GB2092840A - Starting arrangement for a fluorescent lamp - Google Patents

Abstract

Fluorescent lamp 31 is switched on at switch 48. Induced current from coil 34 flows in a starting circuit comprising main circuit wires 36, electrodes 46, which heat up and radiate when said current flows, starting circuit wires 47 and relay 43 which is closed. Radiation emitted from the electrodes 46 impinges on sensor 38 which then allows a switching potential to transistor 39 which, in turn switches on coil 40 of the relay 43, opening the starting circuit. Sudden opening of the starting circuit produces a surge voltage in the coil 34 which is discharged through tube 30. The lamp lights up. <IMAGE>

Description

SPECIFICATION Starting arrangement for a fluorescent lamp This invention relates to fluorescent lamps and more particularly starting arrangements thereof and their methods of operation.

Fluorescent lamps are popular for lighting in industry, commerce and homes. A conventional fluorescent lamp usually comprises a fluorescent tube, a main circuit, a starting circuit and a starting device to control the starting circuit. A common type of starting device comprises a bimetal strip which bends on heating and cooling and thereby closes and opens the starting circuit, and this applies to electrodes of the tube a very high voltage pulse, higher than the normal running voltage, inducing the fluorescent tube to light The bimetal strip takes time to heat up and cool down and its movement is fairly slow. Because of this the switching on and off is often erratic or only partial. Also the pulse delivered to the electrodes of the tube may not be large enough to cause the tube to light. Consequently the tube tends to flicker when switched on or when the starter is faulty.

Generally, a fluorescent tube is a low pressure discharge tube containing a gas mixture consisting of mercury vapour and argon. The inner surface of the tube, which is of glass, is coated with fluorescent material. Under normal atmospheric pressure and temperature, the gas mixture in the tube would be virtually non-conductive. However when at low pressure, and disposed between electrodes to which a high voltage is applied, the gas mixture is ionised and becomes electrically-conductive.

From Paschen's Law, it is known that factors affecting the discharge voltages through gases include: gas pressure and type of gas, the nature of the electrodes, and the distance between the electrodes and the overall structure of the tube. If the nature of the electrodes and type of gas are kept constant, the discharge voltage is dependent on the product of the gas pressure and the distance between the electrodes. Therefore, if the gas pressure is also kept constant, the discharge voltage is directly proportional to the distance between the electrodes.

The large the distance the higher is the discharge voltage.

To enable the invention best to be appreciated, a typical starting device of a conventional fluorescent lamp, and its method of operation will now be described with reference to Figs. 1 and 2 of the accompanying drawing, in which: Fig. lisa diagram illustrating components of the exemplary conventional fluorescent lamp; and Fig. 2 is a diagrammatic view of one of the components ofthe lamp of Fig. 1.

As shown, a fluorescent tube 10 is located byter- minals 11 in circuitry, of a fluorescent lamp 12, having a main circuit comprising an induction coil 13, supply leads 14, and electrodes 15 ofthetube 10.

Electrical flow is either clockwise or anti-clockwise depending on the supply's potential. A starting circuit is defined by the supply leads 14, the electrodes 15, starting circuit leads 16 and a starting device 17.

The starting device 17, which is illustrated in more detail in Fig. 2, comprises two electrodes, of which one is a movable bimetal strip 20 and the other is a fixed electrode 21, said electrodes 20,21 being sealed in a partially evacuated envelope 22. Both the bimetal strip 20 and the fixed electrode 21 are coated with an active agent, such as mercury oxide. In service, the starting device 17 effectively acts as a combined discharge tube and switch. Another type of starting device uses electrodes of different coefficients of expansion.

When the lamp is switched on by means of a switch 18 incorporated in the lead 14, current flows in the starting circuit and discharges through the starter device between the bimetal strip 20 and the fixed electrode 21. Discharge occurs in the starting device 17 ratherthanthefluorescenttube 10 because the distance between the electrodes 20 and 21 is shorter than the distance between the electrodes 15. Heat is produced as a result of the discharge, and this heat causes the bimetal strip 20 to bend and establish contact with the fixed electrode 21. Then, the starting device 17 no longer acts as a discharge tube producing heat; instead it allows flow, through the starting circuit, of a current, larger than that which flowed in the starting circuit, which current pre-heats the electrodes 15 of the fluorescent tube 10 making their active coating incandescent.

Because the bimetal strip 20 is no longer being heated, it coois down, breaks contact with the fixed electrode 21 and opens the starting circuit. This opening of the starting circuit produces a very high surge voltage, from the induction coil 13, according to the equation: e = L.di/dt where e equals the surge voltage produced by the product of the inductance L of the coil 13, which is dependent on the coil's physical characteristics and the rate of change of current flow with respect to time di/dt. The surge voltage across the electrodes 15 causes electrons to be emitted from the electrodes 15, that is to say, electrons are discharged through the tube 10.

When the emitted electrons collide with mercury atoms ultra-violet rays of 253.7nm are produced.

Human eyes can, usually, only detect radiation in the range of 380 to 760nm. Therefore, the radiation from the mercury atoms is invisable. The fluorescent material on the inner surface of the tube absorbs the ultra-violet rays and emits radiation in the visible region of 380 to 760nm.

In general a sudden very high voltage surge must be applied to a fluorescent tube in order to make it light. Therefore, sudden interruption of the starting circuit is essential.

In practice, a fluorescent lamp needs a high running voltage to keep the electrodes incandescent and to allow for energy loss in electron collisions with mercury vapour. However in conventional start ing devices, such as that just described with reference to Figs. 1 and 2 of the drawing, the starting circuit may be closed while the lamp is alight, causing a loss in voltage and the light from the tube to flicker.

An object of the invention is to provide, in a fluorescent lamp, a starting arrangement that will provide a sudden interruption of the starter circuit and that will not produce flickering when the lamp is lit.

Pursuant hereto the present invention provides a fluorescent lamp comprising a fluorescent tube, and a starting circuit for the tube and incorporating a switch adapted to be controlled by a sensor disposed to sense emission from the tube, the starting circuit switch being opened when the sensor is subjected to emission from the tube and closed when not so subjected.

The sensor is preferably a cadmium sulphide resistor which may control the starting circuit switch by means of an electronic switch, preferably a transistor. The transistor may be of the NPN type.

The starting circuit switch may be a relay or like switch and the sensor may be disposed in a direct current control circuit.

The present invention will now be described further, by way of example, with reference to the remaining figure of the accompanying drawing, already referred to, in which: Fig. 3 is a circuit diagram illustrating a preferred embodiment of a fluorescent lamp of the present invention.

A fluorescent tube 30 (similar to the tube 10 of Fig.

1) is located in the circuitry of a fluorescent lamp which is indicated generally at 31. This circuitry comprises a transformer 32 having a primary coil 33 and two secondary coils 34 and 35. The coil 34 is in series with main circuit leads 36, and the coil 35 powers a control circuit 37. The control circuit 37 comprises a sensor38 of light-sensitive cadmium sulphide, an NPN transistor 39, a relay coil 40, a capacitor 41 and a diode 42. The relay coil 40 is part of a reed relay switch 43 comprising the coil 40, and two relay contacts namely a reed contact 44 which responds to magnetic effects of the coil 40 and a fixed contact 45. The relay switch 43 serves to open and close a starting circuit of the lamp 31, which starting circuit comprises the coil 34, the main circuit leads 36, tube electrodes 46, starting circuit leads 47 and relay contacts 44 and 45.The diode 42 produces a half-rectified current in the control circuit while the lamp 31 is on and the capacitor 41 smoothes out peak voltages. The transistor 39 acts as an electronic switch controlled by the sensor 38, as will be explained later in more detail.

Starting with the lamp 31 switched off, the relay 43 will normally be in its closed condition i.e. with the contacts 44 and 45 touching, and therefore the starting circuit is also closed. Upon switching on, e.g. by means of a light switch 48, alternating current flows in the primary coil 33 of the transformer 32. This current induces alternating current in the secondary coils 34 and 35. The current from the secondary coil 34 flows through the starting circuit and heats the electrodes 46. Mercury oxide (or other active agent) on the electrodes 46 is heated and emits radiation and some electrons. The radiation emission from one of the electrodes 46 impinges on the sensor 38 which is located near said electrode 46. The impinging emission causes a fall in the electrical resistance of the sensor 38, thereby allowing an electrical potential to be applied to the base of the transistor 39.This increase in potential switches the transistor 39 on, thereby allowing current to flow through the relay coil 40 creating a magnetic field, which acts on the reed contact 44 pulling it away from the fixed contact 45, thereby suddenly opening the starting circuit. This sudden opening of the starting circuit induces in the secondary coil 34 a surge voltage which is discharged through the tube 30 between the electrodes 46, and the lamp lights.

To summarise, the lamp 31 is switched on and the electrodes 46 heat up and radiate. The radiation impinges on the sensor 38 which then allows a switching potential to the transistor 39. The transistor 39 switches on the coil 40 of the relay 43, opening the starting circuit. This opening of the starting circuit induces a surge in the coil 34 which is discharged through the tube 30 to light the lamp.

Some important advantages of the invention are: (1) that, when the lamp is switched on, the starting circuit is not opened until the electrodes of the tube are pre-heated and ready to light the lamp. With such control the lamp does not flicker on starting.

(2) that a drop in light level from the tube is sensed by the cadmium sulphide resistance which, via the control circuit, closes and opens the starting circuit, producing a high voltage pulse in the fluorescent tube to keep the tube alight and to compensate for energy loss from the tube as light; and (3) speed and accuracy of operation of the control circuit which ensures that the lamp will not flicker.

Claims (10)

1. Afluorescent lamp comprising a fluorescent tube, and a starting circuit for the tube and incorporating a switch adapted to be controlled by a sensor disposed to sense emission from the tube, the starting circuit switch being opened when the sensor is subjected to emission from the tube and closed when not so subjected.

2. A lamp as claimed in claim 1, wherein the sensor is a cadmium sulphide resistor.

3. A lamp as claimed in claim 1 or 2 wherein the sensor serves to control the starting circuit switch by means of an electronic switch.

4. A lamp as claimed in claim 3 wherein the electronic switch is a transistor.

5. A lamp as claimed in claims 3 or 4 wherein the transistor is of the NPN type.

6. A lamp as claimed in any preceding claim wherein the starting circuit switch is a relay or like switch.

7. A lamp as claimed in any preceding claim wherein the starting circuit is controlled by a direct current control circuit in which the sensor is disposed.

8. A lamp substantially as hereinbefore described with reference to and illustrated in Fig. 3 of the accompanying drawing.

9. A method of controlling the starting of a fluorescent lamp comprising a fluorescent tube having a starting circuit including a switch, which comprises sensing emission from the tube by means of a sensor which is operative to open the starting circuit when subjected to emission from the tube and to close the circuit when not so subjected.

10. A method of controlling the starting of a fluorescent lamp substantially as hereinbefore described with reference to and illustrated in Fig. 3 of the accompanying drawing.