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