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

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
[email address is not usable -- followup in the newsgroup]

"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

"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

On Tuesday, February 26, 2013 11:47:54 AM UTC, Arfa Daily wrote:

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 ?

There are various step down switcher topologies, I don't see how we could know which is in use there. I guess you don't have the space to slot in a complete halogen lamp ballast.


NT

On Wednesday, February 27, 2013 1:24:33 AM UTC, Arfa Daily wrote:
wrote in message
...
On Tuesday, February 26, 2013 11:47:54 AM UTC, Arfa Daily wrote:


There are various step down switcher topologies, I don't see how we could
know which is in use there. I guess you don't have the space to slot in a
complete halogen lamp ballast.

I guess there are a couple of 'flavours', but from what I've seen, most seem
to be chip-less self-oscillators that have no primary side smoothing on the
raw DC, and just a transformer at the secondary side. This one certainly
falls into that category. And it *is* effectively a complete halogen lamp
ballast - allbeit an open frame one. The answer to whether there's space to
slot in a 'packaged' off the shelf ballast is yes, but there are a couple of
reservations. Firstly, I would have to re-instate the hold-off relay that's
on the existing board, as an external component. But the potential killer
for doing this, is that the lamp is 24 volt and 150 watts. Most of the
'electronic transformers' readily available, are for 12 v halogens. There
doesn't seem to be many with a 24 v output, and especially at 6 amps ...
This is actually a 'personal' project rather than a paying customer job, so
it's not really important how long it takes to get to the bottom of it. Now
that I've discovered that it does run just fine with a reduced input
voltage, it shouldn't be too difficult to get to the cause. Says he,
confidently ... ! d:-)
Arfa

12v 6A = 72w, a pair of those would surely be quick, cheap & easy.


NT

"Arfa Daily" wrote:

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

Might this thing be for US mains? Also, there used to be converters
which automatically switched between US and UK mains by using two
SCR's and two diodes in a full-wave bridge arrangement for 120V, and
switching off the SCR's so the diodes acted as a half-wave bridge for
240V.

Dave W

wrote in message
...
On Wednesday, February 27, 2013 1:24:33 AM UTC, Arfa Daily wrote:
wrote in message
...
On Tuesday, February 26, 2013 11:47:54 AM UTC, Arfa Daily wrote:


There are various step down switcher topologies, I don't see how we
could
know which is in use there. I guess you don't have the space to slot in
a
complete halogen lamp ballast.

I guess there are a couple of 'flavours', but from what I've seen, most
seem
to be chip-less self-oscillators that have no primary side smoothing on
the
raw DC, and just a transformer at the secondary side. This one certainly
falls into that category. And it *is* effectively a complete halogen lamp
ballast - allbeit an open frame one. The answer to whether there's space
to
slot in a 'packaged' off the shelf ballast is yes, but there are a couple
of
reservations. Firstly, I would have to re-instate the hold-off relay
that's
on the existing board, as an external component. But the potential killer
for doing this, is that the lamp is 24 volt and 150 watts. Most of the
'electronic transformers' readily available, are for 12 v halogens. There
doesn't seem to be many with a 24 v output, and especially at 6 amps ...
This is actually a 'personal' project rather than a paying customer job,
so
it's not really important how long it takes to get to the bottom of it.
Now
that I've discovered that it does run just fine with a reduced input
voltage, it shouldn't be too difficult to get to the cause. Says he,
confidently ... ! d:-)
Arfa

12v 6A = 72w, a pair of those would surely be quick, cheap & easy.


NT


Well, I've fixed the bloody thing now, and I have to say that it had me
fooled. The problem turned out to be a knackered lamp socket. Looking closer
at the specs, it's actually 250 watts not 150 as I first thought, so even
with the forced air cooling it has, the lamp house runs pretty damned hot,
and the lamp socket with it. Basically, the ballast is not a very good
design. It doesn't run at all with no load on it, and as the lampholder was
effectively open circuit, it indeed didn't run, which was the first way that
it fooled me. The second way was that not only does it not run with no load,
it only runs properly with the full designed load. So when I had it out on
the bench, and just had a couple of 50 watt 12 v halogen lamps in series as
a test load, it ran, but only until the mains input voltage got to 160 v on
my variac. Once it went past that, the switching activity progressively
died, until it stopped altogether again.

I had a replacement ceramic holder to hand, as I fix a fair few of these
lighting fixtures, so I grafted it in, and reconnected the lamp wiring to
the ballast. This time, it kicked up and ran the lamp at full intensity
without issue, so that's one to remember if I come across another of this
particular fixture type.

Arfa

"Dave W" wrote in message
...
"Arfa Daily" wrote:

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

Might this thing be for US mains? Also, there used to be converters
which automatically switched between US and UK mains by using two
SCR's and two diodes in a full-wave bridge arrangement for 120V, and
switching off the SCR's so the diodes acted as a half-wave bridge for
240V.

Dave W



No, it's a genuine UK mains only input. It's front end is just a bridge
across the mains supply, not even the sophistication of a filter cap, let
alone any kind of auto voltage sensing. Anyways, I've sorted the problem
now - see reply to meow2222

Arfa

In article ,
"Arfa Daily" writes:
Well, I've fixed the bloody thing now, and I have to say that it had me
fooled. The problem turned out to be a knackered lamp socket. Looking closer
at the specs, it's actually 250 watts not 150 as I first thought, so even
with the forced air cooling it has, the lamp house runs pretty damned hot,
and the lamp socket with it. Basically, the ballast is not a very good
design. It doesn't run at all with no load on it, and as the lampholder was

That's by design, and it's because there's no active regulation,
so if the load is too small, the output voltage would be too high.
That's why a typical 20-60W electronic transformer has a minimum
load output. If you try it with a 10W lamp, it won't run as the
output voltage would be too high.

effectively open circuit, it indeed didn't run, which was the first way that
it fooled me. The second way was that not only does it not run with no load,
it only runs properly with the full designed load. So when I had it out on
the bench, and just had a couple of 50 watt 12 v halogen lamps in series as
a test load, it ran, but only until the mains input voltage got to 160 v on
my variac. Once it went past that, the switching activity progressively
died, until it stopped altogether again.

I had a replacement ceramic holder to hand, as I fix a fair few of these
lighting fixtures, so I grafted it in, and reconnected the lamp wiring to
the ballast. This time, it kicked up and ran the lamp at full intensity
without issue, so that's one to remember if I come across another of this
particular fixture type.

Arfa


--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

"Andrew Gabriel" wrote in message
...
In article ,
"Arfa Daily" writes:
Well, I've fixed the bloody thing now, and I have to say that it had me
fooled. The problem turned out to be a knackered lamp socket. Looking
closer
at the specs, it's actually 250 watts not 150 as I first thought, so even
with the forced air cooling it has, the lamp house runs pretty damned
hot,
and the lamp socket with it. Basically, the ballast is not a very good
design. It doesn't run at all with no load on it, and as the lampholder
was

That's by design, and it's because there's no active regulation,
so if the load is too small, the output voltage would be too high.
That's why a typical 20-60W electronic transformer has a minimum
load output. If you try it with a 10W lamp, it won't run as the
output voltage would be too high.



Yeah, I understand the reasons why. I guess that just condemning it as a
"not very good design", is setting aside the need for it to be cheap. I
suppose that I am 'spoiled' by usually working on properly designed clean
switchers that do have a tight control loop. I mean, at the end of the day,
it's not actually that hard or expensive to do, given that there are tons of
cheap 8 pin controller ICs out there. Thinking of some of the properly
regulated ones that I do work on regularly, they probably actually have less
components in them than this thing does ...

Still, done now, so it can go back on eBay when I have time !

Arfa


Andrew Gabriel
[email address is not usable -- followup in the newsgroup]