Hi allusing a
After a lot of time and research I think I may have the answers you are looking for
---------- Post added at 01:20 ---------- Previous post was at 01:19 ----------
Hi there. I realize this thread is quiet old but I have recently had to investigate repair of these electronic Ballast units as fitted to Thin Light fittings in my Winne. I have reverse engineered the board and can answer some of your questions. Firstly being in the UK I prefer to refer to the board as an inverter because although it replaces the function of a mains ballast and does seem to be referred to commonly as an electronic ballast, it actually operates in a completely different way. The Inverter is a common designed with a small but clever variation. Instead of the normal transformer with three winding (Primary, Secondary and Feedback) the transformer has two extra windings of 8 turns each, which each connect to the ends of the dual tubes that are not linked together and supplies a low voltage to the heater filaments. The secondary high voltage output is connected across these two heater windings so the two ends of the series connected tubes have the High voltage applied across them. The ends of the tubes that are connected together are in series circuit with the feedback voltage output from the transformer and the base of the transistor. This will also produce a small voltage across the heaters. But the main purpose of this arrangement seems to be to inhibit the inverter if a tube fails or is removed. Without feedback the oscillator will stop oscillating.
I also noticed in earlier posts one reply suggested reversing his transistor made his board work, be careful with this one the original poster shows two boards one for the dual tube and one for the singe tube. The single tube circuit board is identical but mirrored so you would have to turn the transistor over for this board. The single tube board is also missing the unneeded windings on the transformer.
So conclusions.
I can confirm almost any high voltage general purpose NPN transistor will do the job, it is not critical. The Tip3055 is ideal, it has the voltage and power handling capabilities required with room to spare and as it is physically lager stays very cool. It is not necessary to isolate the tab when the transistor is mounted into a thin light housing as it is only at 12v and the casing is not otherwise earthed. If your casing is earthed or screwed into a mettle vehicle where contact is made with the chassis, then you will need to use a suitable insulator mounting kit that allows heat transfer or mount on a piece of insulation and use a separate heat sink. Or use a fully encapsulated equivalent.
Now the real problem. A lot has been said about why these units fail, It is not poor design. The two main reasons are normal wear to the insulation between the winding in the High voltage High frequency transformer. Sometimes you may hear a faint high pitched noise. This is caused by the windings in the transformer physically vibrating. This wears the enamel and eventually the insulation on the secondary high voltage output stage is likely to break down.
In fact if you have a unit that you can hear expect it to fail. This is by far the most common reason for these units to fail. Any resister or capacitor in this type of circuit that fails will normally be visibly damaged when it lets the smoke out. The transistor can fail but why should it most likely you will have to replace it because you broke it trying to get the rivet out that holds it to the case. That's why I had to replace mine.
Rewinding the transformer is not difficult (for a qualified person) and I have included the details of the number of turns on each winding on the attached circuit diagram.
Some tips for rewinding
Take the transformer apart carefully. Remove the tape from the graphite cor with a modeling knife and gently wiggle the two U shaped cores to get them out of the bobbin. Retain the two insulation discs that keep the two U shaped cores apart as you must put them back when you reassemble. Keep the windings as even and as tight as possible across the bobbin. The windings should go from one side all the way to the other side by spreading them out if required. The 350 turn secondary winding is the most critical and will require winding several layers back and forth, keep the layers tight together and even, because this coil has as much as 500v across it and if the two end of the winding came into close contact the insulation may fail, it is only good for about 150v. On mine each layer was 50 turns so the maximum difference between turns in contact with each other would be once across and back or 100 turns which is 140v. In reality spikes of up to 1000v can occur but the normal running voltage with the tubes struck is about 300v. A layer of masking tape between each separate set of windings will hold the coil in place reducing the physical movement of the windings and increasing the insulation between separate windings. Finish with insulating tape as tight as you can. Replace the two U shaped graphite cores with the two insulation disks and tape the exposed side together as it originally was. Leave your ends long enough to make it easy to refit in the board. Remove the lacquer from the ends and tin with a soldering iron. This will make it much easier to refit. If you have done a good job not only will your light work but it will be silent in operation.
I did mention two reasons for failure well the second I am afraid is quality. The secondary coil on my transformer was badly wound with bunching at one end this will cause excessive voltage differences between adjacent windings and vibration. Some of the soldering was messy and solder flux had not been removed this attracts moisture and may contain solder spatter causing short circuits or extra loading. So tidy things up if needed.
Finally this information is provided as information only and is not a recommendation to attempt a repair if you are not qualified to do so. There are very high voltages involved so do not attempt to work on any inverter if you are not sure you are qualified. If you do use any of this information it is at your own risk as I am not responsible for any damage or injury resulting from your use of this information. Also I cannot be sure of any changes that the manufacturer may have made between revisions of the boards so the accuracy of this information cannot be guaranteed.
Suggestion regarding rewinding the transformer: Plumbers P.T.F.E tape is very cheap, a good electrical insulator, good at high temperatures and has just the right "stretchiness" to cushion and bind the windings together. It is also thin enough not to significantly increase the winding diameter.
Brian.
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