"tm" wrote in message
...

"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

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

Truth to tell, there's not a lot to see. Nothing very remarkable about it at
all. It's about a half brick size, and looks much like any other typical
little switcher, except no primary or secondary-side filter electros.

I've done a bit more work on it this morning. I shoved it on a variac, and
hooked a 'scope up to it. Oddly, between about 60v and 160v (UK mains up to
240v), it works. It will drive a lamp, and support considerable current
demand. However, as you carry on up beyond 160v, you can watch the activity
around the switching devices progressively 'die', until it stops altogether,
so this is not going to be any kind of easy one ...

Arfa