US2987650A

US2987650A - Gaseous discharge lamp circuit

Info

Publication number: US2987650A
Application number: US842149A
Authority: US
Inventors: Chapman Donald Frederick
Original assignee: Thorn Electrical Industries Ltd
Current assignee: Thorn Electrical Industries Ltd
Priority date: 1958-09-26
Filing date: 1959-09-24
Publication date: 1961-06-06
Anticipated expiration: 1978-06-06

Description

June 6, 1961 D. F. CHAPMAN GASEOUS DISCHARGE LAMP CIRCUIT Filed Sept. 24, 1959 I 0 OOOUUUCWUUOUUOOOg i '4 2 Sheets-Sheet 1 l 00 MWWWOWWWO 0 4 2 3 N u L 2 4 0 UOUOUOOOOOOOOOWOO 00 INVENTOR Denim. FRcoK. CHAPm/w A TTO/ZNE Y June 6, 1961 D. F. CHAPMAN 2,987,650

GASEOUS DISCHARGE LAMP CIRCUIT Filed Sept. 24, 1959 2 Sheets-Sheet 2 N L N.. L

I I /I i i 3 34 B6 3A B J 1 w' L 12 3 4 oooooooooomoooo 000 4r F2 lA/VEN Tok Domm FREDK. HAPMAN ATTORNEY 7 inventionrelates tolcircuits forlstarting and op- ..UP, F- .5 t Q crating one or more gaseous discharge devices, such as fluorescent lamps, from an alternating current supply, such 1 lamps being provided with heatable electrodes, usually 1 filamentary electrodes, at each end. It known that the voltage required to start such lamps is much higher than the vvoltage required for operation. .7

To reduce starting voltage it is well known .to provide initial preheating for the lamp electrodes, such preheating causing emission of .electrons'and .ionisation of the gas filling and mercury vapour and thereby assisting starting. When the lamp has started it may be desirable to reduce or cut off the preheating current supplied to the lamp electrodes in order to reduce power losses and improve the life of the electrodes. Known circuits have used a transformer with primary and secondary windings to provide a power supply to preheat the electrodm In the Quickstart circuit, for example, the primary winding is connected between opposite ends of the lamp and two secondary windings are connccted across the two electrodes respectively. With this circuit a choke ballast is used in series with the supply to ensure stability and limitation of lamp discharge current, and a capacitor is often connected across the supply to improve thelow power factor introduced by the choke.

With such a circuit it hasbeen found that lamp starting may be diflicult if the ambient temperature is low or if atmospheric 7 humidity is high. To overcome these starting difiiculties it has been proposed to use low gas filling pressures inside the lamps and to-coat the glass lamp surface with water-repellent materials. Such modifications to the lamp may increase cost and reduce life.

It is the objector, the present invention to provide an improved circuit for starting and operating gaseous discharge'lamps in which starting difliculties are reduced.

According to the present invention there is provided a circuit for starting and operating one or more gaseous discharge devices having heated electrodes from an alternating current supply, comprising a voltage step-down transformer having a primary winding and at least two secondary windings, the primary winding being connected in series with a ballast choke between supply terminals, two heater current terminals for connection to one lamp electrode being connected in series with one of the secondary windings and a capacitor between the supply terminals, and another of the secondary windings being connected to two further heater current terminals for connection to another lamp electrode.

The invention will be described, by way of example, with reference to the accompanying drawing containing circuit diagrams of a number of embodiments of the invention.

In FIG. 1 there is shown a fluorescent lamp F having filamentary electrodes A and B and an auto-transformer having terminals and taps 1, 2, 3 and 4. The parts 12 and 34 of the auto-transformer constitute secondary windings while the part 2.3 constitutes the primary winding. A ballast choke G is connected in series between the tap 3 and a supply terminal L. The electrode A is connected in series with the secondary winding 12 and acapacitorCbetweensupplyterminalsNandL. The

' electrode B is connected in'parallel with the When a main switch (not shown) connecting the terminals N and L to an AC. supply is closed, a momentary inrush of current occurs through the series circuit comprising capacitor C, winding 1-2, and lamp electrode A. This surge of current occurs because the capacitor is initially discharged and the surge persists for only a few microseconds until the capacitor becomes fully charged. The magnitude of the current surge which occurs when the circuit is switched on depends upona number of factors including the value of the impedance presented by the winding'1-2 in series with the capacitor C. The current surge may be as much as 1000 amps. peak in a circuit such as that of FIG. 1 whenconnected to a 240 volt 50 cycle A.C. supply.

The momentary surge of current induces a momentary high voltage across the Winding 12. Since the windings 2-3 and 3-4 are electromagnetically coupled to the Winding 12 by transformer action, high instantaneous voltages are induced in the former. Because the winding 2-3 has many more turns than the winding 1-2, the voltage across 2-3 is many times that across 1-4. The result is that a very high transient voltage is induced across thewin'ding 23, and hence between the this time before the main discharge has started is, low.

when the circuit is switched on. Such a high voltage, which may approach 2000 volts, although it does not ,start the main discharge in the lamp because at this time the filaments A and B are cold, serves to ionise the gas and vapour in the discharge space of the lamp and enables the main discharge to start at a lower voltage when the filaments have heated up.

The two electrodes A and B now rapidly reach a high temperature and emission of electrons occurs to help ionise the gas and vapour still further. When ionisation is sutficient the lamp will start and a heavy current then 7 flows through the discharge space between the electrodes. This current is limited -by the impedance of the choke G and a voltage drop now occurs across this choke. Voltage between the ends of the lamp is thus reduced after starting and since the primary winding 2-3 is also connected across the lamp the voltage on the winding 23 also drops.

In practical circuits the supply voltage is arranged to be about twice the voltage across the lamp during operation. Thus voltage on the winding 23 drops by about half after the lamp has started. Since the winding 3-4 is coupled to the winding 2-3, the voltage supplied to the electrode B drops after the lamp has started, thereby reducing the current and hence the power losses in this electrode.

When the lamp is operating, current through the electrode A is the vector sum of two antiphase components, i.e. the current through the capacitive branch and the current through the inductively controlled discharge path. Since these currents are anti-phase they tend to cancel each other, with the result that after the lamp has started the current through electrode A drops because of this partial cancellation as well as through the reduction of voltage of the primary winding 2.--3.

During normal operation of the circuit of FIG. 1, the capacitor C serves to improve the power factor. Voltage across the capacitor C after the lamp has started can be shown to be the vector sum of three voltages, namely 'thesupply'voltage, the'voltage across 'the'winding 1-2, and the voltage drop across the electrode A, and this capaoitonvoltage is found to .be approximately equalto the supply voltage.

I By way of example, inone circuit as-shown inIFIG. lthe'lamp F. was a ft. 8,0watt, typ eMCF/U. The

capacitor C was 0157.5 microfaradsjand the'choke G ga, impedance of 210 "ohms at'0.85 arnp., '50 'c./s. The

winding 2'3 was of 1900turns 38 SWG enamelled co per Wire, each secondar'y winding was of 135 T ,tumslZflSWG enamelled copper wire, and the'core'was a f'istack1of No'. 182;N Iaminations' punched from dy- The following values were obtained after switchingon and before the lamp started and after the lamp had The circuits of FIGS. 2- to S idifi'er only .inminor 'reispects from that of FIG. '1.

Thus the connections of 'the electrode A and the capacitor C to the winding 1'-2 are rearranged and the choke G is connected to tap 4 in- -=stead:of to tap 3*. The mode of operation is substantially the same as with 'FIG. 1. shown as having tappings in order to permitoperation on different supply voltages. Moreover a resistor R is shown connected in parallel with the capacitor C in order :In FIG. 5 the'choke G is to-ensurerapid discharge of the capacitor when .the cirvcuit is switched off. 'FA tapped choke G and azresis-tor-R may, of course, be used in anyof the circuits.

In FIG. 6 the only change from FIG. :1 is that the choke Gin FIG. 6 is connected to a further tap 5'. This enables the steady voltage across the lamp during starting to be made substantially greater than the supply voltage. 1 Such a. circuitis, for example, suitable 'for use with an 8 ft. 125 watt fluorescent lamp which requires approximately 350 volts for. satisfactory starting.

InF-IG. 7 the transformer winding is extended beyond the point 4 to a point 6 to which the supply terminal L is connected through the'choke G. This enables the steady voltage across the lamp durin'g starting to be reduced considerably below the supply voltage. This may be advantageous for short fluorescent lamps such as the 2 ft. watt lamp which requires only-about 110 volts for starting.

FIG; 8 shows how the oircuiti ofiFIG; 1 loan zbe modified to operatei'two fluorescent-lamps F land F2 in series.

ILLFIG. 8 the filaments A andiB -of the lamps; F and F respectively are supplied in the same way as thefilaments A- and '-B respectively in FIGs 1;; the filaments B and A are supplied by'i'a' further secondary winding S on the transformer.

The other .circuitscan, of .coursc, be similarly adapted to operate a plurality of lamps.

. p 1.1A circuit for starting and operating at least .one 'g'aseoushischarge device having, heated-electrodes from ,I claim:

an alternating current supply,c'omprising a voltage step- 1 down transformer having aprimary winding: and at least "two secondary windings; a ballastlchoke,.afcapacitor, sup- 7 7 'ply terminals for .coimectibnto a source .of alternating current, first and second pairsko'f heater terminals each pair for connection to a lamp electrodeQmeanscOnnecting saidprimarywinding in series said ,ballast choke between, said. supply terminals, .means connecting said first I pair of heaterterminalsin sexiesiwithone -o f said secondary"windings and saidcapacitonbetween said supply terminals, and meansconnecting saidsecond'pair of heater go ten ninals across. another of said secondary. winding, one of said secondary windings-and said first pair of wherein said primarywindingl said ballast choke, at least heater terminals are connected inseries between said supply'terminals', and means connecting at least oneterminal of each pair of heater terminalsto one of said alterj hating current supply terminals.

.2. [A circuit according to claim .1, wherein at least one .of tsaidsecondary windings is constituted by a part of said primary winding.

3. A circuit according to claim l, .further comprising a resistor connected in parallel with said capacitor.

References Cited in thefile of this patent UNITED STATES PATENTS 2,294,623 Le Brun 'Sept. 1, 1942 2,444,408 1 Larime June 29 1948 2,824,262 Cates Feb. 18, 1958 2,838,714 Moerkens June '10, 1958 2,901,653 Gilmore Aug. 25, 1959