Patent Publication Number: US-2007108920-A1

Title: Current-limit ballast for fluorescent lamp

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an electronic ballast for fluorescent lamp and more particularly to a current-limit ballast, which uses a current-limit circuit to limit the current that starts the fluorescent lamp, preventing damage to the fluorescent lamp.  
      2. Description of the Related Art  
      The electronic ballast of a regular fluorescent lamp is generally comprised of a rectifier circuit, a filter circuit, a pulse transformer, and a choke. The rectifier circuit rectifies AC into DC, which is then filtered by the filter circuit for enabling the pulse transformer to turn on the fluorescent lamp. This design of electronic ballast has numerous drawbacks as follows: 
          1. When started, electric current is rapidly charged to the capacitor of the filter circuit, and the charging current is excessively high.     2. When power resumes after a power failure, the starting current is high and may damage the capacity of the filter circuit.     3. When started to charge the capacitor of the filter circuit in a rush, the voltage and the starting current are high and may darken the ends of the fluorescent lamp, thereby shortening the service life of the fluorescent lamp.     4. The ballast provides no protection during output of power or a short-circuit, and an accident may occur easily.        

     SUMMARY OF THE INVENTION  
      The present invention has been accomplished under the circumstances in provide current-limit ballast, which uses a current-limit circuit to limit the current that starts the fluorescent lamp, preventing damage to the fluorescent lamp. To achieve this and other objects of the present invention, the current-limit ballast comprises a bridge rectifier circuit, a filter circuit, a high-frequency pulse transformer, a choke, and a current-limit circuit electrically coupled to the filter circuit and the high-frequency pulse transformer and formed of a silicon controlled rectifier and a current-limit resistor to limit the range of the current to charge capacitors of the filter circuit for enabling the high-frequency pulse transformer to produce a high-frequency oscillation signal to trigger the silicon controlled rectifier. In an alternate form of the present invention, the current-limit circuit is formed of a TRIAC and a current-limit resistor.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a circuit diagram of a current-limit ballast according to a first embodiment of the present invention.  
       FIG. 2  is a circuit diagram of a current-limit ballast according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION. OF THE PREFERRED EMBODIMENT  
      Referring to  FIG. 1 , a current-limit ballast for use in a fluorescent lamp  6  of 4 W˜40 W at AC220V (or of 4 W˜20 W at 110V) is shown comprising a bridge rectifier circuit  1  formed of four diodes D 1 ˜D 4  for full-wave rectification, a filter circuit  2  formed of capacitors C 2  and C 3 , an oscillation circuit  3  formed of transistors TR 1  and TR 2  and biasing resistors R 2  and R 3 , which transistors TR 1  and TR 2  being connected in parallel to the capacitors C 2  and C 3 , a high-frequency pulse transformer  4 , a choke  5 , and a current-limit circuit  7  electrically coupled to the bridge rectifier circuit  1 , the filter circuit  2  and the high-frequency pulse transformer  4 , and formed of a silicon controlled rectifier SCR, a biasing resistor R 4  and a current-limit resistor R 1 . The silicon controlled rectifier SCR is a triggering diode. The bridge rectifier circuit  1  rectifies AC into DC that passes through the current-limit circuit  7  to charge the capacitors C 2  and C 3  of the filter circuit  2  to the saturated status to further turn on the transistors TR 1  and TR 2  of the oscillation circuit  3 , causing the oscillation circuit  3  to oscillate, and therefore a high frequency is produced and provided to the high-frequency pulse transformer  4 . At this time, the high-frequency pulse transformer  4  is caused to produce a high-frequency pulse to turn on the silicon controlled rectifier SCR of the current-limit circuit  7 , thereby starting the fluorescent lamp  6 .  
      Because the current-limit circuit  7  is formed of a silicon controlled rectifier SCR and a current-limit resistor R 1 , only a limited current is allowed to pass and to further charge the capacitors C 2  and C 3  of the filter circuit  2 . For example, when a 47KΩ 220V current-limit resistor R 1  is used, the current value will be 220V÷47KΩ=0.005 A, therefore the maximum current allowed to pass to the fluorescent lamp  6  is 0.005 A.  
      This embodiment achieves the following advantages: 
          1. Easy to start: The current-limit starting action controls the current under a limited range for starting the fluorescent lamp easily.     2. Extend the life of the fluorescent lamp  6 : The current-limit resistor R 1  of the current-limit circuit  7  prevents transient high current that may damage the fluorescent lamp  6 , and therefore the service life of the fluorescent lamp  6  is prolonged.     3. Prevent damage due to abnormal current: When a short circuit occurred or the fluorescent lamp  6  failed, the silicon controlled rectifier SCR is not oscillated, and the current is limited to a low range, preventing a damage.     4. No flashing: By means of the oscillation circuit  3  and the high-frequency pulse transformer  4 , a high-frequency pulse is provided to drive the fluorescent lamp  6  to give off light without flashing.        

       FIG. 2  shows the current-limit ballast for use in a fluorescent lamp  6  of 40 W (30 W) at AC110V. According to this embodiment, the rectifier circuit  10  is formed of two diodes D 1  and D 2 ; the current-limit circuit  70  is comprised of a TRIAC and a current-limit resistor R 1 . This embodiment achieves the same effect as the aforesaid first embodiment of the present invention.  
      Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention.