Low pressure sodium lamps

Abstract

 A low-pressure vapour electric discharge lamp (L) has a discharge tube filling consisting of neon alone and is connected to a circuit capable of supplying a high frequency current to the lamp. Using a high frequency current means that the lamp can be ignited, even though there is no argon in the discharge tube, contrary to convention. Thus the lamp can be operated a point closer to that of maximum efficacy.

Description

[0001] The present invention relates to low pressure sodium vapour electric discharge lamps, that is to say, lamps of the kind consisting of a sealed elongate discharge tube, containing metallic sodium and a rare gas filling under low pressure, held within a cylindrical outer bulb having on its inner surface a coating transparent to sodium emission but a good reflector of infra-red radiation.

[0002] In conventional sodium lamps, the rare gas filling used in the discharge tube is commonly in the form of a mixture comprising 99% neon and 1% argon. The argon is included since it has a lower excitation potential than neon. This Penning mixture of neon and argon thereby reduces the voltage necessary for igniting the lamp. However, a common cause of lamp failure is argon clean up which results in a gradual increase in the lamp starting voltage and eventual failure to start.

[0003] It is known to apply high-frequency currents to low-pressure sodium lamps. Significant improvements of lamp efficacy have been recorded at frequencies of 200kHz and above. These improvements in efficacy are a result of an increase in arc efficency and a reduction of the electrode losses. However at such frequencies circuit losses can have the effect of reducing the overall efficiency.

[0004] We have now found that reasonably high overall efficiencies can be achieved by the application of high frequency currents in the range 40 to 200kHz inclusive. We have also found that low-pressure sodium lamps operable at high frequencies, that is to say 40kHz and above, operate satisfactorily with a filling gas consisting of neon only, so that the problem of lamp failure due to argon clean up is avoided.

[0005] In accordance therefore with one aspect of the invention, a low-pressure sodium vapour electric discharge lamp of the kind referred to has a discharge tube filling consisting of neon alone and is connected into a circuit capable of supplying a high frequency current to the lamp.

[0006] The use of a high frequency current makes it possible to ignite the lamp despite the absence of argon. Thus it becomes possible to operate the lamp closer to the point of maximum efficacy. No supplementary starting device is required to cause the lamp to ignite.

[0007] According to another aspect of the invention an operating circuit for a low pressure sodium vapour electric discharge lamp comprises a pair of input terminals for connection to an alternating current mains supply, a pair of output terminals for connection to the terminals of a low pressure sodium vapour electric discharge lamp of the kind referred to, and means for deriving from the alternating current mains supply an output frequency in the range 40 to 200kHz at the output terminals, sufficient to start and operate the lamp. Whilst a lamp having a rare gas filling of neon alone can operate satisfactorily on such a circuit, the circuit can also be used to operate lamps having the conventional filling of neon and argon, and whilst, in such a case, the lamps will be subject to the usual argon clean up, this will not significantly affect the lamp operation in the circuit.

[0008] In order to reduce heat losses, the outer bulb of the lamp may be evacuated. Preferably the inner surface of the outer bulb which has an infra-red reflecting film is also coated with a silica anti-reflection film to improve the light output of the lamp. Experiments have shown that this improvement can be of the order of 4-5%.

[0009] The frequencies generated by the lamp operating circuits in accordance with the invention (or their harmonics) may cause interference in certain radio wavebands. To prevent this, an inductance/capacitance network may be incorporated into the supply circuit.

[0010] Tests have been carried out on 26,35,36 and 55 watt lamps at 40kHz and on 66,90,91 and 135 watt lamps at 120kHz. All of these lamp ratings give similar or improved light output at high frequency with lower lamp power consumption. In some cases, the power consumption has been reduced by a third, when gear power losses are taken into account. The lifetime of the lamps is not significantly different from that of lamps operated at conventional frequencies.

[0011] A circuit capable of supplying a high frequency current to a sodium lamp in accordance with the invention is shown in Figure 1 by way of example only.

[0012] The circuit includes a pair of input terminals T for connecting a bridge rectifier B and a smoothing circuit, provided by a capacitor CI and a resistor R1, to an alternating current mains supply. The resistor R1 limits the charging current of capacitance C1.

[0013] The rectified output from the bridge rectifier and smoothing circuit is applied to a sawtooth generator, provided by a resistor R2, a capacitor C2 and a diac D2, which acts as a trigger to start the circuit. On applying the input voltage, the capacitance C2 is charged via R2. When capacitance C2 reaches the break down voltage of diac D2, capacitance C2 passes a short current pulse into the gate circuit of a transistor T2 thus charging the gate capacitance.

[0014] A capacitor C5 connected across the lamp L, and an inductance L1 in series with the lamp L provide a resonant circuit which is triggered into resonance by the sawtooth generator circuit, so as to apply a high frequency voltage across the lamp.

[0015] Initially, the transistor T2 only carries current when diac D2 triggers and then the high frequency self resonance begins (due to feedback via inductances L3 and L4). When resonance is set up, the sawtooth generator shuts down via diode D1 which effectively prevents the diac D2 reaching its breakdown voltage. The oscillator circuit will shut down, should the fuse F fail for any reason.

[0016] By means of the transformer TRI, currents are induced in inductances L2 and L3 once the circuit containing L4 has been triggered. Thus the transistor TI is switched on and supplies a current to L3 during one half of the alternating cycle. In this way, a current is induced in LI from the circuit containing TI during one half of the alternating cycle and from the circuit containing T2 during the other half cycle.

[0017] In normal operation, the high frequency current through the lamp is around 0.5 amps; the circuit may be used to supply a current of up to 200kHz to a 90 or 135 watt lamp.

Claims

1. A low-pressure sodium vapour electric discharge lamp (L) of the kind referred to characterised in that the discharge tube filling consists of neon alone and the lamp is connected into a circuit capable of supplying a high frequency current thereto.

2. A discharge lamp as claimed in Claim 1 in which the outer bulb is evacuated.

3. A discharge lamp as claimed in Claim 1 or 2 in which the inner surface of the outer bulb is coated with infra-red reflecting film.

4. A discharge lamp as claimed in Claim 3 in which the inner surface of the outer bulb is coated with a silica anti-reflection film.

5. An operating circuit for a discharge lamp (L) as claimed in any preceding claim comprising a pair of input terminals (T) for connection to an alternating current mains supply, a pair of output terminals for connection to the terminals of the lamp and means for deriving from an alternating current mains supply an output frequency in the range 40 to 200kHz at said output terminals, sufficient to start and operate the lamp.

6. An operating circuit as claimed in Claim 5 wherein said circuit incorporates an inductance/capacitance network.

Drawing