WARNING: Electronic ballasts are dangerous, not only because they are usually connected directly to the mains but also they generate very high voltages: the one described below has 600V output, much higher than the 110/220V mains voltage.
So by no means try this at home without proper training and certification! Please read Safety Guidelines for High Voltage and/or Line Powered Equipment
I had a fluorescent desktop lamp go dark for no apparent reason. I opened it and found an electronic ballast with the following markings:
Electronic Ballast for Fluorescent Lamp
Input: 120/60Hz 0.38A
Lamp: PL 27W
Output Voltage: 600V
After a few minutes of eye inspection (and nose:) ), I couldn’t find anything with burning marks or any foul smell. The small fuse had continuity along with the connecting wires, which made me think the culprit was the bulb, as the glass was fairly dark near the filaments, although in hindsight the latter came from its age and constant use. The bulb has 27W and 6500Kelvin color temperature. Without much thinking I got a new bulb but when I received it a few days later it didn’t switch on.
So going back to the ballast I went through the circuit, testing the likely culprits in the high voltage section: open/short capacitors, diodes or transistors and finding none, the mystery deepened. I cycled the power and then I went around poking the voltages to find that the mains rectifying circuit was working fine but when prodding with the multimeter probes next to the diac suddenly the bulb flashes! Humm, maybe the diac broke since it’s used to start the circuit, somewhat replacing the old mechanical/thermal starter that pairs reactive ballasts.
Testing a diac is more tricky with just a multimeter so I checked the circuit around it, finding to my surprise an open 510kΩ resistor (R1 in the circuit) with no burning or overheating marks. This resistor is responsible for the charging of capacitor C2 which, when reaching a high enough voltage to overcome the diac breakdown voltage, turns on transistor Q2 to provide the initial kick in the oscillator circuit. Replacing this resistor brought the circuit back to life and now the lamp was working again with no issues.
I took the time to extract the schematic as seen below , which can be downloaded in eagle, png and pdf formats, along with relative placement of the parts on the original pcb.
Some interesting references can be found at