GB2077526A - Supply circuits for discharge lamps - Google Patents

Abstract

The inductive ballast for an inductively operated discharge lamp in a switch start circuit on a.c. supplies is provided with a compensating winding wound on the same magnetic core as the main winding and arranged to give a short-circuit current when the starter is closed substantially the same as the normal operating lamp current. This prevents a high ballast temperature rise when the starter flashes in the condition when the lamp fails to start. The ballast is of particular use with a starter which is designed to remain in a non-flashing closed condition when the lamp fails to start. A circuit using this ballast can be paired with a capacitively ballasted circuit in which no compensating winding is provided on the series inductance and in which the short-circuit current is consequently also practically equal to the normal operating current of the lamp. The size of the compensated inductive ballast can be substantially the same as the ordinary uncompensated inductive ballast. <IMAGE>

Description

SPECIFICATION Reduced short-circuit current This invention relates to a starter switch circuits with inductive ballasts for operating discharge lamps on 100 to 250 volts 50 to 60 c.s. a.c. supplies and is concerned with reducing the short-circuit current in the circuit when the starter switch is in the closed condition, particularly for tubular fluorescent lamps.

The invention will now be described with reference to the drawings in which: FIG. 1 shows a well known inductively ballasted circuit for starting a tubular fluorescent lamp with a glow type starter switch which is connected between the lamp electrodes.

FIG. 2 shows a well known capacitively ballasted circuit for starting a tubular fluorescent lamp also with a glow type starter switch.

FIG. 3 shows a well known capacitively ballasted circuit for starting a tubular fluorescent lamp with a glow starter and also provided with a compensator winding wound on the same core as the series ballast inductance, the compensator winding being connected in series with the starter between the lamp electrodes and arranged to increase the shortcircuit current when the starter switch is closed.

FIG. 4shows an inductively ballasted circuit for a tubular fluorescent lamp started with a glow switch starter and provided according to the invention with a compensator winding wound on the same core as the main ballast winding, the compensator winding being connected in series with the starter between the lamp electrodes and arranged to decrease the short-circuit current when the starter switch is closed.

FIG. 5 shows a Twin Tube circuit arrangement for tubular fluorescent lamps comprising according to the invention of an inductively ballasted circuit provided with a compensator winding designed to decrease the short-circuit current when the glow starter is closed and a capacitively ballasted circuit with a glow starter without a compensator winding.

Fig. 6 shows an inductively ballasted circuit for a tubular fluorescent lamp with a compensator winding according to the invention and employing a 'No-Blink' locked-in glow type starter.

FIG. 7 shows an inductively ballasted circuit for a tubular fluorescent lamp provided with a compensator winding in accordance with the invention and employing a known thermal type starter switch.

The invention is of particular use for tubular fluorescent lamps operated with glow type starter switches and a well known inductively ballasted circuit for such a lamp and starter is indicated in FIG. 1.

Here the lamp 1 is operated in series with a ballast inductance 2 on a supply whose input terminals are 3 and 4 of which 3 is usually the Line in a Line/Neutral a.c. supply. The glow starter 5 is connected between the lamp electrodes 6 and 7 each of which consists of a filament coated with electron emissive material. 8 and 9 are the starter electrodes either one or both of which are bimetallic.

The glass envelope of the starter is filled with gas of suitable composition and at suitable pressure. The starter contacts are normally open and when the supply is connected the voltage across the starter is practically equal to the supply voltage and a glow discharge current usually of the order of 20 mA then flows in the starter. The discharge heats the starter electrodes which are arranged so to bend when heated as to result in contact between the electrodes. The current passing on contact is the shortcircuit current of the ballast 2 and is of the order of 1 A for a fluorescent tube operating on a 240 volt 50 c.s. supply.

The starter is connected so that the current in it also passes through the filamentary lamp electrodes and when the starter contacts are closed the ballast short-circuited current heats the lamp filaments causing them to become emissive. The starter is designed so that the current when the contacts are closed is insufficient to keep the bimetal electrodes in a bent condition and the contacts open. At contact break a voltage surge appears across the inductance 2 and consequently across the lamp and if the surge is large enough the discharge will start in the lamp 1.

A small capacitance 10 of the order of 0.005 mfd is normally connected across the starter terminals and assist in lamp starting as well as providing radio interference suppression. A capacitance 11 is usually connected between the supply terminals to provide power factor correction.

When the lamp has started the voltage across it and consequently across the starter is usually about half the supply voltage and the starter is designed so that the voltage across it during normal lamp operation is too low to maintain a glow discharge or even if there is a small glow the energy provided by it is not enough to bend the bimetals sufficiently to cause contact.

If the lamp has failed so that it will not start the starter will continuously close and re-open giving rise to objectionable flashing often described as 'blinking'. When starter contacts open and the lamp does not start the glow causes the contacts to remake very quickly and with continuous flashing the contacts are closed for the large majority of the time and open for only a small propostion ofthetime. As a result the average current passing during flashing is practically equal to the continuous short-circuit current. The short-circuit current for a tubular fluorescent lamp operated with an inductive ballast on a 240 volt supply is about 30% greater than the normal operating current for which the ballast is designed to have an acceptable temperature rise.

Since the temperature rise of the ballast tends to be proportional to the square of the current the temperature rise with the flashing starter can be substantially greater than under normal operating conditions.

An economic glow type starter can be designed in which continuous flashing with a failed lamp is pre vented by arranging flash for the starter contacts to remain closed after flashing with a failed lamp has lasted for about 30 seconds and the contacts do not then re-open until the supply is switched off. This condition of the starter is known as the 'locked-in' state and the starter can be described as a 'No-blink, Locked-in' type. Although this type of starter eliminates the annoying flashing it has not been used because of the high ballast temperature in the locked-in state although this temperature is not appreciably higher than with the ordinary glow starter in the blinking condition.

The lamp can be operated with a capacitive ballast consisting of a capacitor in series with an inductance instead of with the inductive ballast consisting of an inductance only as shown in FIG. 1. The capacitively ballasted circuit using a glow starter is indicated in FIG. 2. Here the ballast capacitor 12 is in series with the inductor 13 and it is arranged that the impedance of 12 is greater than that of 13 at the supply frequency and at the normal operating lamp current.

The combined impedance of 12 and 13 is the difference between the impedances of 12 and 13 and as a result the circuit has a leading current as compared with the lagging current in the lamp and ballast in the FIG. 1. circuit. The capacitive circuit is often paired with an inductive circuit to give what is known as a Twin Tube arrangement, no power factor correcting capacitor 11 being used in the inductive circuit. The combined input current to the pair of circuits is practically in phase with the supply voltage and close to unity powerfactoris obtained. Afurther advantage is that by mixing the light output from the pair of lamps the stroboscopic flicker is substantially reduced.

The short-circuit current in the capacitive circuit of FIG. 2 tends to be practically the same as the normal operating current. This arises from the harmonics in the voltage across the series inductance 13 introduced by the harmonics in the voltage across the lamp when it is in normal operation and this tends to make the impedance of the inductance 13 greater and the combined impedance of 12 and 13 less during normal operation than in the short-circuit state.

The lower short-circuit current in the capacitive circuit compared with the inductive circuit tends to reduce the electron emission from the lamp filaments during the starting conditions and this tends to reduce filament life and consequently lamp life a little. The reduction will however depend on how often the circuit is switched and provided the frequency of switching is not too great the lamp life will not be reduced to a great extent.

Because of the effect of the decreased short-circuit current on lamp life, where the capacitive circuit is used it is often modified to give increased shortcircuit current in the starter switch circuit. The modified arrangement is indicated in FIG. 3. where an additional coil 14, wound on the same magnetic core as coil 13 is connected in series with the starter switch between the lamp filaments. 14 is wound so that when the starter is closed the combined impedance of 13 and 14 is greater than that of 13 alone but the combined impedance is still below that of 12 and consequently the short-circuit current when the starter is closed during starting is increased. Since there is no current in 14 in normal lamp operation with the starter open the lamp operating current is not affected.

14 is generally described as a compensator and it is usually designed to give a short-circuit current approximately the same as the short -circuit current in the inductive circuit for the same lamp rating. The capacitive circuit even without the compensator is more expensive than the inductive circuit and although the uncompensated circuit will keep the temperature of the inductance during flashing conditions or with a continuously closed starterthe same as during normal lamp operation the FIG. 2. circuit has not been used to avoid a high ballast temperature in starting conditions.

According to the present invention a compensator coil wound on the same magnetic core as the main ballast winding is provided in the inductively ballasted starter switch circuit and is connected in series with the starter switch between the lamp electrodes. The compensator coil is designed so that the combined impedance of the main winding and the compensator winding when the starter switch is closed is greater than that of the main winding alone and the combined impedance is such as to give a short-circuit current with the starter closed which is substantially the same as the normal operating current of the lamp. The normal operating current of the lamp will be unaffected by the compensator winding when the starter switch is open.

A paired circuit arrangement comprising according to the invention of an inductively ballasted circuit with starter switch provided with compensator winding together with a switch start capacitively ballasted circuit without compensator winding will give a short-circuit current substantially equal to the normal lamp operating current in both circuits and will also provide economic and other advantages. No power factor correcting capacitor is connected between the mains input terminals in this paired circuit arrangement.

FIG. 4. shows the inductively ballasted circuit arrangement for a tubular fluorescent lamp with a glow type starter switch provided according to the invention with a compensatorwinding. Here 2 is the main ballast winding and 15 is the compensator winding wound on a common magnetic core. In the case of a tubular fluorescent lamp operated on a 240 volt 50 c.s. supply the turns required for winding 15 to give a short-circuit current with the starter closed substantially equal to the normal operating lamp current is about 10% of the turns used for the main ballast winding 2.

The short-circuit current in the known inductive switch start circuit of FIG. 1 is dependent on the design of the inductive ballast. The impedance of the ballast at the normal operating lamp current must be designed to give the specified normal lamp operating conditions. The short-circuit current will however depend on the degree of magnetic saturation in the ballast core and this is dependent on the magnetic flux density in the core at normal operating conditions. As this flux density increases the short-circuit current is increased, and thus the flux density must be limited in order to avoid a high short-circuit current with consequent high ballast temperature under short-circuit conditions.

In the compensated circuit indicated in FIG. 4. the short-circuit current can however be limited by the design of the compensator winding and practically independently of the magnetic flux density and it is consequently possible to use a higher flux density in the FIG. 4. arrangement than in FIG. 1. It is feasibie to increase the flux density in the FIG. 4 arrangement by about 10% above that used for the arrangement of FIG. 1. and as a result the amount of material in the core and coils of the ballast and the ballast size for the FIG. 4 arrangement can be substantially the same as forth FIG. 1. arrangement.

The compensated inductive circuit of FIG. 4, excluding the power factor correction capacitor 11 can, according to the invention, be paired with the uncompensated capacitive circuit of FIG. 2. to give a Twin Tube arrangement indiciated in FIG. 5. Since no compensator winding is required in the capacitive circuit to obtain the reduced short-circuit current this arrangement is more economic than two FIG. 4. type compensated inductive circuits. In addition the FIG.

5. arrangement gives high overall power factor and less stroboscopic flicker.

A'No-blink' locked-in starter switch can be used instead of the ordinary glow type starter in the circuits of FIGS. 2, 4, and 5. A known 'No-blink' starter switch of this type consists of a glow type starter with a resistive heater coil connected in series with the main starter electrodes, the heater coil being either inside or external to the starter switch envelope. FIG. 6. indicates the arrangement where the inductive circuit with compensator winding according to the invention is used with a 'No-blink Locked-in' starter of this type in which the resistive heater is inside the starter switch envelope.

A known type of starter switch otherthan the glow type consists of two bimetallic strips provided with contacts together with a resistive coil which is arranged to heat one of the bimetal strips. This type of starter, commonly known as a Thermal type, generally operates in air in a non-airtight canister. The starter contacts are normally closed while the heater coil is connected between the lamp and the ballast.

When the supply is connected the ballast shortcircuit current flows in the heater and the starter contacts open giving the surge voltage. When the lamp operates normally the normal lamp current flows in the heater and keeps the starter open.

If the lamp fails to start the starter contacts flash but the period for which the contacts are closed is in general small compared with the open period. As a result the average current during flashing is substantially less than the continuous short-circuit current so that the ballast temperature is substantially less than at continuous short-circuit. A fault can however develop in the starter which could result in continuous short-circuit current flowing with consequent high ballast temperature. To avoid this the compensator winding according to the invention can be used in the inductively ballasted circuit to reduce the short-circuit short-circuit current with the thermal type starter.

The arrangement is according to the invention indicated in FIG. 7. Here the starter 17 has the two bimetal strips 18 and 19 carrying the contacts while 20 is the heater coil.

An additional small capacitance of the order of 0.005 mfd may if necessary be connected directly across the lamp between its terminals where the compensating winding is used according to the invention, and this will assist in radio interference suppression.

Claims (5)

1. An inductive ballast for a discharge lamp in an inductively ballasted circuit with a starter switch on a.c. supplies in which a compensating coil wound on the same magnetic core as the main ballast winding is connected in series with the starter switch between the lamp electrodes, the compensating coil being designed so that the combined impedance of this coil and the main ballast winding when the starter switch is closed is greater than that of the main winding, and the combined impedance is such as to give a short-circuit current when the starter switch is closed which is substantially the same as the normal operating current of the lamp.

2. An inductive ballast according to claim 1 in which an inductively ballasted circuit is paired with a capacitively ballasted switch start circuit, no compensating winding being provided on the ballast in the capacitively ballasted circuit.

3. An inductive ballast according to claim 1 in which the the starter switch is an ordinary glow type starter.

4. An inductive ballast according to claim 1 in which the starter switch is designed to be locked-in in a closed condition after some flashing when the lamp fails to start.

5. An inductive ballast substantially as hereinbefore described with reference to FIGs. 4, 5, 6 and 7 of the accompanying drawings.