GB2330704A

GB2330704A - Semiconductor AC switch units

Info

Publication number: GB2330704A
Application number: GB9822847A
Authority: GB
Inventors: George Heftman
Original assignee: TIMEGUARD Ltd
Current assignee: TIMEGUARD Ltd
Priority date: 1997-10-21
Filing date: 1998-10-20
Publication date: 1999-04-28
Also published as: GB9822847D0

Abstract

A switch unit 1 of an AC power-supply circuit for a lamp 2 includes a triac 7 connected in series with the lamp 2 to regulate current supply to it in dependence upon voltage across a resistor 13. The resistor 13 is connected in series with the AC input 11,12 to a bridge rectifier 10 in a current path that shunts the triac 7. A thyristor 14 is connected across the DC output 16,17 of the rectifier 10 in series with a resistor 15 so that the triac 7 is switched ON or OFF from the shunt-path resistor 13 in dependence upon whether the thyristor 14 is OFF or ON respectively. Gate control of the thyristor 14 is effected from the collector of a transistor 23 which is turned ON or OFF by a control-function signal applied to its base from a controller 9. DC power for the transistor 23 and the controller 9 is supplied from the rectifier 10 across a capacitor 21 that is shunted by a zener diode 22 and charged via a diode 26 and resistor 27 by current drawn from the path through the thyristor 14. The circuit may power an electric motor, for example, of a heating system, instead of the lamp 2.

Description

Switch Units This invention relates to switch units and AC powersupply circuits including them.

The invention is particularly concerned with switch units for use in controlling AC power supply to an electric lamp, motor or other load in accordance with a control function such as for example that exercised by a time controller, a delay timer, a light-level or temperature sensor or a movement detector.

According to the present invention a switch unit which is for connection in series with a load and an AC power source to control power supply to the load in accordance with an applied control-function signal, comprises a triac or other gate-controlled device for regulating current flow to the load through the unit in the series circuit, a bridge or other full-wave rectifier having its AC input connected in a current path that is in shunt with the gate-controlled device, and a further current path which is connected across the DC output of the rectifier and which is switchable between substantially ON and OFF current-flow conditions in dependence upon the applied control-function signal, and wherein the gate of the gate-controlled device is connected to regulate the current flow through the gate-controlled device in dependence upon current flow in the shunt-connected current path.

The shunt-connected path may include resistance, and in these circumstances the gate of the gate-controlled device may be connected to regulate current flow through that device in dependence upon voltage across the resistance.

The said further current path may include a thyristor that is switched between ON and OFF conditions in dependence upon the applied control-function signal. In these circumstances, the control-function signal may be applied to the base of a transistor that has its collector-emitter current path connected in series with resistance, and the gate of the thyristor may then be connected to this resistance to switch the thyristor to its ON or OFF condition in dependence upon whether the transistor is, respectively, non-conductive or conductive.

The switch unit of the invention may be used to replace a mechanical switch in, for example, the provision of automatic control of one or more lamps for lighting a building. As an alternative, it may be used in the control of power supply to an electric motor of, for example, a water pump of a central-heating system or a fan of a warm-air heating system. In any case, DC power required for operation of the unit (and also for operation of control means for providing a controlfunction input to it) may be derived solely from the series connection with the load via two AC-mains lines.

A switch unit in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawing. The drawing shows an AC power-supply circuit including the switch unit, the circuit of the switch unit being shown as connected to control power supply to a load which in this example takes the form of a single filament-lamp (2). The circuit of the switch unit is shown in combination with a controller (9) for providing the control-function input to the switch unit, the controller being represented in block schematic form within the circuit of the switch unit itself.

Referring to the drawing, the switch unit 1 is connected in series with the lamp 2 and an AC-mains supply source 3, with the lamp 2 connected in the mains-return line 4 to the unit 1. The mains-live line 5 is connected within the unit 1 via an inductor 6 (for suppression of mains noise) to a triac 7 that regulates current flow through the unit 1 to power the lamp 2. More particularly, the triac 7 regulates this current flow to turn the lamp 2 on or off according to a control function exercised by the controller 9.

The controller 9, which may take the form, for example, of a time controller, a delay timer, a light-level sensor or a movement detector, is in the present case powered from the mains supply source 3 within the series connection. In this respect, a DC supply to the controller 9 is derived via a bridge rectifier 10 of the unit 1. The AC-input terminals 11 and 12 of the bridge rectifier 10 are connected in series with a resistor 13 in a mains-current path that shunts the triac 7, the terminal 11 being connected to the junction between the inductor 6 and the triac 7, and the resistor 13 to the terminal 12. The gate of the triac 7 is connected to the junction between the resistor 13 and the terminal 12.

A thyristor 14 and a resistor 15 are connected in series with one another across the DC-output terminals 16 and 17 of the bridge rectifier 10. The positive output terminal 16 of the bridge rectifier 10 is also connected via a resistor 18 to a positive supply rail 19 for the controller 9 whereas the negative output terminal 17 is connected to a negative supply rail 20. A reservoir capacitor 21 is connected between the rails 19 and 20 and is shunted by a zener diode 22 that serves to limit the voltage between them.

The control output signal of the controller 9, which is high when the lamp 2 is to be off, and low when it is to be on, is applied to the base of a transistor 23 via a resistor 24. The transistor 23, which is connected between the rails 19 and 20 in series with a collectorcircuit resistor 25, accordingly conducts when the lamp 2 is to be off and is non-conductive when it is to be on.

The gate of the thyristor 14 is connected to the collector of the transistor 23 so that the thyristor 14 is turned to its OFF condition while the transistor 23 conducts, and to its ON condition when the transistor 23 is non-conductive.

While the thyristor 14 is in its OFF condition, the current drawn through the resistor 13 in the AC circuit is solely that required for DC supply via the bridge rectifier 10 to the controller 9 and the conductive transistor 23. The DC current required by the controller 9 may be just a few microamps, and that by the transistor 23 (being determined by the voltage between the rails 19 and 20 and the value of the resistor 25) may be typically 200 microamps. Accordingly, while the thyristor 14 is in its OFF condition, current flow in the resistor 13 to the bridge rectifier 10 is low so that the gate input to the triac 7 is low also, holding the triac 7, and consequently the lamp 2, turned off. Although power supply to the lamp 2 via the triac 7 is turned off, there is nonetheless low-current flow through the lamp 2 to maintain the DC supply to the controller 9 and transistor 23.

Change of the control output signal of the controller 9 from high to low, causes the transistor 21 to become nonconductive. This triggers the thyristor 14 to change to its ON condition causing increased output-current drain on the bridge rectifier 10. The increased current drain results in increase of current flow through the resistor 13 and leads to triggering of the triac 7 into conduction so as thereby to turn on the lamp 2. More particularly, while the thyristor 14 remains in its ON condition, the triac 7 is triggered to conduct during each half-cycle of the AC mains supply. The triggering takes place when the voltage (of either sense) across the resistor 13 attains a threshold magnitude (determined by the resistance value of the resistor 13) during that half-cycle.

The voltage across the resistor 15 reaches a peak value just before the triac 7 triggers during each half-cycle, and a diode 26 connected to the junction between the thyristor 14 and resistor 15 is used for re-charging the capacitor 21 towards this value. In this respect, the diode 26 is connected in series with a resistor 27 to the positive rail 19 so as to conduct and charge the capacitor 21 as required from current flow through the thyristor 14. The resistor 27 prevents high transient currents from flowing through the thyristor 14 in the event that the circuit is activated mid-way through a half-cycle of the AC mains supply.

Although the invention has been described above with reference to the drawing in the context of powering a lamp, it may be used otherwise. For example, it may be used in the control of an electric motor-driven waterpump of a central-heating installation. In this case, and utilising the arrangement shown in the drawing, the motor of the pump is connected in circuit in place of the lamp 2. The controller 9 might then function, as where the lamp 2 is involved, as a time programmer, or might, more particularly, involve a thermistor or other temperature-responsive device such that the motor is energised to drive the pump according to ambient temperature.

The electronic switch unit described has the significant advantage that it can be connected in the power supply to a load without the need for a separate connection or connections to supply power to the switch unit itself; the switch unit is powered without any such connection irrespective of whether power to the load is on or off.

Accordingly, the switch unit can replace directly a manually- or other mechanically-operated switch without requiring any change of, or addition to, existing wiring.

Furthermore, there are the advantages that the controller can be powered from the switch unit itself, and that the switch unit requires very little power for operation.

The low-power requirement is important in enabling the switch unit to be used with a wide range of loads, but is especially so in enabling it to be used with loads, such as low-energy lamps, that do not have low resistance in the off, unenergised state.

Claims

Claims: 1. A switch unit which is for connection in series with a load and an AC power source to control power supply to the load in accordance with an applied control-function signal, comprising a triac or other gate-controlled device for regulating current flow to the load through the unit in the series circuit, a bridge or other fullwave rectifier having its AC input connected in a current path that is in shunt with the gate-controlled device, and a further current path which is connected across the DC output of the rectifier and which is switchable between substantially ON and OFF current-flow conditions in dependence upon the applied control-function signal, and wherein the gate of the gate-controlled device is connected to regulate the current flow through the gatecontrol led device in dependence upon current flow in the shunt-connected current path.
2. A switch unit according to Claim 1 wherein said shunt-connected current path includes resistance and the gate of the gate-controlled device is connected to regulate current flow through that device in dependence upon voltage across the resistance.
3. A switch unit according to Claim 1 or Claim 2 wherein said further current path includes a thyristor that is switched between ON and OFF conditions in dependence upon the applied control-function signal.
4. A switch unit according to Claim 3 wherein resistance is connected in series with the thyristor across the DC output of the rectifier.
5. A switch unit according to Claim 3 or Claim 4 wherein the control-function signal is applied to the base of a transistor, the collector-emitter current path of the transistor is connected in series with resistance, and the gate of the thyristor is connected to this resistance to switch the thyristor to its ON or OFF conditions in dependence upon whether the transistor is, respectively, non-conductive or conductive.
6. A switch unit according to Claim 5 wherein a reservoir capacitor is connected in series with resistance across the DC output of the rectifier to establish DC supply for the transistor.
7. A switch unit according to Claim 6 wherein current for charging the capacitor is drawn from said further current path via a diode and resistance in series.
8. A switch unit according to Claim 6 or Claim 7 wherein a zener diode is connected in shunt with the capacitor to limit voltage across the capacitor.
9. A switch unit according to any one of Claims 1 to 8 in combination with a DC-powered controller, wherein DC power for the controller is derived via the rectifier.
10. A switch unit substantially as hereinbefore described with reference to the accompanying drawing.
11. An AC power-supply circuit for a load, including a switch unit according to any one of Claims 1 to 10 wherein the switch unit is connected in the power-supply circuit with the triac or other gate-controlled device in series with the load and an AC power source, to control power supply to the load in accordance with the applied control-function signal.
12. An AC power-supply circuit according to Claim 11 wherein the load is a lamp or electric motor.