Fluorescent Lamp Circuit Driven Initially At Lower Voltage And Higher Frequency

Abstract

A circuit for fluorescent lamps includes an inverter which incorporates means such as temperature dependent elements or switches whereby the voltage from the inverter is lower but at a higher frequency initially than during normal running. This inverter feeds a plurality of lamps, each having associated circuit elements which are resonant or semi-resonant at the higher frequency, to facilitate the lamps conducting.

Description

The present invention relates to circuits including fluorescent lamps.

The term fluorescent lamps is intended to cover all lamps wherein a discharge takes place in a gaseous medium and all other elements having a negative resistance characteristic and requiring to be rendered conductive by a combination of voltage and current.

One way of firing a fluorescent lamp is to have one or more electron-emitting filaments. When the lamp conducts the, or each, filament is subject to ion bombardment which can result in the filament overheating especially if it is fed with heating current, the overheating in combination with the ion bombardment is harmful to the life of the lamp. It is therefore desired to reduce or prevent heating current through the filament when the lamp is conductive. There are many known arrangements and in one of them two filaments are connected in series in a circuit including an inductor and a capacitor which are resonant at a frequency near the supply frequency, the capacitor being shunted by the lamp when the lamp conducts so that the circuit loses its near-resonant property and becomes a greater impedance reducing the current through the filaments. This type of circuit may include an inverter supplying the lamp at a comparatively high frequency say 1,000 c/s so that the capacitor and inductor can be relatively small.

According to the present invention there is provided a circuit comprising an element such as a fluorescent lamp which has to be conditioned to conduct, circuit elements connected, one in series and one in parallel, with the first-mentioned element and designed to resonate at a resonant frequency and an inverter for supplying the lamp and said circuit elements with a voltage at a frequency near the resonant frequency, the improvement residing in that said inverter incorporates means whereby it can deliver a reduced voltage at a frequency nearer the resonant frequency to facilitate conditioning of the first mentioned element.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings.

IN THE DRAWINGS

FIG. 1 is a diagrammatic representation of a fluorescent lamp and its associated circuit elements,

FIG. 2 is an exemplary inverter for supplying current to the lamp of FIG. 1 and employing temperature dependent circuit elements,

FIG. 3 is an exemplary inverter for supplying current to the lamp of FIG. 1 employing switches, and

FIG. 4 is an exemplary high power inverter for supplying current to a plurality of lamps as shown in FIG. 1.

In FIG. 1 cathode filaments 11 of a fluorescent lamp 12 are connected in a series resonant circuit comprising an inductor 13 and a capacitor 14 across a supply which is shunted by a capacitor 15 designed to bring the power factor of the current taken from the supply to near unity when the lamp is running normally. The lamp (i.e. the ion conduction path thereof) is connected across the capacitor 14.

The value of the inductor 13 is such as to provide the desired ballast after the lamp is conductive and the value of the capacitor 14 is chosen so the capacitor and the inductor resonate at a frequency above the designed supply frequency.

At switch-on, a heavy current is drawn through the inductor 13 and heats the filaments 11. Due to the near resonance of the inductor 13 and the capacitor 14, a voltage higher than the supply voltage appears across the capacitor 14 and the lamp 12 and a higher current than normal heats the filaments. These are ideal conditions for the lamp to conduct. If the circuit were at resonance, the current and voltage magnification effects would be too large (unless the circuit was heavily damped leading to operational inefficiency) and the lamp would be prone to damage. When the lamp conducts the effective resistance across the lamp between the filaments 11 drops to a low value, damping the circuit so that its resonant properties become negligible and the circuit behaves substantially as if the inductor 13 was in series with the lamp and has a low power factor. The capacitor 15 takes a leading current to increase the power factor and is chosen to bring the power factor when the lamp is conducting to any chosen value, normally as near unity as is economical.

An inverter shown in FIGS. 2 and 3 can be used to supply a plurality of lamps which would each have its own near resonant circuit. The main advantage of the invention is in fact when the inverter is used to supply a plurality of lamps. For example, before a fluorescent lamp conducts, the lamp and its associated circuit elements take a much heavier current than in the running condition. When a semi-conductor inverter starts to supply a large number of fluorescent lamps, the pre-conducting current represents a heavy temporary load on the inverter, and the inverter has to be uneconomically designed to cope with this temporary load.

This current is reduced to about normal running current in the practice of the present invention by having the inverter initially delivering a voltage reduced to below that at which the lamp would start to conduct at operational frequency, but at a frequency nearer resonance so that the lamp will strike even at the reduced supply voltage because the voltage across the lamp is increased by the voltage magnification effects of the semi-resonance.

When sufficient time has elapsed for the lamps to strike, the voltage and frequency are adjusted to their running values either abruptly or gradually.

A switch can be used to modify the performance of the inverter for example by introducing or removing circuit elements such as capacitors, resistors or inductors or by altering tappings on transformers and can be operated manually, or automatically in response to time, temperature of a component or current. Instead of switches, temperature dependent circuit elements can be used so that the inverter, before it reaches its normal running frequency, is made to sweep through a range of frequencies at progressively increasing voltages. By suitable use of temperature dependent elements, it can be more or less guaranteed that random manufacturing differences between the lamps will cause them to fire one after another and they will then run feebly until the inverter runs at its normal frequency and voltage.

FIG. 2 shows a phase-shift inverter relying on temperature dependent circuit elements which can be normal resistors associated with bi-metallic switches using the heat developed by a resistor which is permanently in circuit and shunting another resistor either when they are hot or are cold to give the desired resistance temperature dependency. The collector of a transistor 20 is connected to a supply line through a load resistor 21 and to the base of the same transistor through three phase shifting stages, each comprising a capacitor 22 and a temperature dependent resistor 23. The temperature dependent resistors 23 have an initially low resistance (so the frequency of the inverter is high) which increases when the resistors warm up so the frequency is reduced. The voltage applied to the base is a smaller proportion of the collector voltage when the resistance is low than when the resistance increases. Thus the collector voltage is smaller as well. Therefore the collector voltage increases as the resistors warm up and the frequency decreases.

The base and the emitter of the transistor are connected to ground by resistors 24 and 25 respectively. The resistor 25 is shunted at least in part by a capacitor 26 to provide the desired bias for the transistor. It is possible for one or both of these resistors 24 and 25 to be temperature dependent, for example if resistor 24 has a large mass and a large positive temperature coefficient it is possible to alter frequency and voltage within different time spans so that the frequency is reduced and then the voltage increased, the voltage increase taking longer than the frequency reduction.

To avoid the inverter frequency being affected by the lamp circuits, it is advisable to use a buffer amplifier 27 with an output transformer 28.

FIG. 3 shows a tuned inverter relying on switches for voltage and frequency changes. A transistor 31 has a tuned collector circuit 32 comprising a tapped inductor 32 and a capacitor 33. The collector can be connected to any one of the tappings of the inductor 32 by a multi-position switch 34. A feed back coil 35 is electromagnetically coupled to the inductor 32. Initially the inductor 32 should have a low inductance. Again a buffer amplifier 36 is provided but this time a tapped transformer 37 is used as the output load. A multi-positioned switch 38 is used to select the voltage output. The switches 34 and 38 are controlled manually or by a temperature, current or time sensitive device represented by a block 39.

FIG. 4 shows a high power inverter. An astable multivibrator 41 having its frequency adjustable by a variable resistor 42 and a switched resistor chain 43 is used to control a pair of power thyristors 44. The output is fed to a transformer 45 having a variable tapping selector 46. The wave form is improved by an inductor 47 and capacitors 48. Protective diodes or zener diodes 49 are used to eliminate undesired pulses. The tapping selector 46 and the switched resistor chain are operated as before in response to temperature of a component, time, current or manually.

If one lamp fails to fire at reduced voltage (or if a lamp conducting at reduced or full voltage is switched off), it will fire at full voltage when this is re-applied.

The invention is particularly applicable to installations where the cable size is important, such as in coal or other mines, because power factor correction capacitors which then must be positioned near or in the lamp fittings take a heavy leading current and this is uncompensated by any lagging current before the lamps conduct, giving very unfavorable load conditions to the cables and to the inverter.

The following is an example of a possible sequence of outputs of, say, a 110 volts-output inverter which normally runs at 1,000 cycles per second and which, feeds a circuit having a resonant frequency of 1,500 cycles per second.

The inverter is switched on and runs at 1,500 cycles per second yielding an output voltage of about 30 volts. Due to the resonance of the circuit, a larger current flows and a larger voltage appears across each lamp than would be expected from the small voltage. Not all the lamps may conduct however and the inverter design is such as to increase the voltage and reduce the frequency progressively or stepwise to 110 volts and 1,000 cycles per second respectively during which time the remainder of the lamps fire randomly one after another.

By the time the inverter has reached its running condition all lamps have fired in random sequence, ensuring that any transient surges associated with firing will not be experienced by the inverter simultaneously.

The starting and running frequencies (i.e., 1,500 and 1,000 cycles per second) fix the characteristics of the frequency regulating circuit elements. After these are fixed, the voltage regulating circuit elements can be decided. If the timing is to be automatic rather than manual, due regard is paid to obtaining the desired timing in the selection of all circuit elements.

Claims

I claim:

1. A circuit comprising an element such as a fluorescent lamp which has to be conditioned to conduct, circuit elements connected, one in series and one in parallel, with the first-mentioned element and designed to resonate at a resonant frequency and an inverter for supplying the lamp and said circuit elements with a voltage at a frequency less than the resonant frequency, the improvement residing in that said inverter incorporates means whereby it can deliver a reduced voltage at a frequency nearer the resonant frequency to facilitate conditioning of the first-mentioned element.

2. A circuit as claimed in claim 1 wherein said first-mentioned element is a gaseous discharge device having at least one filament and the said circuit elements are a capacitor and an inductor connected in series with each other and with the said at least one filament with the capacitor being connected across said discharge device so it is shunted by the device when the device conducts.

3. A circuit as claimed in claim 2 in which said means are temperature dependent elements.

4. A circuit as claimed in claim 2 in which said means are switches.

5. A circuit as claimed in claim 2 in which said inverter comprises an astable multivibrator having switched means for varying the pulse repetition frequency, thyristors rendered conductive by said multivibrator, and output means including means for varying the output from said thyristors.