GB1575745A

GB1575745A - Starting means for high pressure sodium and high pressure mercury halide discharge lamps

Info

Publication number: GB1575745A
Application number: GB272077A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-04-11
Filing date: 1978-04-11
Publication date: 1980-09-24

Description

(54) STARTING MEANS FOR HIGH PRESSURE SODIUM AND HIGH PRESSURE MERCURY HALIDE DISCHARGE LAMPS (71) I, JULIUS CATES of 69 Kingsway, Wembley, Middlesex, HA9 7QP, a British Subject, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to the use of a resistor in series with a bimetallic thermal type starter switch for operating High Pressure Sodium (HPS) and High Pressure Mercury Halide lamps on 50 c.s. to 60 c.s. a.c.

supplies, where the starter is incorporated in the lamp and depends on heat from the discharge lamp for it, the starter, to be kept in the open circuit condition when the lamp is alight.

Considering firstly the HPS lamps the majority of these are filled with Xenon at a cold (25"C) pressure of 15 to 20 torr and these lamps require a voltage pulse of about 3 Kv peak for starting at 250C. A starting aid is consequently required for operating these lamps on nominal 240v or 41 5v a.c. supplies.

Other HPS lamps are filled with Neon at a cold filling pressure of about 20 torr with a small addition of Argon or Krypton. The peak voltage pulse needed for such lamps is substantially reduced to about 400v but a starting aid is still required for such lamps on 240v a.c. and is even desirable on 415v supplies.

The invention will now be described with reference to the drawings in which: Fig. 1 shows a known electrical circuit for starting and operating a discharge lamp which can also be used for the invention.

Fig. 2 shows a modification according to the invention of the circuit of Fig. 1 Fig. 3 shows an existing arrangement of starter switch and heater coil in a High Pressure Sodium lamp.

Fig. 4 shows an arrangement of starter switch and heater coil in the High Pressure Sodium lamp according to the invention.

Fig. 5 shows a cross-section of Fig. 4 indicating the positioning of the heater coil and the bimetal strip of the starter.

A well known starting aid often used for HPS lamps consists of a bimetallic type starter switch the construction and mode of operation of which can be described with .reference to Fig. 1. The switch in Fig 1 comprises a bimetal strip 1 carrying at one extremity the contact 2 and connected at its other extremity to one end of a wire heating coil 3. The other end of 3 is connected to the terminal 4.

The contact 2 is normally touching the contact 5 which is attached to one end of a fixed conducting member 6 the other end of which is connected to a terminal 7. The terminals 4 and 7 of the starter are connected to the input leads of the HPS lamp 8. This type of starter is usually described as a thermal type to distinguish it from another type of bimetallic starter which is operated by a glow discharge between bimetal electrode strips and is conventionally referred to as a glow starter.

The lamp 8 is connected in series with its ballast 9 across the supply terminals 10 and 11. A capacitor 12 is usually also connected across the supply terminals for power factor correction but other than this it has no effect on the operation of the lamp, ballast or starter switch. 9 is usually an inductor but can consist of an inductor in series with a capacitor.

The starter is usually mounted inside the outer jacket of the lamp and is situated above the inner discharge tube. The outer jacket is evacuated to a hard vacuum.

The resistance of the heater coil in existing HPS starters of this type has been very small in comparison with the impedance at the supply frequency of the ballast 9, while the resistance of the bimetal strip and the member 6 are negligible. and consequently when the contacts 2 and 5 are closed the HPS lamp is practically short-circuited and the current flowing is equal to the short-circuit current of the ballast. The contacts are initially closed and heat from the coil 3 due to current in it warms the bimetal strip causing it to bend in a downward direction (Fig.1) so that contact between 2 and 5 is broken.

A voltage pulse is produced across the ballast inductor and consequently across the lamp at the break and if the pulse is sufficiently large the lamp will start. The starter is designed and situated so that when the lamp is alight heat from the discharge tube keeps 1 in a bent position, the starter contacts remain open and no current passes in the heater coil.

The break occurs in a fairly hard vacuum and there is consequently little attenuation of the pulse. The magnitude of the pulse voltage is dependent on the product of the inductance and the rate of alteration of the current at break. The actual current broken depends on the point in the a.c. cycle at which the break occurs but the average value of the current broken depends on the rms current heating resistance 3. The value of the inductance needed to ballast the lamp depends on the lamp wattage rating and the supply voltange and frequency. The largs majority of HPS lamps operate on 240v 50 or 60 c.s. and in the case for example of the widely used 400w rating on a 240v 50 c.s. supply the inductance is 0.15 Henry and its short-circuit current at the nominal supply v. is 5.5A.

The peak voltage surge produced at break with the thermal starter in the existing 400w arrangement is of the order of 20 Kv. This voltage is unnecessarily high for lamp starting and is so high as often to cause insulation breakdown in the inductance with resultant failure of the ballast and also often of the lamp. Pulses of similar magnitude are found with HPS lamps of other wattage ratings.

If the peak surge volts can be kept below about 5Kv this will be satisfactory as regards ballast insulation and the peak must also be kept at least to 2.5 to 3 Kv to ensure rcliable starting for Xenon filled lamps. One of the objects of the present invention is to modify the existing circuit so that the pulse voltage is reduced to an acceptable safe value. At the same time the modification also speeds the operation of the starter and this is also a desirable improvement.

According to the invention these objectives are achieved by increasing the resistance in the starter branch of the circuit i.e.

between terminals 4 and 7 including the starter switch. so that the rms current which is broken under starting conditions when the lamp is not alight is substantially less than the short-circuit current of the ballast, and also substantially less than the normal operating current of the lamp and ballast while the normal operating conditions of the lamp and ballast are unaffected. The resistance is designed to give as low a current during starting as possible consistent with the obJective of the invention, while an upper limit to the current during starting is, according to the invention, one sixth the normal operating current of the lamp.

According to the invention therefore there is provided a High Pressure Sodium discharge lamp or a High Pressure Mercury Halide discharge lamp in which a bimetallic strip thermal type starter switch comprising a pair of contacts in series with a resistor, part or the whole of which forms the heater coil for the strip, is connected electrically across a discharge tube and mounted in the vicinity of the discharge tube within an outer envelope surrounding the discharge tube and which switch is kept in the open position when the lamp is alight entirely by heat from the discharge tube, no current then passing through the resistor, the resistor having a value so that the current through it at contact break of the switch and before the lamp has started is less than one sixth of the normal operating lamp current.

The factors involved in the modification may be considered by way of example for the 400w HPS lamp operated on a 240v 50 c.s.

supply. The normal operating current of the 400w lamp is 4A and the impedance of the inductive ballast is 46 ohms at 4A 50 c.s. It is found that if the current broken is reduced to 0.15A the peak voltage pulse is reduced to an acceptable value of 2.5 to 3 Kv.

The voltage across the ballast 9 at 0.15A is negligibly small and during starting conditions when the starter contacts are closed and the lamp is not alight the voltage across the resistor in the starter branch is practically equal to the supply voltage. The resistance required to give 0.15A at 240v is 1.6 K ohms and since the power consumption in it is equal to the product of the current and voltage across it this power is 36w.

In the existing arrangement the heater coil is fairly close to the bimetal strip and the losses in the heater at the ballast short-circuit current of 5.5A are of the order of 5w. If the heater was similarly close to the bimetal in the modified arrangement the heating effect on the bimetal due to the increased heater wattage would be excessive and it would consequently be necessary to increase the separation of the heater from the bimetal.

Alternatively the required resistance in the starter branch in the modified arrangement could be obtained by keeping the heater coil approximately in its existing position.

limiting its resistance so that it consumed about 5w as in the existing arrangement and adding additional resistance in the starter circuit to make the total resistance in the circuit equal to the 1.6 K ohms required. The modified circuit arrangement is then as indicated in Fig 2 where 13 is the additional resistor and is connected between 3 and 4.

Resistor 13 could of course be connected between 6 and 7. Resistor 13 is situated sufficiently remote from strip 1 to have little heating effect on it. The resistance of 3 in Fig 2 arrangement to consume 5w is 200 ohms while the resistance of 13 is then 1.4 K ohms and its consumption 31w.

Fig. 3 indicates the top portion of a form of the 400w HPS lamp incorporating a thermal starter in accordance with the existing arrangement where the current broken is the short-circuit current of the ballast. The lamp is cylindrical in shape with an overall diameter of 52mm and an overall length of 285 mm. In Fig 3 as in Figs 1 and 2, 1 is the bimetal strip while 2, 3, 4, 5, 6, and 7 also represents the same items as in Figs 1 and 2.

The top part of the inner discharge lamp 8 is shown in Fig. 3. 14 is an insulating base with metallic inserts to which the starter is attached. Base 14 is fixed to a mounting structure 15 which also provides electrical connection to the bottom electrode of lamp 8.

The upper end of structure 15 is connected to a lead 16 which passes through the pinch 17 at the top of the outer jacket 18. The top electrode of the inner discharge tube is connected by the flexible lead 19 to another lead 20 in the pinch. The lamp leads are brought out to the lamp cap 21. 22 is a mica disc which provides a baffle which deflects heat from the lamp away from the cap.

In the modified arrangement in which the resistance of the heater coil is increased to the full 1.6 K ohms, the heater could be conveniently mounted on the underside of the mica baffle. On account of its high wattage consumption the heater coil would be physically larger than in the existing arrangement and it may be convenient to divide it into two or more substantially equal parts connected together. Each part could consist of a wire resistance wound on a cylindrical insulating base and the resistors would be connected in series. Fig. 4 indicates the modified arrangement in which the resistance is divided into two substantially equal resistors 23 and 24 which are cylindrical in shape. Fig 5 gives a plan view of the resistors mounted on the underside of the mica disc and includes a plan view of the bimetal strip 1.

In the existing arrangement indicated in Fig 3 the heater coil is usually cylindrical in shape and is generally sufficiently robust to make it unnecessary to wind it on a supporting insulating base. The coil is also usually designed to operate at a temperature corresponding to dull red heat with the starting current through it. In the modified arrangement where the starting current is substantially reduced and the heater wire diameter correspondingly less the heater coil could also be wound without a supporting insulating base providing it is sufficiently robust and could also be designed to operate at a dull red heat. Where the heater is divided into a number of parts as for example indicated in Fig 4 the coils could also be wound without a supporting base if they are robust enough.

If the modified arrangement is as in Fig. 2 and the heater coil 3 is in the existing position indicated in Fig 3, the additional resistor 13 could be conveniently mounted on the mica disc but on the disc surface remote from the discharge tube to minimise its heating effect on strip 1. The additional resistance could conveniently consist of two or more substantially equal resistors and in the case of two resistors their physical size would be practically the same as the resistors 23 and 24 indicated in Figs 4 and 5.

The additional resistors could each be wound without a supporting base if they are sufficiently robust. Suitable tungsten filament lamps could also be used for these additional resistors. The heater coil 3 in Fig 2 is likely to be robust enough to be wound without a supporting base.

The diameter of the wire in the heater coil and additional resistors can be very much reduced by reducing the current in the coil from 5.5A to 0.15A and also the thickness and size of the bimetal strip can be reduced.

The heat capacity of the components of the starter will consequently be reduced and as a result the bimetal will tend to heat up more quickly and the contacts open up sooner than in the existing arrangement so that the second objective of the invention is achieved.

The opening time of the starter in the existing arrangement is 1 to 2 minutes after switch-on whereas in the modified arrangement it will be only about 15 seconds. The speed of operation of the starter will tend to be greater in the Fig 2 arrangement than in Fig 4 since it is unlikely that the heater coil 3 will need to be wound on an insulating base in Fig 2 arrangement and its heat capacity will be at a minimum.

If the fully operating lamp is extinguished by a downward surge in the supply voltage it will take some time for the starter to cool sufficiently for it to reclose. This time may be 10 to 15 minutes in the existing arrangement but will be considerably less in the modified arrangement on account of reduced heat capacity.

There is no current in the starter when the lamp is in normal operation and current passes only during starting conditions. The resistor 3, 13,23 and 24 are in general required to last only for the life of the lamp, which is of the order of 5000 hours. and the proportion of the lamp life during which current passes in the starter resistors is very small. Consequently the wattage consumed in the resistor can be substantially greater than that for which it is normally rated and an acceptable life of the resistor can still be achieved.

The current at break for the 400w HPS lamp could in fact be increased above 0.15a without endangering ballast insulation due to excessive voltage surge and this would tend to improve starting reliability for the Xenon filled lamps at the higher gas filling pressures.

The resistor losses would then however be increased and consequent increase in resistor size might make it more difficult to accommodate the resistors in the lamp.

On the other hand, since the starting voltage of the Neon filled HPS lamp is much lower than the Xenon lamp the current at break for the Neon lamp could be considerably reduced to about 0.06A. A resistance of 4 K ohms is required to give this current at 240v mains and the power consumption in it would be 14 watts so that its size would be much less than the 1.5 K ohm resistor needed for the Xenon lamp.

HPS lamps of several different wattage ratings of both the Xenon and Neon filled types are available and the existing internal starter arrangement can be modified similarly to the 400w rating described above. In general the modified current at break will be such as to make the ratio of the modified current to the normal lamp current the same as for the 400w lamp using the same type of gas filling.

If the current at break was increased to improve starting reliability for the Xenon filled lamps it could be increased to a value which might give a voltage pulse which could tend to be marginal as regards endangering ballast insulation. In this case an attenuator could be connected across the lamp to limit the peak voltage as described in Patent Specification 1 461 055. The attenuator could consist of two Voltage Dependent Resistors which could be mounted in the lamp cap as indicated in Patent 1 461 055.

According to the present invention the starter switch resistance is arranged so that the rms current broken when the starter contacts open and the lamp is not alight does not exceed one sixth of the normal operating lamp current.

In some types of discharge lamps where a starter switch is employed to produce a voltage pulse at current break it is found that starting reliability can be improved by connecting a small capacitance of the order of 0.05 mfd across the lamp. This capacitance attenuates the pulse voltage but improves starting reliability by broadening the pulse.

Such a capacitor. even if it is only of the order of 0.005 mfd has a deleterious effect on starting reliability of the Xenon filled HPS lamp where it is not therefore used and is thus not shown in Figs l and 2. A small capacitance of about 0.005 mfd could be used for the Neon filled lamps without harmful affects but since it gives little improvement in reliability it is seldom connected.

Considering now the Mercury Halide lamps, a lamp of this type consists of a quartz discharge tube with Argon gas filling together with Mercury and also metallic Halides such as Sodium Iodide. The outer envelope may be evacuated to a fairly hard vacuum or filled with gas at a fairly high pressure to provide adequate insulation in the outer envelope for the pulse voltage pro duced by the starter switch.

The wattage ratings in which these lamps are available are in general the same as for the HPS lamps. The overall dimensions of the two types of lamps of the same wattage rating are practically the same as are also the normal operating lamp currents.

The starting voltage of the Halide lamps is considerably lower than that for the Xenon filled HPS lamps. Where a bimetallic thermal starter is used for the Halide lamp it is found that starting reliability is often improved by connecting s small capacitance across the lamp as described above. The capacitance used is generally sufficient to attenuate the pulse voltage to a value low enough to avoid danger to ballast insulation. The objective in providing modification in the internally connected bimetallic thermal starter in the Halide lamps is consequently mainly to reduce the size of the starter and to increase its speed of operation.

The value of the capacitance across the lamp would need to be reduced owing to reduction in the current broken in the modified arrangement and this would be advantageous in reducing the physical size of the small capacitor. The modifications required for a Halide lamp of a particular wattage rating will be practically the same as for the HPS lamp of the same wattage rating.

WHAT I CLAIM IS: 1. A High Pressure Sodium discharge lamp or a High Pressure Mercury Halide discharge lamp in which a bimetallic strip thermal type starter switch comprising a pair of contacts in series with a resistor, part or the whole of which forms the heater coil for the strip, is connected electrically across a discharge tube and mounted in the vicinity of the discharge tube within an outer envelope surrounding the discharge tube and which switch is kept in the open position when the lamp is alight entirely by heat from the discharge tube, no current then passing through the resistor, the resistor having a value so that the current through it at contact break of the switch and before the lamp has started is less than one sixth of the normal operating lamp current.

2. A discharge lamp as claimed in claim 1 in which the whole of the resistor forms the heater coil wherein the resistor is divided into two or more substantially equal parts supported at a suitable distance from the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. which it is normally rated and an acceptable life of the resistor can still be achieved. The current at break for the 400w HPS lamp could in fact be increased above 0.15a without endangering ballast insulation due to excessive voltage surge and this would tend to improve starting reliability for the Xenon filled lamps at the higher gas filling pressures. The resistor losses would then however be increased and consequent increase in resistor size might make it more difficult to accommodate the resistors in the lamp. On the other hand, since the starting voltage of the Neon filled HPS lamp is much lower than the Xenon lamp the current at break for the Neon lamp could be considerably reduced to about 0.06A. A resistance of 4 K ohms is required to give this current at 240v mains and the power consumption in it would be 14 watts so that its size would be much less than the 1.5 K ohm resistor needed for the Xenon lamp. HPS lamps of several different wattage ratings of both the Xenon and Neon filled types are available and the existing internal starter arrangement can be modified similarly to the 400w rating described above. In general the modified current at break will be such as to make the ratio of the modified current to the normal lamp current the same as for the 400w lamp using the same type of gas filling. If the current at break was increased to improve starting reliability for the Xenon filled lamps it could be increased to a value which might give a voltage pulse which could tend to be marginal as regards endangering ballast insulation. In this case an attenuator could be connected across the lamp to limit the peak voltage as described in Patent Specification 1 461 055. The attenuator could consist of two Voltage Dependent Resistors which could be mounted in the lamp cap as indicated in Patent 1 461 055. According to the present invention the starter switch resistance is arranged so that the rms current broken when the starter contacts open and the lamp is not alight does not exceed one sixth of the normal operating lamp current. In some types of discharge lamps where a starter switch is employed to produce a voltage pulse at current break it is found that starting reliability can be improved by connecting a small capacitance of the order of 0.05 mfd across the lamp. This capacitance attenuates the pulse voltage but improves starting reliability by broadening the pulse. Such a capacitor. even if it is only of the order of 0.005 mfd has a deleterious effect on starting reliability of the Xenon filled HPS lamp where it is not therefore used and is thus not shown in Figs l and 2. A small capacitance of about 0.005 mfd could be used for the Neon filled lamps without harmful affects but since it gives little improvement in reliability it is seldom connected. Considering now the Mercury Halide lamps, a lamp of this type consists of a quartz discharge tube with Argon gas filling together with Mercury and also metallic Halides such as Sodium Iodide. The outer envelope may be evacuated to a fairly hard vacuum or filled with gas at a fairly high pressure to provide adequate insulation in the outer envelope for the pulse voltage pro duced by the starter switch. The wattage ratings in which these lamps are available are in general the same as for the HPS lamps. The overall dimensions of the two types of lamps of the same wattage rating are practically the same as are also the normal operating lamp currents. The starting voltage of the Halide lamps is considerably lower than that for the Xenon filled HPS lamps. Where a bimetallic thermal starter is used for the Halide lamp it is found that starting reliability is often improved by connecting s small capacitance across the lamp as described above. The capacitance used is generally sufficient to attenuate the pulse voltage to a value low enough to avoid danger to ballast insulation. The objective in providing modification in the internally connected bimetallic thermal starter in the Halide lamps is consequently mainly to reduce the size of the starter and to increase its speed of operation. The value of the capacitance across the lamp would need to be reduced owing to reduction in the current broken in the modified arrangement and this would be advantageous in reducing the physical size of the small capacitor. The modifications required for a Halide lamp of a particular wattage rating will be practically the same as for the HPS lamp of the same wattage rating. WHAT I CLAIM IS:

1. A High Pressure Sodium discharge lamp or a High Pressure Mercury Halide discharge lamp in which a bimetallic strip thermal type starter switch comprising a pair of contacts in series with a resistor, part or the whole of which forms the heater coil for the strip, is connected electrically across a discharge tube and mounted in the vicinity of the discharge tube within an outer envelope surrounding the discharge tube and which switch is kept in the open position when the lamp is alight entirely by heat from the discharge tube, no current then passing through the resistor, the resistor having a value so that the current through it at contact break of the switch and before the lamp has started is less than one sixth of the normal operating lamp current.

bimetallic strip.

3. A discharge lamp as claimed in claim 1 in which part of the resistor forms the heater coil wherein that part of the resistor which does not form part of the heater coil is mounted so that it does not affect the bimetal strip thermally.

4. A discharge lamp as claimed in claims 1 and 3 wherein that part of the resistor which does not form part of the heater coil is divided into two or more substantially equal parts.

5. A discharge lamp as claimed in claims 1, 3 and 4 wherein that part of the resistor which does not form part of the heater coil consists of a Tungsten filament lamp or two or more Tungsten filament lamps the resistances of the filament lamps being substantially equal where two or more are used.

6. A discharge lamp as claimed in any preceding claim wherein a voltage surge attenuator is mounted inside the lamp and connected electrically across the discharge tube.

7. A discharge lamp substantially as hereinbefore described with reference to and as illustrated in the Figs 1, 2, 4 and 5 of the accompanying drawings.