GB2120470A - Improvements in or relating to discharge lamp circuits - Google Patents

Abstract

Photoprinting discharge lamps, in particular the known Graph X-lamp, are required to allow low minimum exposure times so that they are required to reach operating power quickly. It is proposed to achieve this by holding the lamp current at a value above the preferred running current until a preset lamp voltage is reached. For this purpose the circuit may include a choke ballast (13, 14) and diode means (15, 16) to cause the current in the ballast to be unidirectional in starting and thereby to cause the ballast to saturate and have a low impedance. Switch means (17, 18) is connected to means (19, 20) responsive to the lamp voltage, to short circuit the diode means when the running voltage is approached. This allows the ballast to desaturate so that its impedance increases to the value for the preferred running current. A conventional ignitor (22) starts the lamp. <IMAGE>

Description

SPECIFICATION Improvements in or relating to discharge lamp circuits The present invention relates to circuits for starting discharge lamps, in particular, but not exclusively, photoprinting discharge lamps, for example the lamps known as Graph-X.

The Graph-X lamp comprises a discharge lamp designed to have its major radiation emission in bands suitable for photoprinting, which is generally either at 420 nm or 380 nm but perhaps other wavelengths such as 365 nm. The effect is partly achieved by suitable design of the lamp and choice of the fill and partly by placing the lamp in a dichroic reflector such as the reflector disclosed in British Patent Application No. 81 27693.

Other requirements placed on the lamp include minimum and maximum exposure times, with the lamp hot and cold respectively and it is an object of this invention to provide a starting circuit which facilitates meeting these requirements.

According to the present invention there is provided a lamp circuit for a discharge lamp, the circuit including means for increasing the lamp current during starting of the lamp and means responsive to the lamp voltage to cause the means for increasing the lamp current to cease to be operative when the lamp voltage reaches a desired value.

In order that the invention may be clearly understood and readily carried into effect it will now be described by way of example with reference to the accompanying drawings of which: Figure 1 shows the arc tube of a 250 watt Graph-X lamp, Figure 2a shows a circuit in accordance with this invention suitable for the lamp of Figure 1, Figure 2b shows an alternative to the circuit to Figure 2a, Figure 3a and 3b shows waveforms used to explain the operation of the circuit of Figure 2, and Figure 4a, 4b, 4c and 4d show alternative forms of part of the circuit of Figure 2.

The requirements for radiation at 420 nm or 380 nm mentioned hereinbefore are met in existing technology, for example in a known 800 watt Graph-X lamp by using gallium or iron based halide lamps respectively.

The minimum exposure time, which is obtained when the lamp is hot, has proved to be more problematical and it is now proposed to achieve this with the selection of the appropriate operating power, in one case 250 watts.

The maximum exposure time, when the lamp is cold is dependent on two factors, one of which is the time required to achieve an arc tube wall temperature which is sufficient to vaporise the gallium or iron halide inclusions. This is directly proportional to the mass of quartz used to fabricate the arc tube. Thus if the mass is smaller the wall temperature will rise more quickly. It is therefore proposed to make the lamp both much smaller than the conventional lamp and more highly loaded. For one example of a the 250 watt lamp the total mass, including the pinch seal areas, is about 3 grams compared with about 20 grams for the known 800 watt lamp. The wall loading is about 80 watts/cm2.

This 250 watt lamp is illustrated in Figure 1. In conventional manner it includes an arc tube 1, in this example double ended, with two pinch seals 2. Two electrodes 3 carrying conventional overwinds 4 are mounted at opposing ends, electrical connection thereto being via molybdenum foils 5 sealed in the pinch seals 2 and lead wires 6, in this example modified versions of the known 'hair pin' lead.

In this example the quartz arc tube 1 is of generally oval shape and is of 1 mm wall thickness, the inside surface being indicated by the broken line 7. As indicated the lamp is unconventionally small, the arc length (between the tips of the electrodes 3) being approximately 10 mm, the minor diameter, Y, being about 15 mm, the major diameter, X, being about 20 mm and the overall lamp length, L, being approximately 40 mm.

The other factor, mentioned hereinbefore as affecting maximum exposure time, is the power dissipated by the lamp during its run-up period.

This is a function of the lamp current and the voltage drop across the lamp.

When the discharge is initiated the lamp voltage drop is low, for the 250 watt lamp of Figure 1 about 10-1 5 volts, and to develop a high initial power in the lamp a high current is required.

However when the lamp has warmed up the lamp voltage drop is higher, about 70 volts for the 250 watt lamp, and lamp current should be reduced to a normal or operating value if the lamp is not to be overrun.

Figure 2 shows a lamp circuit in accordance with the invention including means for increasing the lamp current and means sensitive to the lamp voltage effectively to disconnect the means for increasing the current. The lamp 10 is connected across the live and neutral power supply terminals 11 and 12. In series with the lamp in conventional manner is a choke ballast provided in this example by two chokes 13 and 14 in parallel circuits. These are conventional iron-cored chokes, in this example for the 250 watt Graph-X lamps the chokes are those conventionally used for 150 watt high pressure sodium lamps.In each of the parallel circuits, in series with the choke therein, is a diode 1 5, 1 6 and connected across each diode is a switch 17, 18. In this example the switches are ganged to be operated together in response to a relay 1 9 the coil of which is energised by the rising lamp voltage. This coil may preferably be connected in a diode bridge 20 to ensure smooth positive action. The coil may be designed to operate when the lamp voltage reaches 55 volts RMS. Alternatively, in a circuit shown in Figure 2b, an electronic voltage sensing circuit 21 could be used, this circuit incorporating a reference voltage device and energising the relay coil through a solid state switch. This alternative would allow more accurate control of the switching voltage and also adjustment of the voltage if desired.

Also included in Figure 2a and 2b is an ignitor 22 operated by a switch 23. The ignitor operates in the conventional manner and the actual details of its circuit are not relevant to the present invention.

With the lamp running and the switches closed there is provided a short circuit across each diode and operation is conventional with a lamp current waveform generally sinusoidal as shown in Figure 3a.

However if the switches are opened the diodes result in current for one half cycle only passing through choke 13 and for the other half cycle only passing through choke 14. These conventional iron-cored chokes are known to be closed to saturation when in operation and removal of the reversed current of the opposing half cycle causes each choke to saturate thus reducing the choke impedance and increasing the choke current so that the current waveform is as in Figure 3b.

Clearly if only one diode is provided the current increase can be achieved on one half cycle only.

The effect is believed to result from the fact that the diode reduces the current through and the voltage across the respective choke to zero in each alternate half cycle so that at the start of the next half cycle the current and voltage rise from zero together. The causes current doubling in a non-saturating choke and in the chokes used for this invention, which are chosen to saturate at the currents achieved, the current rises to several times its normal level.

In the operating arrangements the voltage sensor 20 is arranged to cause the relay to open switches 1 7, 1 8 below a suitable trigger voltage (which should be below the minimum operating voltage of the lamp, i.e. about 55 volts for a nominal 70 volts, 250 watt Graph-X lamp) and to close them at higher voltages. Thus when the lamp is first ignited the lower lamp voltage drop causes the diodes to be in circuit thus increasing the lamp current. When the voltage exceeds the set level the diodes are short circuited bringing the choke impedance up to its normal level and reducing the lamp current This is the effect desired for fast wa#m-up.

It should be noted that if the relay contacts fail to open the choke will rapidly overheat and in such equipment a means for preventing continuous operation in this mode is desirable. Such protection may readily be devised by an experienced circuit engineer.

Clearly other arrangements of chokes and diodes are possible to achieve the same effect. For the choke parts of the lamp circuit only, some of these are shown in Figure 4a 4d. Operation of these circuits is similar to that of Figure 2 and will be readily understood.

It will be appreciated that the invention may be used with lamps of other powers to the 250 watt lamp of Figure 1, with appropriate changes of the trigger voltage. It may be also used with lamps other than the Graph-X lamp if they have similar warm-up voltage characteristics and if it is required to reduce the time delay until the operating power is reached.

Alternative forms of the circuit may be devised.

For example two chokes, each with a respective diode in parallel, may be used with the two diode/choke parallel circuits being in series with each other. The two diodes would clearly be connected in opposition. However although it is considered that such an arrangement would work it is not preferred, for practical reasons.

Claims (14)

1. A lamp circuit for a discharge lamp, the circuit including means for increasing the lamp current during starting of the lamp and control means responsive to the lamp voltage to cause the means for increasing the lamp current to cease to be operative when the lamp voltage reaches a desired value.

2. A circuit according to Claim 1 including a choke ballast and in which the means for increasing the lamp current comprises means for reducing the choke impedance.

3. A circuit according to Claim 2 in which the means for reducing the choke impedance comprises means allowing only unidirectional current to flow therethrough.

4. A circuit according to Claim 3 in which the means allowing unidirectional current is a diode means and the control means includes means for short circuiting said diode means.

5. A circuit according to Claim 4 in which the choke ballast comprises two individual chokes in parallel and the diode means includes a diode in series with each choke connected to pass current of one polarity in one choke and the other polarity in the other choke when in circuit.

6. A circuit according to Claim 5 in which the choke ballast comprises two individual chokes connected in parallel and the diode means is operative to provide unidirectional current in one said choke only.

7. A circuit according to any preceding claim in which the control means includes means sensitive to the lamp voltage for operation thereof.

8. A circuit according to Claim 7 in which the means sensitive to the lamp voltage includes relay means operating the control means.

9. A lamp circuit for a discharge lamp, the circuit including a saturable choke ballast arranged not to be at saturation during normal lamp running, means for causing the choke ballast to saturate during starting of the lamp to increase the current therethrough and control means responsive to the lamp voltage to allow the choke ballast to desaturate when the lamp voltage reaches a desired level.

10. A circuit according to Claim 9 in which the means for causing the choke ballast to saturate includes means for causing current in the choke ballast to flow substantially only in one direction.

11. A circuit according to Claim 10 in which the means for causing current to flow in one direction includes diode means.

12. A lamp circuit for a discharge lamp, the circuit being substantially as herein described with reference to Figures 2 to 4 of the accompanying drawings.

13. A discharge lamp in combination with a circuit according to any preceding claim.

14. A photoprinting discharge lamp in combination with a circuit according to any of Claims 1-12.