Tuesday, October 5, 2010

Compact Florescent Lamp Repair

This is Shiharan Choudhury and here I will show you how to repair CFL bulbs. Well, now it’s been a long time that I have quit repairing CFL bulbs, because of profit margin, but I thought of sharing all my years of experience in the field of repairing CFL bulbs, for the sake of utility.
By a study of the market, most of the CFL bulbs are guaranteed for a period of 1year.But, like all electronics, CFL eventually fails. Decrying the throw away mentality and by going through this small report, you can successfully repair CFL bulbs.
Now here is how CFL bulbs work?
Compact fluorescent lamps have some benefits in comparison with classic light bulbs. It is lower power consumption (to 80%) and much longer lifetime (5 to 15 times). Disadvantages are longer starts mainly at more expensive types, impossibility to use darker and price.
   Fluorescent lamps are available usually in these color temperatures:

       *Warm white (2700K)
       *Cool white (4000K)
       *Daylight (6000K)
Most often we meet with "warm white", which is close to classic bulb and which is most pleasant to people. Compact fluorescent lamp use vacuum pipe similar to classic strip lamp and princip of energy transformation to light is same. Tube has on both ends two electrodes faced with Barium. Kathode has high temperature about 900 degree Celsius and generates many electrons which are accelerated by voltage between electrodes and hits atoms of Argon and Mercury. There arise low temperature plasm. Overflowing energy mercury radiate in a UV light form. Inner side of tube is faced with luminophore, which transform UV light in to the visible light. Tube is powered by alternating current, so that function of electrodes (cathode and anode) is still changing. Because there are used switched converter, which works on tens of kilohertz, that CFL lamp doesn't "blink" in comparison to classic strip tube lamp. Converter, which is present in a screw cap, substitute classic ballast with a starter.
Principle of function we explain on a LUXAR 11W lamp. Circuit contains supply section, which includes interference suppressor L2, fuse F1, bridge rectifier from 1N4007 diodes and filtering capacitor C4. Starting section includes D1, C2, R6 and diac. D2, D3, R1, R3 have protect function. Other parts have normal operation function.
 Lamp start
R6, C2 and DIAC mades first pulse to base of transistor Q2 and cause his opening. After start is this section blocked by diode D1. After every opening of Q2 is discharged C2. There is not possible to collect enough energy for reopening of diac. Next are transistors excitated over very small transformer TR1. It consists of ferrite ring with three windings (5 to 10 coils). Now are filaments powered over capacitor C3 from voltage rises from resonant circuit from L1, TR1, C3 and C6. Than the tube lights up is resonation frequency specified by capacity of C3, because he has much lower capacity than C6. In this moment is voltage on a C3 over 600V in a relation to used tube. During start is peak collector current about 3 to 5 times bigger than during normal operation. When the tube is damaged, there are hazard of transistor destroying.
Normal operation
When the gas is ionisated in a pipe, C3 will be practically shorted and thanks to this frequency goes down and changer is now drived only by C6 and changer generates much lower voltage but enough to keep the light on. In a normal situation, when transistor opens, that current to TR1 increasing until his core is saturated and next his feedback to base drop away and transistor closes. Now opens second transistor which is excitated by reversly connected windind of TR1 and all process repeats.
Mechanical construction
Lamp is usually compounded of two parts. One is plastic cover with holes for pipe and bills. Tube is agglutinated to it. Second much bigger piece has slots for bills from the inner side. Inside is printed circuit board with components and wires from tube. From the upper side of PCB are wires to top of lamp, where are soldered or stamped to the contact. Both plastic parts are clicked to himself and sometimes glued. Usually you can carefully leverage with a small screwdriver sequently to round to the gap between both plastic pieces for releasing of glue. Next you must leverage more to the opening lamp. For closing of lamp you can only click both plastic pieces to himself. Look at photo of opened lamp.

Common failure is broken capacitor C3. it is possible mainly at cheap lamps, where are used cheaper components for lower voltage. Whet the pipe doesn't lights up on time, there are risk of destroying transistors Q1 and Q2 and next resistors R1, R2, R3 and R5. When lamp starts, changer is very overloaded and transistors usually doesn't survive longer temperature overloading. When the pipe serve out, electronics is usually destroyed too. When the pipe is old, there can be overburned one of filaments and lamp doesn't lights up anymore. Electronics usually survives. Sometimes can be pipe broken due to internal tension and temperature difference. Most frequently lamp fails, when power on.
Repair of CFL
Repair of electronics usually means change of capacitor C3 if he is brobek. When burns fuse, probably will be damaged transistors Q1, Q2 and resistors R1, R2, R3, R5. You can replace fuse with resistor R5. Failures can be multiplied. For example, when is shorted capacitor there can be thermally overloaded transistors and will be destroyed. Best transistors for replacing of original types are MJE13003, but it is not easy to find them. I replaced them with BD129, but they are not available now. There exists other variants like a 2SC2611, 2SC2482, BD128, BD127, but I am not sure if they will be long-life. Original transistors are not available on our market. If doesn't matter size of case TO220, it's possible to use transistors MJE13007.
CFL bulbs basically fail due to breakdown of some very basic electronic components, which are cheap, easily avliable, and easy to test. The components are Electrolytic Capacitors, Transistors, Diodes, Transformer, and Resistors.
The most difficult part of the repair work is to open the enclosure. After the enclosure is taken off it’s found that it has the CFL, Inverter Circuit, and the Housing of the circuit that contains the screwbase.Now the repairing starts with a visual inspection.
If you find any bulged electrolytic capacitor then desolder it out from the circuit and replace it with newer one having same or close capacitance measure, and a voltage rating at least or above 250volts.
Check the transistors. Not by a visual inspection, but by testing them using an ohm-meter. In some cases it might not work due to the base drive circuit. So it’s always recommended to desolder out the transistors, and then test them.
Test the inductor/transformer for open circuit, using an ohm-meter.
Check the diodes for forward resistance reading. Make sure that it has no or negligible resistance reading when it is tested reversely. Also check for open circuit.
Have a close look at the resistors. If they appear to be charred, then desolder it out and test it for its resistance reading. In some cases it’s found that resistors open circuit.30% of the failure of CFL bulbs is caused by Open Circuit Resistor that is connected to the mains supply line. It’s basically a low ohm, 0.5 watts resistors.
And if all the above components are found to be ok, then have a continuity test on the Vacuum Tube. If it’s found open circuit then, you cannot do anything further, but can obviously quit the repair work.
                                                Here is the view of a repaired CFL bulb                                     

        Mains Voltage is dangerous, and could be lethal. Avoid Direct Contact.
And incase if you have any doubt or problem pleasw contact me,by mailing me or by calling.

                                              All the Best!

       Shiharan Choudhury
(Electronic Repairer & Designer)
        +91 9479964871
        +91 9479964872