Need help with electronic ballast

Discussion in 'The Projects Forum' started by stevenmahoney, Feb 23, 2013.

  1. stevenmahoney

    Thread Starter New Member

    Jan 25, 2013
    10
    0
    I am trying to repair an electronic ballast and do not understand some of its circuits. Can anyone help me?

    1) R4 suppose to regulate the brightness of HMI light. It looks like to me as a voltage divider (I am not even sure why transistors Q3 and Q4 were used, as they do not do anything). The voltage difference from highest to lowest at pin 3 of U2A when I rotate it is 1V. However, there is hardly any voltage on pin 2, so I am confused as to how that comparator should be operating. Also, the connection to Q9, Q10 transistors is not clear... Especially when resistor R19 is so low (.1 om).

    2) U4 produces a square wave. I do not understand how it turned into an oscillator. Positive bias via capacitors C1 and C2? If so, how would it work?

    3) Pin 4 of U4 produces narrow square waves (pink on diagram). Is that suppose to be like this? U4 is a quad cmos NOR gate. Am I correct to assume that somehow the comparator U2A suppose to change the frequency or width of the signal on pin 4? This mechanism is also a mystery...

    4) What is the purpose of transistors Q7 and Q8? How they suppose to work?

    Any help (the more detailed the better) would be appreciated. Really want to get to the bottom of it... :)

    Steven

    Detailed schematics with oscilloscope clippings are in the attached files.

    [​IMG]

    [​IMG]
     
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  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Electronic ballasts are basically push-pull inverters. Repairs are suauly a simple task, as the prime cause of faults are electro caps gone bad, which can be tested by replacement, and the other main fault is blown output FETs which can usually be seen as blown or test shorted, possibly in conjunction with a blown fuse or fusible resistor.

    What is the symptom of your "faulty" ballast?

    Also you should be aware that they are very cheaply made in enormous quantities and is probably much more cost efficient to replace than repair...
     
  3. stevenmahoney

    Thread Starter New Member

    Jan 25, 2013
    10
    0
    This particular ballast had all output transistors (Q5-Q10) bad and transistors Q1, Q2. I replaced them, as well as ICs and optocoupler. Checked most of capacitors with Sencore LC103 and found none bad. My task is not only to fix this ballast, but also understand how it works.
     
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