Arfa Daily wrote in message
...
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





failed bleed resistor ?

CFL circuit similar ?
http://www.nxp.com/documents/applica...te/AN00048.pdf