GB2060287A - Lamp starting circuits - Google Patents

Abstract

A lamp starting circuit for a high or low pressure sodium lamp has a thyristor Th1 connected in series in a resonant circuit including a capacitor C1 and a primary winding L1 of a transformer arranged to supply high voltage pulses generated by switching of the thyristor Th1 to a lamp. To prevent this part of the circuit from continuing to operate if the lamp fails to ignite after a suitable time during which the high voltage pulses are supplied to it, the thyristor Th1 is connected to a timer circuit which holds the thyristor Th1 off after a predetermined time starting at switching on of the starting circuit. The timer circuit includes a capacitor C3 arranged to be charged to a breakdown voltage of a zener diode Z2 unless the lamp strikes before this voltage is reached. When the zener diode Z2 breaks down, it passes current to a switching transistor Tr1 which when conducting switches off the thyristor Th1 until the supply is disconnected. <IMAGE>

Description

SPECIFICATION Lamp starting circuits This invention relates to starting circuits for electric discharge lamps such as high pressure sodium lamps, low pressure sodium lamps and high pressure mercury-iodide lamps (MBI lamps), although it is not limited to use with those lamps exclusively.

The main purpose of this invention is to provide a starting circuit which operates for the minimum period of time consistent with igniting the lamp, so as to retain the advantages of quick electronic ignition while avoiding the disadvantages of needlessly stressing lamp ballast components and wiring and risking generation of radio interference which arises from continuous operation of the electronic lamp starting circuit for a prolonged period when a lamp fails to strike.

Although known lamp starting circuits normally include means for switching out the starting circuit if the lamp substantially immediately ignites, a lamp may not be in condition to ignite instantly, especially if has just extinguished after running, so that it is hot with consequent high gas pressure and restrike voltage, or a lamp may be aged or faulty in which case the known ordinary electromechanical or electronic starters will switch continuously until the supply is disconnected.

According to the present invention there is provided a lamp starting circuit as defined in claim 1 hereinafter.

The invention will now be described in more detail, solely by way of example, with reference to the accompanying drawings in which Figs. 1 to 3 are circuit diagrams of three different embodiments of the invention, and figs. 4 to 7 are circuit diagrams of four different timer circuits for use in embodiments of the present invention.

A lamp starting circuit embodying the invention is shown in the schematic diagram of Fig. 1 and includes a timer circuit formed by two zener diodes Z1/Z2' a rectifying diode Dr, a transistor Tra, a charging capacitor C3 and resistors R1, R2, R3 and R5 connected as shown between a ballast inductor L in a live a.c. supply line 11, and a neutral a.c. supply line 12.

In the first mains supply half cycle when the supply line 11 is negative, no current can flow in the branch between points (b) and (d) due to the diode D,. Thus no reverse current flow and voltage appear on the transistor Tr,. In the next half cycle there is a positive voltage on the supply line 11 and a thyristor Th, and the transistor Tr, can conduct if triggered. The thyristor Th, fires at a point determined by the breakdown voltage of the zener diode Z,, the number of times it fires or number of half cycles it fires in being determined by the time over which the transistor Tr, is not conducting.

The starting circuit of Fig. 1 includes a step-up transformer with primary and secondary windings L, and2, to generate high voltage electric pulses for striking the lamp arc. The secondary winding L2 is connected in series between the ballast inductor L and the lamp.

Voltage pulses are generated in the primary winding L, of the transformer by means of a resonant circuit formed by a capacitor C, with the primary winding L1, a resistor Re and a diode D2.

In the first half cycle of mains frequency when the supply line 11 is negative, the capacitor C, charges via the diode D2 to, for example, 350 volts. On the reversal of the supply polarity, the thyristor Th, can conduct at a predetermined firing voltage when the zener diode Z, conducts. Conduction of the thyristor increases the voltage across the capacitor C, to, for example, 400 volts, and generates a resonance voltage by discharging the capacitor C, into the primary winding L1, a capacitor C2 providing a low impedance path for the high frequency pulse produced by the sudden flow of the current in the capacitor C,, and the resultant high voltage appearing only at the lamp end (c) of the secondary winding L2 and not across the control circuit between the points (b) and (d).

Thus the capacitor C2 protects the control circuit as well as other components such as the ballast inductor and wiring. The resistor R6 ensures that there is substantially a single pulse at each triggering of the thyristor Th,.

The magnitude of the output pulse, the switch off point for the starting circuit once the light ignites, and the duration of the train or trains of high voltage starting pulses generated are determinable by selection of the zener breakdown voltage values and the duration of non-conduction of the transistor Tor,.

In the circuit of Fig. 1 the duration of the high voltage starting pulse train is determined bathe values of the resistor R2 and capacitor C3 in the timer circuit. When the voltage across the capacitors C3 rises to a value predetermined by the breakdown voltage of the zener diode Z2, the zener diode Z2 breaks down and the transistorTr, conducts and prevents the thyristor Th, from firing, and generation of the high voltage starting pulse is stopped. The voltage at the cathode of the zener diode Z2 remains then at the breakdown value until the main supply is switched off.

When the supply is switched off, the capacitor C3 discharges into the resistor R5 and the circuit resets.

If the lamp strikes before the predetermined charging time of the capacitor C3, the voltage between the points (a) and (b) falls to the lamp (or tube) running voltage which is below the breakdown voltage of the zener diode Z1 The capacitor C3 therefore discharges into the resistor Zs and the circuit resets.

The resistors R3 and R4 in this case form a network with the resistor R1 which prevents the voltage across the transistor Tr, from exceeding its working voltage and allows a low voltage transistor to be used as the transistorTr1.

Fig. 2 is a diagram of a modification of Fig. 1. High voltage pulses are generated by a transformer (ferrite assembly) across the supply lines 11 and 12, a low impedance path for the pulses being formed by the capacitor C2. A further capacitor (not shown) may be connected between the points (b) and (d) to provide another low impedance path for pulses if necessary.

Fig. 3 is a diagram of a transformerless modification of Fig. 1 in which the resonant high voltage pulse is formed by the capacitor C,, the resistor R6 and the ballast inductor L. This circuit is suitable for starting a SOX lamp of, for example, 35 watts.

A further modification indicated by broken lines in Fig. 3 has a ferrite inductor L, between points (b) and (c) to form the resonant circuit which generates high voltage pulses. The capacitor C2 is included to form a low impedance path to protect all components to the left of the points (b) and (d).

As an alternative to the transistor timer circuit used in Figs 1,2 and 3, other embodiments may employ an integrated circuit.

Fig. 4 is a schematic diagram of part of the lamp starting circuit with a timer circuit which uses an astable flip-flop for switching a train of pulses on and off for relatively accurately predetermined periods, with a short on period followed by a long off period. The high voltage pulse generating part of this starting circuit includes a thyristorTh1 and a diode D2 and may be as described hereinbefore with reference to any one of Figs. 1, 2 and 3.

A standard 555 IC timer is used, connected for astable operation. The predetermined on and off periods are determined by the values of resistors Ra and Ra and a capacitor C5. A diode D4 permits the duty cycle 'on period' to be less than the 'off period'. For example, the 'on period' may be half a minute and the 'off period' two minutes. When the output of the IC timer is switched high a diode D3 at the output pin 3 of the IC timer is reversed biased and the thyristor Th1 is fired during positive mains frequency half cycles by current through a resistor R1. When the output of the IC timer is switched low, the diode D3 is forward biased and current flows from the resistor R, into the diode D3 so that the thyristorTh1 is held off.

A resistor R7, a zener diode Z3 and a capacitor C4 form a network to maintain a stable supply voltage for the IC timer, before the lamp strikes. When the lamp strikes however, the voltage between the points (b) and (d) falls to the lamp (tube) running voltage and below the breakdown voltage of the zener diode Z, so that the starting circuit is inactive.

Fig. 5 is a lamp starting circuit similar to that of Fig. 4 except that the transistor circuit of Fig. 1 is added and gives a maximum starting operation of, for example, 1 5 minutes. A resistor R10 is connected to the collector of the transistorTr1 and to the reset pin 4 of the IC timer. The IC timer behaves as described above in connected with Fig. 4 but the combination of the resistor R2 and the capacitor C3 has a long time constant so that after the on-off periods determined by the operation of IC timer, the circuit is finally switched off when the transistor Tr, conducts and pulls the voltage at the reset pin 4 of the IC timer to the low value, unless the lamp strikes beforehand and the zener diode Z, stops conducting. The circuit thus performs three distinct functions.

Fig. 6 is a similar circuit to that of Fig. 5 but employs an IC timer instead of the transistor and IC combined of Fig. 5. In Fig. 6, the IC used is a 556 (i.e. a dual 555 IC) with one side connected as an astable flip-flop operating as in Fig. 4, and the other side connected as a mono-stable with a long time constant. The output of the mono-stable finally switches the "toggling" astable off. The resistor R2 and the capacitor C3 determine the time constant of the mono-stable operation.

Fig. 7 indicates a similar arrangement to Fig. 6 except that a single IC timer with four NAND gates is used to perform as in Fig. 6. Gates 1 and 2 form the astable flip-flop, while gate 4 acts as the monostable.

Practical Circuit Values Typical values for circuits to operate high pressure sodium lamps requiring a 4.5 KV pulse (i.e.

those lamps in the range from 1 00 to 400 watts on 240 volts) are as follows, aithough other values can be used for different lamps.

Fig. 1 Ferrite core assembly; E core: Siemens type E42/1 J Coil: 10/60 turns Z,: 200 volts type PL200Z SSC Z2: 5.8 volts type PL200Z SSC D,/D2: PV 127 SSC Th,: TL107-4 Tr,: BC 184L R,: 150K # R2: 2.7M # R3; 4.7K # R4 1KQ R5: 2.7M 8 R6: 3.3 # C,: 0.33 microfarads C2: 0.022 microfarads C3: 220 microfarads 6 volts Fig. 2 Ferrite core assembly E core: Siemens type 34/26 Coil: 50/400 turns Other values as above.

Fig. 3 Typical values to operate a 35 watt low pressure sodium lamp are the same as above excluding the ferrite core assembly. A ferrite choke wound on Neosid type 34/26 with 150 turns can be used in the modified version of Fig. 3.

A large combination of operating periods is practical to cover a number of different conditions. The following values for the timer circuits of Figs 4, 5, 6 and 7 give a pulse train period of 4 to 5 seconds on and approximately one minute off. The period of operation of this on-off sequence is approximately 17 to 20 minutes duration in Figs. 5 to 7.

Fig. 4 IC: 555 Z3: 10 volts PL 102 (SSC Ltd.) C4: 50 microfarads 10V DD4: IN4148 R9: 47 K Q R8: 680KQ C5: 100 microfarads 10V Fig. 5 IC: 555 R2: 2M Q C3: 470 microfarads Fig. 6 R2: 2MQ C3: 470 microfarads R9. 47K # R8. 680K # All other values as above Fig. 7 All values as used above.

In use, the starting circuits of Figs. 1 to 7 are incorporated in a lamp operating circuit having a ballast inductor such as the ballast inductor L in Figs. 1 and 3 and a power factor correction capacitor (not shown) connected across the input terminals of the operating circuit.

The frequencies of the starting pulses generated by the circuits of Figs. 1 and 3 for high pressure sodium lamps (SON) are of the order of 300 KHz. The frequency of the starting pulses generated by the circuit of Fig. 3 as unmodified for a low pressure sodium lamp (SOX lamp) is of the order of 300Hz.

Claims (7)

1. A starting circuit for an electric discharge lamp, the circuit including controllable switching means arranged to enable application of starting voltage to a discharge lamp, and means for controlling the controllable switch means, the controlling means including timing circuitry adapted to so limit operation of the switching means as to restrict the duration of application of starting voltage.

2. A starting circuit according to claim 1, wherein the timing circuity includes an astable circuit arranged to so control the switching of the switching means that starting voltage is applied, in use, to a discharge lamp only during one state of the astable circuit.

3. A starting circuit according to claim 2, wherein the timing circuitry includes means for inhibiting the astable circuit after a predetermined time from an initial application of power to the starting circuit.

4. A starting circuit according to claim 3, wherein the means for inhibiting the astable circuit comprises a mono-stable circuit.

5. A starting circuit according to any preceding claim, wherein the switching device is a thyristor and the starting circuit includes a high frequency resonant circuit adapted to be completed by conduction of the thyristor.

6. A starting circuit according to claim 1 and substantially as described hereinbefore with reference te Fig. 1 or 2 or 3 of the accompanying drawings.

7. A starting circuit according to claim 1 in which the timing circuitry is substantially as described hereinbefore with reference to any one of Figs. 4 to 7 of the accompanying drawings.