Abstract:
A method for using an electronic ballast circuit configured to operate a high intensity discharge (HID) lamp. Multiple light emitting diodes (LEDs) are attached to the current output of the electronic ballast circuit, and current is driven from the current output to light said LEDs. Optionally, prior to driving current through the LEDs, the impedance of the current output is sensed; and the current is driven through the LEDs to light the LEDs upon detection of an impedance significantly lower than an impedance characteristic of the HID lamp. Ignition appropriate to ignite the high intensity discharge is not performed when LEDS are attached to the current output. Alternatively, a signal is provided to disconnect the LEDs during a high voltage output for ignition of the high intensity discharge (HID) lamp.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a ballast used to drive high intensity discharge lamps and, more particularly to the same HID ballast being used for a bank of interconnected light emitting diodes (LEDs). 
     2. Description of Related Art 
     A high-intensity discharge (HID) lamp produces light by means of an electric arc between tungsten electrodes housed inside a fused quartz or fused alumina arc tube. The tube is filled with both gas and metal salts. The gas facilitates an initial strike or ignition of the arc. Once the arc is started, the arc heats and evaporates the metal salts forming a plasma, which greatly increases the intensity of light produced by the arc and reduces its power consumption. In typically 1 to 2 minutes, a low powered 70 W HID lamp warms up to produce its rated light output. When the HID lamp is initially cool, an ignition voltage of 4000 volts for instance is typically required to ignite the HID lamp. A re-ignition for the same lamp when the lamp is still hot, may require up to 20,000 volts for re-ignition to occur. The re-ignition when the lamp is still hot may also require a different frequency or phase characteristic for the ignition voltage to avoid risk of blowing up the HID lamp. Ballasts and lamps with hot re-strike capability are much more expensive then ballasts and lamps without hot re-strike capability. 
     After ignition, the HID ballast provides alternating current to the lamp at low voltage, e.g. 20-100 Volts. The physical properties of the HID lamp typically determine the operating voltage across the HID lamp. 
     There are two types of HID ballasts, generally termed “low” and “high” frequency ballasts. The “low frequency ballast” includes a rectifier circuit which rectifies the alternating current of the power line to direct current. The direct current is input to a circuit that performs “power factor correction” (PFC). “Power factor” is a figure of merit indicating to what extent the current and the voltage are in phase. The PFC circuit is followed by a “buck converter” providing a current source and performing a DC-DC step down conversion. The “buck converter” is followed by a full-wave bridge operating as an “inverter” outputting a low frequency, e.g. 160 Hz. square wave as input to the discharge lamp. 
     The “high frequency ballast” includes a rectifier circuit followed by a PFC circuit followed by either a “half bridge” or a “full bridge” circuit operated at high frequency, 100 kHz. or greater. The ignition method used in high frequency ballasts may include resonant ignition, using a high frequency sine wave or semi-resonant ignition using pulses superimposed on the peaks of a high frequency sine wave. 
     Modern HID ballasts are microprocessor controlled, ie. circuit blocks include transistor switches, e.g. gates of MOSFETS, which are controlled by a microprocessor. 
     HID lamps are widely used for illumination in public areas because of the high efficiency available, e.g 100-140 lumens/watt. However, under a drop of mains voltage, when hot re-strike is not used or unavailable, HID lamps remain off for five to ten minutes while they cool down before re-ignition. While HID lamps are in the process of cooling down, other lighting must be used which supplies sufficient light just after the mains voltage comes back on. Quartz-halogen lamps are often used for emergency lighting which are lit while the HID lamps are cooling down and waiting for re-ignition. The quartz-halogen lamps require different wiring and fixtures from the HID lamps. 
     Thus there is a need for and it would be advantageous to have a system and method for providing emergency lighting during the time period after a drop in mains voltage and before re-ignition of the HID lamps without requiring use of different circuitry, additional infrastructure or hot re-strike capability. 
     The ballast used to ignite and operate an HID lamp is very different from and should not be confused with the ballast and starter used to operate a fluorescent lamp. A fluorescent lamp uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that causes a phosphor to fluoresce, producing visible light. The mercury atoms in the fluorescent tube must be ionized before an arc can “strike” within the tube. A combination filament/cathode at each end of the lamp in conjunction with a mechanical or automatic switch initially connects the filaments in series with the ballast and thereby preheat the filaments prior to striking the arc. The preheating typically takes between 2 to 3 seconds which is followed by striking of the warmed mercury vapor inside the fluorescent lamp. The strike is performed after preheating the lamp to avoid damage to the fluorescent lamp. The strike is typically performed by using another controlled circuit portion of the fluorescent ballast circuit known as a starter. The peak voltage of the pulse provided by the starter is used to strike the warmed mercury vapor inside the fluorescent lamp and is typically 1200 to 1500 volts. Light produced by the fluorescent lamp after application of the starter circuit is virtually instantaneous. A typical 40 W 48″ fluorescent tube, starts at 400-650 Volts and has about a 93V working voltage. High frequency ballasts for fluorescent lamps run at 20-60 kHz. Fluorescent lamps immediately re-ignite if turned off. 
     BRIEF SUMMARY 
     According to embodiments of the present invention there is provided a lighting system including an electronic ballast circuit configured to operate a high intensity discharge (HID) lamp. The electronic ballast circuit has a current output and an impedance sensor connected to the current output. Multiple light emitting diodes (LEDs) are connected to the current output of the electronic ballast circuit. The electronic ballast circuit includes an ignition circuit configured to ignite an HID lamp (if connected) and an impedance sensor adapted to sense impedance of the current output. The ignition circuit is activated only when the sensed impedance is characteristic of the HID lamp (prior to ignition) and not characteristic of the LEDs. 
     The lighting system may include a second electronic ballast configured to operate a high intensity discharge (HID) lamp. The second electronic ballast shares an input of mains power with the electronic ballast. After momentary failure of the mains power, the electronic ballast and LEDs are adapted to provide emergency lighting while the high intensity discharge lamp (HID) connected to the second ballast is cooling down (and waiting for re-ignition). 
     The lighting system may further include a switch connected to the electronic ballast, the HID lamp and the LEDs. The switch selects either the HID lamp or the LEDs for drawing current from the ballast circuit. The switch is configured to select the LEDs for drawing current when the HID lamp is not operable such as during a time period after a momentary failure of mains electrical power. The ballast circuit is typically controlled by a microprocessor. The microprocessor may have an output configured to control the switch. Alternatively, the HID lamp and the LEDs may be driven simultaneously by the current output of the electronic ballast circuit. 
     According to the present invention there is provided a method for using an electronic ballast circuit configured to operate a high intensity discharge (HID) lamp. Multiple light emitting diodes (LEDs) are attached to the current output of the electronic ballast circuit. and current is driven from the current output to light said LEDs. 
     Optionally, prior to driving current through the LEDs, the impedance of the current output is sensed; and the current is driven through the LEDs to light the LEDs upon detection of an impedance significantly lower than an impedance characteristic of the HID lamp. Ignition appropriate to ignite the high intensity discharge lamp is not performed when LEDS are attached to the current output. Alternatively, a signal is provided to disconnect the LEDs during the high voltage output for ignition of the high intensity discharge (HID) lamp. 
     A rectifier and a parallel capacitor may be disposed between the current output and the LEDs. The capacitor is adapted to protect the LEDs from being damaged by an ignition pulse intended to ignite the HID lamp. 
     According to the present invention there is provided an electronic ballast circuit configured to operate a high intensity discharge (HID) lamp. The electronic ballast circuit includes an ignition circuit for providing an ignition pulse to ignite the HID lamp, an inverter circuit for providing current to the HID lamp and a current output configured for connection to multiple light emitting diodes (LEDS). 
     The electronic ballast may include an impedance sensor on the current output. The impedance sensor is configured to sense an impedance of the current output. The ignition circuit is activated when the impedance is characteristic of the LEDs and not characteristic of the HID lamp. 
     A microprocessor typically controls the ballast. The microprocessor may include a signal output adapted to disconnect the LEDs during the ignition pulse and to connect the LEDs only while the inverter circuit is providing the current. 
     The ballast circuit may include a switch connected to the HID lamp and the LEDs. The switch is configured to select the LEDs for drawing current when the HID lamp is not operable during a time period after a momentary failure of mains electrical power. 
     A rectifier typically a full-wave bridge rectifier may be disposed between the current output and the LEDs. A capacitor may be parallel connected between the direct current output of the rectifier and the LEDs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIG. 1  shows a ballast circuit with an input connected to an alternating current (AC) power supply and an output connected to a high-intensity discharge (HID) lamp, according to an embodiment of the present invention. 
         FIG. 2  shows the same ballast circuit as in  FIG. 1  connected to a bank of light emitting diodes (LEDs), according to an embodiment of the present invention. 
         FIG. 3  shows a method according to an embodiment of the present invention, the method using the ballast of  FIGS. 1 and 2 . 
         FIG. 4   a  shows a circuit according to an embodiment of the present invention for switching output of ballast between HID lamp and LEDs. 
         FIG. 4   b  shows a method according to an embodiment of the present invention for providing emergency lighting using the circuit of  FIG. 4   a.    
         FIG. 5  shows a system according to another embodiment of the present invention, the system including multiple ballast circuits which power HID lamp and/or bank of LEDs both at the same time. 
         FIG. 6  shows a circuit according to another embodiment of the present invention for operating both HID lamp and LEDs simultaneously from a single ballast circuit of  FIGS. 1 and 2 . 
         FIG. 7   a  shows a circuit according to yet another embodiment of the present invention for operating both HID lamp and LEDs simultaneously from a single ballast circuit of  FIGS. 1 and 2 . 
         FIG. 7   b  shows a method according to an embodiment of the present invention using the circuit of  FIG. 7   a.    
         FIG. 8  shows yet another embodiment of a circuit, according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures. 
     Before explaining embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
     By way of introduction, embodiments of the present invention are directed to the use of existing high intensity discharge ballasts for driving light emitting diodes. One application of the present invention is to provide emergency lighting instead of quartz-halogen lamps when hot-re-strike capability is unavailable or too expensive to implement. Other applications may be include decorative fixtures with a mixture of colors. 
     Referring now to the drawings,  FIG. 1  shows a ballast circuit  100  with an input connected to an alternating current (AC) mains power  104  and an output connected to a high-intensity discharge (HID) lamp  112 . Ballast circuit  100  typically includes a rectifier circuit  102 , a power factor control circuit  104 , an inverter circuit  106 , and ignition circuit  108  under monitor and control of microprocessor  114 . Ballast circuit  100  may be a high frequency ballast or a low frequency ballast which provides a controlled AC current output. For a high frequency ballast  100 , the AC output of inverter  106  is sinusoidal with a frequency typically of 100 kHz or more. Low frequency ballast  100  outputs a square wave at about 160 Hertz. An optional communications interface  112  may be connected to microprocessor  114  to enable programming and/or reprogramming of ballast operation parameters, output current (I) and/or voltage (V) of ballast  100  for example. 
     Rectifier  102  has a mains electricity input  104 . Input  104  is typically a 120/240 root mean square (RMS) alternating current (AC) voltage with a frequency of 60/50 Hertz. Rectifier  102  rectifies mains electricity input  104  to produce a direct current (DC) output which is input into power factor correction (PFC) circuit  104 . The DC output of PFC  104  is connected to the input of inverter circuit  106 . Inverter  106  may be a “half bridge” or a “full bridge” inverter circuit. Ignition circuit  108  is connected in parallel to the AC output of inverter  106 . An impedance sensor (a current and/or voltage sensor)  110  is shown connected to the output to HID lamp  112 . 
     Reference is now made to  FIG. 2  which shows the same ballast circuit  100  as  FIG. 1  now connected to a bank of light emitting diodes, according to an embodiment of the present invention. Ballast circuit  100  typically includes rectifier circuit  102 , power factor control circuit  104 , inverter circuit  106 , and ignition circuit  108  under monitor and control of microprocessor  114 . Impedance sensor (current and/or voltage sensor)  110  is shown connected to the output to HID lamp  112 . An optional select pin  118  is configured as an additional input and/or an output to/from microprocessor  114 . Unlike  FIG. 1 , ballast  100  has its current output connected to a bank of light emitting diodes (LEDs)  118  suitably interconnected in series and/or in parallel in forward bias. 
     Reference is now to  FIG. 3  which shows a method  301  according to an embodiment of the present invention. Method  301  uses ballast  100  which is configured to operate HID lamp  112  as shown in  FIG. 1 . Typically a configuration of ballast  100  to operate HID lamp  112  involves details of an ignition pulse to be applied to lamp  112  and a maximum level of current to be supplied to lamp  112  during a normal mode of operation of HID lamp  112 . The normal mode of operation of lamp  112  occurs after ignition during and after warm up of lamp  112 . The nominal value of voltage (V) which appears across lamp  112  and the nominal level of current to be supplied to lamp  112  during the normal mode is used to determine the number of LEDs and their respective interconnections to form bank of LEDs  118 . 
     A LED has a typical forward bias volt drop 3.2 volts, given that voltage (V) is sinusoidal for a high frequency balance or square wave for a low frequency ballast, the LEDs operate at 50% duty cycle. If it is desired to operate at 100% duty cycle a full wave rectifier may be inserted between the output of ballast  100  and the bank of LEDs. 
     As an example, the number (n) of LEDs needed to form a serial string if peak voltage of V=100 volts is given by:
 
 n =(0.318 ·V )/forward volt drop LED=(0.318·100)/3.2≈100 LEDs
 
     Serial strings of LEDs may be connected in parallel to form the bank of LEDs  118 . Typically, in order to insure current division among the serial strings of LEDs, a small resistive element is connected in series with each string. The maximum forward current of a serial string is used to determine the number of parallel connected strings to draw the maximum current (I) output of ballast  100 . 
     Referring now to method  301  of  FIG. 3 , when bank of LEDs  118  is attached (step  303 ) to the output of ballast  100  as shown in  FIG. 2 , impedance sensor  110  for instance applies a current (I) and monitors (step  305 ) the voltage (V) across LEDs  118 . The impedance or voltage value is conveyed to microprocessor  114  (as analog signal or digital data). Microprocessor  114  determines that a low impedance load (i.e. LEDs  118 ) (decision block  307 ) is connected to the output of ballast  100  and normal operation (step  311 ) of lighting using LEDs  118  controlled by microprocessor  114  continues without prior ignition which may damage LEDs  118 . Normal operation (step  311 ) typically may involve using the initial impedance value and measured voltage (V) and/or current (I) in step  307  to determine the level of maximum current output (I) of ballast  100  to supply LEDs  118 . Thereafter, normal operation (step  311 ) continues with LEDs  118  under output current control. 
     When a HID lamp  112  is attached (step  303 ) to the output of ballast  100  as shown in  FIG. 1 , sensor  110  monitors impedance (step  305 ) of HID lamp  112 . The impedance (current and/or voltage) is conveyed to microprocessor  114 . Microprocessor  114  determines that a high impedance load (i.e. non-ignited HID lamp  112 ) (step  307 ) is connected to the output of ballast  100 . The ignition of HID lamp  112  is then performed in step  309 . Once HID lamp  112  is ignited using ignition circuit  108 , normal operation (step  311 ) of lighting using HID lamp  112  continues. Normal operation (step  311 ) typically involves allowing for HID lamp  112  to warm up so as to produce maximum intensity of light. 
     Reference is now made to  FIG. 4   a  which shows a circuit  400  according to an embodiment of the present invention for switching output of ballast  100  between HID lamp  112  and LEDs  118 . Alternating current (AC) mains supply  104  is connected to the input of ballast  100 . The output of ballast  100  is connected to either the input of rectifier  402  or across HID lamp  112  using switch SW 1 . By way of example, switch SW 1  includes two single pole double pole double throw (SPDT) switches which are mechanically linked together. Alternatively switch SW 1  may have just one (SPDT) switch which is used to switch the live output of ballast  100  with the neutral output of ballast  100  connected to the neutral inputs of lamp  112  and rectifier  402 . Switch SW 1  may be activated/deactivated by input from an input select  418  to switch SW 1  which may be used to manually select which of the two light sources HID  112  or LEDS  118  are to be powered. Alternatively, or in addition switch select  418  may be connected to select pin  118  of microprocessor  114 . 
     Rectifier  402  is preferably a full wave rectifier which has an output connected to bank of LEDs  118 . The use of rectifier  402  in circuit  400  makes serial strings of LEDs  118  active for the whole of period of voltage (V) and current (I) or 100% duty cycle. 
     Reference is now made to  FIG. 4   b  which shows a method  401  according to an embodiment of the present invention for providing emergency lighting using circuit  400 . With mains  104  applied to ballast  100 , switch SW 1  applies the output of ballast  100  to the input of HID lamp  112  (step  403 ) and HID lamp  112  is ignited and turned on. Switch SW 1  is controlled by microprocessor  114  via select line  418 . When a power failure of mains  104  occurs or mains  104  is turned off, HID lamp  112  turns off also. Switch SW 1  changes position (to its normal power-off position) and the output of ballast  100  is applied to the input of rectifier  402  (step  405 ). Once mains  104  is back on, LEDs  118  are now turned on and a previously programmed time delay of typically 5-10 minutes is initiated by microprocessor  114  (step  407 ). During the time delay, LEDs  118  are now on and HID lamp  112  cools down. After the time delay, switch SW 1  changes position turning LEDs  118  off and applies the output of ballast  100  to the input of HID lamp  112 . The output of ballast  100  applied to the input of HID lamp  112  ignites and turns on HID lamp  112  (step  409 ). 
     Reference is now made  FIG. 5  which shows a system  500  according to another embodiment of the present invention. System  500  is includes multiple ballast circuits  100  which selectably power HID lamp  112  and/or bank of LEDs  118  both at the same time. 
     Alternating current (AC) supply  104  is connected across the input of identical ballasts  100   a  and  100   b  respectively. The output of ballast  100   a  is connected across HID lamp  112 . The output of ballast  100   b  is connected across the input of rectifier  402 . The output of rectifier  402  is connected across bank of LEDs  118 . In system  500 , when mains power turns off and immediately turns on again, LEDs  118  provide sufficient emergency light while HIDs are cooling and waiting for re-ignition. Ballasts  100   a  and  100   b  are fully identical and select pin  118  is not required. 
     Reference is now made  FIG. 6  which shows a circuit  600  according to another embodiment of the present invention for operating both HID lamp  112  and LEDs  118  simultaneously from a single ballast circuit  100 . AC supply  104  is connected across the input of ballast  100 . The output of ballast  100  is connected either across HID lamp  112  and the input of rectifier  402  using switch SW 2 . Switch SW 2  may be activated/deactivated by input select  618  provided by select output  118  of microprocessor  114 . Switch SW 2  has a single pole switch which connects the live output of ballast  100  to the live input of rectifier  402 . The neutral output of ballast  100  connects directly to the neutral input of rectifier  402 . The output of rectifier  402  is connected across bank of LEDs  118 . 
     In the operation of circuit  600 , SW 2  is closed only after HID  112  is ignited. In this way, rectifier  402  and LEDs  118  are not exposed to high ignition voltage. Switch SW 2  normally closes and connects LEDs  118  not under mains power. Ballast  100  tests output impedance and senses the high impedance of HID lamp  112 . Ignition proceeds and switch SW 2  closes and connects LEDs only after ignition. If mains power fails, then LEDs  118  are connected. Ignition is attempted only after a time delay after power on as in method  400  of  FIG. 4   a.    
     Reference is now made  FIG. 7   a  which shows a circuit  700  according to another embodiment of the present invention. AC mains supply  104  is connected across the input of ballasts  100 . The live output of ballast  100  is connected to one side of HID lamp  112  and the other side of lamp  112  connecting to the common node of single pole double throw switch SW 3 . The neutral output of ballast  100  connects to one node of switch SW 3  and the neutral input of rectifier  402 . The other node of switch SW 3  connecting to the live input of rectifier  402 . Switch SW 3  may be activated/deactivated by an input select  718  provided from microprocessor  114 . The output of rectifier  402  is connected across bank of LEDs  118 . 
     Reference is now made to  FIG. 7   b  which shows a method  701  according to an embodiment of the present invention using circuit  700 . With mains  104  applied to ballast  100  switch SW 3  applies the output of ballast  100  across HID lamp  112  by virtue of one end of lamp  112  being applied to the neutral output of ballast  100 . With one end of lamp  112  being applied to the neutral output of ballast  100  HID, lamp  112  is ignited and turned on (step  703 ). Switch SW 3  is controlled by microprocessor  114  via select line  718 . When SW 3  changes position, HID lamp  112  remains on and LEDs  118  are turned on by virtue of HID lamp  112  being connected in series with LEDs  118  via rectifier  402  (step  705 ). 
     When a power failure of mains  104  occurs or mains  104  is turned off, HID lamp  112  and LEDs  118  turn off also and switch SW 3  changes position. Once mains  104  is back on, a time delay of typically 5-10 minutes is initiated by microprocessor  114  (step  707 ). During the time delay, LEDs  118  are on and HID lamp  112  is off and cools down. After the time delay HID lamp  112  is re-ignited and once again switch SW 3  changes position which turns LEDs  118  on by virtue of HID lamp  112  being connected in series with LEDs  118  via rectifier  402  (step  709 ). 
     Reference is now made to  FIG. 8  which illustrates a circuit  800 , according to an embodiment of the present invention.  FIG. 1  shows a ballast circuit  100  with an input connected to an alternating current (AC) mains power  104  and an output connected to a rectifier  402  which is typically a full-wave bridge rectifier. The DC output from rectifier is filtered by a parallel-connected capacitor  404  of typically low capacitance. Rectifier  402  has an output connected to bank of LEDs  118 . During operation of circuit  800 , if the energy of the ignition pulse is sufficiently small, capacitor  404  may protect LEDs  118  from being damaged by the ignition pulse. 
     The definite articles “a”, “an” is used herein, such as “a LED”, “a switch” have the meaning of “one or more” that is “one or more LEDs” or “one or more switches”. 
     Although selected embodiments of the present invention have been shown and described, it is to be understood the present invention is not limited to the described embodiments. Instead, it is to be appreciated that changes may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and the equivalents thereof.