> Has anyone seen any good write-ups on, or figured out for themselves,
>> just
>> how the self-oscillating dirty - i.e. smoothing cap-less - switchers that
>> you find in use as lamp ballasts (or 'electronic transformers' that they
>> seem to be sometimes known as) work ?
>>
>> I recently acquired a fairly sophisticated disco lighting fixture that
>> has
>> such a supply to run the 24v 150 watt lamp. It seems to have two main
>> switching devices, which I suspect are FETs or IGBTs, but can't tell for
>> sure so far, as the numbers have been ground off them. On the mains side,
>> it
>> has a bridge but no filter cap, so dirty DC is applied directly to the
>> collector / drain of one of the devices. No sign of any control IC, but
>> there is a small vertical sub-pcb that has a few small transistors and
>> other
>> bits and bobs on it. It does have a preset pot on the main board, which
>> I'm
>> guessing sets the output voltage. There are lots of 1 and 2 watt
>> resistors
>> scattered around, as well as quite a lot of diodes, but overall, the
>> topology is not one that I immediately recognise as being any type of
>> common
>> switcher. The output to the lamp is taken straight from the secondary
>> side
>> transformer. No rectification or filtering.
>>
>> Thus far, I've only quickly run an ESR meter over the small electros on
>> there, but nothing particularly bad looking amongst them. It just sits
>> there, pretty dead. Nothing exploding or smoking. Fuse intact. Nothing
>> obviously short on the 'power' side of things. Probably going to finish
>> up
>> being an open polyester cap or some such, but it would be nice to
>> understand
>> a bit more, what kicks these designs off, and keeps them running.
>>
>> Arfa
Can you put up some pictures somewhere? You should be able to dope out the switch as a fet or IGBT with an ohm meter.
tm