighting circuit, normally for the purposes of economy of power consumption.

It is one of the objects of the present invention to provide a form of time-delay switch that has advantages over existing forms of such switch.

According to the present invention a time-delay switch having a pair of terminals for connecting it to a power supply in series with a load, comprises a relay having contacts which are closed while the relay is un-energised for establishing a path between the terminals for power supply to the load, and which are open for interrupting said path while the relay is energised, and a control circuit which includes capacitance that is charged from current flowing between said terminals in shunt with said path while said path is interrupted, said control circuit being selectively operable to interrupt energisation of the relay transitorily, thereby closing said contacts to complete said path, for a period that is dependent on discharge of said capacitance.

2 Where the timedelay switch of the invention is for use with an alternating-current power supply, the control circuit may include a bridge-rectifier or other rectifying means that is connected between said terminals in shunt with said path for supplying direct current to charge the capacitance and to energise the relay. current- limit ing impedance means may be connected in series with the bridgerectifier across the terminals, and in these circumstances selective operation of the control circuit (which may be effected by manual operation of a push- button) may provide transitory bypassing of this impedance means as well as interruption of said path.

The increase of current-flow said rectifying means consequent upon the bypassing of the impedance means may be used for charging the capacitance to an increased voltage level and for interrupting energisation of the relay. More particularly in the latter respect, an energisation coil of the relay may be connected in series with a transistor device across the output terminals of the bridge-rectifier, in shunt with said capacitance. In these circumstances, the transistor device may be switched to a non-conducting condition so as to interrupt energisation of the relay coil, in response to charging of the capacitance to the increased voltage level. Furthermore, the transistor device may be held in the non-conducting condition until the capacitance has discharged to a substantial extent, before reverting to its conducting condition to restore energisation of the relay.

A time-delay switch in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawing which shows the time-delay switch as used in the context of controlling supply of alternating-current power to a lamp load; 3 modification features of the circuit are indicated in dotted line in the drawing.

Referring to the drawing, the time-delay switch 1 is connected to an alternating-current mains-supply source 2 in series with a filament lamp 3 for supplying operating power to the lamp 3 while the switch 1 is in its ON state, and to interrupt supply while the switch 1 is in its OFF state. As with conventional systems of this nature, there is no neutral line, and the switch 1 has just two terminals 4 and 5 for connection respectively to the live-feed line 6 from the source 2 and the switchedlive line 7 to the load 3.

The switch 1 includes a relay 8 having contacts 9 which in the unenergised state of the relay 8 (as shown) are closed and which are connected in series with a manuallyoperable push-button switch 10 between the terminals 4 and 5. While the switch 10 remains un-operated, the contacts 9 establish a direct -connection path between the terminals 4 and 5 to def ine the ON state of the switch 1. However, connection of the source 2 to the terminals 4 and 5 through the lamp 3, results in energisation of the relay 8 with consequent opening of the contacts 9 to interrupt the direct-connection path between the terminals 4 and 5 and accordingly turn the switch 1 to its OFF state.

Energisation of the relay 8 results from direct current supplied to its coil 11 from a bridge-rectifier 12. The rectifier 12 is connected in series with a capacitor 13 and low-value resistor 14 across the terminals 4 and 5 in shunt with the path through the serial ly- connected contacts 9 and switch 10. The capacitor 13, which is shunted by a high-value discharge resistor 15, limits current-flow through the rectifier 12, and thus through the lamp 3 in the OFF state of the switch 1, to about 4 6 mA. This is well below the threshold for powering emission of light by the lamp 3, but results in sufficient direct-current voltage across the output terminals 17 and 18 of the bridge-rectifier 12 to energise the relay coil 11 in series with the source drain path of a field-effect transistor (FET) 16.

The gate of the FET 16 is connected to the junction of two Zener diodes 19 and 20 that are connected across the terminals 17 and 18 in series with fixed and variable resistors 21 and 22 respectively. Both diodes 19 and 20 conduct in their reverse sense during the OFF state of the switch 1 and the current passed is just sufficient to hold the FET 16 in its ON condition. The relay 8 therefore remains energised to hold the contacts 9 open and maintain the switch 1 is in its OFF state in which there is a small current-flow through the lamp 3 but no effective power supply to it.

When the push-button of the switch 10 is depressed, the serially-connected capacitor 13 and resistor 14 are bypassed so that current-flow through the rectifier 12 increases significantly. The voltage across the terminals 17 and 18 of the rectifier 12 increases correspondingly, causing the charge of a capacitor 23 connected across the terminals 17 and 18 to increase rapidly towards near-peak voltage of the source 2. The increase in voltage causes a Zener diode 24 connected in series with two resistors 25 and 26, across the capacitor 23, to conduct in its reverse sense. The current-flow through the diode 24 causes a transistor 27, which has its base connected to the junction between the diode 24 and the resistor 25, to turn ON.

The collector of the transistor 27 is connected to the junction of the diodes 19 and 20 with the gate of the FET 16 so that when the transistor 27 turns ON, the diode 19 stops conducting. The consequent switching OFF of the FET 16 breaks supply of energisation current to the coil 11 of the relay 8 causing the contacts 9 to switch back to their closed condition. Once the push-button switch 10 has been released, the closed contacts 9 re-establish the direct- connection path between the terminals 4 and 5 for power supply to the lamp 3. Release of the pushbutton switch 10 also terminates by-pass of the seriallyconnected capacitor 13 and resistor 14, so current through the rectifier 12 is again limited and the voltage across the terminals 17 and 18 falls. However, owing to the charged condition of the capacitor 23, the diode 24 continues to conduct in the reverse sense to maintain the transistor 27 ON and the FET 16 in its OFF condition.

Discharge of the capacitor 23 is mainly via the resistors 21 and 22, the diode 20 and the transistor 27, holding the FET 16 OFF and at a rate determined by the setting of the resistor 22. After a period dependent on this setting, the voltage across the diode 24 falls to below its Zener threshold, so that the diode 24 ceases to conduct and the transistor 27 accordingly turns OFF. The diode 19 then returns to conduction in its reverse direction with the diode 20, so that the FET 16 turns ON to restore energisation of the relay 8 and open the contacts 9. Opening of the contacts 9 returns the switch 1 to its OFF state, and terminates power supply to the lamp 3 until the push-button 12 is operated again.

The lamp 3 is powered to emit light only while the switch 1 is in its ON state. This period is determined by the time taken for the capacitor 23 to discharge, but since the discharge current is required solely for holding the transistor 27 ON and the FET 16 in its OFF condition, the period can readily be quite long. It is very significant in this regard that the relay 8 remains un-energised during the ON state of the switch 1. There is 6 accordingly no requirement, as otherwise would be the case, for energisation of the relay 8 to be sustained during the ON state by the capacitor-charge. If the capac itor- charge were required to sustain energisation, there would be severe limitation for practical purposes on the length of time f or which the switch 1 could be held in its ON state.

With the switch 1 of the present invention, energisation current for the relay 8 is derived from the mains supply 2 while the lamp 3 is OFF. This imposes no material limitation on the circuit or its operation so that the relay 8 and other components of the switch I can be of standard, low-cost form.

The time-delay switch 1 can be readily housed within a standard wall box used for light switches. A neon lamp 28 for exposure through a window of the facia-plate of the mounting, is connected in series with a resistor 29 across the terminals 4 and 5 to indicate whether the lighting circuit is powered and, by its glow in the dark, the location of the push-button 12.

The low-value resistor 14 in series with the capacitor 13 provides a degree of protection for the switch 1 against high-voltage surges; a current- limiting resistor may be used in place of the capacitor 13 and the resistors 14 and 15, but the capacitor is preferred in the interests of reducing heat output. Also, as indicated in dotted line in the drawing by way of modification of the circuit of the switch 1, a resistor 30 connected in series with Zener diodes 31 and 32, may be added in shunt with the capacitor 23 to protect the capacitor 23 against overcharging, and a diode 33 may be connected across the relay coil 11 to limit backswing in the coil 11 when the FET 16 is turned OFF.

7 claims:

1. A time-delay switch having a pair of terminals for connecting it to a power supply in series with a load, comprising a relay having contacts which are closed while the relay is un-energised for establishing a path between the terminals for power supply to the load, and which are open for interrupting said path while the relay is energised, and a control circuit which includes capacitance that is charged from current flowing between said terminals in shunt with said path while said path is interrupted, said control circuit being selectively operable to interrupt energisation of the relay transitorily, thereby closing said contacts to complete said path, for a period that is dependent on discharge of said capacitance.

Claims

2. A time-delay switch according to Claim 1 wherein selective operation of

the control circuit is effected by manual operation of a push-button.

3. A time-delay switch according to Claim 1 or Claim 2 for use with an a 1 ternat ing- current power supply, wherein the control circuit includes rectifying means connected between said terminals in shunt with said path for supplying direct current to charge the capacitance and to energise the relay.

4. A time-delay switch according to Claim 3 wherein said rectifying means is a bridge-rectifier.

5. A time-delay switch according to Claim 3 or Claim 4 including impedance means connected in series with said rectifying means for limiting current-flow therein, and wherein the selective operation of the control circuit 8 provides transitory by-passing of said impedance means as well as interruption of said path.

6. A time-delay switch according to claim 5 wherein increase of currentflow in said rectifying means consequent upon the by-passing of said impedance means results in charging of the capacitance to an increased voltage level and interruption of energisation of the relay.

7. A time-delay switch according to Claim 6 wherein an energisation coil of the relay is connected in series with a transistor device in shunt with said capacitance.

8. A time-delay switch according to Claim 7 wherein the transistor device is switched to a non-conducting condition so as to interrupt energisation of the relay coil, in response to charging of the capacitance to the increased voltage level.

9. A time-delay switch acqording to Claim 8 wherein the transistor device is held in the non-conducting condition until the capacitance has discharged to a substantial extent, before reverting to its conducting condition to restore energisation of the relay.

10. A time-delay switch according to any one of Claims 5 to 9 wherein said impedance means is a current- limiting resistance.

11. A time-delay switch according to any one of Claims 5 to 9 wherein said impedance means comprises capacitance.

12. A time-delay switch having a pair of terminals for connecting it to an alternatingcurrent power supply in series with a load, substantially as hereinbefore described with reference to the accompanying drawing.

9 13. A time-delay switch according to Claim 12, modified as hereinbefore described with reference to features indicated in dotted line in the accompanying drawing.