GB2079551A - Improvements in dimming of fluorescent lights - Google Patents

Abstract

The invention provides a switch 20 which may be incorporated within the housing of a dimmable fluorescent tube to apply the full mains voltage automatically to one terminal of the ballast 12 and hence heater current to the lamp filaments when a variable voltage from a dimmer switch 10' is detected on a second input of the dimming ballast. The automatic remote switching by the switch 20 of the invention enables a dimming ballast 12 to operate with only two wires 11, 16 connected to the dimmer circuit. <IMAGE>

Description

SPECIFICATION Improvements in dimming of fluorescent lights The present invention relates to the dimming of fluorescent lights.

When dimming a fluorescent tube, it is necessary to vary the voltage so as to reduce the current flowing through the tube but at the same time it is important to maintain the full voltage across the heaters. Consequently, a tube with a conventional ballast cannot be used as this would reduce the heater voltage and it is necessary instead to use a dimming ballast which has separate connections to maintain the heaters at full voltage while the current through the tube is reduced.

All dimming ballasts necessarily require at least three input connections namely a neutral line, a live line at the full mains voltage and a variable line controlled by the dimmer switch.

The dimmer control switch therefore also requires three connections namely an input live mains wire and the two output lines leading to the dimming ballast of which the first is the switched mains voltage for the heaters and the second is the variable line for the tube current.

When converting an existing installation to dim fluorescent tubes, apart from replacing the ballasts and the wall-mounted control switches, it has hitherto been necessary to alter the wiring to provide an additional wire.

This would usually prove the most difficult and expensive part of the conversion since it would often require making a channel in the wall leading to the control switch thus necessitating redecoration.

The present invention seeks to enable a fluorescent light having a dimming ballast to be controlled without the need to alter the existing wiring.

In accordance with one aspect of the present invention, there is provided a switch which is operative to apply to one input of a dimmable ballast of a fluorescent tube, the full mains voltage in response to the presence on a line connecting a dimmer circuit to a second input of the dimmable ballast of a voltage exceeding a predetermined value.

Thus, the dimmer switch need only have leading from it to the ballast a single line carrying the variable voltage and the full mains voltage is automatically switched on by the switch when the dimmer circuit is in operation.

According to a second aspect of the invention, there is provided a switch having first and second input lines for connection to the live and neutral conductors of a mains supply, respectively, a third input line for connection to a variable voltage and an output line, the switch comprising a switching circuit operative to connect the live conductor of the mains supply to the output line in response to the voltage on the third input line exceeding a predetermined threshold.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a prior art wiring arrangement for a fluorescent tube with a dimmable ballast; Figure 2 shows the wiring of an installation using a switch in accordance with the invention; and Figure 3 shows a detailed circuit diagram of the arrangement of Fig. 2, omitting the dimmer circuit.

In Fig. 1, a dimmer switch 10 has an input line 11 connected to the live wire of a mains supply and two output lines which are connected to a dimmable ballast 1 2. Output line 14 carries the mains voltage whenever the dimmer switch 10 is ON and output line 1 6 carries a variable voltage. By variable voltage it is meant that the voltage is phase-angle controlled in the usual manner to interrupt the mains voltage intermittently and thereby vary the power transmitted to the load which, in this case, consists of the ballast and the fluorescent tube. The ballast is additionally connected to the tube and to the starter in a conventional manner, but this is not indicated in the drawing in the interest of clarity.

It will be seen that there are three wires leading to the dimmer switch 10 and since an existing installation would only have two wires leading to a conventional on-off switch, the insertion of a dimmer would require altering the wiring.

The arrangement of the invention shown in Fig. 2 has the same ballast 1 2 but the dimmer switch 10' has only two wires leading to it, namely the mains input line 11 and the variable output line 16. In order to provide the live voltage at the appropriate terminal of the ballast 12, an automatic heater switch 20 of the invention is connected to the live and neutral lines and also to the variable line 1 6.

When a voltage is detected on the variable line 16, the switch 20 closes to apply the full mains voltage to the input of the ballast 1 2.

Fig. 3 shows the internal construction of the automatic heater switch and its connection to the ballast and the fluorescent tube.

A thyristor 30 forms part of the dimmer switch and receives firing pulses from a phase-angle control circuit so that the line 1 6 carries the variable voltage. This line is connected in series through the fluorescent tube 32 and one part 1 2a of the ballast to the neutral line 1 3. The automatic heater switch 20, as shown in Fig. 2, has four connection terminals, the first which is connected to the live wire of the mains supply is designated 24, the terminal connected to the variable voltage is designated 26, the neutral connection terminal is designated 27 and the output leading to the second part 1 2b of the ballast, supplying the heaters, is designated 29.

A capacitor 51, resistor 52, zener diode 53, diode 54 and capacitor 55, form a conventional power supply circuit connected across line and neutral of the mains supply to rectify, smooth and limit the mains voltage to provide a power rail for the internal electrical circuitry of the automatic heater switch. The variable voltage is applied by way of an input circuit consisting of a capacitor 61 and a resistor 62 to a detector 63 which half-wave rectifies the signal. The detected signal is smoothed by a circuit comprising resistor 66 and capacitors 67, 68 and applied to the input of an electronic switch which is formed of the transistors Q1 and 02. When the detected and smoothed voltage developed across resistor 69 is below a pre-deternained level both transistors are off but when this voltage exceeds the switching threshold both transistors are switched on. This applied a d.c. voltage to the triggering electrode of triac 75 which is connected between the ballast 1 2b and the live terminal of the mains supplies. Thus the full mains voltage is applied across the part 1 2b of the ballast to apply the full heater voltage to the tube heaters for as long as a voltage exceeding the threshold of the electronic switch 0, 02 is present at the input terminal 26 of the automatically heater switch. If desired a variable resistor may be provided to adjust the switching threshold of the electronic switch.

Claims (8)

1. A switch which is operative to apply to one input of a dimmable ballast of a fluorescent tube, the full mains voltage in response to the presence on a line connecting a dimmer circuit to a second input of the dimmable ballast of a voltage exceeding a predetermined value.

2. A switch having first and second input lines for connection to the live and neutral conductors of a mains supply, respectively, a third input line for connection to a variable voltage and an output line, the switch comprising a switching circuit operative to connect the live conductor of the mains supply to the output line in response to the voltage on the third input line exceeding a predetermined threshold.

3. A switch as claimed in Claim 2, wherein the switching circuit is electronic and is connected to the third input line by way of a detector and a smoothing circuit.

4. A switch as claimed in Claim 2 or Claim 3, wherein the electronic switching circuit is connected to the triggering electrode of a triac connected between the live terminal of the mains supply and the output line.

5. A switch as claimed in Claim 2, 3 or 4, wherein a power supply circuit is connected across the first and second input lines to provide a d.c. voltage for the operation of the electronic switching circuit.

6. A switch constructed substantially as hereinbefore described, with reference to and as illustrated in Figs. 2 and 3 of the accompanying drawings.

CLAIMS (12 June 1981)

6. A fluorescent light installation which comprises a fluorescent tube, a dimming ballast and a dimmer circuit for varying the current flowing through the tube and a switch operative to apply to one input of the dimmable ballast the full main voltage in response to the presence on the line connecting the dimmer circuit to a second input of the dimmable ballast of a voltage exceeding a predetermined value whereby two wires need be connected to the dimmer circuit.

7. A dimmable fluorescent light which comprises a flurescent tube, a housing for the fluorescent tube and a dimmable ballast and a switch arranged within the housing, the said switch being operative to apply to one input of the dimmable ballast of the fluorescent tube the full main voltage in response to the presence on a second input of the dimmable ballast of a voltage exceeding a predetermined value.

8. A switch constructed substantially as hereinbefore described with reference to and as illustrated in Figs. 2 and 3 of the accompanying drawings.