"Arfa Daily" wrote in message
news


"Andrew Gabriel" wrote in message
...
In article ,
"Arfa Daily" writes:
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 ?

They're capacitor-less because the load doesn't care, and it
makes their power factor much nearer to 1 without needing any
additional power factor correction circuitry. Some of them
will also work with phase control dimmers. This is the design
of most 12V electronic lighting transformers.

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.

Power semiconductors can explode without showing it on the device
package. Look carefully around the component for remains of the
ejected guts, often just a dark mark on the inside of a case lid,
or similar. Often ejected where the plastic package bonds to the
heatsink tab, or where the leads enter the package.

--
Andrew Gabriel

You're right about this being the 'common' design for these lighting
'transformers'. I've repaired a few over the years, but as far as I can
recall, they've all had problems with semiconductors having let go their
magic smoke. I do this stuff all day every day, so am pretty good at
spotting such things, if not with the Mk I eyeball, then with the Mk IV
AVO - that's an 8 of course ! :-)

However, the problem with this one seems to be 'passive', so I guess I'm
going to have to fault-find it properly. Understanding the topology -
which although similar, does not seem to follow quite, any 'normal'
switcher - would help. Have you seen any schematics anywhere for one of
these controller-less dirty types ? I've now removed it from the light so
I can better get at it. I'll try and hook it up to a variac later, and see
what I can find. I'm sure it must have a startup supply somewhere as a
beginning. It's got that sort of 'feel' to it when you have a conventional
switcher with the startup resistor gone high or open ...

Arfa

Hmmm. Now it gets a whole lot more complicated. With a mains supply between
about 60v and 160v, it works. It will light a lamp, and support considerable
amounts of current. However, as you go above that voltage, you can watch on
the 'scope, the activity around the switching devices progressively die,
until it stops altogether. I think that the next thing I am going to do is
look for the 'potted down' supply to that little sub board, and see if it
ever stabilizes at any figure, or just keeps going up and up. I've spotted a
fairly meaty 220k resistor straight off the bridge, heading in that
direction, so I guess that might be a good place to start.

Arfa