BI-LEVEL BALLAST CIRCUIT FOR OPERATING HID LAMPS

Abstract: A bi-level ballast network is disclosed for operating an HID lamp (16) at either a low power or standby level and at a high power or full light level. The network includes an unswitched capacitor (C1) connected to the transformer ballast coil and a switched capacitor (C2) connected to a solid state relay (SSR) circuit (26), the electronic components providing the switching therein being back-to-back SCR's (23, 25). When the SSR (26) is operated switch contacts (24, 28) connected to the switched capacitor (C2) open or close at the next subsequent zero crossing of the applied ac source voltage. This changes the capacitance of the network to increase the power to the lamp or decrease the power thereto depending on the operating conditions prior to the SSR (26) being operated. Current, voltage and dvdt protection devices (22, 36, 38) are also provided.

Description: THIS INVENTION relates to a ballast circuit for operating an HID lamp and more specifically to such a ballast circuit capable of operating either in a reudced energy standby mode or a full light output mode. High intensity discharge (HID) lamps include mercury vapour lamps, metal halide lamps, and high pressure sodium lamps, each requiring a ballast circuit operating therewith to accommodate to the pre-strike and post-strike conditions of the lamp. Conventionally magnetic transformer ballasts have been employed to provide the voltage and current compensation required. Even the most sophisticated circuits, however, cannot instantaneously cause such a lamp to go from a cold start to a high level operation and, therefore, unlike incandescent lamps or even fairly rapidly starting low pressure discharge lamps (for example, fluorescent lamps), if the operating requirements are such as to make desirable a fast full light output condition, it is required to keep such lamps operating at a dimming output level to begin with. Moreover, a dimming condition is often desirable in any event to provide emergency lighting to the areas serviced by the lamps. Dimming controls suitable for non-HID lamps are notoriously unsuited for HID lamps that must have a continuous voltage and current condition maintained and without prolonged phase reversals, characteristic of many of such devices, since this will cause HID lamps to extinguish. Extinguishment of an HID lamp requires a subsequent long start up time, as discussed above. Many other techniques employed are not efficient in that although providing reduced power to the lamp, there is no reduction of power overall, excess power not directed to the lamp being wasted in heat loss or the like. Nevertheless, various techniques have been employed to provide dimming, perhaps the most successful being the employment of careful removal of some of the applied voltage to a lamp each half cycle without causing lamp extinguishment. A circuit that does this is shown in U.S. Patent 4482844. However, a range of dimming operations such as shown in the circuit of U.S. Patent 4482844 is usually not required, such circuit having a large number of components to accomplish this feat. It is usually satisfactory for a ballast circuit to operate either at a full light output level or at a reduced output light level. Moreover, although all or nearly-all electronic ballasts have been designed and made available in recent years, it is still recognized that constant wattage auto-­ transformer (CWA) ballasts and regulated lag ballasts are still highly favoured for their dependable operation. Heretofore, a relatively simple and efficient bi-level ballast circuit operating with a conventional magnetic transformer ballast has not been available. It is an aim of the present invention to provide an improved bi-level operating ballast circuit for operating with a magnetic transformer ballast to efficiently operate an HID lamp at a reduced energy, standby mode and alternatively to operate such lamp at a high, full light level mode. Accordingly, the present invention provides a bi-­ level ballast circuit for operating an HID lamp alternatively in a reduced energy standby mode and a full light output mode, comprising: a magnetic transformer ballast having an input connection to an ac power source; an unswitched capacitor connected in series with the output of the transformer ballast and with the lamp to ensure at least a reduced power level applied to the lamp; a switched capacitor connected to the unswitched capacitor and the lamp; and control switching means connected to the switched capacitor and connectable to a control power source for increasing the total capacitance and thereby the total power applied to the lamp to operate the lamp at a full light output level, the switching means including closing and opening switch contacts connected to the switched capacitor that operate only at the time the voltage level of the applied ac power source passes through zero. Embodiments of the present invention provide an improved bi-level ballast circuit HID operation including controlled switching that occurs at zero voltage crossings, thereby minimizing disruptive, often harmful results. Preferred circuits are described hereinafter for connection to several different types of HID lamps. In each case, however, the circuit includes a conventional magnetic transformer connected to an unswitched capacitor, which, in turn, is connected to the lamp (or lamp circuit) for operation at a first level. A switched capacitor connected to a control switching means, preferably incorporating a solid state relay (SSR) having back-to-back SCR's, is controllably switched into combination with the unswitched capacitor to provide a second level of power operation for the lamp. The switch-in (or switch-out) occurrences are automatically timed to occur at a zero crossing point of the applied source voltage and, therefore, applies or removes the switched capacitor only when the voltage level is not able to cause excessive spiking or surging by the switched capacitor being partly or fully charged. In order that the invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a simplified schematic diagram of a bi-­ level ballast circuit in accordance with a preferred embodiment of this invention suitable for operating a 150, 250, or 400 watt, high pressure sodium vapour lamp; Figure 2 is a simplified schematic diagram of a bi-­ level ballast circuit in accordance with a preferred embodiment of this invention suitable for operating a 1000 watt high pressure sodium vapour lamp; and Figure 3 is a simplified schematic diagram of a bi-­ level ballast circuit in accordance with a preferred embodiment of this invention suitable for operating a 175-1000 watt metal halide or mercury vapour lamp. Now referring to the drawings and first to Figure 1, a simplified schematic diagram of a first preferred embodiment is shown. A step-up magnetic transformer ballast, normally a constant wattage auto-transformer, has a primary coil 10 connected to an ac source, nominally at 110-120 volts, 60 hertz. A tap 12 therefrom is connected to unswitched capacitor C1, which, in turn, is connected in series with secondary coil 14 of the transformer ballast. The secondary coild is connected to a lamp 16, the return connection to the lamp being to common line 18, which is also connected to primary coil 10. Lamp 16 in this embodiment can be a 150, 250, or 400 watt, high pressure sodium vapour lamp. A starter device 20 is connected across a portion of secondary coil 14 and across lamp 16, in conventional fashion, the tapped portion of the coil providing an operating voltage differential for the starter device. Also connected to tap 12 is a connection to thermistor 22, which, in turn, is connected to a first terminal 24 of a solid state relay (SSR) 26. The second terminal 28 of SSR 26 is connected to switched capacitor C2, which, in turn, is connected to the junction of C1 and secondary coil 14 of the circuit to lamp 16. Solid state relay 26 includes back-to-back SCR's 23 and 25 across terminals 24 and 28 and a control input 30 to input terminals 32 and 34, control input 30 leading to a manual control switch (not shown) to connect the control input to a voltage source, either ac or dc, to cause relay firing circuit 27, included within SSR 26, to gate on or off SCR's 23 and 25. Finally, a metal oxide variator 36 is connected across terminals 24 and 28 of SSR 26 and, in a heavy duty version of the circuit, a choke coil 38 is connected across thermistor 22. Operation of control input 30 to SSR 26 results in the effective closing or opening of the electronic switch in the form of back-to-back SCR's 23 and 25 connected as part of the internal network of the SSR from terminal 24 to 28. However, the switch does not operate instan­ taneously with the operation of the control switch. The voltage level of the AC source applied to coil 10 must pass through zero either in its descending progression or its ascending progression for the switching to occur. When the SSR is operated "closed", a full-wave rectified voltage results, each SCR operating in an alternate half-­ wave rectifying mode. When the SSR is operated "open", the SCR's are both held in their open conditions. As will be explained below, the switching timing is important to the operation of the overall circuit. Now turning to operation of the illustrated circuit, it is first assumed that the internal switch of SSR 26, just described and connected between terminals 24 and 28, is closed, thereby effectively putting C1 and C2 in parallel (except for protection devices 22, 36 and 38). Hence, the capacitance of the parallel combination is at a value higher than the capacitance value of either capacitor alone, and specifically at a value higher than that of capacitor C1. Together with the primary and secondary coils, this parallel capacitance combination enables a higher current to be supplied to lamp 16, thereby causing it to operate at a normal energy consumption level or in its full light mode. Removal of the control input to SSR 26 causes the switch across terminals 24 and 28 at the next zero crossing of the applied voltage to remove capacitor C2 from the circuit. Thus, capacitor C1, together with the primary and secondary ballast coils, but without capacitor C2, now supply lamp 16 with a substantially lower level current to produce reduced light output from lamp 16. The lamp operates in this mode until the control input to SSR 26 is again operated to put the conditions back into the full energy consumption mode once again. Again, switching occurs not instantaneously with operation of the manual control, but at the next zero crossing occurrence of the applied ac source voltage or the voltage applied to the primary coil of the ballast. Operation in this manner prevents capacitor C2 from being switched in or out of the circuit while partly or fully charged and therefore prevents spikes or surges from being applied to lamp 16 and other ballast components. Thermistor 22 resides in series with the SSR to protect it from current surges appearing on the line. Metal oxide varistor 36 resides in parallel with the SSR to protect it from voltage surges. Choke 38 protects the SSR for dvdt, or half-cycle switching, correction. Now referring to Figure 2, a preferred embodiment of the circuit connections for operating a 1000 watt high pressure sodium lamp is shown. Like parts in common with Figure 1 are shown with the same numbers for convenience. However, the bi-level switching components of the circuit are connected differently from Figure 1. Capacitor C2 is in series with C1, instead of parallel. The junction of thermistor 22 and choke 38 is connected to a junction point between capacitors C1 and C2 and terminal 28 of SSR 26 is connected to the junction between capacitor C2 and coil 14. Thus, except for the protection devices, the electronic switch terminals 24 and 28 of the SSR are in parallel with capacitor C2. In operation in the reduced energy consumption mode of this circuit, the electronic switch is operated open to effectively place capacitor C2 in operation. Thus, the coils and capacitor C1 alone determine the operating current applied to lamp 16 when the electronic switch is closed. In this cae, capacitor C1 is sized to provide the full energy consumption current operation. When terminals 24 and 28 are effectively switched open at a zero crossing of the applied ac source voltage, as described above, then capacitor C2 is put in series with capacitor C1, thereby reducing the total capacitance to that required to provide the standby or reduced energy consumption mode. Thermistor 22, varistor 36, and choke 38 provide the protection described above. When switched capacitor C2 is in series with unswitched capacitor C1, as is the case with the Figure 2 circuit, choke 38 is in series with the SSR to provide dvdt correction. Figure 3 is a circuit showing the bi-level ballast network embodying the invention connected in a preferred manner for operating a 175-1000 watt metal halide or mercury vapour lamp. The differences between the Figure 2 and Figure 3 circuits pertain to the absence of coil 14 and starter device 20 from the Figure 3 circuit and the addition of coil 40. Coil 40 precedes capacitor C1. This coil provides voltage step-up for required operation (instead of coil 14 shown in Figures 1 and 2). Otherwise the two are the same. With respect to the inventive network components, there are no differences. While several preferred embodiments of the invention have been shown, it will be understood that the invention is not limited thereto. Many modifications may be made, which will become apparent to those skilled in the art. For example, the operating control connected to SSR 26 has been described as a manual control. Alternatively, it can readily be automated, such as by a time clock or other device, if desired. In addition, HID lamp wattages other than those specifically described may be operated in an energy saving mode employing the techniques herein described. The features disclosed in the foregoing description, in the claims and/or in the accompanying drawings may, both separately and in any combination thereof, be material for realising the invention in diverse forms thereof.

Claims: 1. A bi-level ballast circuit for operating an HID lamp (16) alternatively in a reduced energy standby mode and a full light output mode, comprising: a magnetic transformer ballast (10, 14, or 10, 40) having an input connection to an ac power source; an unswitched capacitor (C1) connected in series with the output of the trans­ former ballast (10, 14 or 10, 40) and with the lamp (16) to ensure at least a reduced power level applied to the lamp; a switched capacitor (C2) connected to the unswitched capacitor and the lamp; and control switching means (26) connected to the switched capacitor (C2) and connectable (30) to a control power source for increasing the total capacitance and thereby the total power applied to the lamp to operate the lamp at a full light output level, the switching means (26) including closing and opening switch contacts (24, 28) connected to the switched capacitor that operate only at the time the voltage level of the applied ac power source passes through zero. 2. A bi-level ballast circuit in accordance with claim 1, wherein the switched capacitor (C2) is connected in series with the unswitched capacitor (C1) and the switch contacts (24, 28) of the switching means (26) are connected in parallel with said switched capacitor, the contacts being operated closed for increasing the total capacitance to the lamp. 3. A bi-level ballast circuit in accordance with claim 1, wherein the switched capacitor (C2) is connected in parallel with the unswitched capacitor (C1) and the switch contacts (24, 28) of the switching means (26) are connected in series with the switched capacitor (C2), the contacts being operated closed for increasing the total capacitance to the lamp. 4. A bi-level ballast circuit in accordance with any one of claims 1 to 3, wherein the control switching means (26) includes two back-to-back SCR's in a solid state relay device. 5. A bi-level ballast circuit in accordance with any preceding claim, wherein a thermistor (22) is connected in series with the switch contacts (24, 28) of the switching means (26) for current surge protection. 6. A bi-level ballast circuit in accordance with any preceding claim, wherein a varistor (36) is connected in parallel with the switch contacts (24, 28) of the switching means (26) for voltage surge protection. 7. A bi-level ballast circuit in accordance with any preceding claim, wherein a choke (38) is connected in series with the switch contacts (24, 28) of the switching means (26) for dvdt correction. Amended claims in accordance with Rule 86(2) EPC. 4. A bi-level ballast circuit in accordance with any one of claims 1 to 3, wherein the control switching means (26) includes two back-to-back SCR's in a solid state relay device. 5. A bi-level ballast circuit in accordance with any preceding claim, wherein a thermistor (22) is connected in series with the switch contacts (24, 28) of the switching means (26) for current surge protection. 6. A bi-level ballast circuit in accordance with any preceding claim, wherein a varistor (36) is connected in parallel with the switch contacts (24, 28) of the switching means (26) for voltage surge protection. 7. A bi-level ballast circuit in accordance with any preceding claim, wherein a choke (38) is connected in series with the switch contacts (24, 28) of the switching means (26) for dvdt correction. 8. A bi-level ballast circuit in accordance with any preceding claim, wherein said control power source includes a manual control switch.

Inventors: CARL, MICHAEL J.
Applicants: WIDE-LITE INTERNATIONAL CORPORATION
Application year: 1989
Main classification: H05B41
IPC codes: H05B41