On Friday, 2 February 2018 14:04:25 UTC, Terry Casey wrote:
In article ,
says...

Someone suggested earlier that a 100W lamp could be wired
across the open ends.

That seemed a far more practical solution.


Yes - a lamp limiter - common practice for enthusiasts
renovating old radios and TVs - especially TVs.

They usually have a bulb and bulbholder mounted on their work
bench in series with the mains supply with a bypass switch to
short out the lamp when they are satisfied that no damage will
result.

20mm glass fuses may be unsuitable if it's mains fusing, they have low rupturing capacity. A light bulb is a nondestructive option.
Of course if you put a 3A fuse in the plug you don't need one in the charger - purely for testing I mean.


NT

On 02/02/2018 14:37, wrote:
On Friday, 2 February 2018 14:04:25 UTC, Terry Casey wrote:
In article ,
says...

Someone suggested earlier that a 100W lamp could be wired
across the open ends.

That seemed a far more practical solution.


Yes - a lamp limiter - common practice for enthusiasts
renovating old radios and TVs - especially TVs.

They usually have a bulb and bulbholder mounted on their work
bench in series with the mains supply with a bypass switch to
short out the lamp when they are satisfied that no damage will
result.

20mm glass fuses may be unsuitable if it's mains fusing, they have low rupturing capacity. A light bulb is a nondestructive option.
Of course if you put a 3A fuse in the plug you don't need one in the charger - purely for testing I mean.


NT

Update:
So fuse arrived today, soldered in to the charger, switched on and POP!
That will be for the bin then as now beyond my understanding of electronics.
At least it never exploded this time.

Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.

In article 481d0fbe-e0ce-451b-a273-
, says...


20mm glass fuses may be unsuitable if it's mains fusing, they have low rupturing capacity. A light bulb is a nondestructive option.
Of course if you put a 3A fuse in the plug you don't need one in the charger - purely for testing I mean.


Yes. There are usually two failure modes. In one, the wire
vapourises, coating the inside of the glass jet black or, in
more violent cases, the entire guts of the fuse disappear,
leaving only the two end caps. In such cases, I've never found
any trace of the glass or wire anywhere!

I've never seen one where half the glass and wire remain but
that glass does look unusually thick

--

Terry

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On Fri, 2 Feb 2018 05:42:39 -0800 (PST), wrote:

On Thursday, 1 February 2018 22:58:47 UTC, Archibald Tarquin Blenkinsopp wrote:
On Thu, 1 Feb 2018 14:21:39 -0800 (PST), tabbypurr wrote:
On Thursday, 1 February 2018 21:52:28 UTC, Archibald Tarquin Blenkinsopp wrote:
On Thu, 1 Feb 2018 13:28:38 -0800 (PST), tabbypurr wrote:
On Thursday, 1 February 2018 21:05:05 UTC, Archibald Tarquin Blenkinsopp wrote:
On Thu, 1 Feb 2018 12:45:45 -0800 (PST), tabbypurr wrote:
On Thursday, 1 February 2018 20:12:30 UTC, Archibald Tarquin Blenkinsopp wrote:
On Thu, 01 Feb 2018 08:33:20 GMT, Harry Bloomfield
wrote:
ss used his keyboard to write :

Charger for a mobility scooter. One charger not working, I opened it up and
as per image below it looks like some kind of fuse has blown.
Text on the board looks like 3A/2500AR.

Can anyone confirm and identify what I would need to replace.
I have limitations on electronic stuff but could probably manage to solder
this part if I can find a replacement.

https://imgur.com/a/QTZ0e

From what I can see in the image, it looks like a wire ended fuse which
has exploded. That suggests a serious fault.

Also from what I can see, it looks like a fairly crude charger.
transformer, bridge, then possibly a relay switching the charge current
on and off, driven by a basic voltage across the battery sensing
circuit.

Cobblers!

One does not need an NTC thermistor in a "crude" charger.

I would have serious doubts that terminal Voltage plays a big part in
the control feedback. The control is via current monitoring so a more
stable source of reference would be needed.

There would be no piddly little rectifiers in a "crude" charger
either, they would be after the tranny and would need to handle a few
Amps, assuming the charge process was to take less than 24 hours.

Incidentally an exploded fuse is a fairly minor fault from a
diagnostics perspective. Generally a few seconds with a DMM will find
the problem.

As to what happens when the little plastic thing with legs = 3 is
found to be S/C, is a completely different story.

AB

I'm puzzled by your comments. What sort of lead acid charger is not voltage regulated?


NT

The statement was "terminal Voltage".

Direct Voltage regulation isn't really an option. Have you tried
charging a 12Volt lead acid battery from 12Volts?

Have you tried charging from 14Volts?


Can you suggest a few drawbacks to using a supply regulated to give
either Voltage, or even one in between those values?

The internal resistance of a battery is dependent on a number of
things, age, temperature and the amount of charge stored, therefore
the terminal Voltage of the battery isn't the best starting point for
control.

AB

I can only conclude you don't understand lead acid charging.


NT
What aspect?

I must admit the chemistry is not something that I have dealt with for
some considerable number of years.

I am however keen to learn, just how would an expert such as your good
self approach charger design?

AB

Lead acids are charged with voltage regulated chargers. There are 1 stage or better chargers have more stages. The charger must of course have overcurrent limit too, but it is the battery itself that limits i much of the time.

As opposed to NiCd & NiMH which require current controlled chargers.


NT
That's deep!

So you have Voltage regulation?

Why do you need an overcurrent limit? You already have Voltage
regulation, so how can the current become excessive?


When you say the battery limits the current "much of the time", just
how much of the time?

At what point does the battery stop limiting the current and why
doesn't your Voltage regulation just reduce the Voltage to the point
where you are not reliant on the environmental variables of the
surroundings to influence current consumption?

AB

the keywords you're looking for a lead acid battery charging


NT

If it makes it easier for you.......

When you say the lead acid battery battery limits the current "much of
the time", just
how much of the time?


At what point does the lead acid battery stop limiting the current
and why doesn't your Voltage regulation just reduce the Voltage to the
point where you are not reliant on the environmental variables of the
surroundings to influence current consumption?

I trust this helps?

AB

On Fri, 2 Feb 2018 14:04:22 -0000, Terry Casey
wrote:

In article ,
says...

Someone suggested earlier that a 100W lamp could be wired
across the open ends.

That seemed a far more practical solution.


Yes - a lamp limiter - common practice for enthusiasts
renovating old radios and TVs - especially TVs.

They usually have a bulb and bulbholder mounted on their work
bench in series with the mains supply with a bypass switch to
short out the lamp when they are satisfied that no damage will
result.

I'm glad you said enthusiast!

In my dim and distant, working as a TV Engineer, another fuse went in
first.

Some of those spring type antisurges had an impressive delay time, far
longer than the regulator, feed rectifiers and printed circuit track.

:-)

AB

On Fri, 2 Feb 2018 15:29:34 +0000, ss wrote:

snip

Update:
So fuse arrived today, soldered in to the charger, switched on and POP!

As some predicted it might. ;-)

That will be for the bin then as now beyond my understanding of electronics.

Before you do ... do you or a friend have a Digital Multimeter (or if
you have anything with a battery in it, get one yourself anyway). Put
it on Diode test, or the lowest value of ohms and test both ways
across all four of the diodes (with it disconnected from the mains of
course). If all is well with the diodes you should see a reading in
one direction and different in the other (equally on all 4 diodes),
but not a short circuit across any. Any showing as a short circuit are
probably faulty and can be replaced easily and cheaply. (You might
also just desolder one end of each diode that shows as a short and
test it like that in case the short is coming from elsewhere).

It's not difficult to do and might only cost pence if it turns out to
be a diode (or two).

At least it never exploded this time.

;-)

Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.

So time to test the diodes in the other charger. ;-)

Cheers, T i m

On Fri, 2 Feb 2018 06:37:03 -0800 (PST), wrote:

On Friday, 2 February 2018 14:04:25 UTC, Terry Casey wrote:
In article ,
says...

Someone suggested earlier that a 100W lamp could be wired
across the open ends.

That seemed a far more practical solution.


Yes - a lamp limiter - common practice for enthusiasts
renovating old radios and TVs - especially TVs.

They usually have a bulb and bulbholder mounted on their work
bench in series with the mains supply with a bypass switch to
short out the lamp when they are satisfied that no damage will
result.

20mm glass fuses may be unsuitable if it's mains fusing, they have low rupturing capacity. A light bulb is a nondestructive option.
Of course if you put a 3A fuse in the plug you don't need one in the charger - purely for testing I mean.


NT
You crafty little devil you!!

And here was me thinking that the light bulb was to help you see the
fuse blow.

20mm fuses come in a wide variety of flavours, they do not have to be
glass, but even glass ones can be low rupturing capacity.


If you wish to share your wisdom with a few hundred thousand
manufacturers that use them for mains fusing, I'm sure they will be
thankful.

AB

On 02/02/2018 16:05, T i m wrote:
On Fri, 2 Feb 2018 15:29:34 +0000, ss wrote:

snip

Update:
So fuse arrived today, soldered in to the charger, switched on and POP!

As some predicted it might. ;-)

That will be for the bin then as now beyond my understanding of electronics.

Before you do ... do you or a friend have a Digital Multimeter (or if
you have anything with a battery in it, get one yourself anyway). Put
it on Diode test, or the lowest value of ohms and test both ways
across all four of the diodes (with it disconnected from the mains of
course). If all is well with the diodes you should see a reading in
one direction and different in the other (equally on all 4 diodes),
but not a short circuit across any. Any showing as a short circuit are
probably faulty and can be replaced easily and cheaply. (You might
also just desolder one end of each diode that shows as a short and
test it like that in case the short is coming from elsewhere).

It's not difficult to do and might only cost pence if it turns out to
be a diode (or two).

At least it never exploded this time.

;-)

Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.

So time to test the diodes in the other charger. ;-)

Cheers, T i m

Thanks Tim, yes I have a multi meter but basically use it for continuity
and mains V checking, I`ll try what you suggest, its about time I got
some better use out of it.

On Fri, 02 Feb 2018 16:05:24 +0000, T i m wrote:

On Fri, 2 Feb 2018 15:29:34 +0000, ss wrote:

snip

Update:
So fuse arrived today, soldered in to the charger, switched on and POP!

As some predicted it might. ;-)

That will be for the bin then as now beyond my understanding of electronics.

Before you do ... do you or a friend have a Digital Multimeter (or if
you have anything with a battery in it, get one yourself anyway). Put
it on Diode test, or the lowest value of ohms and test both ways
across all four of the diodes (with it disconnected from the mains of
course). If all is well with the diodes you should see a reading in
one direction and different in the other (equally on all 4 diodes),
but not a short circuit across any. Any showing as a short circuit are
probably faulty and can be replaced easily and cheaply. (You might
also just desolder one end of each diode that shows as a short and
test it like that in case the short is coming from elsewhere).

It's not difficult to do and might only cost pence if it turns out to
be a diode (or two).

At least it never exploded this time.

;-)

Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.

So time to test the diodes in the other charger. ;-)

Cheers, T i m

While prodding around under the board, stick the meter across the legs
of the device attached to the heatsink.

There should be a high reading from possibly the center pin to the
other two. If Q1 is a transistor that is!

The bit that has the Volts on will go to the ferrite transformer.

All three legs reading low Ohms to each other is a fault and may be
more likely than a blown rectifier.

Having said that, if you have a dodgy plug connection or are using one
of those adapters using copper foil to make contact, it's yer diodes.

If the transistor is S/C, it may just be that but its worth checking
the low Ohms resistors in the emitter. These often fail when the full
whack of current goes through Q1.

Finally if the "transistor" is S/C, it is also worth checking the
shaping network on the collector.

On this diagram the items are R12 and C9.

https://www.bing.com/images/search?v...x=0&ajaxhist=0

The resistors here frequently went o/c and this led to a very short
lifespan for Q3 in the diagrams case, or Q1 for your board.

On a final note, it would appear that Q1 is a bog standard transistor,
but all sorts of things get included in epoxy packages these days.

On the plus side, it may have a part number stamped on :-)


The last SMPS I bought from Ebay was less than £5-00.

When I repaired them for a local industrial electronics company, the
repair would cost the client forty time that sum.

Happy days :-)


AB

On Fri, 02 Feb 2018 15:29:34 +0000, ss wrote:

====snip====

Update:
So fuse arrived today, soldered in to the charger, switched on and POP!

Just as I expected and tried to advise you of this most likely outcome.
Once that safety fuse has blown, you can guarantee that the cause was an
HT switching transistor[1] going short circuit for one reason or another
which may well involve a fault in any of the components in the driving
and control circuitry. Just blindly replacing a blown HT switching
transistor is far from guaranteeing a successful repair.

That will be for the bin then as now beyond my understanding of
electronics.

And mine, too (and I'm far more knowledgeable of SMPSU basics than you
[2]).

At least it never exploded this time.

Probably just a matter of "dumb luck". :-)


Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.

From what I can recall, that looks a much better bet.

--
Johnny B Good

On Friday, 2 February 2018 15:29:46 UTC, ss wrote:
On 02/02/2018 14:37, tabbypurr wrote:
On Friday, 2 February 2018 14:04:25 UTC, Terry Casey wrote:
In article ,
says...

Someone suggested earlier that a 100W lamp could be wired
across the open ends.

That seemed a far more practical solution.


Yes - a lamp limiter - common practice for enthusiasts
renovating old radios and TVs - especially TVs.

They usually have a bulb and bulbholder mounted on their work
bench in series with the mains supply with a bypass switch to
short out the lamp when they are satisfied that no damage will
result.

20mm glass fuses may be unsuitable if it's mains fusing, they have low rupturing capacity. A light bulb is a nondestructive option.
Of course if you put a 3A fuse in the plug you don't need one in the charger - purely for testing I mean.


NT

Update:
So fuse arrived today, soldered in to the charger, switched on and POP!
That will be for the bin then as now beyond my understanding of electronics.
At least it never exploded this time.

Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.

worth replacing the 4 mains diodes if you know how, those are the most likely cause.


NT

On Friday, 2 February 2018 16:14:27 UTC, Archibald Tarquin Blenkinsopp wrote:
On Fri, 2 Feb 2018 06:37:03 -0800 (PST), tabbypurr wrote:
On Friday, 2 February 2018 14:04:25 UTC, Terry Casey wrote:
In article ,
says...

Someone suggested earlier that a 100W lamp could be wired
across the open ends.

That seemed a far more practical solution.


Yes - a lamp limiter - common practice for enthusiasts
renovating old radios and TVs - especially TVs.

They usually have a bulb and bulbholder mounted on their work
bench in series with the mains supply with a bypass switch to
short out the lamp when they are satisfied that no damage will
result.

20mm glass fuses may be unsuitable if it's mains fusing, they have low rupturing capacity. A light bulb is a nondestructive option.
Of course if you put a 3A fuse in the plug you don't need one in the charger - purely for testing I mean.


NT
You crafty little devil you!!

And here was me thinking that the light bulb was to help you see the
fuse blow.

20mm fuses come in a wide variety of flavours, they do not have to be
glass, but even glass ones can be low rupturing capacity.


If you wish to share your wisdom with a few hundred thousand
manufacturers that use them for mains fusing, I'm sure they will be
thankful.

AB

I'm sure they know what they're doing just fine. You plainly don't.


NT

On Fri, 2 Feb 2018 14:04:22 -0000, Terry Casey
wrote:


Yes - a lamp limiter - common practice for enthusiasts
renovating old radios and TVs - especially TVs.

In article ,
says...

I'm glad you said enthusiast!

In my dim and distant, working as a TV Engineer, another fuse went in
first.


The same when I was in the trade but remember, we weren't
powering up sets that hadn't seen the light of day, let alone
power, for 20, 30 years or more!

Usually a Variac is used as well as the main smoothing, at
least, usually needs reforming so needs to be powered up
gradually.

I don't dabble myself but do follow a couple of forums where
most members do. It is surprising how often the smoothing
reforms OK - a bonus as you can't whistle up a replacement
from RS Components these days!

In fact, before even switching on, there is a visual
inspection and the suppressor capacitor across the incoming
mains supply snipped out!

--

Terry

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http://www.avg.com

On 02/02/2018 18:52, wrote:
Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.
worth replacing the 4 mains diodes if you know how, those are the most likely cause.

Readings from diodes, hope it makes sense:

https://imgur.com/a/zxgxH

On 02/02/2018 16:05, T i m wrote:
On Fri, 2 Feb 2018 15:29:34 +0000, ss wrote:

snip

Update:
So fuse arrived today, soldered in to the charger, switched on and POP!

As some predicted it might. ;-)

That will be for the bin then as now beyond my understanding of electronics.

Before you do ... do you or a friend have a Digital Multimeter (or if
you have anything with a battery in it, get one yourself anyway). Put
it on Diode test, or the lowest value of ohms and test both ways
across all four of the diodes (with it disconnected from the mains of
course). If all is well with the diodes you should see a reading in
one direction and different in the other (equally on all 4 diodes),
but not a short circuit across any. Any showing as a short circuit are
probably faulty and can be replaced easily and cheaply. (You might
also just desolder one end of each diode that shows as a short and
test it like that in case the short is coming from elsewhere).

It's not difficult to do and might only cost pence if it turns out to
be a diode (or two).

At least it never exploded this time.

;-)

Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.

So time to test the diodes in the other charger. ;-)

Cheers, T i m

Readings from diodes, hope it makes sense:

https://imgur.com/a/zxgxH

On Fri, 02 Feb 2018 18:00:36 +0000, Johnny B Good wrote:

On Fri, 02 Feb 2018 15:29:34 +0000, ss wrote:

====snip====

Update:
So fuse arrived today, soldered in to the charger, switched on and POP!

Just as I expected and tried to advise you of this most likely outcome.
Once that safety fuse has blown, you can guarantee that the cause was an
HT switching transistor[1] going short circuit for one reason or another
which may well involve a fault in any of the components in the driving
and control circuitry. Just blindly replacing a blown HT switching
transistor is far from guaranteeing a successful repair.

That will be for the bin then as now beyond my understanding of
electronics.

And mine, too (and I'm far more knowledgeable of SMPSU basics than you
[2]).

At least it never exploded this time.

Probably just a matter of "dumb luck". :-)


Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.

From what I can recall, that looks a much better bet.

Oops! Forgot to add the [notes]

[1] or, if you're very lucky, a shorted bridge rectifier diode.

[2] I may know the basics but most of the drive and control circuitry in
modern day SMPSUs will be integrated into a custom controller chip or two
which would need to be identified and a data sheet tracked down before
one can even begin to think about attempting to diagnose and repair such
a unit. Unless it's truly impossible to substitute the failed SMPSU with
another off-the-shelf unit, it's normally much cheaper to replace rather
than repair an existing one.

--
Johnny B Good

On 02/02/2018 19:52, ss wrote:


Readings from diodes, hope it makes sense:

https://imgur.com/a/zxgxH

Something wrong with number one.
They should be similar.
Check for any derbies from the explosion on the tracks/components that
may be shorting it,

Then consider unsoldering/cutting it out and testing as a bare device to
make sure its the faulty device before trying a new one.

On Friday, 2 February 2018 19:52:07 UTC, ss wrote:
On 02/02/2018 18:52, tabbypurr wrote:
Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.
worth replacing the 4 mains diodes if you know how, those are the most likely cause.

Readings from diodes, hope it makes sense:

https://imgur.com/a/zxgxH

Well, sort of. Normally on the diode range the meter reports the Vf of the diode. However the bottom right one is clearly very different to the others, so it looks like they're suspect. One shorted diode could be enough to take out a fuse.


NT

On 02/02/2018 20:37, dennis@home wrote:
On 02/02/2018 19:52, ss wrote:


Readings from diodes, hope it makes sense:

https://imgur.com/a/zxgxH

Something wrong withÂ* number one.
They should be similar.
Check for any derbies from the explosion on the tracks/components that
may be shorting it,

Then consider unsoldering/cutting it out and testing as a bare device to
make sure its the faulty device before trying a new one.


OK.
On diodes 1 & 2 showing the readings 157.7 & 1.4 on the underside the
solder is connecting the 2 diodes, is this correct? Image below.

https://imgur.com/a/KM4b6

Roger Hayter brought next idea :
I think that would only work for trickle charging. If you want to
charge at a decent rate then the battery has to be charged at a somewhat
higher voltage, overlapping what would be the fully charged voltage.

I agree entirely, but my experience with a permanently on trickle
charge, will dry a battery out eventually.

In article -
september.org, lid says...

In article ,
says...

In my dim and distant, working as a TV Engineer, another fuse went in
first.


The same when I was in the trade but remember, we weren't
powering up sets that hadn't seen the light of day, let alone
power, for 20, 30 years or more!


Actually, I think I ought to clarify this. If the fuse had
just gently parted companydue to an overload, after a quick
visual check it was easiest to replace the fuse, switch on and
look for smoke or possibly a valve anode starting to glow red
as the set warmed up. If you were lucky, this might point you
in exactly the right direction.

On the other hand, if the fuse had blown due to an obvious
short, as here, it would be a waste of time to just replace it
without carriying out an investigation first.


--

Terry

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http://www.avg.com

In article c1ee650b-fb8a-40ed-a58d-ec06aff2d137
@googlegroups.com, says...


Well, sort of. Normally on the diode range the meter reports the Vf of the diode. However the bottom right one is clearly very different to the others, so it looks like they're suspect. One shorted diode could be enough to take out a fuse.


The OP didn't say anything about the diode range and his
reading results clearly states that they were taken on the 200
ohm range of his meter.

They clearly show that D1 is short circuit.

QED

--

Terry

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http://www.avg.com

On 01/02/2018 23:06, ss wrote:
On 01/02/2018 21:34, Fredxx wrote:
The earths are ok, no burn marks.
I dont think the blasted fuse is in a fuse holder as such as when I
check on ebay they look similar ie as if they are in a holder.

I have known a suppression type capacitor to go short and the the
corresponding fuse to go in an old TV. In my case there was little
left of a conventional holder.

In this case the fuse is in series with the thermister.

I suspect the problem is whatever was plugged into CN3.

CN3 connects to the on/off switch on the outer casing.

Ah, then check the blue cap between the fuse and earth, see if it's a short.

If not I might be tempted to put another fuse in and wear ear defenders
and eye protection while switching it on.

On Fri, 2 Feb 2018 21:37:25 -0000, Terry Casey
wrote:

In article -
september.org, lid says...

In article ,
says...

In my dim and distant, working as a TV Engineer, another fuse went in
first.


The same when I was in the trade but remember, we weren't
powering up sets that hadn't seen the light of day, let alone
power, for 20, 30 years or more!


Actually, I think I ought to clarify this. If the fuse had
just gently parted companydue to an overload, after a quick
visual check it was easiest to replace the fuse, switch on and
look for smoke or possibly a valve anode starting to glow red
as the set warmed up. If you were lucky, this might point you
in exactly the right direction.

On the other hand, if the fuse had blown due to an obvious
short, as here, it would be a waste of time to just replace it
without carriying out an investigation first.

Granted,
Apart from the grey polyester o.1uF dangling across the
back of the power switch.

The end was often blasted off!!!


When I started in the trade I dilligently replaced em.

Anyway it was more fun to see the flash.

AB

On 02/02/18 19:56, Johnny B Good wrote:
[1] or, if you're very lucky, a shorted bridge rectifier diode.


That seems virtually certain.

[2] I may know the basics but most of the drive and control circuitry in
modern day SMPSUs will be integrated into a custom controller chip or two
which would need to be identified and a data sheet tracked down before
one can even begin to think about attempting to diagnose and repair such
a unit. Unless it's truly impossible to substitute the failed SMPSU with
another off-the-shelf unit, it's normally much cheaper to replace rather
than repair an existing one.

No: By and large the chips don't blow, There is usually a chopper power
MOSFET and that is what takes the strain if a diode goes bad.

Replace diode, 450V cap and power FET and fuse and it should work




--
Socialism is the philosophy of failure, the creed of ignorance and the
gospel of envy.

Its inherent virtue is the equal sharing of misery.

Winston Churchill

The Natural Philosopher laid this down on his screen :
No: By and large the chips don't blow, There is usually a chopper power
MOSFET and that is what takes the strain if a diode goes bad.

Replace diode, 450V cap and power FET and fuse and it should work

I agree, but rather than simply replacing the cap, I would test it
first.

On 03/02/18 07:27, Harry Bloomfield wrote:
The Natural Philosopher laid this down on his screen :
No: By and large the chips don't blow, There is usually a chopper
power MOSFET and that is what takes the strain if a diode goes bad.

Replace diode, 450V cap and power FET and fuse and it should work

I agree, but rather than simply replacing the cap, I would test it first.

If you have the ability to do so, yes, and can spare the time..


I'd estimate that a driver would cover all those parts - and take less
than half an hour to do.

Why bother testing?

When years ago I worked in a comany producing rather shoddy audio power
amps, the girls who fixed them had zero theoretical knowledge. They were
taught to replace all of the output transistors, the drivers, the fuses,
and the power resistors if they looked burnt.


about 15 mninutes with almost completely unskilled labour...

The transistors were then tested later by an engineer on a transistor
tester, and good ones put back as 'replacment parts' in the repair
department.

I cant recall whether that tester was not in fact a simple home made
device checking that they werent short circuit and exhibited some gain...


--
No Apple devices were knowingly used in the preparation of this post.

On Friday, 2 February 2018 20:53:45 UTC, ss wrote:
On 02/02/2018 20:37, dennis@home wrote:
On 02/02/2018 19:52, ss wrote:


Readings from diodes, hope it makes sense:

https://imgur.com/a/zxgxH

Something wrong withÂ* number one.
They should be similar.
Check for any derbies from the explosion on the tracks/components that
may be shorting it,

Then consider unsoldering/cutting it out and testing as a bare device to
make sure its the faulty device before trying a new one.


OK.
On diodes 1 & 2 showing the readings 157.7 & 1.4 on the underside the
solder is connecting the 2 diodes, is this correct? Image below.

https://imgur.com/a/KM4b6

The input is a bridge rectifier, ie the 4 diodes are connected in a ring.


NT

In article 319606a6-c15b-4094-9063-
, says...

The input is a bridge rectifier, ie the 4 diodes are connected in a ring.


Agreed. But, given the OP's stated lack of electronics
knowledge it might be helpful to explain that the bridge can
be visualised as being one diode on each side of a square with
the AC input to diagonally opposite corners. The other corners
are the positive and negative outputs.

On the PCB, the square has been squashed into a thin rectangle
with the AC input to the central connections to the pairs of
diodes like this (must be viewed using a fixed width font):


D1 D3
!-----!-----!-----!-----!
-ve ! AC ! +ve
!-----!-----!-----!-----!
D2 D4
--

Terry

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Update:
So fuse arrived today, soldered in to the charger, switched on and POP!
That will be for the bin then as now beyond my understanding of electronics.
At least it never exploded this time.

Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.

worth replacing the 4 mains diodes if you know how, those are the most likely cause.


NT

I have taken out the doide that reads faulty. I assume its a straight
forward order, its a IN5399 any other rating to consider for ordering.

Is it worth replacing any other parts at this stage? I will replace the
fuse again.

On Sat, 03 Feb 2018 05:21:56 +0000, The Natural Philosopher wrote:

No: By and large the chips don't blow, There is usually a chopper power
MOSFET and that is what takes the strain if a diode goes bad.

True, but some older ones use power bipolar junction transistors for a
chopper.


Replace diode, 450V cap and power FET and fuse and it should work

I should mention at this point that SMPS units are highly dangerous to
work on live. Some have voltages close to 800V at more than enough
amperage to kill. If you cannot avoid live testing then FFS use an
isolation transformer and keep one hand behind your back all the time
you're prodding around.



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protocols constitutes acceptance of this condition.

On Sat, 03 Feb 2018 12:12:56 +0000, Archibald Tarquin Blenkinsopp wrote:

The switching device would be short in a number of cases and mosfet or
bipolar the cause was all too often the RC snubber network across the
primary. The purpose of it was to remove the high energy spikes which
are a natural result of switching an inductive load at speed.

There's also an LC mains filter prior to the rectifier and that's highly
prone to failure as well. When an SMPS fires up from cold, the initial
surge current can be huge, 'cos it looks like a short circuit at switch-
on.


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On Saturday, 3 February 2018 14:00:49 UTC, ss wrote:

Update:
So fuse arrived today, soldered in to the charger, switched on and POP!
That will be for the bin then as now beyond my understanding of electronics.
At least it never exploded this time.

Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.

worth replacing the 4 mains diodes if you know how, those are the most likely cause.


NT

I have taken out the doide that reads faulty. I assume its a straight
forward order, its a IN5399 any other rating to consider for ordering.

Is it worth replacing any other parts at this stage? I will replace the
fuse again.

1N5399 defines the ratings (not IN), so it's that simple. It just needs to go in the right way round, as per the original, line at the same end.

Other parts? We don't know is the only definitely correct answer. Anything else would result in no end of time wasting here. You might or might not need other parts.

When you power it up if you do so via a 100w light bulb it'll spare diode & fuse if anything else is faulty.


NT

On Sat, 3 Feb 2018 14:13:22 -0000 (UTC), Cursitor Doom
wrote:

On Sat, 03 Feb 2018 12:12:56 +0000, Archibald Tarquin Blenkinsopp wrote:

The switching device would be short in a number of cases and mosfet or
bipolar the cause was all too often the RC snubber network across the
primary. The purpose of it was to remove the high energy spikes which
are a natural result of switching an inductive load at speed.

There's also an LC mains filter prior to the rectifier and that's highly
prone to failure as well. When an SMPS fires up from cold, the initial
surge current can be huge, 'cos it looks like a short circuit at switch-
on.

Rubbish!

The NTC limits the switch on surge. The filter would be more to stop
hash getting out from the PSU than anything coming the other way.

It isn't prone to failiure either. The coil has very few turns of
comparitively thick wire, so in the extremely unlikely event of a
shorted turn or two, you would never know the difference.

Assuming your neigbours don't listen to long wave on their wireless
that is.

The input capacitors are x or y rated and are not prone to the
failiures that the old polyester devices were subject to.

I personally have never ever replaced a coil or cap on the mains input
of an SMPS,

Now a transorb is a different matter, I have replace a good number,
but the similarity to a capacitor tends to end after body shape.

AB

On Sat, 3 Feb 2018 12:19:31 -0000, Terry Casey
wrote:

In article 319606a6-c15b-4094-9063-
, says...

The input is a bridge rectifier, ie the 4 diodes are connected in a ring.


Agreed. But, given the OP's stated lack of electronics
knowledge it might be helpful to explain that the bridge can
be visualised as being one diode on each side of a square with
the AC input to diagonally opposite corners. The other corners
are the positive and negative outputs.

On the PCB, the square has been squashed into a thin rectangle
with the AC input to the central connections to the pairs of
diodes like this (must be viewed using a fixed width font):


D1 D3
!-----!-----!-----!-----!
-ve ! AC ! +ve
!-----!-----!-----!-----!
D2 D4

Nice explanation Terry.

Cheers, T i m

On Sat, 3 Feb 2018 14:00:35 +0000, ss wrote:



Update:
So fuse arrived today, soldered in to the charger, switched on and POP!
That will be for the bin then as now beyond my understanding of electronics.
At least it never exploded this time.

Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.

worth replacing the 4 mains diodes if you know how, those are the most likely cause.


NT

I have taken out the doide that reads faulty.

Have you tested it again out of circuit OOI? I ask just in case
something else around the diode was giving you that reading (although
it is unlikely).

I assume its a straight
forward order, its a IN5399 any other rating to consider for ordering.

Nope, that should do it. ;-)

Is it worth replacing any other parts at this stage?

Whilst not necessarily related but I might be tempted to change all 4
diodes once I was there.

I will replace the
fuse again.

I think you will have to. ;-)

Cheers, T i m

On 03/02/2018 16:20, T i m wrote:
I will replace the
fuse again.
I think you will have to.;-)

Ha ha yes of course, thought I better mention before before someone else
did. Fortunately they came in packs of 4.

On Sat, 03 Feb 2018 06:38:34 -0800, tabbypurr wrote:

On Saturday, 3 February 2018 14:00:49 UTC, ss wrote:

Update:
So fuse arrived today, soldered in to the charger, switched on and
POP! That will be for the bin then as now beyond my understanding of
electronics.
At least it never exploded this time.

Ok now waiting on replacement fuse & holder for the other charger,
fingers crossed.

worth replacing the 4 mains diodes if you know how, those are the
most likely cause.


NT

I have taken out the doide that reads faulty. I assume its a straight
forward order, its a IN5399 any other rating to consider for ordering.

Is it worth replacing any other parts at this stage? I will replace the
fuse again.

1N5399 defines the ratings (not IN), so it's that simple. It just needs
to go in the right way round, as per the original, line at the same end.

Other parts? We don't know is the only definitely correct answer.
Anything else would result in no end of time wasting here. You might or
might not need other parts.

When you power it up if you do so via a 100w light bulb it'll spare
diode & fuse if anything else is faulty.

Unfortunately, that's likely to lead to even more component failure if
you've failed to identify and replace *all* of the faulty components
since the rapid destruction of the safety fuse will have limited the
damage in the original failure event by timely suppression of the
destructive transients before they could do any further damage.

A typical SMPSU is effectively made up from a selection of components
arranged as a fleet of "Accidents waiting to happen". Almost every
component in an SMPSU can lead to a catastrophic cascade of faults with
very few of them resulting in a non-catastrophic failure depending on
whether they'd obligingly failed open or short circuit as required to
allow such a non-destructive failure event.

Considering the workings of a SMPSU gives me the heebee jeebies every
time[1]. It's rather like an electronic form of Jenga or Russian Roulette
where 5 bullets have been loaded into the "Six Shooter Revolver" where
the victim is that obligatory safety fuse whose primary role is to "Put
out fires before they get a chance to take hold and destroy the whole
forest.".

SMPSUs *can* be repaired but unless it's one integrated into the main
PCB of an expensive piece of kit, it's usually far cheaper to simply
replace what has now become a commodity item with a new replacement unit.

One normally wouldn't even bother *attempting* to repair a ten quid
commodity PC ATX PSU these days. The only time I broke this rule of thumb
(don't get sucked into wasting precious time on a folly) was when a
customer returned their PC after a completely successful repair after
blowing the safety fuse in its ATX PSU by accidentally nudging the dual
voltage switch into the 110v setting when they'd been setting it up on
returning home.

This particular failure mode gave me pause to reconsider the benefit of
opening it up to do some basic testing of the HT module with a view to
simply try a replacement fuse. Said tests suggested there was a
possibility that the fuse had blown in time to prevent damage so it was
duly replaced and the offending and redundant voltage selector switch was
disconnected. Contrary to my expectations, the repair proved successful.

Prior to that, I've repaired a custom AT PSU (NEC Powermate II) that had
become so reluctant to restart that I needed to utilise a hair-dryer to
persuade it to spring back into life (it was a Novell Netware 3.11
fileserver box). This involved replacing a small 100mF 16v capacitor that
had dried out from the heat of an adjacent plastic power transistor.

Much earlier than that, I'd managed a successful repair of a 1970s Gould
5v 10A SMPSU which had blown one of its 1N4000 series bridge rectifier
diodes along with its user serviceable glass fuse. I can't remember the
exact number of the diode, only that it was a 700PIV rated 1N4000 series
diode. I remember the PIV rating rather than the precise part number only
because it struck me as rather odd that Gould should have chosen a diode
with a barely sufficient PIV rating for the job.

A bridge rectifier needs to have a PIV rating equal to or greater than
double the peak voltage of the maximum permitted mains supply (2 times
root 2 of 265 = 2 x 375v = 750v) to avoid the need to provide current
spike limiting resistors when used with a smoothing capacitor. The 700v
rating was barely enough to cover mains voltage excursions up to 247v
which aren't uncommon events where I live (urban underground supply).

Gould could so easily have used 1N4000 series diodes with 800 and even
1000 volt PIV ratings which begged the question, "Why not?" which then
led me to conclude that this may have been a deliberate choice based on
the concept of "Better to have a ten cent diode sacrificially fail to
protect a more difficult to replace ten dollar HT switching transistor".

This was reinforced by the fact that the diodes were far more accessible
than the HT switching transistors and the fact that the SMPSU would have
cost far more than a typical Hi-Fi set up of the day, maybe even as much
as a small capacity motorbike!

Today, thanks to consumer demand driven mass production, the once cheap
'n' cheerful 50Hz mains transformer bridge rectifier capacitively
smoothed low voltage DC supply is now both inefficient and heavy and the
most expensive in the cost of its raw materials that the new "Cheap
'n'Cheerful" has become the ubiquitous SMPSU. Only the most canny of
futurists with his wits at full stretch would have foreseen this dramatic
turn around in the time of just a single generation (Thanks to the PC
revolution, commoditised SMPSUs became a fact of life in the late 8os to
early nineties).

[1] The only other example of a PSU circuit diagram (a DC to DC converter
actually) giving me the heebee jeebies was seeing the use of thyristors
in place of switching transistors relying on secondary thyristors to
discharge a capacitor across the primary switching thyristors in order to
turn them back off. Predictably, fuse blowing events were a fairly common
failure mode with this novel use of thyristors on DC supplies. :-)

--
Johnny B Good

On Sat, 03 Feb 2018 17:09:08 +0000, ss wrote:

On 03/02/2018 16:20, T i m wrote:
I will replace the fuse again.
I think you will have to.;-)

Ha ha yes of course, thought I better mention before before someone else
did. Fortunately they came in packs of 4.

Assuming you're discovering and replacing one faulty component at a
time, a pack of 4 fuses will allow you to replace as many as sigma 4 (10)
components in total before you run out of fuses. If you carry on the
procedure using another pack of fuses, you could land up replacing
components a total of 36 times (8+7+6+5+4+3+2+1).

I think your good fortune is that they *only* come in packs of four. I
can't see you investing in yet another pack to continue your "Suck it and
see" approach to repair by component substitution unless you really do
have nothing better to do with your spare time.

If you've seen any of my previous follow ups, you'll know exactly where
I'm coming from on this. :-)

--
Johnny B Good

Assuming you're discovering and replacing one faulty component at a
time, a pack of 4 fuses will allow you to replace as many as sigma 4 (10)
components in total before you run out of fuses. If you carry on the
procedure using another pack of fuses, you could land up replacing
components a total of 36 times (8+7+6+5+4+3+2+1).

I think your good fortune is that they *only* come in packs of four. I
can't see you investing in yet another pack to continue your "Suck it and
see" approach to repair by component substitution unless you really do
have nothing better to do with your spare time.

If you've seen any of my previous follow ups, you'll know exactly where
I'm coming from on this. :-)

I agree with you but I have to work within my limitations so the fuse
and diodes are about the extent of it then it will head to the bin. It
is too late in life to start learning the `trade` so I have to follow as
best I can the advice given. Mechanical is my thing mostly, like just
started watch repairing which wont be a problem for me as I can
physically see whats wrong, unlike electronics.
DIY I will always give something a go before binning.(with safety in mind)