AU628129B2 - A control circuit - Google Patents

Description

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62 3 129 FORM S F Ref: 84677 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Class Int. Class Application Number: PJ2610 Lodged: 7 February 1989 sea* a Accepted: Publi shed: Priori ty: Related Art: 9B Name and Address of Applicant: Actual Inventor: Address for Service: Trestoto Pty Limited Lot 120 Aylmerton Road Aylmerton New South Wales 2575

AUSTRALIA

Donat Witold Majewski Spruson Ferguson, Patent Attorneys, Level 33 St Martins Tower, 31 Market Street, Sydney, New South Wales, 2000, Australia Complete Specification for the invention entitled: A Control Circuit The following statement is a full description of this invention, including the best method of performing it known to me/us REPRINT OF RECEIPI B0 i 2 36 07/02/90 5815/2 i -i e r

ABSTRACT

The present invention discloses a dimmer circuit for use with gas discharge lamp(s). The circuit finds greatest application with fluorescent lamps but is not limited thereto. Filament heating and series or parallel operation are disclosed. The power dissipation and flickering problems of prior art circuits are substantially overcome.

The circuit uses a conventional low cost ballast and a separate high voltage pulse transformer.

**00 **0 :0 BJG*778* *8D «0 0 400 BJG/77R8D 00 0 The present invention relates to dimming circuits for gas discharge lamps and to fluorescent lamps, in particular, however, the present invention is not restricted to such lamps and is applicable to other types of discharge lamps.

The object of the present invention is to provide a dimming circuit which enables discharge lamps to be easily started and the lamp intensity controlled over a broad range from very low intensities, or even off, to substantially 100% intensity. Such a dimming circuit finds particular application in modern office blocks where it is intended to 10 control the amount of energy consumed by electric lighting in accordance 0 9 with the degree of ambient lighting provided by sunlight.

A preferred feature of the present invention is to reduce the bulk of prior art gas discharge dimming circuits and also do away with the need for a grounded metal plate adjacent to the lamp thereby giving rise to the ability to design new types of luminaires for dimming applications.

In accordance with the present invention there is disclosed a gas discharge lamp dimming circuit comprising a gas discharge lamp connected in a first series connection with a current limiting ballast and a •o thyristor controlled dimmer across an AC mains supply, a first impedance (n a po4ra((4( co-rtecv-orl ic/(ft connec edAin par&allel i th said lamp and connected in a second series connection with said dimmer across said supply, and a high voltage pulse *see* transformer having a secondary winding connected in said first series connection and a primary winding connected in said second series connection, and wherein said ballast has a second impedance connected in parallel therewith, said second impedance having a low impedance to high voltage high frequency pulses produced by said pulse transformer. A charge dissipation circuit is also disclosed.

Embodiments of the present invention will now be described with \reference to the drawings in which: 0O Figs. 1 to 3 inclusive each represent prior art dimming circuits, Fig. 4 is a circuit diagram of a first embodiment of the present invention, Fig. 5 illustrates various alternative comiponent positions for the circuit of Fig. 4, Fig. 6 is a circuit diagram of a second embodiment of the present invention, Fig. 7 represents various alternate component positions for the circuit of Fig. 6, .10 Fig. 8 illustrates a circuit diagram of a third embodiment of the present invention, Fig. 9 illustrates various alternate circuit component positions for the embodiment of Fig. 8, Fig. 10 illustrates various realisations of circuit components of Figs. 4 to 9 inclusive, Fig. 11 is a circuit diagram of a further embodiment of the present invention, and Fig. 12 illustrates modifications to the pulse transformer primary S winding circuit.

Discharge lamps in general, and fluorescent lamps in particular, are normally started by the application of a high voltage across the lamp. This high voltage ionises the gas within the lamp and an arc is established. A current through the lamp (which has a negative resistance) is then controlled by some device such as a ballast.

To reduce the magnitude of the voltage required to initially ignite the lamp, a filament heating transformer FT is normally used to bring the filaments at the ends of the lamp to emission temperature.

The heating of the filaments also prevents "cold" starting of the lamp and improves lamp life. The filament heating is generally maintained during normal lamp operation.

3 BJG/7788D j L_/Ij _ILIYIIL___PILI~li- lK. ~i Such a circuit is commonly referred to an "uncompensated RS circuit" and forms the basis of the standard prior art fluorescent lamp dimming circuit which is illustrated in Fig. 1.

The dimmer D is typically a phase controlled triac or SCR dimmer.

Preferably a resistor RB is provided as an option to ensure that the triac of the dimmer D has a passive load thereby ensuring reliable turn on of the dimmer D for each half cycle of the AC mains supply.

The prior art circuit illustrated in Fig. 1 has a number of disadvantages. In particular, where a number of lamps are connected in parallel there is non-uniformity of light output from the lamps controlled by the one dimmer particularly at low level dimming. At low Se°: level dimming the triac of the dimmer D is turned on late each half cycle where the instantaneous mains voltage is sometimes not sufficient fee: to ignite some lamps. The net result is that some lamps are on, some lamps are attempt:'g to start (thereby flickering) and some lamps are completely out. This is most unsatisfactory.

p *oe It is known 7o achieve a significant improvement in dimming by use of "pulse dimming". There are a number of ways in which this is achieved with the two most common methods being illustrated in Figs. 2 and 3.

In both the arrangements of Figs. 2 and 3, the current controlling ballast B acts as an autotransformer to produce a high frequency high voltage pulse each half cycle to ensure that the lamp is reliably ignited.

In Figs. 2 and 3, the high voltage high frequency pulse is produced as follows: capacitor C1 is initially uncharged, the triac in dimmer D is turned on at some particular time during each half cycle depending on the dimmer setting, and the instantaneous supply voltage is then applied across the turns N1 of ballast B and capacitor Cl. Since capacitor C1 has a relatively low impedance, most of the available -4 BJG/7788D i ,1 1_ i I mains voltage initially appears across the turns N1 and is thus transformed to a much higher voltage which appears across the turns N2.

This high voltage is sufficient to start the lamp L and the mains current then flows through windings N2 only (in Fig. 2) and through both windings N1 and N2 (in Fig. The process is repeated each half cycle. Sometimes a small resistor is connected in series with capacitor C1 to shape the high voltage pulse.

The circuit arrangements of Figs. 2 and 3 have some disadvantages in that the ballast B must have insulation which is sufficient to be able to withstand the high voltage pulse generated each cycle. As a consequence, the ballast B generally has to be larger than the commonly used "slimline" ballast. Consequently the ballast B is more expensive.

Furthermore, the charge remaining on the capacitor C1 at the end of each half cycle sometimes causes misfiring of the triac. This is particularly evident at low level dimming and manifests itself by flashing and flickering of the lamp (or lamps where two or more lamps .0 are operated in parallel). In the event of the lamp(s) not firing immediately, the capacitor C1 can have a charge equal to, or even greater than, the instantaneous supply voltage. Clearly, any charge on capacitor C1 of the same polarity as the mains voltage inhibits the turn on process of the triac.

S• This misfiring is usually overcome by providing a relatively low resistance R2 in parallel with the capacitor C1 to discharge the capacitor. The resistance R2 also provides the triac with an adequate turn on current. In addition, a resistor R1 and capacitor C2 connected in series are normally provided to damp the oscillations around the loop formed by the lamp L the capacitor C1 and the ballast B. Not only does this reduce radio interference, but it also helps to reduce the amount of charge on capacitor Cl and also to reduce the amount of power that has to be dissipated in resistor R2 to eliminate the charge on capacitor C2.

-7 _8 Even so, the amount of power that has to be dissipated by resistor R2 is substantial and can amount to several watts per lamp.

Turning now to Fig. 4, one form of circuit arrangement in accordance with the present invention is illustrated therein. Windings N1 and N2 form the primary and secondary winding respectively of a small transformer which is preferably wound on a ferrite core. Ballast B is a standard fluorescent light ballast which is of both of low cost and also of small bulk. Impedances Z1 and Z2 are preferably small capacitors.

The dimmer D is a low cost prior art triac dimmer.

"0 One function of the impedance Z1 is to provide a low impedance path to the initial current flowing at the moment the dimmer triac has fired. At this time the lamp L appears as an open circuit and the impedance Z1, by being essentially a short circuit results in most of the instantaneous supply voltage appearing across the primary winding Nl of the pulse transformer because the capacitance within impedance Z1 is uncharged. This voltage is transformed to a higher voltage by winding N2 and this high voltage pulse appears across, and starts, the lamp L.

Both impedances Z1 and Z2 provides a low impedance path for the high frequency pulse current around the loop formed by impedance Z1 the' lamp L, winding N2 and the parallel connection of impedance Z2 and ballast B.

Once the high frequency, high voltage pulse starts the lamp, the mains current then flows through the series circuit of dimmer D, windings N1 and N2, ballast B and the lamp L.

Also shown in Fig. 4 is a non-essential improvement in the form of a series connected resistor R4 and capacitor C4 which are preferably connected in series with each other and in parallel across the primary winding Ni. The resistor R4 and capacitor C4 provide a damping and pulse shaping circuit to improve lamp starting each half cycle.

-6- BJG/7788D

A

Fig. 5 illustrates modifications which can be made to the basic circuit of Fig. 4 by positioning the primary winding N1 at any one of the five positions illustrated. Although not specifically illustrated in Fig. 5, the resistor R4 and capacitor C4 c,4n be connected in parallel with the primary winding Nl at each of the five positions illustrated in Fig. Fig. 6 illustrates an alternative arrangement to that of Fig. 4 in that the ballast B and its by-pass impedance Z2 are connected in series with the dimmer D. This has the advantage that the high voltage pulse produced by winding N2 is only applied across the lamp L and impedance Z and is not applied to the lamp ballast. Non-essential resistor R4 and capacitor C4 function as before. Again, Fig. 7 illustrates modifications possible to the circuit of Fig. 6 in that the primary winding Nl of the pulse transformer (together with resistor R4 and capacitor C4 if desired) can be located at any one of the six possible positions indicated.

.4 Fig. 8 illustrates a still further variation to the circuits of Figs. 4 and 6 in that the primary and secondary windings of the pulse transformer are not directly connected in series but are, of course, still magnetically coupled. Again Fig. 9 illustrates the various s. alternate locations of the primary winding of N1.

As indicated in Fig. 10, the impedance Z2 can be just a capacitor or it can be a capacitor in parallel with a resistor, or a capacitor in parallel with a series connected resistor and further capacitor, or a capacitor in parallel with a varistor or a combination of similar such components. The essential function of impedance Z2 is to prjvide a by-pass for the high frequency pulse current so that the ballast B is not subjected to excessively high voltages. As also indicated in Fig.

the impedance Z1 can consist either of a capacitor C10 alone, or a capacitor C10 connected in series with a resistor RIO. With either of -7- BJG/7788D these arrangements, the circuits of Figs. 4 to 9 suffer somewhat from misfiring of the triac as previously mentioned. This misfiring can be overcome by the methods described above in relation in Figs. 2 and 3.

That is to say the resistors R1 and R2 and capacitor C2 can be provided as a preferred feature as indicated by broken lines in Figs. 5, 7 and 9 in order to ameliorate the misfiring problem.

However, this misfiring problem can be overcome by the circuit arrangement for Z1 illustrated in Fig. 10 and including diodes D1 and D2. This arrangement provides the necessary low impedance path at the moment the triac turns on and also provides a low impedance path to complete the pulse loop current through winding N2, the lamp L, and the parallel connection of the ballast B and impedance Z2.

The operation of the two arm circuit arrangement for impedance Z1 illustrated in Fig. 10 is as follows. The triac in the dimmer D is turned on at some time during, say, the positive half cycle of the mains supply, the time of turn on being dependent on the dimmer setting. The capacitor C10 is uncharged and current flows via diode D1 and capacitor C10. As a result most of the instantaneous supply voltage appears across winding N1 and the high voltage pulse is produced by transformer action and the lamp L ignited as previously described.

The purpose of the diode D1 is to "isolate" any charge on capacitor C10 and prevent it from subtracting from the available voltage and interferring with the triac turn on. This is particularly important at low level dimming because the lamp L might not ignite instantly and the voltage on the capacitor C10 would be in opposition to the supply voltage at that moment.

The charge on capacitor C10 is now able to dissipate via resistor RIO which, in consequence, can be a relatively large magnitude and thus a relatively small power ratiig is required. The capacitor C10 has more than just a single half cycle to accomplish this discharge. The same -8i process is repeated during the next negative half cycle but via diode D2, capacitor C20 and resistc- It will be apparent to those skilled in the art that the arrangement of Figs. 4 to 10 can also be applied where filament heating is desired. The circuit of Fig. 11 illustrates this arrangement.

Fig. 12 shows a still further modification in that a small inductor L1 is connected in series with the primary winding Nl. The inductor L1 is preferably air cored or provided with a ferrite core.

The combination of primary winding N1 and inductor L1 can be positioned at any of the locations for N1 illustrated in Figs. 4-9 inclusive.

Alternatively, the inductor Ll and primary winding Nl need not be adjacent to each other but instead separate from each other but still connected in any one of the various locations illustrated for N1. The purpose of the inductor L1 is to provide some energy storage and waveshaping to optimise lamp starting each half cycle.

It will be apparent to those skilled in the art that the above described arrangements overcome a number of problems. In particular, the large wattage dissipation in resistor R2 of the circuits of Figs. 2 and 3 and resistor RB of the circuit of Fig. 1 is overcome whilst the 20 problem of potential misfiring at low dimming intensities is substantially overcome by the preferred two armed circuit of impedance S Z1* Zl. Furthermore, standard slimline ballasts, and consequently slimline *oo.: luminaires, can be used together with standard economical dimmers.

These can operate satisfactorily with very little, if any, extra passive loading per lamp in the form of resistors R3 in Fig. 11 for example.

In addition, the dimming range possible is very wide and for most lamps extends from 0% to 100% of light output. Furthermore, in existing non-pulse dimming installations the dimming can be easily upgraded to pulse dimming making use of the existing ballast and filament heater transformer.

-9- *4 0* In addition, existing pulse dimming installations can be improved by replacing their capacitors and resistors used to produce the pulse with the above described charge dissipating circuit arrangements.

The foregoing describes only some embodiments of the present invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the present invention.

For example, the winding N2 can be inserted into the neutral leg supplying the lamp or even split into two windings located one to either side of the lamp L so that a portion of the high voltage pulse appears I0 on each side of the lamp L. If it is desired to operate lamps in series, the winding N2 can be interposed between the series connected lamps.

A further disclosure of the present invention is the use of the two arm circuit arrangement for impedance Z1 including diodes D1 and D2 in the prior art circuits of Figs. 2 and 3 in place of capacitor C1 thereby giving improved dimming and lower power consumption as a result.

A still further disclosure of the present invention is that the high starting voltage pulse achievable means that it is not necessary to provide a starting aid such as a grounded metal plate adjacent to the

S.

S

S

lamp.

It will also be clear to those skilled in the art how to extend the above described circuits to operate two or more lamps in parallel.

For example, a single dimmer D can be used to operate 2 or more lamps in parallel. To achieve this result, the circuit of Fig 4 is duplicated for each lamp save that only one dimmer D is provided, and the winding N1 of each pulse transformer is connected to the single dimmer D as a phase controlled active. All the neutrals are connected together.

BJG/7788D .04

Claims (14)

1. A gas discharge lamp dimming circuit comprising a gas discharge lamp connected in a first series connection with a current limiting ballast and a thyristor controlled dimmer across an AC mains supply, a a p rO((ie cO(iAec4 4 0A i;- first impedance connected, -n-parzale-l.-ci~-h said lamp and connected in a second series connection with said dimmer across said supply, and a high voltage pulse transformer having a secondary winding connected in said first series connection and a primary winding connected in said second series connection, and wherein said ballast has a second impedance connected in parallel therewith, said second impedance having a low impedance to high voltage high frequency pulses produced by said pulse transformer.

2. A circuit as claimed in claim 1 wherein said lamp, said ballast and said secondary winding are directly connected in series, said first impedance is directly connected in parallel across said direct series connection, and said primary winding and said dimmer are connected in series with said direct parallel connection. S

3. A circuit as claimed in claim 1 wherein said lamp and said o secondary winding are directly connected in series, said first impedance is directly connected in parallel across said direct series connection and said primary winding, ballast and dimmer are connected in series with said direct parallel connection.

4. A circuit as claimed in any one of claims 1 to 3 wherein said gas discharge lamp is provided with filament heaters, and wherein a filament heating transformer having a primary and at least two secondary windings is provided, said filament heating transformer primary winding being connected across said mains supply and each of said filament heaters being connected across a corresponding one of said filament heating transformer secondary windings.

A circuit as claimed in any one of claims 1 to 4 wherein at least 11 BJG/7788D two of said gas discharge lamps are connected together in series in said first series connection.

6. A circuit as claimed in claim 5 wherein said pulse transformer secondary winding comprises two separate windings, said lamps and separate windings being interposed in said first series connection.

7. A circuit as claimed in any one of claims 1 to 4 wherein at least two of said discharge lamps are operated in parallel, each of said lamps being located in a corresponding first series connection.

8. A circuit as claimed in claim 7 wherein a single dimmer is used to operate said parallel connected discharge lamps, said dimmer having one terminal thereof connected to the active side of said AC mains supply, ear... and each of said first series connections branching from the other terminal of said dimmer and terminating at the neutral side of said AC supply.

9. A circuit as claimed in any one of the preceding claims wherein said first impedance is selected from the group consisting of a capacitor, a series connected capacitor and resistor and a two arm circuit arrangement which comprises, for each arm, a series connected diode and capacitor with a bleed resistor being connected in parallel with said capacitor, said arms being connected in parallel with said diodes oppositely poled.

10. A circuit as claimed in any one of the preceding claims wherein said second impedance is selected from the group consisting of a capacitor, a capacitor connected in parallel with a resistor, a capacitor connected in parallel with a series connected resistor and capacitor, and a capacitor connected in4.rri--.with a varistor.

11. A circuit as claimed in any one of the preceding claims wherein said pulse transformer primary winding is positioned at any location in said second series connection.

V c12. A circuit as claimed in claim 11 wherein a series connected Y- 12 0 resistor and capacitor are connected in parallel with said pulse transformer primary winding.

13. A circuit as claimed in claim 11 or 12 wherein an inductor is connected in series with said pulse transformer primary winding.

14. A gas discharge lamp dimming circuit substantially as described herein with reference to any one of Figs. 4 to 12 of the accompanying drawings. DATED this THIRD day of JUNE 1992 Trestoto Pty Ltd eo oooo oe oo o oo e o oo oee o oo *oo *e oo Patent Attorneys for the Applicant SPRUSON FERGUSON -13- HRF/7696W II0 Z