NZ330486A - Dimmer circuit for discharge lamp with current bypass during dimmer switching action - Google Patents

Description

New Zealand No. 330486 International No. PCT/ TO BE ENTERED AFTER ACCEPTANCE AND PUBLICATION Priority dates: 21.05.1997; Complete Specification Filed: 21.05.1998 Classification:^) H05B41/40.16 Publication date: 24 September 1998 Journal No.: 1432 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention: Dimmer circuits for discharge lamps Name, address and nationality of applicant(s) as in international application form: TRESTOTO PTY LIMITED, an Australian company of No. 12 Gantry Place, Braemar, New South Wales 2575, Australia No: Date: NEW ZEALAND Patents Act 1953 COMPLETE SPECIFICATION DIMMER CIRCUITS FOR DISCHARGE LAMPS We, TRESTOTO PTY LIMITED an Australian company of No. 12 Gantry Place, Braemar, New South Wales 2575, Australia do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement -1- (followed by la) DIMMER CIRCUITS FOR DISCHARGE LAMPS This invention relates to dimmer circuits for discharge lamps. The dimmer circuits are particularly suited to High Intensity Discharge (HID) lamps.

HID lamps include metal halide, high pressure sodium and mercury vapour types. HID lamps are widely used in industrial and outdoor applications. It is common to require a variable intensity from such lamps, commonly known as "dimming". Dimming of a HID lamp is achieved by reducing the power consumed by the lamp. This can be achieved by reduction of the supply voltage and/or current through the lamp.

A characteristic feature of HID lamps is that even on a momentary interruption of the lamp current the lamp will extinguish. When a lamp has been operating for some time its internal gas temperature will be such that, if it extinguishes, the arc can only be restruck on application of an extremely high voltage, typically in the range of tens of thousands of volts. In general, to re-ignite an HID lamp after such an outage requires the lamp to cool for up to 30 minutes.

Known dimmer circuits include switching devices diat tend to introduce an interruptions in lamp current, and thus are generally unsatisfactory in operation.

It is an objective of the present invention to provide dimmer circuits for HID lamps that overcomes or at least ameliorate this problem.

Accordingly, the invention broadly discloses a dimmer circuit for one or more HID lamp having switching means to control the power consumed by the lamp(s) and shunt circuit means to provide a continuous current path on any activation of the switching means.

In a particular embodiment the dimmer circuit comprises an inductive circuit element having two or more taps to vary the power consumed by said lamp, switching means to switch between said taps and shunt circuit means to provide a continuous current path for the full period of any activation of the switching means.

The shunt circuit means can comprise a capacitive element in parallel with a resistive element, both connected across the DC terminals of a bridge rectifier circuit whereby the capacitive element acts as an initial short-circuit path and subsequently a relatively low impedance path for the current flow and the resistive element discharges the capacitive element following completion of activation of the switching means. There also can be a respective said shunt circuit for each said tap.

The invention further discloses a lighting system having a dimming capability, said lighting system comprising a discharge lamp circuit and a dimmer circuit as disclosed above.

Embodiments of the invention have the advantage of being a simple circuit construction and therefore having a low manufacturing unit cost.

Embodiments of the invention now will be described with reference to the accompanying drawings in which: Fig. 1 shows a first dimmer circuit arrangement; Fig. 2 shows a second dimmer circuit arrangement; Fig. 3 shows a third dimmer circuit arrangement; Fig. 4 shows a further dimmer circuit arrangement that is similar to Fig. 1; Fig. 5 shows yet another dimmer circuit arrangement; Fig. 6 shows an alternative shunt circuit configuration; and Figs. 7 A and 7B illustrate current waveforms during switching.

Referring to Fig. 1, there is shown a dimmer circuit 10, constituted between input AC supply terminals A-N and output terminals M-N, having connection with a discharge lamp circuit(s) 15. The discharge lamp circuit 15 typically will comprise a ballast and HID lamp (not shown). The input supply A-N is applied to the primary winding of an auto-transformer 20 having a full supply voltage tap X and reduced voltage tap Y on the secondary side. For example, the input supply may be 240 Vrms and the Y tap voltage be 220 Vmis. The two contacts of a relay-activated switch Si provide alternate connection between points X and Z, or Y and Z.

A first shunt circuit 22 is provided across circuit points X and Z. In a similar manner a second shunt circuit 24 is provided across circuit points Y and Z. Each shunt circuit comprises a full-wave rectifier bridge including diodes DpD4 having a parallel ■> c. ■ 3 ■ 4S \) ^ ' : "■ I o RC network (Rj, Cj, and R2, C2 respectively) between tlie DC terminals of the bridge. The rectifier bridge acts to isolate the RC network from the load once switching of the power has been completed.

For the switched position of contact switch Sj shown in Fig. 1 where Z connects to X, the first shunt circuit 22 is short-circuited, hence Cj is in an uncharged state and the full power supply voltages applied to the lamp discharge circuits) 15. When it is desired to dim the discharge lamp(s) by applying the reduced supply voltage present at tap Y, the switch contact S] is required to make the alternative connection between Z and Y. Immediately the contact Sj breaks from the terminal of circuit point X, the capacitor Cj is initially a short-circuit as it is uncharged allowing a shunt conduction path via the respective limbs of the diode bridge. Once the contact Sj 'makes' with the terminal associated with circuit point Y the conduction path resumes between points Y and Z and so to the discharge lamp(s) 15. In this manner, continuous current can be provided to the discharge lamp circuit(s) 15 whilst switching between two supply voltage levels. An additional effect of this shunt path is that arcing between the contact Sj on a respective terminal is reduced. Arcing otherwise causes a reduction in the lifetime of the switch. Examples of the continuous current flow are seen in Figs. 7A and 7B which relate to switching from low to high power, and from high to lower power respectively. As seen the transition is continuous with no break in the AC current being supplied to the lamp.

Once the switching operation is completed, capacitor Cj has the potential difference X-Z appearing across it, and reaches a steady state voltage level substantially equal to the peak voltage between X-Z. Any charge appearing on capacitor C2 before the switching operation was completed is dissipated through resistor R2 by virtue of circuit points Y and Z being short-circuited. In that case, the second shunt circuit 24 is in a condition to provide a shunt path by virtue of capacitor C2 acting as an immediate short-circuit on the next switching event.

In the example of a single HID lamp in a typical installation operating with the supply voltages noted above, the resistors Rj and R2 may be chosen to have a value in %-7 fe " ^ \ n v.' V*' ;the KQ to MQ, and the capacitors Cj and C2 may have values in the range of 10's ^F. Those skilled in the art will appreciate that if other supply voltages and/or loads are used, component values may also vary. ;Fig. 1 further shows in a dashed representation, two additional capacitors C3 and C4 respectively connected in parallel with the shunt circuits 22,24. These circuit elements may be provided if desired as a form of quenching current path during the time immediately flowing breaking of contact Sj from the terminal of circuit point X until the diodes in the bridge become forward biased. This will further reduce the incidence of arcing between the terminal and the switch Sj. The quenching capacitors preferably should be quite small and may be less than one hundredth the capacitance value of the bridge capacitors Cj and C2. ;Fig. 2 shows a dimmer circuit 11 providing for four-level voltage reduction dimming. Circuit elements common with those shown in Figure 1 have like reference numerals. The shunt circuits in particular are shown in an abbreviated form, in this case 22,24,26 and 28. The auto-transformer 20' has three taps Y1-Y3 to provide for four voltage levels that can be applied between the output terminals M-N. Each tap point YJ-Y3 has an associated relay activated switch with contacts S1-S3 and is associated with a respective second, tiiird and fourth shunt circuit 24-28. Operation of the switch contacts Si-S3 can be manual or sequenced (by such as a programmable logic controller) to achieve the desired output voltage level at circuit point Z. Each switching operation proceeds in the same manner as described with reference to Fig. 1. ;Fig. 3 shows a further dimmer circuit arrangement 12 that operates to reduce the power consumed by varying the in-line impedance of a circuit element associated with a HID lamp 16. In particular, the conventional inductive ballast provided to regulate current flow through the HID lamp is provided with a tap B so that a different impedance is seen depending upon whether the switch contact Sl is in contact with the terminals of circuit points X or Y. Operation of the first and second shunt circuits 22,24 and the switching contact Sj proceeds as described for Fig. 1. If the active supply is taken from tap B a reduced impedance is present and the available ;■z" n H o w- ;- 5 • J ^ ;current, and thus power that can be consumed by the HID lamp 16, increases as therefore does the luminous output. That is, the switch contact Sj in position X relates to nominal operation, and in position Y relates to dimmed operation. A conventional power factor correcting capacitor 32 also is shown. ;Fig. 4 shows a dimmer circuit arrangement 13 that is very similar to that shown in Fig. 1. The circuit 13 is suited to providing a dimming operation for one or more discharge lamp circuits 15 of the type having a leading power factor by virtue of the series capacitor component therein. The auto-transformer 20 is configured to have an air gap so that it draws a predetermined excitation current and provides for power factor correction of the leading current due to the discharge lamp circuits 15. ;Fig. 5 shows a yet further dimmer circuit arrangement 14, in this case again similar to the circuit of Fig. 1 and providing for a parallel arrangement of independent dimming of separate discharge lamp circuits 15,15' again in the manner described with reference to Fig. 1, utilising a single tap auto-transformer 20. Such a circuit arrangement also can be extended to provide for more than two dimming levels as shown with reference to Fig. 2. ;Fig. 6 shows an alternative configuration of a shunt circuit 36 which may be substituted for the shunt circuits 22,24,26 and 28 used in the embodiments described above. The circuit 36 operates in essentially the same fashion as the circuit 24 with substantially the same effect. As before resistors R1 and Rl' ensure the corresponding capacitor CI and CI1 are discharged when no potential difference exists between X (or Y) and Z. When switching commences CI and CI1 are initially short circuits that permit current to flow at least until the switch again becomes closed. ;Dimmer circuits embodying the invention also can be beneficially used for other forms of lamp, including high pressure sodium types, fluorescent and incandescent lamps. ;Numerous alterations and modifications can be made without departing from the basic inventive concept. For example, a varistor can be used in association with the diode bridge to limit the voltage appearing across the capacitor. Furthermore, a ;35048* combination of voltage reduction and impedance changing can be included in the one dimmer circuit, such as may be achieved by combining the embodiments shown in Figs. 1 and 3.

Yet further, the auto-transformer 20 may be configured to boost or reduce the nominal supply voltage to match the optimum lamp operational requirements. The auto-trans former, at the same time, remains in control of the dimming operation as previously described.

Other embodiments may provide that the relay activated switch S] be replaced by solid state switching devices such as thyristors, transistors, IGBTs, GTOs, etc. to perform the switching function. In the case of Fig. 1, for example, the single switch S] would be replaced by two solid state switching devices, one between X and Z, and the other between Y and Z.

Yet other embodiments may include series inductance between Z and M of, say Fig. 1, to suppress peak currents during switching, in particular when the discharge lamp circuit(s) include shunt power factor correction capacitor(s). Further embodiments may include capacitors and inductors to suppress harmonics. -1= ?30 4

Claims (16)

WHAT WE CLAIM IS: O ^

1. A dimmer circuit for one or more lamps, said circuit including switching means configured to control the power consumed by the lamp(s), and shunt circuit means arranged to provide a continuous current path on any activation of said switching means.

2. A circuit according to claim 1 wherein said shunt circuit means comprises a plurality of circuit networks, at least one of said networks being associated with each one of a corresponding plurality of power levels selectable by said switching means.

3. A circuit according to claim I or 2 further comprising an inductive circuit element having two or more taps arranged to the power consumed the Iamp(s), said switching means being configured to switch between said taps, said shunt circuit means providing a continuous current path to said lamp(s) for the full period of any activation of said switching means.

4. A circuit according to claim 1, 2 or 3 wherein said shunt circuit means includes at least one network comprising at least one parallel resistor-capacitor combination and isolation means for isolating said combination from said lamp(s) when said switching means is in a steady state position, said isolation means creating said continuous current path during activation of said switching means.

5. A circuit according to claim 4 wherein said network comprises a capacitive element connected in parallel with a resistive element to at least one terminal of said isolation means.

6. A circuit according to claim 5 wherein the capacitive element acts as an initial short circuit path and subsequently a relatively low impedance path for current flow to said lamp(s) during activation of said switching means, and said resistive element acts to discharge said capacitive element following completion of activation of said switching means at which time said isolation means isolates said combination from said lamp(s).

7. A circuit according to claim 5 or 6 wherein said isolation means comprises a full wave bridge rectifier circuit and said combination is connected across the DC terminals thereof.

8. A circuit according to claim 5 or 6 wherein said network includes a first sub-network connected in parallel to a second sub-network, each said sub-network including a parallel connected resistor-capacitor combination arranged in series with a diode forming part of said isolation means, said diodes being oppositely biased so that said network conducts current for substantially an entire cycle.

9. A circuit according to any one of claims 4 to 8 further including at least one capacitive element connected in parallel with each said shunt circuit means and configured to contribute to said continuous current path during a change in conduction state of said isolation means.

10. A circuit according to any one of the preceding claims wherein said lamp(s) comprise high intensity discharge lamp(s).

11. A circuit according to any one of claims 1 to 9 wherein said lamp(s) is selected from the group consisting of high pressure sodium lamps, fluorescent lamps and incandescent lamps.

12. A dimmer circuit comprising: an auto-transformer having connections for coupling to an AC mains supply and a plurality of output taps defining different power levels of AC output for supplying a discharge lamp system; switching means for selectively coupling one of said taps to said discharge lamp system; and shunt circuit means associated with each of said taps, said shunt circuit means being isolated from a conduction path to said discharge lamp circuit when said switching means connects to the corresponding one of said taps, and forming a conduction path to said discharge lamp system during an activation of said switching means when disconnecting from said corresponding one tap and prior to connection with another one of said taps. . 9 . — ^ L (j 10 po X.:-- "^

13. A circuit according to any one of the preceding claims wherein said switching means comprises at least one switching element associated with each selectable power level.

14. A dimmer circuit substantially as described herein with reference to any one of Figs. 1 to 5 of the drawings.

15. A dimmer circuit substantially as described herein with reference to any one of Figs. 1 to 5, and Fig. 6 of the drawings.

16. A discharge lamp system incorporating a dimmer circuit according to claim 14 or 15. TRESTOTO PTY LIMITED by its Attorney DON HOEONS & ASSOCIATES