GB2023951A - Fluorescent lamp starting circuits - Google Patents

Abstract

A starter circuit to control the operation of a fluorescent lamp includes a semi-conductor switching device, typically a silicon controlled rectifier 10, arranged to be connected between the filaments 3 and 6 of the lamp 4 during starting to allow current flow through the filaments 3 and 6 only during alternate supply half cycles and to apply the supply voltage between the filaments 3 and 6 during the other alternate half cycles. The semiconductor switching device 10 is preferably gated by a gate signal generator including a diode 12 in series with a potential divider network formed by resistors 13 and 14 the junction 15 of which is coupled through a Zener diode 16 to the gate of the switching device 10. The output of the Zener diode 16 is preferably coupled to the gate of the switching device 10 through a capacitor 18 to inhibit switching of the device 10 in the event of the fluorescent tube 4 being faulty. <IMAGE>

Description

SPECIFICATION Fluorescent lamp starting circuits This invention relates to starting circuits for fluorescent lamps.

A conventional fluorescent lamp comprises a glass tube which contains mercury vapour and is coated on its inner surface with a fluorescent phosphor. At each end of the tube is an oxide-coated tungsten filament to act as a hot cathode. In use of the lamp, one side of an a.c. supply main is connected via a ballast choke to one end of one filament and the other side of the supply is connected to one end of the other filament. A glow discharge switch is connected between the other filament ends.

The glow discharge switch comprises bimetallic strip contacts enclosed in a bulb containing an inert gas. When the supply is first switched on, the contacts are open. A glow discharge forms in the gas between the contact strips, which are thereby warmed so that the contacts close. The filament circuit of the lamp is therefore completed and the filaments start to warm up. The closing of the glow switch contacts extinguishes the glow discharge in the switch, and the contacts therefore cool down and reopen. The opening of these contacts breaks the filament current which is flowing through the choke, and a high voltage is induced in the choke by the high rate of decrease of the current. If the filaments are hot enough, the high voltage is sufficient to cause breakdown in the mercury vapour, and the lamp lights.The discharge current in the lamp is then sufficient to keep the filaments hot.

However, it usually happens in practice that the lamp does not strike the first time that the glow switch contacts reopen. There is a flash of light from the lamp, but a continuous discharge is not achieved. The filaments then start to col due to the opening of the flow switch contacts; the glow discharge is formed again between the contacts; and the cycle is repeated.

Again the lamp may not strike when the switch contacts reopen. In fact, several switch cycles may be needed before the lamp finally lights properly.

The several flashes of light from the lamp over a period of some seconds before stable operation of the lamp is achieved are annoying to the user.

It is an object of the present invention to provide an improved starting circuit for a fluorescent lamp.

According to the invention, a starting circuit to control the starting of a fluorescent lamp comprises a semi-conductor switching device arranged to be connected between the filaments of the lamp and operative during starting to allow current flow through the filaments only during alternate supply half cycles and to apply the supply voltage between the filaments during the other alternate half cycles.

Preferably the switching device comprises a silicon controlled rectifier (SCR) connected between first and second conductors for connection to the respective filaments, and the starting circuit further comprises a gate signal generator connected betwen said conductors for gating the SCR on during said first-mentioned alternate supply half cycles.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying circuit diagram.

Referring to the diagram, a starting circuit 1 in accordance with the invention has a terminal 2 connected to one end of a filament 3 of a fluorescent lamp 4, and a terminal 5 connected to one end of a filament 6 of the lamp.

A choke 7 is connected between the other end of the filament 3 and a line 8 of an a.c. supply. The other end of the filament 6 is connected to the other line 9 of the supply.

The circuit 1 comprises a silicon controlled rectifier 10, the anode and cathode of which are connected, respectively, to the terminals 2 and 5. The gate electrode of the SCR 10 is connected to the cathode via a resistor 11. A gating signal is derived from a potential divider comprising a diode 12 and resistors 13 and 14 connected in series between the terminals 2 and 3. The signal is fed from a juncton 15 between the resistors 13 and 14, via a zener diode 16, a diode 17 and a capacitor 18 to the gate of the SCR 4. A discharge resistor 19 is connected across the capacitor 18.

To cause operation of the circuit, the supply lines 8 and 9 are energised. In those half cycles of the supply voltage when the terminal 2 is positive with respect to the terminal 5, current will flow through the diode 12 and the resistors 13 and 14, and a signal will be fed to the SCR gate, via the capacitor 18, to trigger the SCR 10. However, this signal does not start as soon as the terminal 2 goes positive, because the potential at the junction 15 must first rise until it exceeds the break-over voltage of the zener diode 16. There is, therefore, a delay between the going positive of the anode of the SCR and the application of the gate signal.The length of this delay is preselected so that the resulting mean current flowing through the SCR 10, and hence through the filaments 3 and 6, in each positive half cycle, as controlled by the conduction period of the SCR and by the impedance of the choke 7 and the filaments 3 and 6, does not exceed the allowable level for the filaments.

In successive positive half cycles the filaments warm up. In the alternate half cycles when the terminal 2 is negative with respect to the terminal 5, the SCR is cut off and no filament current flows. The full supply voltage plus a voltage induced in the choke 7 by switching off of the filament current is applied between the filaments. When the filaments have warmed up sufficiently, this voltage in a subsequent negative half cycle will cause the lamp to fire. This may not result in immediate continuous running of the lamp and the lamp may produce a flash of light, but continuous running will be achieved in a relatively small number of cycles later.

Since the supply frequency is 50Hz (or 60Hz), the time taken for even a substantial number of switching cycles is very small, and any flashing which occurs, and the subsequent proper striking of the lamp, will all appear to the user to be continuous, and running of the lamp will be achieved quickly after first switching on the supply. None of the objectionable, relatively long term, initial flashing of the lamp which is evident when using conventional lamp starters can be seen when the present circuit is used.

When the lamp is running properly, the voltage across the terminals 2 and 5 will be the voltage across the discharge in the mercury vapour between the lamp filaments. This will be considerably less than the supply voltage, and the remainder of the supply voltage will be dropped across the choke 7.

Hence, the voltage across the potential divider 12, 13,14 will be low, and no further gating signals will be fed to the SCR 10. The rectifier will therefore remain cut off, and a substantially open circuit will appear between the terminals 2 and 5, the only current path being that through the relatively high impedance of the potential divider 12, 13 and 14 and then only in the positive half cycles. The filaments 3 and 6 will be kept hot by the discharge current, just as with conventional starting circuits.

If the lamp filaments are intact but the lamp is otherwise faulty, the lamp will not strike, even after numerous successive switching cycles of the SCR 10. During the positive half cycles the capacitor 18 will gradually charge until it is fully charged. No more charging current will then flow as a gating signal via the gate/cathode circuit of the SCR. Hence, the SCR will no longer fire, and the flow of lamp filament current will cease. As long as the supply is still connected to the lamp circuit, the capacitor 18 will remain in this charged state and the SCR 10 will remain cut off. Hence, the continual, usually somewhat irregular, flashing of faulty lamps which is experienced when conventional starting circuits are used, is avoided by use of the present circuit. The tube will just remain de-energised.

Disconnection ofthe mains supply will allow the capacitor 18 to discharge through the resistor 19, and the circuit is therefore reset ready for further operation when the mains supplied is re-applied.

The starting circuit 1 may be mounted in a similar can to that used to house a conventional B.S.3772 glow discharge type of starter circuit. The conventional starter circuit and that in accordance with this invention are completely compatible in use and mounting them in similar cans enables them to be interchanged in practice.

Claims (9)

1. A starting circuit to control the starting of a fluorescent lamp, comprising a semi-conductor switching device arranged to be connected between the filaments of the lamp and operative during starting to allow current flow through the filaments only during alternate supply half cycles and to apply the supply voltage between the filaments during the other alternate half cycles.

2. A starting circuit according to claim 1, in which the switching device comprises a silicon controlled rectifier connected between first and second conductors which are arranged to be connected to the respective filaments, and the starting circuit further comprises a gate signal generator connected between the conductors for gating the silicon controlled rectifier on during the alternate supply half cycles.

3. A starting circuit according to claim 2, in which the first and second conductors are connected to terminals which are arranged to be compatible with the standard starter circuit socket of a fluorescent lamp fitting.

4. A starting circuit according to claim 2 or claim 3, in which the gate signal generator includes a diode in series with a potential divider, and a Zener diode connected in series with the output from the potential divider.

5. A starting circuit according to claim 4, in which the gating signal output from the Zener diode is applied to the gate of the silicon controlled rectifier through a capacitor, so that, in th event of the lamp being faulty, the capacitor charges up after repeated switching cycles to maintain the flow through the silicon controlled rectifier switched off during all subsequent half cycles.

6. A starting circuit according to claim 5, including a high resistance resistor connected in parallel with the capacitor which allows the capacitor to discharge through the high resistance resistor when the mains supply is removed from the starter circuit and hence allows the starter circuit to reset.

7. A starter circuit according to claim 1, constructed substantially as described with reference to the accompanying drawing.

8. A fluorescent light fitting including a starter circuit in accordance with any one of the preceding claims.

9. A fluorescent light including a starter circuit in accordance with any one of claims 1 to 7.