Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1   Report Post  
Posted to sci.electronics.repair
external usenet poster
 
Posts: 120
Default SMPS wall wart failure.

In article ,
says...

I have a Netgear cable modem. It is powered by a Jentec Technology AF1205-B
power adapter. The adapter failed and I replaced it with another adapter but
of course I was curious what the fault was. I opened up the case and found a
bad (bulging) 1,000uF 10V filter cap. I did some other circuit tests to make
sure there were no shorts because I was getting extremely low ESR readings
(less than 0.1 ohms) around the bad cap, Eventually I found an SMD zener
diode that was shorted. There are no identifying marks on the diode other
than its color which is the standard looking orange. I am fairly sure it's a
zener because the pc board has it marked as ZD-2. Its location in the
circuit is in parallel with the 5 volt power supply diode that comes off of
the secondary of the switching transformer. The power diode has two sets of
numbers. The top row is marked 540 (perhaps an SB540?) on the bottom row is
marked 849. That diode is ok. Only the zener is shorted. I was wondering if
there was some ballpark zener voltage that I could use to replace the old
one. It's always good to have a spare 5V supply.

Thanks for your replies.


THat is a protection device.. could be a MOV but it sure is a protection
device design to clamp if voltage exceeds, that is, if it is truely
across the output..
The caps are bad and caused higher than desired voltage from the
switching signal, thus shorted the protection device. Basically, it did
its job.

You may be able to get by using a 5 watt zener but you still need to
replace the caps.

btw, it seems a very common problem for the switching wallwarts to fail
in this manner for routers, I replaced my 3 times, now I have a
transformer 60hz type and works great...

Jamie

  #2   Report Post  
Posted to sci.electronics.repair
external usenet poster
 
Posts: 579
Default SMPS wall wart failure.

M Philbrook wrote:
In article ,
says...

I have a Netgear cable modem. It is powered by a Jentec Technology
AF1205-B power adapter. The adapter failed and I replaced it with
another adapter but of course I was curious what the fault was. I
opened up the case and found a bad (bulging) 1,000uF 10V filter cap.
I did some other circuit tests to make sure there were no shorts
because I was getting extremely low ESR readings (less than 0.1
ohms) around the bad cap, Eventually I found an SMD zener diode that
was shorted. There are no identifying marks on the diode other than
its color which is the standard looking orange. I am fairly sure
it's a zener because the pc board has it marked as ZD-2. Its
location in the circuit is in parallel with the 5 volt power supply
diode that comes off of the secondary of the switching transformer.
The power diode has two sets of numbers. The top row is marked 540
(perhaps an SB540?) on the bottom row is marked 849. That diode is
ok. Only the zener is shorted. I was wondering if there was some
ballpark zener voltage that I could use to replace the old one. It's
always good to have a spare 5V supply.

Thanks for your replies.


THat is a protection device.. could be a MOV but it sure is a
protection device design to clamp if voltage exceeds, that is, if it
is truely across the output..
The caps are bad and caused higher than desired voltage from the
switching signal, thus shorted the protection device. Basically, it
did its job.

You may be able to get by using a 5 watt zener but you still need to
replace the caps.

btw, it seems a very common problem for the switching wallwarts to
fail in this manner for routers, I replaced my 3 times, now I have a
transformer 60hz type and works great...

Jamie


Hi Jamie,

I did replace the faulty cap. As far using a 5 watt zener diode, the
original one is 3mm long and 1.4mm in diameter. That would seem to be in the
under 1 watt category. Also, please see my corrections to my original post.
The zener diode is paralleled by the filter cap, not the power diode.

Thanks for your reply.
--
David Farber
Los Osos, CA


  #3   Report Post  
Posted to sci.electronics.repair
external usenet poster
 
Posts: 4,045
Default SMPS wall wart failure.

On Sun, 7 Jun 2015 12:04:01 -0700, "David Farber"
wrote:

The zener diode is paralleled by the filter cap, not the power diode.


If the zener is there strictly for protection, the power supply should
work normally without it. Have you tried using it without the zener?

If it's in parallel with the output, it's likely some voltage slightly
over 5V, such as 5.6v or 6.8v. If the zener is expected to short upon
overvoltage, I would guess(tm) that a low power zener would be more
appropriate than one that can handle the power.

I've had my share of 5v 2A power supplies blow up. I don't recall if
they were made by Jentec, but when I replaced the usual bulging
capacitor(s), most would not recover. I never bothered to
troubleshoot further and just recycled them. Thanks for the hint
about the zener.

--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
  #4   Report Post  
Posted to sci.electronics.repair
external usenet poster
 
Posts: 6,772
Default SMPS wall wart failure.


Also, please see my corrections to my original post.
The zener diode is paralleled by the filter cap, not the power diode.


Although strictly speaking it is correct no matter which way round you say
it, that still comes across sort of wrong, as though the zener has some
primary function, and that of the filter cap is secondary. It would be
better to say that the filter cap is paralleled by the zener, implying that
it's the zener that has the secondary function. Probably just me splitting
hairs though ... :-)

Arfa



Thanks for your reply.
--
David Farber
Los Osos, CA


  #5   Report Post  
Posted to sci.electronics.repair
external usenet poster
 
Posts: 269
Default SMPS wall wart failure.

"Arfa Daily" wrote in message
...

Also, please see my corrections to my original post.
The zener diode is paralleled by the filter cap, not the power diode.


Although strictly speaking it is correct no matter which way round you say
it, that still comes across sort of wrong, as though the zener has some
primary function, and that of the filter cap is secondary. It would be
better to say that the filter cap is paralleled by the zener, implying
that it's the zener that has the secondary function. Probably just me
splitting hairs though ... :-)

Arfa



Thanks for your reply.
--
David Farber
Los Osos, CA





1. Cap develops ESR.
2. High frequency AC waveform appears on the desired DC output.
3. Circuit interprets as low DC - attempts to compensate.
4. Actual DC runs away. In the Panasonic example, the 14 volt line would go
to 35 volts.
4. Zener shorts.
5. Ideally this throws off the "Q" of the circuit so much that it simply
stops oscillating. Power supply stops but does not blow up.

"Ideally".

The old Panasonic switchers still usually fried the primary circuit
switching transistors in this scenario.

Remove the shorted zener, replace the cap, observe the voltage at that
point, replace with an appropriate zener.

For example, Panasonic put an 18 volt zener across the 14 volt line.

If your rated output is 5 volts, maybe a 6 volt zener.

Mark Z.



  #6   Report Post  
Posted to sci.electronics.repair
external usenet poster
 
Posts: 579
Default SMPS wall wart failure.

Arfa Daily wrote:
Also, please see my corrections to my original post.
The zener diode is paralleled by the filter cap, not the power diode.


Although strictly speaking it is correct no matter which way round
you say it, that still comes across sort of wrong, as though the
zener has some primary function, and that of the filter cap is
secondary. It would be better to say that the filter cap is
paralleled by the zener, implying that it's the zener that has the
secondary function. Probably just me splitting hairs though ... :-)

Arfa



Thanks for your reply.
--
David Farber
Los Osos, CA


Hi Arfa,

I agree with you regarding the phrasing. Your way does sound better and it
is splitting hairs. (-:

I was wondering what would happen if the choke were placed before the
capacitor and zener diode. Wouldn't the choke filter out the spikes and then
make the zener diode unnecessary or at least less likely to short?

Thanks for your reply.
--
David Farber
Los Osos, CA


  #7   Report Post  
Posted to sci.electronics.repair
external usenet poster
 
Posts: 1,249
Default SMPS wall wart failure.

David Farber wrote:


I was wondering what would happen if the choke were placed before the
capacitor and zener diode.


** A choke would present a high impedance in series with each current pulse from the switching tranny - causing a big drop in the voltage appearing on the electro cap.


Wouldn't the choke filter out the spikes and then
make the zener diode unnecessary or at least less likely to short?


** The zener (probably 6.2 V) conducted heavily and failed short when the electro went high ESR and caused the peak output voltage to go high.

An electro that has developed high ESR cannot smooth the current pulses being delivered by the switching tranny and diode, so the output wave has continuous high peaks with a low *average* value. The control loop responds to the low average and tries to correct it by making each current pulse stronger, which only makes things worse.

In short, the output electros in a SMPS are critical to it operation and in many cases there in nothing to prevent the output voltage going high when they wear out.

I have seen serious damage done to 5V logic when this happens.



.... Phil



  #8   Report Post  
Posted to sci.electronics.repair
external usenet poster
 
Posts: 579
Default SMPS wall wart failure.

Phil Allison wrote:
David Farber wrote:


I was wondering what would happen if the choke were placed before the
capacitor and zener diode.


** A choke would present a high impedance in series with each current
pulse from the switching tranny - causing a big drop in the voltage
appearing on the electro cap.


Wouldn't the choke filter out the spikes and then
make the zener diode unnecessary or at least less likely to short?


** The zener (probably 6.2 V) conducted heavily and failed short when
the electro went high ESR and caused the peak output voltage to go
high.

An electro that has developed high ESR cannot smooth the current
pulses being delivered by the switching tranny and diode, so the
output wave has continuous high peaks with a low *average* value. The
control loop responds to the low average and tries to correct it by
making each current pulse stronger, which only makes things worse.

In short, the output electros in a SMPS are critical to it operation
and in many cases there in nothing to prevent the output voltage
going high when they wear out.

I have seen serious damage done to 5V logic when this happens.



... Phil


Hi Phil,

I was forgetting about the pulsing action. I guess I was thinking about a
standard AC transformer operating in the 60 Hz range. I imagine it would
hold true that whatever the transformer pulse/sinusoidal frequency is, the
choke belongs at the end of the line.

Thanks for your reply.
--
David Farber
Los Osos, CA


  #9   Report Post  
Posted to sci.electronics.repair
external usenet poster
 
Posts: 579
Default SMPS wall wart failure.

Jeff Liebermann wrote:
On Sun, 7 Jun 2015 12:04:01 -0700, "David Farber"
wrote:

The zener diode is paralleled by the filter cap, not the power diode.


If the zener is there strictly for protection, the power supply should
work normally without it. Have you tried using it without the zener?

If it's in parallel with the output, it's likely some voltage slightly
over 5V, such as 5.6v or 6.8v. If the zener is expected to short upon
overvoltage, I would guess(tm) that a low power zener would be more
appropriate than one that can handle the power.

I've had my share of 5v 2A power supplies blow up. I don't recall if
they were made by Jentec, but when I replaced the usual bulging
capacitor(s), most would not recover. I never bothered to
troubleshoot further and just recycled them. Thanks for the hint
about the zener.


Hi Jeff,

As you suggested, I installed a 5.6v zener and it works fine now. The output
voltage is a solid 5.17v.

Thanks for your reply.
--
David Farber
Los Osos, CA


  #10   Report Post  
Posted to sci.electronics.repair
external usenet poster
 
Posts: 579
Default SMPS wall wart failure.

Mark Zacharias wrote:
"Arfa Daily" wrote in message
...

Also, please see my corrections to my original post.
The zener diode is paralleled by the filter cap, not the power
diode.


Although strictly speaking it is correct no matter which way round
you say it, that still comes across sort of wrong, as though the
zener has some primary function, and that of the filter cap is
secondary. It would be better to say that the filter cap is
paralleled by the zener, implying that it's the zener that has the
secondary function. Probably just me splitting hairs though ... :-)

Arfa



Thanks for your reply.
--
David Farber
Los Osos, CA





1. Cap develops ESR.
2. High frequency AC waveform appears on the desired DC output.
3. Circuit interprets as low DC - attempts to compensate.
4. Actual DC runs away. In the Panasonic example, the 14 volt line
would go to 35 volts.
4. Zener shorts.
5. Ideally this throws off the "Q" of the circuit so much that it
simply stops oscillating. Power supply stops but does not blow up.

"Ideally".

The old Panasonic switchers still usually fried the primary circuit
switching transistors in this scenario.

Remove the shorted zener, replace the cap, observe the voltage at that
point, replace with an appropriate zener.

For example, Panasonic put an 18 volt zener across the 14 volt line.

If your rated output is 5 volts, maybe a 6 volt zener.

Mark Z.


Hi Mark,

As soon as I found the short, those marvelous Panasonic VCR power supplies
came to mind. I installed a 5.6 volt zener and it works fine now.

Thanks for your reply.
--
David Farber
Los Osos, CA




  #11   Report Post  
Posted to sci.electronics.repair
external usenet poster
 
Posts: 6,772
Default SMPS wall wart failure.



"Phil Allison" wrote in message
...
David Farber wrote:


I was wondering what would happen if the choke were placed before the
capacitor and zener diode.


** A choke would present a high impedance in series with each current
pulse from the switching tranny - causing a big drop in the voltage
appearing on the electro cap.


Wouldn't the choke filter out the spikes and then
make the zener diode unnecessary or at least less likely to short?


** The zener (probably 6.2 V) conducted heavily and failed short when the
electro went high ESR and caused the peak output voltage to go high.

An electro that has developed high ESR cannot smooth the current pulses
being delivered by the switching tranny and diode, so the output wave has
continuous high peaks with a low *average* value. The control loop
responds to the low average and tries to correct it by making each current
pulse stronger, which only makes things worse.

In short, the output electros in a SMPS are critical to it operation and
in many cases there in nothing to prevent the output voltage going high
when they wear out.

I have seen serious damage done to 5V logic when this happens.



... Phil


David - see Phil's answer regarding the choke being first. And +1 on his
observations about serious damage occurring when a cap goes high ESR and the
control loop 'lets go' in its efforts to correct for what it sees as the low
output voltage. Notable among these cases are cheap Chinese DVD players and
set-top boxes, but I have also seen it happen on equipment from what you
would normally consider to be 'reputable' manufacturers ...

Arfa

  #12   Report Post  
Posted to sci.electronics.repair
external usenet poster
 
Posts: 1,249
Default SMPS wall wart failure.

Arfa Daily wrote:



In short, the output electros in a SMPS are critical to it's operation and
in many cases there is nothing to prevent the output voltage going high
when they wear out.

I have seen serious damage done to 5V logic when this happens.



David - see Phil's answer regarding the choke being first. And +1 on his
observations about serious damage occurring when a cap goes high ESR and the
control loop 'lets go' in its efforts to correct for what it sees as the low
output voltage. Notable among these cases are cheap Chinese DVD players and
set-top boxes, but I have also seen it happen on equipment from what you
would normally consider to be 'reputable' manufacturers ...


** The examples I had in mind were Yamaha Multi FX processors like the SPX90 and a few relatives.

The SMPS was on its own PCB and there was enough heat to dry out the electros on the DC rails - particularly the 5V one. That rail could rise to nearly double voltage and take out a whole bunch of 74LS ICs and others that were strictly 5V.

The same PCB had the dreaded "yellow glue" dobbed all over the place eating the leads off small electros, zeners and resistors in the control loop.

After a full clean up and fitting all new electros, I added a SCR crow bar to the 5V output to protect the main board in future.



..... Phil




  #13   Report Post  
Posted to sci.electronics.repair
external usenet poster
 
Posts: 6,772
Default SMPS wall wart failure.



"Phil Allison" wrote in message
...
Arfa Daily wrote:



In short, the output electros in a SMPS are critical to it's operation
and
in many cases there is nothing to prevent the output voltage going high
when they wear out.

I have seen serious damage done to 5V logic when this happens.



David - see Phil's answer regarding the choke being first. And +1 on his
observations about serious damage occurring when a cap goes high ESR and
the
control loop 'lets go' in its efforts to correct for what it sees as the
low
output voltage. Notable among these cases are cheap Chinese DVD players
and
set-top boxes, but I have also seen it happen on equipment from what you
would normally consider to be 'reputable' manufacturers ...


** The examples I had in mind were Yamaha Multi FX processors like the
SPX90 and a few relatives.

The SMPS was on its own PCB and there was enough heat to dry out the
electros on the DC rails - particularly the 5V one. That rail could rise
to nearly double voltage and take out a whole bunch of 74LS ICs and others
that were strictly 5V.

The same PCB had the dreaded "yellow glue" dobbed all over the place
eating the leads off small electros, zeners and resistors in the control
loop.

After a full clean up and fitting all new electros, I added a SCR crow bar
to the 5V output to protect the main board in future.



.... Phil


Seems like a good move ...

Arfa

Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
SMPS wall wart failure. Phil Allison[_3_] Electronics Repair 5 June 8th 15 02:01 AM
SMPS wall wart failure. Gareth Magennis Electronics Repair 1 June 7th 15 11:09 AM
SMPS failure query N_Cook Electronics Repair 6 April 23rd 14 09:04 AM
Wall wart in a box The Medway Handyman UK diy 14 November 9th 09 07:34 PM
A wall-wart alternative [email protected] Home Repair 60 January 17th 06 06:17 AM


All times are GMT +1. The time now is 01:29 PM.

Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 DIYbanter.
The comments are property of their posters.
 

About Us

"It's about DIY & home improvement"