Repairing small switch mode power supply
 

I have a Lidl worklight of the type that uses a panel of 150 LED's.
It worked fine for a while but one day it flickered and went out.

As I'm one of those who likes to see what makes things tick, rather then
send it back I had a look inside the housing where there is a small Switch
Mode Power Supply (SMPS).
Photo he
http://freespace.virgin.net/enigma.1666/index.htm

The mains input fuse on the board has blown but nothing is burnt.

I'm hoping someone can advise on the likely faulty component.

Thanks for any help.

Roger R

Repairing small switch mode power supply
 

"Roger R" wrote in message
...
I have a Lidl worklight of the type that uses a panel of 150 LED's.
It worked fine for a while but one day it flickered and went out.

As I'm one of those who likes to see what makes things tick, rather then
send it back I had a look inside the housing where there is a small Switch
Mode Power Supply (SMPS).
Photo he
http://freespace.virgin.net/enigma.1666/index.htm

The mains input fuse on the board has blown but nothing is burnt.

I'm hoping someone can advise on the likely faulty component.

Thanks for any help.

Roger R



If the fuse is blown violently i.e. blackened inside, check D1,2,3,4 for
short circuit. If they are ok, then likely that IC1 has failed. If it has,
that may not be the end of the story. Has the supply got any s.m. components
on the back of the board ? If so, and the chip has failed, there will
probably be s/c zeners / diodes amongst them. If the fuse has failed gently,
you might just be lucky and have something like a s/c secondary side diode -
red one top left, black one bottom left - although if the chip has got any
'inteligence', usually secondary side overloads will result in a cycling
shutdown rather than excess primary side current.

Arfa

Repairing small switch mode power supply
 

In article , Roger R
writes

The mains input fuse on the board has blown but nothing is burnt.

I'm hoping someone can advise on the likely faulty component.

Agreed with Arfa. If the mains fuse is silvered/blackened on the inside
of the glass (hard to tell from the photo), it's a short in the primary
side, most likely one of D1/2/3/4. D2[obscured] looks like it's been
farted about with, have you had someone look at this supply already?

There's no primary-side switching transistor, which is unusual. Maybe
it's inside IC1, but your picture isn't clear enough for me to read the
markings on IC1 to look up the spec.

IC1 also looks slightly brown on the surface, as if it's overheated.
It's certainly a suspect.

There also appear to be two D2s, unless the green capacitor C4 is
obscuring another number of diode D2x, the one I refer to as
D2[obscured] above.

Nasty cheap design, intended to just get you past the warranty period
before expiring.

--
Fred Bloggs

Repairing small switch mode power supply
 

On Sep 18, 12:10*pm, "Roger R" wrote:
I have a Lidl worklight of the type that uses a panel of 150 LED's.
It worked fine for a while but one day it flickered and went out.

As I'm one of those who likes to see what makes things tick, rather then
send it back I had a look inside the housing where there is a small Switch
Mode Power Supply (SMPS).
Photo hehttp://freespace.virgin.net/enigma.1666/index.htm

The mains input fuse on the board has blown but nothing is burnt.

I'm hoping someone can advise on the likely faulty component.

Thanks for any help.

Roger R

If for some reason you dont get the smpsu fixed, it could be run
without it by substituting a simple CR supply, reconfiguring the LEDs
and making sure the LEDs were sufficiently insulated.


NT

Repairing small switch mode power supply
 

On Fri, 18 Sep 2009 13:27:48 +0100, Fred Bloggs wrote:
There's no primary-side switching transistor, which is unusual.

I thought that, too. It is off-board somewhere, or soldered onto the
reverse side of the PCB (which would also be unusual)?

But yes, that or rectifier diodes. I'm not sure if that's some kind of
thermistor near the fuse; if so it *might* have gone bad, although that's
less likely.

Worth checking the main smoothing capacitor, too; whilst it may not have
shorted, it may have gone bad and be the cause of failure in whatever
component *has* made the fuse blow.

It could be a fault on the LV side, too - but checking the obvious on the
HV side should be quick and easy and is more likely I think.

your picture isn't clear enough for me to read the
markings on IC1 to look up the spec.

Looked like a prefix of V1F / VIF / VTF / VLF and then -22A to me,
but none of those cough up anything via Google unfortunately. Seems
unusual that an IC would no heatsink could do the job of switching,
though. Unless the heatsink fell off, which is why it's cooked :-)

cheers

Jules

Repairing small switch mode power supply
 

"Fred Bloggs" wrote in message
...
In article , Roger R
writes

The mains input fuse on the board has blown but nothing is burnt.

I'm hoping someone can advise on the likely faulty component.

Agreed with Arfa. If the mains fuse is silvered/blackened on the inside
of the glass (hard to tell from the photo), it's a short in the primary
side, most likely one of D1/2/3/4. D2[obscured] looks like it's been
farted about with, have you had someone look at this supply already?

There's no primary-side switching transistor, which is unusual. Maybe
it's inside IC1, but your picture isn't clear enough for me to read the
markings on IC1 to look up the spec.

Not unusual these days Fred. There are many 8 pin (usually actually only 7
pins but a 'standard' 8DIP package) 'switchers-on-a-chip' now. They are used
extensively in LCD TV smps to implement the PFC front-end or standby
supplies. The TNY266 is a typical example of this sort of chip. Modern FETs
have such low resistance switched drain-source channels, that very little
power is dissipated in the device itself. These supplies can achieve
efficiencies in excess of 90%. So as long as you can make sure that there is
enough insulation resistance around the package pins to which the switching
FET is internally connected - and that's why they have the 'missing' pin 7 -
then using them in a switcher man enough to drive those LEDs, is something
that they can easily cope with.

At the moment, I'm still erring towards the problem being one of the
front-end reccies though, as *usually*, although not always, s/c failure of
those chips is very evident in that all of the magic smoke will have escaped
from the gaping hole in the top ... :-)

Arfa

Fred Bloggs

Repairing small switch mode power supply
 

"Jules" wrote in message
ail.com...
On Fri, 18 Sep 2009 13:27:48 +0100, Fred Bloggs wrote:
There's no primary-side switching transistor, which is unusual.

I thought that, too. It is off-board somewhere, or soldered onto the
reverse side of the PCB (which would also be unusual)?


Thanks to the replies,
To answer your points:

The fuse glass is perfectly clear, no violent blow. But it is blown.

There are no components located elsewhere and nothing on the back (not even
tiny surface mounts).

As noted there are two occurrences of D2 marking on the board.
-D2 not disturbed - nothing at all has been touched on the board.

The bridge rectifier diodes D1-4 are type 1N4007.
Diode D2 (2) also looks like its 1N4007, but can't really be seen.
Diode D3 is different type FR10...something

The small diodes seem to measure ok - like 640 something one way and nothing
the other.

The large Ultra high speed rectifier diode D7 (bottom left of the blue
transformer) is type UF5404

IC1- ( VIPer22AS) brown writing - just a trick of the light I think - not
burnt.
http://tinyurl.com/lnrne3

- I am reluctant to unsolder any component to test as the unit is still
under warranty and I can send or take it back, but I hope to identify the
fault, satisfy my curiosity and save the trouble of returning it

It doesn't look like the large capacitor has failed.

My suspect is an insulation breakdown in the chopper transformer.
If so, I won't be able to get one and will have to send it back.

My website pictures updated.

Roger R

Repairing small switch mode power supply
 

In article ,
Roger R wrote:
It doesn't look like the large capacitor has failed.

They don't always blow apart. ;-)

--
*Why is the third hand on the watch called a second hand?

Dave Plowman London SW
To e-mail, change noise into sound.

Repairing small switch mode power supply
 

"Roger R" wrote in message
...

"Jules" wrote in message
ail.com...
On Fri, 18 Sep 2009 13:27:48 +0100, Fred Bloggs wrote:
There's no primary-side switching transistor, which is unusual.

I thought that, too. It is off-board somewhere, or soldered onto the
reverse side of the PCB (which would also be unusual)?


Thanks to the replies,
To answer your points:

The fuse glass is perfectly clear, no violent blow. But it is blown.

There are no components located elsewhere and nothing on the back (not
even
tiny surface mounts).

As noted there are two occurrences of D2 marking on the board.
-D2 not disturbed - nothing at all has been touched on the board.

The bridge rectifier diodes D1-4 are type 1N4007.
Diode D2 (2) also looks like its 1N4007, but can't really be seen.
Diode D3 is different type FR10...something

The small diodes seem to measure ok - like 640 something one way and
nothing
the other.

The large Ultra high speed rectifier diode D7 (bottom left of the blue
transformer) is type UF5404

IC1- ( VIPer22AS) brown writing - just a trick of the light I think - not
burnt.
http://tinyurl.com/lnrne3

- I am reluctant to unsolder any component to test as the unit is still
under warranty and I can send or take it back, but I hope to identify the
fault, satisfy my curiosity and save the trouble of returning it

It doesn't look like the large capacitor has failed.

My suspect is an insulation breakdown in the chopper transformer.
If so, I won't be able to get one and will have to send it back.

My website pictures updated.

Roger R



So, if the fuse hasn't failed violently, but is definitely open, have you
tried just replacing it ? Not unknown for fuses to fail for no apparent
reason. May just have been a metal fatigue thing, or a defectively
manufactured fuse, or even a short term event like a surge or spike on the
mains. It is virtually unknown for an input fuse on a switcher to fail in
this way, if there is genuinely anything wrong with the supply ...

Arfa

Repairing small switch mode power supply
 

In message . com, Jules
writes
On Fri, 18 Sep 2009 13:27:48 +0100, Fred Bloggs wrote:
There's no primary-side switching transistor, which is unusual.

I thought that, too. It is off-board somewhere, or soldered onto the
reverse side of the PCB (which would also be unusual)?
There is, it's in the ST Viper Chip, it's pretty much all the control
gear and power bits for a small SMPSU. I'd replace the chip, the diodes
D1-4 and check the rest of the diodes, primary and secondary.

Jules


--
Clint Sharp

Repairing small switch mode power supply
 

Roger R formulated the question :
Thanks to the replies,
To answer your points:

The fuse glass is perfectly clear, no violent blow. But it is blown.

In that case my first step would be to replace the fuse and see what
happens - it just be a faulty fuse which has failed.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk