SMPS design ...

[Arfa Daily]

<snip>

Are the 330pF multilayer ceramic that could have metalisation creep /
cracks/ unreliable /leaky from humidity. I would replace them with a
different brand/construction

It would probably be worth doing some further checks on those caps to make
sure that they are not any part of the problem - if indeed there actually is
one. I'm not so sure now that I haven't somehow led myself up the garden
path, as I said elsewhere in the thread.

The actual type of cap is a high voltage pulse tolerant disc ceramic. I
think that these are single layer, aren't they ? Usually, if they do
anything, they get an arc track across them, and ultimately the plastic
coating splits. These are showing no signs of distress in any way. As to
changing them for a different type, I would consider that to be a big no. I
would never advocate using substitute parts anywhere in the primary side of
a switcher, particularly where high voltages and the magnetics are involved.
These things work by the skin of their teeth in the first place, and any
parts that I replace, are always sourced to be either dead ringers for the
originals wherever possible, or at worst, something of identical
construction and spec, from a different manufacturer.

Arfa

 

[Phil Allison]

"Arfa Daily"

"Phil Allison"
No, it's regulated DC.


** With just two transistors and one mosfet ?

That is what you have told us.

And SFA else.


You shouldn't take things quite so literally, Phil. I was talking about
the primary side only when I said that, but yes, it pretty much has a
pair of small TO92 transistors, and a TO220 mosfet. Obviously, it also
has input filtering, a bridge, made of 4 discrete diodes, a main filter
cap, and assorted R and C to make those two little transistors into an
oscillator. About 20 components altogether. The secondary side is
exactly like any other fixed voltage typical design, and there is, of
course, a perfectly normal 6 pin opto for regulation feedback. Better ?


** All that needed to be in the first post, plus the operating frequency.

Now it seems the whole story was BS anyhow.


Well, actually, it didn't, as it was irrelevant to the situation.

** The one YOU alone knew about and WE did not !!!

You fucking pommy retard.

How the mosfet receives its drive is neither here nor there

** Was NOT the issue - you bullshitting pommy turd.

Whether the SMPS was a normal regulated, DC type WAS !!!

With only 3 active devices mentioned, it seemed unlikely.

You fucking pommy retard.

As long as it has a pulse width modulated 'square' wave of sufficient
amplitude to fully drive the gate, how that waveform is produced is of no
consequence.

** Your qualification above was also MISSING from the story - fuckhead.

As to the operating frequency, yes, I probably should have stated this,
but most supplies of this sort of size, operate between 40 kHz and 80 KHz,
as would be understood by anyone who works with them regularly.

** FFS - you tenth witted MORON !!

We are considering a FAULTY design scenario here.

So NO such wild assumptions can be made - at all !!!

My god you are STUPID !!!

And why do you think it's all bullshit ?

** Errr - you just told us you were wrong about the mosfet heating issue.

You god almighty, pommy retard.




.... Phil

 

[petrus bitbyter]

"Arfa Daily" <arfa.daily@ntlworld.com> schreef in bericht
news:l5Knq.8102$Qv2.4777@newsfe29.ams2...

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:j7jb95$j6l$1@dont-email.me...
Arfa Daily <arfa.daily@ntlworld.com> wrote in message
news04nq.10308$tk5.3106@newsfe13.ams2...
I've been given a number of switchers to look at, by a company that I do
other work for. The one that concerns me here, is an open frame type,
single
12 volt output at, I would guess, 3 - 4 amps. It appears to be a very
simple
design, in that the chopper drive circuit is discrete, employing two
bipolar
transistors as an astable. The output of this is fed pretty much
directly
to
the gate of a single FET. In the drain of the FET, is a single primary
winding up to the raw rail from the input bridge.

Across that primary, is a network comprising a 330pF 2 Kv disc ceramic
cap,
and two 3 watt cement-body resistors, all in series. On every one of the
examples sent to me, the two resistors are chalky and very discoloured
to
the point where you can't read the bands. On some of them, one of the
resistors is open. Of the remaining resistors, they all seem to go
around
150 ohms, so I'm taking that to be the original value, based on the fact
that this type of resistor doesn't usually go low, and some of them have
gone open. Make no mistake, these resistors look like they run very hot
normally, to the point where the solder on their joints has crystalised,
and
on some, scorch damage has been done to the print, and the substrate.

Now here's the bit that I am finding puzzling. If you take one where one
or
both of the resistors has gone open, the supply works just fine. Loaded
up
to a couple of amps, it runs cool and the regulation is good. The
switching
FET is barely breaking a sweat, as you would expect. So I went ahead and
replaced the resistors with a pair of 150 ohm 3 watt types that look
pretty
much identical to the originals - even down to the blue body colour. The
cap
checks ok for value and leakage. With the resistors in place, the supply
still works just fine, except that it now runs pretty hot, even when
unloaded. The FET is a lot hotter than it was before. The resistors get
well
hot, as I was expecting, given the condition of the originals, but with
the
supply loaded up to a couple of amps, they get very hot, and the FET
becomes
uncomfortably hot as well.

As far as I can make out, doing some on-line reading about SMPS design,
this
network across the transformer primary, is a simple snubber (as opposed
to
a
clamp or combination clamp and snubber as is also sometimes found in
this
position). Texts suggest that its purpose is to limit the level of
voltage
spikes at the switching point, to keep the switching device operating
within
its SOA and reduce dissipation, which seems a fair enough comment.
However,
quite the reverse appears to be true. The whole supply seems a lot
happier
with that network 'not there' as it effectively is, when the resistors
are
open.

So has anyone got any good thoughts as to what is going on here ? I've
done
a great deal of repairs to switchers over the years, and am well versed
with
the principles of operation and repair, but I freely admit that I am not
a
designer in this field, so I'm at a bit of a loss as to whether it's
just
generally a poor design, or whether there's something else wrong that
I'm
missing. As those components were originally designed in, and are
clearly
faulty now, they need to be replaced, but the fact that the supply seems
to
run less efficiently when they are in place, feels altogether
counter-intuitive

Arfa



What happens if you double the R and halve the C ?

Don't know. However, these are a commercial item that I am repairing for a
company, and as they are, is as they were designed, and I guess the
company that wants me to mend them, would want them left as designed and
approved.

As it happens, today I got back to doing some more work on them, and this
time, the situation didn't seem half as bad, which I also can't figure. I
used the same mains isolation transformer to run them, and exactly the
same load - a couple of low voltage halogen lamps totaling 40 watts.
Today, the FET got no hotter under these conditions, than it did with no
load. In fact, it stayed quite cool. Replacement resistors still ran hot,
as I'm sure that they must be expected to, given that they are rated at 3
watts each, but not so hot that you would feel uncomfortable about them
over dissipating. This has left me a bit non-plussed. Something must be
different between what I was doing Monday, and what I did today, but I
can't figure what.

As to them generating high levels of RF, there is certainly no evidence on
a 'scope, of any RF on the switching waveform. There are a couple of
radios on in the workshop all the time, one of which is an HF radio
usually on 10 metres, and the other is a weather sat VHF one. Neither
showed any signs of picking up anything nasty whilst any of the supplies
was running.

The nominal switching frequency is around 50 kHz but on the rising edge,
there is a very tall very narrow spike when the resistors are burnt out.
When they are replaced, the spike is still there, but quite a lot smaller,
so I guess that the purpose of the network is to reduce the level of that
spike to get it down within the ratings of the switching FET. Because the
spike is very fast and narrow, I guess that the 330pF will have a much
smaller Xc to that component of the waveform.

Arfa

Didn't you walk right into the problem? It seems that by a yet unknown cause
the circuit sometimes runs into some state that makes it oscillate or in
some other way fries the resistors. This kind of intermitted faults are the
most difficult to solve as most of the times there seems to be no problem.
Even your observations with the scope may mislead you as connecting the
probe may change the circuit enough to change its behavior.

petrus bitbyter

 

[Ian Field]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9gc2qeFlseU1@mid.individual.net...

"Arfa Daily"
"Phil Allison"
No, it's regulated DC.


** With just two transistors and one mosfet ?

That is what you have told us.

And SFA else.


You shouldn't take things quite so literally, Phil. I was talking about
the primary side only when I said that, but yes, it pretty much has a
pair of small TO92 transistors, and a TO220 mosfet. Obviously, it also
has input filtering, a bridge, made of 4 discrete diodes, a main filter
cap, and assorted R and C to make those two little transistors into an
oscillator. About 20 components altogether. The secondary side is
exactly like any other fixed voltage typical design, and there is, of
course, a perfectly normal 6 pin opto for regulation feedback. Better ?


** All that needed to be in the first post, plus the operating
frequency.

Now it seems the whole story was BS anyhow.


Well, actually, it didn't, as it was irrelevant to the situation.


** The one YOU alone knew about and WE did not !!!

You fucking pommy retard.


How the mosfet receives its drive is neither here nor there


** Was NOT the issue - you bullshitting pommy turd.

Whether the SMPS was a normal regulated, DC type WAS !!!

With only 3 active devices mentioned, it seemed unlikely.

You fucking pommy retard.


As long as it has a pulse width modulated 'square' wave of sufficient
amplitude to fully drive the gate, how that waveform is produced is of no
consequence.

** Your qualification above was also MISSING from the story - fuckhead.


As to the operating frequency, yes, I probably should have stated this,
but most supplies of this sort of size, operate between 40 kHz and 80
KHz, as would be understood by anyone who works with them regularly.

** FFS - you tenth witted MORON !!

We are considering a FAULTY design scenario here.

So NO such wild assumptions can be made - at all !!!

My god you are STUPID !!!


And why do you think it's all bullshit ?


** Errr - you just told us you were wrong about the mosfet heating
issue.

You god almighty, pommy retard.




... Phil

Such eloquence.

 

[Ian Field]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:lr3oq.5208$UN2.1898@newsfe04.ams2...

"Jim Yanik" <jyanik@abuse.gov> wrote in message
news:Xns9F8477002E528jyaniklocalnetcom@216.168.3.44...
"Arfa Daily" <arfa.daily@ntlworld.com> wrote in
news:JKKnq.3161$V_3.67@newsfe06.ams2:



You shouldn't take things quite so literally, Phil. I was talking
about the primary side only when I said that, but yes, it pretty much
has a pair of small TO92 transistors, and a TO220 mosfet. Obviously,
it also has input filtering, a bridge, made of 4 discrete diodes, a
main filter cap, and assorted R and C to make those two little
transistors into an oscillator. About 20 components altogether. The
secondary side is exactly like any other fixed voltage typical design,
and there is, of course, a perfectly normal 6 pin opto for regulation
feedback. Better ?

Arfa



I've been wondering about how the regulating feedback loop works.
How does it modify the multivibrator's output?


--
Jim Yanik

Don't know, Jim. I don't have any schematics for it, as is ever the case,

Can't remember the last time I had a schematic for a PSU I was repairing, I
frequently trace them out by hand.

If its only an astable driving a MOSFET (or thereabouts) and you have a few
to fix, it shouldn't be too much of a hardship for the gain.

 

[Ian Field]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:MB3oq.5714$_p4.409@newsfe19.ams2...

snip


Are the 330pF multilayer ceramic that could have metalisation creep /
cracks/ unreliable /leaky from humidity. I would replace them with a
different brand/construction



It would probably be worth doing some further checks on those caps to make
sure that they are not any part of the problem

The low capacitance, high voltage disc-ceramics sometimes used for flyback
tuning in TV LOPT stage have been known to fail, ut its usually visible.

Sometimes the ceramic disc cracks and is usually visible because the resin
dip also cracks with it, sometimes they track around the edge of the ceramic
disc and burn a little "volcano" through the coating.

 

[Arfa Daily]

"Ian Field" <gangprobing.alien@ntlworld.com> wrote in message
news:Chdoq.686$bI5.366@newsfe10.ams2...

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9gc2qeFlseU1@mid.individual.net...

"Arfa Daily"
"Phil Allison"
No, it's regulated DC.


** With just two transistors and one mosfet ?

That is what you have told us.

And SFA else.


You shouldn't take things quite so literally, Phil. I was talking
about the primary side only when I said that, but yes, it pretty much
has a pair of small TO92 transistors, and a TO220 mosfet. Obviously,
it also has input filtering, a bridge, made of 4 discrete diodes, a
main filter cap, and assorted R and C to make those two little
transistors into an oscillator. About 20 components altogether. The
secondary side is exactly like any other fixed voltage typical design,
and there is, of course, a perfectly normal 6 pin opto for regulation
feedback. Better ?


** All that needed to be in the first post, plus the operating
frequency.

Now it seems the whole story was BS anyhow.


Well, actually, it didn't, as it was irrelevant to the situation.


** The one YOU alone knew about and WE did not !!!

You fucking pommy retard.


How the mosfet receives its drive is neither here nor there


** Was NOT the issue - you bullshitting pommy turd.

Whether the SMPS was a normal regulated, DC type WAS !!!

With only 3 active devices mentioned, it seemed unlikely.

You fucking pommy retard.


As long as it has a pulse width modulated 'square' wave of sufficient
amplitude to fully drive the gate, how that waveform is produced is of
no consequence.

** Your qualification above was also MISSING from the story - fuckhead.


As to the operating frequency, yes, I probably should have stated this,
but most supplies of this sort of size, operate between 40 kHz and 80
KHz, as would be understood by anyone who works with them regularly.

** FFS - you tenth witted MORON !!

We are considering a FAULTY design scenario here.

So NO such wild assumptions can be made - at all !!!

My god you are STUPID !!!


And why do you think it's all bullshit ?


** Errr - you just told us you were wrong about the mosfet heating
issue.

You god almighty, pommy retard.




... Phil


Such eloquence.

Indeed. I really don't know how the antipodean twat gets through life with
so little between his ears. Still, par for the course, I suppose. As I said,
just Philip spoiling for a fight, as ever ...

Arfa

 

[Arfa Daily]

"Ian Field" <gangprobing.alien@ntlworld.com> wrote in message
news:4ldoq.5595$gy7.2297@newsfe28.ams2...

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:lr3oq.5208$UN2.1898@newsfe04.ams2...


"Jim Yanik" <jyanik@abuse.gov> wrote in message
news:Xns9F8477002E528jyaniklocalnetcom@216.168.3.44...
"Arfa Daily" <arfa.daily@ntlworld.com> wrote in
news:JKKnq.3161$V_3.67@newsfe06.ams2:



You shouldn't take things quite so literally, Phil. I was talking
about the primary side only when I said that, but yes, it pretty much
has a pair of small TO92 transistors, and a TO220 mosfet. Obviously,
it also has input filtering, a bridge, made of 4 discrete diodes, a
main filter cap, and assorted R and C to make those two little
transistors into an oscillator. About 20 components altogether. The
secondary side is exactly like any other fixed voltage typical design,
and there is, of course, a perfectly normal 6 pin opto for regulation
feedback. Better ?

Arfa



I've been wondering about how the regulating feedback loop works.
How does it modify the multivibrator's output?


--
Jim Yanik

Don't know, Jim. I don't have any schematics for it, as is ever the case,


Can't remember the last time I had a schematic for a PSU I was repairing,
I frequently trace them out by hand.

If its only an astable driving a MOSFET (or thereabouts) and you have a
few to fix, it shouldn't be too much of a hardship for the gain.

The company have now come back to me, and accepted the quotes that I have
given them for quantity repair on this supply, so it will now be worth
spending the time to trace it out.

Arfa

 

[Arfa Daily]

"Ian Field" <gangprobing.alien@ntlworld.com> wrote in message
news:sqdoq.860$gS1.157@newsfe17.ams2...

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:MB3oq.5714$_p4.409@newsfe19.ams2...
snip


Are the 330pF multilayer ceramic that could have metalisation creep /
cracks/ unreliable /leaky from humidity. I would replace them with a
different brand/construction



It would probably be worth doing some further checks on those caps to
make sure that they are not any part of the problem

The low capacitance, high voltage disc-ceramics sometimes used for flyback
tuning in TV LOPT stage have been known to fail, ut its usually visible.

Sometimes the ceramic disc cracks and is usually visible because the resin
dip also cracks with it, sometimes they track around the edge of the
ceramic disc and burn a little "volcano" through the coating.

Yes, that's the type of cap that it is, and those modes of failure are my
experience of them also. These show no signs of anything like that, and
check ok for leakage - although not actually tested for this at high
voltage - and capacitance.

Arfa

 

[Phil Allison]

"Arfa Daily"


** Get utterly fucked

- you stupid, bullshitting, pommy TROLL

 

[Arfa Daily]

Didn't you walk right into the problem? It seems that by a yet unknown
cause the circuit sometimes runs into some state that makes it oscillate
or in some other way fries the resistors. This kind of intermitted faults
are the most difficult to solve as most of the times there seems to be no
problem. Even your observations with the scope may mislead you as
connecting the probe may change the circuit enough to change its behavior.

petrus bitbyter

Yes, there may indeed be some kind of intermittent problem. I will put them
all back up again on Monday, and see how they perform this time. As to my
'scope muddying the waters, it's generally pretty well behaved in that
respect. It's a high quality 100 MHz job, which is always used with a x 10
low capacitance probe. If that is having much of an effect on the circuit,
then it must for sure be a pretty poor design. To be honest, I still think
that the problem lies with me somehow. Something that I did differently
between the first and second times that I tried them, but I'm not sure what
....

Arfa

 

[Ian Field]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9geii5F9l4U1@mid.individual.net...

"Arfa Daily"


** Get utterly fucked

- you stupid, bullshitting, pommy TROLL

Missing your dingo mommy & daddy?

 

[Ian Field]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:Uuooq.6890$UN2.5702@newsfe04.ams2...

"Ian Field" <gangprobing.alien@ntlworld.com> wrote in message
news:sqdoq.860$gS1.157@newsfe17.ams2...

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:MB3oq.5714$_p4.409@newsfe19.ams2...
snip


Are the 330pF multilayer ceramic that could have metalisation creep /
cracks/ unreliable /leaky from humidity. I would replace them with a
different brand/construction



It would probably be worth doing some further checks on those caps to
make sure that they are not any part of the problem

The low capacitance, high voltage disc-ceramics sometimes used for
flyback tuning in TV LOPT stage have been known to fail, ut its usually
visible.

Sometimes the ceramic disc cracks and is usually visible because the
resin dip also cracks with it, sometimes they track around the edge of
the ceramic disc and burn a little "volcano" through the coating.


Yes, that's the type of cap that it is, and those modes of failure are my
experience of them also. These show no signs of anything like that, and
check ok for leakage - although not actually tested for this at high
voltage - and capacitance.

Arfa

They sometimes get very hot and discolour, that probably means either a
fault causing RF noise going into the snubber or sometimes bad design.

 

[petrus bitbyter]

"Arfa Daily" <arfa.daily@ntlworld.com> schreef in bericht
news:zDooq.16012$tk5.4043@newsfe13.ams2...

Didn't you walk right into the problem? It seems that by a yet unknown
cause the circuit sometimes runs into some state that makes it oscillate
or in some other way fries the resistors. This kind of intermitted faults
are the most difficult to solve as most of the times there seems to be no
problem. Even your observations with the scope may mislead you as
connecting the probe may change the circuit enough to change its
behavior.

petrus bitbyter


Yes, there may indeed be some kind of intermittent problem. I will put
them all back up again on Monday, and see how they perform this time. As
to my 'scope muddying the waters, it's generally pretty well behaved in
that respect. It's a high quality 100 MHz job, which is always used with a
x 10 low capacitance probe. If that is having much of an effect on the
circuit, then it must for sure be a pretty poor design. To be honest, I
still think that the problem lies with me somehow. Something that I did
differently between the first and second times that I tried them, but I'm
not sure what ...

Arfa

Off course you may have made a mistake. Humans make mistakes, even I
sometimes do
But if so, what on earth fried those resistors the first time?

petrus bitbyter

 

[josephkk]

On Sat, 22 Oct 2011 13:25:36 +0100, "Ian Field"
<gangprobing.alien@ntlworld.com> wrote:

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9geii5F9l4U1@mid.individual.net...

"Arfa Daily"


** Get utterly fucked

- you stupid, bullshitting, pommy TROLL


Missing your dingo mommy & daddy?

They disowned him for gloriously obvious reasons.

>=P

 

[Arfa Daily]

"petrus bitbyter" <petrus.bitbyter@hotmail.com> wrote in message
news:4ea31dff$0$482$e4fe514c@dreader26.news.xs4all.nl...

"Arfa Daily" <arfa.daily@ntlworld.com> schreef in bericht
news:zDooq.16012$tk5.4043@newsfe13.ams2...




Didn't you walk right into the problem? It seems that by a yet unknown
cause the circuit sometimes runs into some state that makes it oscillate
or in some other way fries the resistors. This kind of intermitted
faults are the most difficult to solve as most of the times there seems
to be no problem. Even your observations with the scope may mislead you
as connecting the probe may change the circuit enough to change its
behavior.

petrus bitbyter


Yes, there may indeed be some kind of intermittent problem. I will put
them all back up again on Monday, and see how they perform this time. As
to my 'scope muddying the waters, it's generally pretty well behaved in
that respect. It's a high quality 100 MHz job, which is always used with
a x 10 low capacitance probe. If that is having much of an effect on the
circuit, then it must for sure be a pretty poor design. To be honest, I
still think that the problem lies with me somehow. Something that I did
differently between the first and second times that I tried them, but I'm
not sure what ...

Arfa

Off course you may have made a mistake. Humans make mistakes, even I
sometimes do
But if so, what on earth fried those resistors the first time?

petrus bitbyter

Every example that I've seen so far, has the resistors badly discoloured and
the print and substrate scorched. On some, one of the resistors has been
open, so the network has not been doing the job it was put there for, at
all. They are 3 watt resistors, and even when the supply *appears* to be
running correctly, they get hot enough for you to say that they are probably
dissipating a good 3 watts, and maybe a bit more, so I would guess that you
would have to say that from that angle, it's a badly designed bit of
circuitry. I am fairly convinced that the purpose of the network is to
attenuate the big spike that occurs on the leading edge of the switching
waveform. This would tie in with why they have used about the biggest film
resistors they could get, rather than using a higher power wirewound type,
which would have a fair bit of inductance. I would also surmise that they
have used two x 150 ohm resistors rather than a single 330 ohm, to try to
spread the dissipation a bit.

When you replace the resistors, they still run hot, with no visible signs on
the 'scope of any 'RF' on the waveform, so you'd have to say that it *is*
working correctly. What led to this thread in the first place was that when
I was initially evaluating these supplies for the company that wants them
repairing, after I replaced the resistors, they ran very hot when the supply
was loaded, but seemed to just run 'acceptably' hot when it was idling.
Likewise, when loaded, the switching FET got very hot, but was almost cold
at idle.

However, when I next tried them - same conditions for i/p voltage and load,
as far as I was aware - they now seemed to be working much better in that
the resistors were just acceptably hot for all conditions, loaded or not,
and the FET remained cool also. So this has now led me to believe that it
must have been something I was doing differently - and wrongly - when it was
running very hot. So, a mistake ? Yes, probably. As you say, we all make
them, and this has got to be one of the easiest trades for doing it in.

I've just heard from the company that they are sending a bunch more up this
week, so if there's some more of this type amongst them, I'll have some more
'untouched' ones to see what happens this time.

Arfa

 

[Michael A. Terrell]

Arfa Daily wrote:

"petrus bitbyter" <petrus.bitbyter@hotmail.com> wrote in message
news:4ea31dff$0$482$e4fe514c@dreader26.news.xs4all.nl...

"Arfa Daily" <arfa.daily@ntlworld.com> schreef in bericht
news:zDooq.16012$tk5.4043@newsfe13.ams2...




Didn't you walk right into the problem? It seems that by a yet unknown
cause the circuit sometimes runs into some state that makes it oscillate
or in some other way fries the resistors. This kind of intermitted
faults are the most difficult to solve as most of the times there seems
to be no problem. Even your observations with the scope may mislead you
as connecting the probe may change the circuit enough to change its
behavior.

petrus bitbyter


Yes, there may indeed be some kind of intermittent problem. I will put
them all back up again on Monday, and see how they perform this time. As
to my 'scope muddying the waters, it's generally pretty well behaved in
that respect. It's a high quality 100 MHz job, which is always used with
a x 10 low capacitance probe. If that is having much of an effect on the
circuit, then it must for sure be a pretty poor design. To be honest, I
still think that the problem lies with me somehow. Something that I did
differently between the first and second times that I tried them, but I'm
not sure what ...

Arfa

Off course you may have made a mistake. Humans make mistakes, even I
sometimes do
But if so, what on earth fried those resistors the first time?

petrus bitbyter



Every example that I've seen so far, has the resistors badly discoloured and
the print and substrate scorched. On some, one of the resistors has been
open, so the network has not been doing the job it was put there for, at
all. They are 3 watt resistors, and even when the supply *appears* to be
running correctly, they get hot enough for you to say that they are probably
dissipating a good 3 watts, and maybe a bit more, so I would guess that you
would have to say that from that angle, it's a badly designed bit of
circuitry. I am fairly convinced that the purpose of the network is to
attenuate the big spike that occurs on the leading edge of the switching
waveform. This would tie in with why they have used about the biggest film
resistors they could get, rather than using a higher power wirewound type,
which would have a fair bit of inductance. I would also surmise that they
have used two x 150 ohm resistors rather than a single 330 ohm, to try to
spread the dissipation a bit.

When you replace the resistors, they still run hot, with no visible signs on
the 'scope of any 'RF' on the waveform, so you'd have to say that it *is*
working correctly. What led to this thread in the first place was that when
I was initially evaluating these supplies for the company that wants them
repairing, after I replaced the resistors, they ran very hot when the supply
was loaded, but seemed to just run 'acceptably' hot when it was idling.
Likewise, when loaded, the switching FET got very hot, but was almost cold
at idle.

However, when I next tried them - same conditions for i/p voltage and load,
as far as I was aware - they now seemed to be working much better in that
the resistors were just acceptably hot for all conditions, loaded or not,
and the FET remained cool also. So this has now led me to believe that it
must have been something I was doing differently - and wrongly - when it was
running very hot. So, a mistake ? Yes, probably. As you say, we all make
them, and this has got to be one of the easiest trades for doing it in.

I've just heard from the company that they are sending a bunch more up this
week, so if there's some more of this type amongst them, I'll have some more
'untouched' ones to see what happens this time.

Are you using metal or carbon film resistors? You could have a
resonance with a cap & metal film.


--
You can't have a sense of humor, if you have no sense.

 

[Ian Field]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:rq6dnSPqhbfurzvTnZ2dnUVZ_h-dnZ2d@earthlink.com...

Arfa Daily wrote:

"petrus bitbyter" <petrus.bitbyter@hotmail.com> wrote in message
news:4ea31dff$0$482$e4fe514c@dreader26.news.xs4all.nl...

"Arfa Daily" <arfa.daily@ntlworld.com> schreef in bericht
news:zDooq.16012$tk5.4043@newsfe13.ams2...




Didn't you walk right into the problem? It seems that by a yet
unknown
cause the circuit sometimes runs into some state that makes it
oscillate
or in some other way fries the resistors. This kind of intermitted
faults are the most difficult to solve as most of the times there
seems
to be no problem. Even your observations with the scope may mislead
you
as connecting the probe may change the circuit enough to change its
behavior.

petrus bitbyter


Yes, there may indeed be some kind of intermittent problem. I will put
them all back up again on Monday, and see how they perform this time.
As
to my 'scope muddying the waters, it's generally pretty well behaved
in
that respect. It's a high quality 100 MHz job, which is always used
with
a x 10 low capacitance probe. If that is having much of an effect on
the
circuit, then it must for sure be a pretty poor design. To be honest,
I
still think that the problem lies with me somehow. Something that I
did
differently between the first and second times that I tried them, but
I'm
not sure what ...

Arfa

Off course you may have made a mistake. Humans make mistakes, even I
sometimes do
But if so, what on earth fried those resistors the first time?

petrus bitbyter



Every example that I've seen so far, has the resistors badly discoloured
and
the print and substrate scorched. On some, one of the resistors has been
open, so the network has not been doing the job it was put there for, at
all. They are 3 watt resistors, and even when the supply *appears* to be
running correctly, they get hot enough for you to say that they are
probably
dissipating a good 3 watts, and maybe a bit more, so I would guess that
you
would have to say that from that angle, it's a badly designed bit of
circuitry. I am fairly convinced that the purpose of the network is to
attenuate the big spike that occurs on the leading edge of the switching
waveform. This would tie in with why they have used about the biggest
film
resistors they could get, rather than using a higher power wirewound
type,
which would have a fair bit of inductance. I would also surmise that they
have used two x 150 ohm resistors rather than a single 330 ohm, to try to
spread the dissipation a bit.

When you replace the resistors, they still run hot, with no visible signs
on
the 'scope of any 'RF' on the waveform, so you'd have to say that it *is*
working correctly. What led to this thread in the first place was that
when
I was initially evaluating these supplies for the company that wants them
repairing, after I replaced the resistors, they ran very hot when the
supply
was loaded, but seemed to just run 'acceptably' hot when it was idling.
Likewise, when loaded, the switching FET got very hot, but was almost
cold
at idle.

However, when I next tried them - same conditions for i/p voltage and
load,
as far as I was aware - they now seemed to be working much better in that
the resistors were just acceptably hot for all conditions, loaded or not,
and the FET remained cool also. So this has now led me to believe that it
must have been something I was doing differently - and wrongly - when it
was
running very hot. So, a mistake ? Yes, probably. As you say, we all make
them, and this has got to be one of the easiest trades for doing it in.

I've just heard from the company that they are sending a bunch more up
this
week, so if there's some more of this type amongst them, I'll have some
more
'untouched' ones to see what happens this time.


Are you using metal or carbon film resistors? You could have a
resonance with a cap & metal film.

There's allways low inductance thick film resistors, but they have to be
generously rated to minimise heating - they were a constant hassle in the
video O/P stage in one of the TCE CTVs, running too hot and repeated thermal
cycling took its toll on the pins attachment pads.

 

[Michael A. Terrell]

Ian Field wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:rq6dnSPqhbfurzvTnZ2dnUVZ_h-dnZ2d@earthlink.com...

Arfa Daily wrote:

"petrus bitbyter" <petrus.bitbyter@hotmail.com> wrote in message
news:4ea31dff$0$482$e4fe514c@dreader26.news.xs4all.nl...

"Arfa Daily" <arfa.daily@ntlworld.com> schreef in bericht
news:zDooq.16012$tk5.4043@newsfe13.ams2...




Didn't you walk right into the problem? It seems that by a yet
unknown
cause the circuit sometimes runs into some state that makes it
oscillate
or in some other way fries the resistors. This kind of intermitted
faults are the most difficult to solve as most of the times there
seems
to be no problem. Even your observations with the scope may mislead
you
as connecting the probe may change the circuit enough to change its
behavior.

petrus bitbyter


Yes, there may indeed be some kind of intermittent problem. I will put
them all back up again on Monday, and see how they perform this time.
As
to my 'scope muddying the waters, it's generally pretty well behaved
in
that respect. It's a high quality 100 MHz job, which is always used
with
a x 10 low capacitance probe. If that is having much of an effect on
the
circuit, then it must for sure be a pretty poor design. To be honest,
I
still think that the problem lies with me somehow. Something that I
did
differently between the first and second times that I tried them, but
I'm
not sure what ...

Arfa

Off course you may have made a mistake. Humans make mistakes, even I
sometimes do
But if so, what on earth fried those resistors the first time?

petrus bitbyter



Every example that I've seen so far, has the resistors badly discoloured
and
the print and substrate scorched. On some, one of the resistors has been
open, so the network has not been doing the job it was put there for, at
all. They are 3 watt resistors, and even when the supply *appears* to be
running correctly, they get hot enough for you to say that they are
probably
dissipating a good 3 watts, and maybe a bit more, so I would guess that
you
would have to say that from that angle, it's a badly designed bit of
circuitry. I am fairly convinced that the purpose of the network is to
attenuate the big spike that occurs on the leading edge of the switching
waveform. This would tie in with why they have used about the biggest
film
resistors they could get, rather than using a higher power wirewound
type,
which would have a fair bit of inductance. I would also surmise that they
have used two x 150 ohm resistors rather than a single 330 ohm, to try to
spread the dissipation a bit.

When you replace the resistors, they still run hot, with no visible signs
on
the 'scope of any 'RF' on the waveform, so you'd have to say that it *is*
working correctly. What led to this thread in the first place was that
when
I was initially evaluating these supplies for the company that wants them
repairing, after I replaced the resistors, they ran very hot when the
supply
was loaded, but seemed to just run 'acceptably' hot when it was idling.
Likewise, when loaded, the switching FET got very hot, but was almost
cold
at idle.

However, when I next tried them - same conditions for i/p voltage and
load,
as far as I was aware - they now seemed to be working much better in that
the resistors were just acceptably hot for all conditions, loaded or not,
and the FET remained cool also. So this has now led me to believe that it
must have been something I was doing differently - and wrongly - when it
was
running very hot. So, a mistake ? Yes, probably. As you say, we all make
them, and this has got to be one of the easiest trades for doing it in.

I've just heard from the company that they are sending a bunch more up
this
week, so if there's some more of this type amongst them, I'll have some
more
'untouched' ones to see what happens this time.


Are you using metal or carbon film resistors? You could have a
resonance with a cap & metal film.

There's always low inductance thick film resistors, but they have to be
generously rated to minimise heating - they were a constant hassle in the
video O/P stage in one of the TCE CTVs, running too hot and repeated thermal
cycling took its toll on the pins attachment pads.

Carbon film is less inductive. Carbon comp would be ideal, but most
EEs these days don't know they exist. Low power SMD metal film
resistors do a lot better at UHF and Microwave frequencies. We used
them at 10 GHz, with no problems. Larger, high power metal film on a
round core are a spiral of metal.


--
You can't have a sense of humor, if you have no sense.

 

[Ian Field]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:cKOdnVfqb_OHuDrTnZ2dnUVZ_oqdnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:rq6dnSPqhbfurzvTnZ2dnUVZ_h-dnZ2d@earthlink.com...

Arfa Daily wrote:

"petrus bitbyter" <petrus.bitbyter@hotmail.com> wrote in message
news:4ea31dff$0$482$e4fe514c@dreader26.news.xs4all.nl...

"Arfa Daily" <arfa.daily@ntlworld.com> schreef in bericht
news:zDooq.16012$tk5.4043@newsfe13.ams2...




Didn't you walk right into the problem? It seems that by a yet
unknown
cause the circuit sometimes runs into some state that makes it
oscillate
or in some other way fries the resistors. This kind of intermitted
faults are the most difficult to solve as most of the times there
seems
to be no problem. Even your observations with the scope may
mislead
you
as connecting the probe may change the circuit enough to change
its
behavior.

petrus bitbyter


Yes, there may indeed be some kind of intermittent problem. I will
put
them all back up again on Monday, and see how they perform this
time.
As
to my 'scope muddying the waters, it's generally pretty well
behaved
in
that respect. It's a high quality 100 MHz job, which is always used
with
a x 10 low capacitance probe. If that is having much of an effect
on
the
circuit, then it must for sure be a pretty poor design. To be
honest,
I
still think that the problem lies with me somehow. Something that I
did
differently between the first and second times that I tried them,
but
I'm
not sure what ...

Arfa

Off course you may have made a mistake. Humans make mistakes, even I
sometimes do
But if so, what on earth fried those resistors the first time?

petrus bitbyter



Every example that I've seen so far, has the resistors badly
discoloured
and
the print and substrate scorched. On some, one of the resistors has
been
open, so the network has not been doing the job it was put there for,
at
all. They are 3 watt resistors, and even when the supply *appears* to
be
running correctly, they get hot enough for you to say that they are
probably
dissipating a good 3 watts, and maybe a bit more, so I would guess
that
you
would have to say that from that angle, it's a badly designed bit of
circuitry. I am fairly convinced that the purpose of the network is to
attenuate the big spike that occurs on the leading edge of the
switching
waveform. This would tie in with why they have used about the biggest
film
resistors they could get, rather than using a higher power wirewound
type,
which would have a fair bit of inductance. I would also surmise that
they
have used two x 150 ohm resistors rather than a single 330 ohm, to try
to
spread the dissipation a bit.

When you replace the resistors, they still run hot, with no visible
signs
on
the 'scope of any 'RF' on the waveform, so you'd have to say that it
*is*
working correctly. What led to this thread in the first place was that
when
I was initially evaluating these supplies for the company that wants
them
repairing, after I replaced the resistors, they ran very hot when the
supply
was loaded, but seemed to just run 'acceptably' hot when it was
idling.
Likewise, when loaded, the switching FET got very hot, but was almost
cold
at idle.

However, when I next tried them - same conditions for i/p voltage and
load,
as far as I was aware - they now seemed to be working much better in
that
the resistors were just acceptably hot for all conditions, loaded or
not,
and the FET remained cool also. So this has now led me to believe that
it
must have been something I was doing differently - and wrongly - when
it
was
running very hot. So, a mistake ? Yes, probably. As you say, we all
make
them, and this has got to be one of the easiest trades for doing it
in.

I've just heard from the company that they are sending a bunch more up
this
week, so if there's some more of this type amongst them, I'll have
some
more
'untouched' ones to see what happens this time.


Are you using metal or carbon film resistors? You could have a
resonance with a cap & metal film.

There's always low inductance thick film resistors, but they have to be
generously rated to minimise heating - they were a constant hassle in the
video O/P stage in one of the TCE CTVs, running too hot and repeated
thermal
cycling took its toll on the pins attachment pads.


Carbon film is less inductive. Carbon comp would be ideal, but most
EEs these days don't know they exist. Low power SMD metal film
resistors do a lot better at UHF and Microwave frequencies. We used
them at 10 GHz, with no problems. Larger, high power metal film on a
round core are a spiral of metal.

Why do you think I suggested thick film!