Diagnosing a fluorescent light circuit

Discussion in 'The Projects Forum' started by anku94, Nov 22, 2010.

  1. anku94

    Thread Starter New Member

    Nov 22, 2010
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    Hi guys

    I've got two fluorescent lights of the same design, one works and the other does not.

    The problem is somewhere in the circuit board. It's a small board, semicircular, something like half a coin. It has a small transformer, two capacitors, a few resistors, diodes, 2-4 transistors, and what looked like a wounded ferrite coil, some sort of inductor/choke coil, I'd guess.

    So I found the fuse on the circuit burnt up. I joined the fuse wire and it tripped/short-circuited again.

    So how do I diagnose which component is short-circuited ?

    A thin portion of a 67000(0) Ω resistor had peeled of, a quick visual inspection revealed. It looked like a standard carbon resistor. Other parts seem to be perfectly fine.

    Normally, I'd say that the resistor is faulty, but then I thought how can a faulty resistor cause a short-circuit. Rather, it would break the circuit.

    From the post, you must have surmised I'm an absolute amateur, I know the basic function of a capacitor, inductor, transformer and a resistor quite properly, have some understanding of diodes and very little understanding of transistors. And absolutely no understanding of what all of these are doing in that circuit.

    So, can anybody suggest me how to locate the defective part ?

    And oh, I don't have a multimeter. :p

    PS : Let me know if you need pics, I thought that a pic of an assembled circuit wouldn't present much info, so I didn't post one.

    Thanks a lot.
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Here in the US, the compact flourescents aren't worth salvaging since they are getting close to the same price as incandescent bulbs. Not sure how much CFLs cost in India, though.

    There should be a small transformer on the board as well, check the continuity on it, if present.

    If working with something other than a CFL ballast, post pictures, well, post pictures either way.
     
  4. anku94

    Thread Starter New Member

    Nov 22, 2010
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    Oh CFLs are cheap alright. $2-$3 depending on the wattage. This one is a designer light for cabinet. Cost 10 times a CFL, so I really don't intend to throw if just coz of a faulty 1 cent resistor.

    Here's a pic.

    [​IMG]

    Notice the center resistor in the non-functioning circuit, there seems to be something wrong with it.

    And yes, there's a transformer too.

    So, should I check it for continuity ? I'm asking this as I don't have a multimeter and I'll have to create a light bulb circuit with a pencil cell or something, so don't wanna do it unless necessary. :)
     
  5. anku94

    Thread Starter New Member

    Nov 22, 2010
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    Err...guys ?
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Your first investment should be in a Good Multimeter, there are several options, but you usually can't go wrong with a Fluke, I have several Fluke 87s, but anything that will do a diode test to 4V for LEDs, can measure capacitance down to nanofarads, and has True RMS readings would be best.

    They cost around 50-100x what the bulb costs, but will be used in the future for MUCH more. Others may have suggestions for other manufacturer DMMs that are economical, I don't have much experience with them to really give advice though.

    From your image, it would appear that at least four resistors and 1 capacitor may have big issues. This could be due to the switching transistor failing shorted. The Fuse appears to be the line became so hot it desoldered itself from the board.

    The discolored components have had an excessive current load, and I cannot identify the device with the metal heatsink tab, but would guess it is at fault.


    CFLs shouldn't be mounted in an enclosure, or mounted upside down. The circuit needs a lot fo cooling.
     
  7. anku94

    Thread Starter New Member

    Nov 22, 2010
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    So I'll try replacing those components and others at random. The device with metal heatsink is a npn transistor [Part number 13003]

    I can't really get a multimeter with my student's pocket money. But I'd like to buy a good one once I'm in college, saving up for it. :)

    Thanks for the info. :)
     
    Last edited: Nov 26, 2010
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