Abstract:
A lamp ignition circuit provides a high voltage electrical pulse to ignite a gas discharge lamp. A non-linear filter element within a charge circuit regulates the voltage of the ignition pulse such that the ignition pulse remains within a prescribed voltage range over a wide variety of conduit lengths between the lamp and the lamp ignition circuit. This allows for the ballast and lamp ignition circuit to be mounted either close to the lamp or far from the lamp without modifying the ignition circuitry.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   None. 
   BACKGROUND OF THE INVENTION 
   This invention relates generally to ballasts used to power gas discharge lamps. More particularly, this invention pertains to circuits used in conjunction with a magnetic ballast to ignite a gas discharge lamp. 
   Gas discharge lamps require a high voltage pulse of electricity for ignition. The design of the lamp determines the voltage requirements for the ignition pulse, and there is typically a minimum and maximum voltage requirement for the ignition pulse. After a gas discharge lamp is ignited, the lamp presents a negative resistance. Therefore, a ballast is used to control and limit the amount of current going to the lamp after ignition. In many commercial lighting environments, the ballast and ignition circuit (sometimes referred to as a “starter” circuit) are connected to the lamp using electrical wires placed in a conduit. This arrangement creates a parasitic capacitance which increases with increased conduit length. The larger the parasitic capacitance, the greater the load affecting the amplitude of an ignition pulse from a lamp starter circuit. The conduit length actually installed in the field is variable, so the amount of parasitic capacitance associated with the conduit is variable. A starter circuit which can simply and reliably provide ignition pulses having a voltage within the prescribed range over a wide variety of conduit lengths is desirable. 
   Many circuits have been developed to deliver ignition pulses to lamps over varying starter-circuit-to-lamp conduit lengths. For example, U.S. Pat. No. 6,522,088 describes a starter circuit having a voltage clamping device connected between the two leads to the lamp. The ballast circuitry is capable of generating an ignition pulse having a voltage in excess of the prescribed range for the lamp. Due to the higher voltage of the ignition pulse, a longer conduit length between the lamp and the ballast circuitry is possible. If the longer length is used, the parasitic capacitance reduces the voltage of the ignition pulse to within the prescribed range. The voltage clamping device has an impedance which varies with voltage such that if the voltage exceeds the clamping voltage, the impedance drops and thereby lowers the voltage of the ignition pulse delivered to the lamp. The voltage clamping device is typically comprised of two varistors connected in series wherein the combined clamping voltage of the two varistors is near the maximum voltage acceptable for the lamp. Unfortunately, using a clamping device in the starting circuit adds cost which is disadvantageous in the highly competitive lighting industry. Also, the clamping device may be required to dissipate significant energy when clamping high voltage ignition pulses. This decreases reliability of the device. 
   Publication No. JP2005251722 describes a device having a second starting device positioned close to the lamp when the conduit length between the first starting device and the lamp is long. When the conduit between the first starting device and the lamp is short, a second starting device is not used. This provides for a wider range of acceptable conduit lengths between the first starter device and the lamp. 
   U.S. Pat. No. 6,396,220 describes circuitry with a first and a second reactive energy source. The first reactive energy source generates ignition pulses for longer conduit lengths, and the second reactive energy source generates ignition pulses for shorter conduit lengths. A switch is provided so that either the first or the second reactive energy source is utilized. There are several embodiments wherein different components of the ignition circuitry are switched on and off, but in all embodiments a switch is used to select between components which generate ignition pulses having different voltages. 
   In the highly competitive field of lighting electronics cost and reliability are important considerations. Costs can be reduced by using fewer components and/or using components designed for lower voltages. Also, as a general rule, the fewer components used, the more reliable the system. Therefore, a system using fewer components and/or components designed for lower voltage is preferred. 
   BRIEF SUMMARY OF THE INVENTION 
   The lamp ignition circuit of the present invention includes an ignition pulse source, wherein the ignition pulse source includes a ballast and a charge circuit. An ignition pulse is directed through a conduit to a lamp, and also through the charge circuit back to a power source. High impedance in the charge circuit maximizes the ignition pulse to the lamp, and reduced impedance in the charge circuit lowers the ignition pulse voltage at the lamp. There is a non-linear filter element in the charge circuit wherein the impedance of the non linear filter element varies with both frequency and voltage. The impedance of the non-linear filter element increases with higher frequencies, and the impedance decreases with higher voltages once a clamping voltage has been exceeded, regardless of the frequency. The ignition pulse voltage at the lamp is maintained within a prescribed range by a lowering of the non-linear filter element impedance when the conduit length is short, such that part of the ignition pulse is diverted through the charge circuit. When the conduit length is long, the impedance of the non-linear filter element remains high, so the ignition pulse voltage at the lamp is maximized. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a schematic of one embodiment of a lamp circuit in accordance with the present invention, shown in combination with a ballast and gas discharge lamp. 
       FIG. 2  is a schematic of a preferred embodiment of the lamp circuit of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The lamp circuit  10  shown in  FIG. 1  includes a gas discharge lamp  12 , such as a commonly used high intensity discharge (HID) lamp. Gas discharge lamps require a high voltage pulse for ignition. Typically this high voltage pulse, also herein referred to as an ignition pulse, has a permissible, prescribed range specific to each type of lamp. There will be a minimum voltage as necessary to ignite the lamp, and a maximum voltage rating that prevents the lamp from being damaged by the ignition pulses. As an example, a typical metal halide lamp may have a prescribed voltage range with a minimum required voltage of 3000 volts and a maximum permissible voltage of 4000 volts. 
   Another characteristic of the lamp  12  is that the ignition pulse necessary for igniting the lamp has a much higher peak voltage than the voltage used for operating the lamp  12  after ignition. The lamp  12  is connected to the lamp circuit  10  by a pair of wires typically enclosed in a conduit, which is also herein referred to as a line  14  or a conduit  14 . In some applications, the wire  14  can be enclosed within a protective conduit, but the term conduit  14  as used herein refers to the wires delivering operating power to the lamp, regardless of whether the wires are enclosed in a protective housing or not. The conduit  14  has a conduit length  16  measured or defined between the lamp circuit  10  and the lamp itself  12 . The conduit length  16  used by the end user varies, and can be long, short, or intermediate in length. As is well known in the art, the conduit  14  introduces a parasitic capacitance which increases as the conduit length  16  increases. Therefore, as the conduit length  16  increases, an ignition pulse voltage correspondingly decreases because the pulse is affected by the relatively low impedance of the parasitic capacitance. 
   The lamp circuit  10  includes first and second output terminals  18  and  20  respectively. The AC power source  28  is connected to terminal  20  through line  26 . The ballast  22  has an input line  30  connected to AC power source  28  and an output line  24  connected to terminal  18 . The ballast  22  can be a reactor ballast, a transformer ballast, an autotransformer ballast, or any other type of ballast functional to power a gas discharge lamp. 
   The lamp circuit  10  further includes a charge circuit  32  connected to the ballast  22  and to lines  24  and  26  at nodes C and A respectively. The charge circuit  32  includes a non-linear filtering element  34 , a resistor  36 , and a capacitor  38 . The non-linear filtering element  34  is connected between node A and resistor  36 . 
   One embodiment of the non-linear filtering element  34  is shown in  FIG. 2  and includes an inductor  40  connected in parallel with a voltage-clamping device  42 . 
   The resistor  36  is connected in series with the non-linear filtering element  34  and, at node B, with the combination of capacitor  38  and a bilateral voltage triggered switch  48 . The capacitor  38  has a first terminal  44  connected to node B and a second terminal  46  connected to node C. A first terminal of switch  48  is connected to node B, and a second terminal of switch  48  connected to an intermediate point  50  on the inductive element of ballast  22 . 
   The impedance of the non-linear filtering element  34  varies in a non-linear fashion, and depends on both pulse frequency and peak voltage, such that the impedance of the charge circuit  32  also varies in a non-linear fashion. The impedance of the non-linear filtering element  34  is high at the ignition pulse frequencies, but also decreases with increased peak voltage. This decrease in impedance with increased voltage does not occur until after a specified threshold voltage has been exceeded. The decrease in impedance with increased voltage occurs regardless of the frequency. 
   A SIDAC (Silicon Diode for Alternating Current) can be used as the bilateral voltage triggered switch  48 . A SIDAC, bi-directional thyristor breakover diode, or more simply a bi-directional thyristor diode, is technically specified as a bilateral voltage triggered switch. A SIDAC remains non-conducting until the applied voltage meets or exceeds its rated breakover voltage. Once entering this conductive state, the SIDAC continues to conduct, regardless of voltage, until the applied current falls below its rated holding current. At this point, the SIDAC returns to its initial non-conductive state to begin the cycle once again. 
   Referring to the preferred embodiment shown in  FIG. 2 , one manner of constructing the non-linear filtering element  34  is to connect an inductor  40  and a voltage-clamping device in parallel. In a preferred embodiment, a single varistor  42  may be used as the voltage clamping device. The impedance of inductor  40  increases when the frequency of the current increases. Therefore, the inductor  40  presents a low impedance to current from the AC power source  28  and high impedance to the short ignition pulses rich with high frequency content. The varistor  42  has a clamping voltage, and acts effectively as an open circuit where the peak voltage across the non-linear filtering element  34  is less than the clamping voltage. The impedance of the non-linear filtering element  34  at this point is thus equal to the inductor impedance, and remains very high until the clamping voltage is reached. Once the clamping voltage is reached, the impedance of the varistor  42  drops. Because the inductor  40  and the varistor  42  are connected in parallel, once the clamping voltage has been reached, the impedance of the non-linear filtering element  34  decreases, regardless of the frequency. 
   The lamp circuit  10  generates ignition pulses until the lamp  12  is ignited. The ignition pulses are generated by an ignition circuit  52  which is a functional combination of ballast  22 , the charge circuit  32 , and switch  48 . It is within the knowledge of persons of ordinary skill in the art to select components for ignition circuit  52  to be capable of producing ignition pulses at a voltage exceeding the minimum voltage of the prescribed range for the lamp  12 . The non-linear filtering element  34  in the charge circuit  32  prevents the ignition pulse voltage from exceeding the maximum prescribed value for the lamp  12 . 
   The energy for the ignition pulses is provided by AC power source  28 . The power source  28  is generally a 60 Hz AC commercial power source. The 60 Hz frequency is low enough for the impedance of the inductor  40  in the non-linear filtering element  34  to remain low, which allows the 60 Hz current to easily pass through the non-linear filtering element  34 . The 60 Hz current charges the capacitor  38  through resistor  36 . 
   The ignition pulse is triggered by the switch  48 . The bilateral voltage-triggered switch  48  remains open until a breakover voltage is reached. Once a voltage exceeding the breakover threshold is present, the switch  48  closes and effectively becomes a short circuit. The switch  48  remains closed until the current drops below a pre-determined value. When the power source  28  begins charging the capacitor  38 , the voltage at the switch  48  is below the breakover threshold and the switch  48  remains open. As the capacitor  38  is charged, the voltage at the switch  48  builds until the voltage exceeds the breakover threshold and the switch  48  closes. The capacitor  38  then discharges through the switch  48  and ballast  22 . As this discharge current pulse passes through a segment or portion of the inductor in ballast  22 , the voltage is stepped-up to a high voltage, short ignition pulse to be sent to the lamp  12 . 
   The magnitude of the ignition pulse voltage at the lamp  12  depends on the effective loading on the lamp ignition circuit provided by the lamp  12 , the conduit  16  and the charging circuit  32 . If the conduit length  16  is long, the parasitic capacitance is high and the lamp conduit  14  presents a lower impedance load for the ignition pulse circuit  52 . This can result in a lower ignition voltage at the lamp  12 . 
   The resistor  36  and the non-linear filtering element  34  primarily determine the effective impedance of the charge circuit  32  that is presented to the ignition pulse circuit  52 . The clamping voltage of the non-linear filtering element  34  is selected such that its impedance for the ignition pulse is high when the conduit length  16  is long, and so that its impedance for the ignition pulse is lower when the conduit length  16  is short. The impedance of the inductor  40  in the non-linear filtering element  34  is high for short ignition pulses rich with high frequency content. Therefore, when the conduit length  16  is long, the inductor  40  and the varistor  42  both have high impedance, which presents a lower effective load on the ignition pulse circuit  52 . The impedance from the parasitic capacitance from the long conduit length  16  combined with the large impedance from the non-linear filter element  34  produces an ignition pulse voltage within the prescribed range for the lamp  12 . 
   If the conduit length  16  is short, the parasitic capacitance of the conduit  14  is small, so the impedance of the conduit  14  is relatively high. This high impedance results in a relatively low load for the ignition pulse circuit  52 . Because the clamping voltage is exceeded, the impedance of the varistor  42  in the non-linear filtering element  34  drops. The reduced impedance from the non-linear filtering element  34  produces a larger load for the ignition pulse. This serves to reduce the voltage of the ignition pulse at the lamp  12  to a voltage below the maximum. The high impedance and low parasitic capacitance from the relatively short conduit length  16  indirectly is responsible for a lower impedance in the non-linear filtering element  34  and the charge circuit  32 , so the total load for the ignition pulse circuit  52  is somewhat balanced for both long and short conduit lengths  16 . Therefore, the non-linear filtering element  34  prevents the ignition pulse voltage at the lamp  12  from exceeding the prescribed range by lowering the non-linear filtering element  34  impedance when the conduit length  16  is short. Therefore, the ignition pulse circuit  52  of the lamp circuit  10  provides ignition pulses to the lamp  12  within the prescribed range over a wide variety of conduit lengths  16 . 
   Placing the non-linear filtering element  34  in the charge circuit  32  allows for a lower cost lamp circuit  10  comparing to the circuit described in U.S. Pat. No. 6,522,088. The voltage seen by the non-linear filtering element  34  during ignition pulse is lower than the ignition pulse voltage itself. Therefore, the clamping voltage of the varistor  42  in the non-linear filtering element  34  can be lower than if the varistor  42  were exposed to the whole voltage of the ignition pulse as it is in U.S. Pat. No. 6,522,088. Therefore, the clamping voltage of the varistor  42  is less than the maximum value of the prescribed voltage range of the lamp. The lower clamping voltage allows for the economical use of a single varistor  42  as the voltage clamping device. Using a single varistor  42  with a lower clamping voltage reduces the overall cost of the lamp circuit  10  comparing to that of the circuit described in U.S. Pat. No. 6,522,088 where two varistors are needed in practical application. 
   If the lamp  12  ignites, the lamp  12  presents a very low impedance. The voltage between nodes C and A ( FIG. 2 ) drops then to a level lower than the break-over voltage of the bilateral voltage triggered switch  48 . As a result, no more ignition pulses are generated as long as the lamp  12  remains lit. If the lamp  12  fails to ignite, the lamp  12  acts as an open circuit and the charge circuit  32  repeatedly generates ignition pulses until the lamp  12  ignites. 
   The present invention also includes a method of igniting a gas discharge lamp  12  over a variable conduit length  16 . The method includes providing a lamp circuit  10  which is connected to a power source  28 . An ignition pulse circuit  52  within the lamp circuit  10  generates a high voltage ignition pulse. Ignition pulses are repeatedly generated until the lamp ignites. A non-linear filtering element  34  clamps the voltage of the high voltage pulse below an allowed maximum voltage for the lamp  12 . The non-linear filtering element  34  has an impedance that varies in a non-linear manner. The non-linear filtering element  34 , and therefore the charge circuit  32 , has an impedance which increases with increased frequency, and the impedance decreases when a clamping voltage is exceeded regardless of the frequency. The non-linear filtering element  34  could be comprised of, but is not limited to, an inductor  40  and a varistor  42  connected in parallel. 
   Thus, although there have been described particular embodiments of the present invention of a new and useful Lamp Circuit with Controlled Ignition Pulse Voltages over a Wide Range of Ballast-to-Lamp Distances, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.