SMPS wall wart failure.

[David Farber]

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.

--
David Farber
Los Osos, CA

 

[N_Cook]

On 07/06/2015 04:41, David Farber wrote:

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.

Could it be a VTS, ie designed to go short circuit in overvoltage
situation? Perhaps the ovelay component character set does not allow for
VTS .

 

[Gareth Magennis]

"N_Cook" wrote in message news:ml0qcb$rh8$1@dont-email.me...

On 07/06/2015 04:41, David Farber wrote:

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.

Could it be a VTS, ie designed to go short circuit in overvoltage
situation? Perhaps the ovelay component character set does not allow for
VTS .





I had to look that up.

You mean TVS.



Gareth.

 

[Phil Allison]

David Farber wrote:

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.

** Never seen a rectifier diode and zener in *parallel* before.

Only purpose I can think of is the main diode is a Schottky type and needs protecting from excess reverse voltage spikes.

Schottys range at about 30V up so maybe try a 27V zener.



.... Phil

 

[N_Cook]

On 07/06/2015 09:53, Gareth Magennis wrote:

"N_Cook" wrote in message news:ml0qcb$rh8$1@dont-email.me...

On 07/06/2015 04:41, David Farber wrote:
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.


Could it be a VTS, ie designed to go short circuit in overvoltage
situation? Perhaps the ovelay component character set does not allow for
VTS .





I had to look that up.

You mean TVS.



Gareth.

Brain-fart, I end up typing VTS a lot in another subject area of marine
flooding issues, standing for (Southampton) Vessel Traffic Services, and
also Voltage Transient sounds more appropriate than Transient Voltage

 

[Phil Allison]

Arfa Daily wrote:

I think it's maybe a 'lost in translation' thing Phil. Although he does
suggest that the 'zener' -if indeed it is one - is in parallel with the
rectifier diode, I think the intention was to convey that it is in parallel
with the 5 volt rail, immediately *after* the rectifier. So I guess that
would be in parallel with the main filter cap, depending on whether there is
a choke before or after it ... ??

** That was my first idea - main electro goes belly up, feedback loop loses control and fries the safety zener.

Standard failure mode of many SMPSs.

But the OP's post is very specific.

Maybe the zener shorting the main diode fried that electro rather quickly.


..... Phil

 

[Arfa Daily]

"Phil Allison" <pallison49@gmail.com> wrote in message
news:cb6f774e-22d4-4abb-bd6a-68a2192f46ce@googlegroups.com...

David Farber wrote:

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.


** Never seen a rectifier diode and zener in *parallel* before.

Only purpose I can think of is the main diode is a Schottky type and needs
protecting from excess reverse voltage spikes.

Schottys range at about 30V up so maybe try a 27V zener.



... Phil

I think it's maybe a 'lost in translation' thing Phil. Although he does
suggest that the 'zener' -if indeed it is one - is in parallel with the
rectifier diode, I think the intention was to convey that it is in parallel
with the 5 volt rail, immediately *after* the rectifier. So I guess that
would be in parallel with the main filter cap, depending on whether there is
a choke before or after it ... ??

Arfa

 

[M Philbrook]

In article <ml0ed9$vou$1@dont-email.me>, farberbear.unspam@aol.com
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

 

[David Farber]

Arfa Daily wrote:

"Phil Allison" <pallison49@gmail.com> wrote in message
news:cb6f774e-22d4-4abb-bd6a-68a2192f46ce@googlegroups.com...
David Farber wrote:

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.


** Never seen a rectifier diode and zener in *parallel* before.

Only purpose I can think of is the main diode is a Schottky type and
needs protecting from excess reverse voltage spikes.

Schottys range at about 30V up so maybe try a 27V zener.



... Phil



I think it's maybe a 'lost in translation' thing Phil. Although he
does suggest that the 'zener' -if indeed it is one - is in parallel
with the rectifier diode, I think the intention was to convey that it
is in parallel with the 5 volt rail, immediately *after* the
rectifier. So I guess that would be in parallel with the main filter
cap, depending on whether there is a choke before or after it ... ??

Arfa

Apologies folks. Looks like I misread the pc traces. The anode of the power
diode is only in common with the secondary of the transformer. The cathode
connects to one end of the zener diode, the filter cap (which I replaced),
then goes to a choke, then to the output wires.

Checking further, I've discovered that the shorted zener diode was in
parallel with the filter cap. That explains the low, in circuit ESR reading.
The negative side of the filter cap and zener are also connected back to the
secondary of the transformer. I imagine the zener is used to protect the cap
and output circuit from voltage spikes?

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

 

[John-Del]

On Sunday, June 7, 2015 at 11:47:39 AM UTC-4, David Farber wrote:

Apologies folks. Looks like I misread the pc traces. The anode of the power
diode is only in common with the secondary of the transformer. The cathode
connects to one end of the zener diode, the filter cap (which I replaced),
then goes to a choke, then to the output wires.

Checking further, I've discovered that the shorted zener diode was in
parallel with the filter cap. That explains the low, in circuit ESR reading.
The negative side of the filter cap and zener are also connected back to the
secondary of the transformer. I imagine the zener is used to protect the cap
and output circuit from voltage spikes?

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

Basic crowbar zener. Meant to protect the device powered by the supply in case the supply runs away.

 

[David Farber]

M Philbrook wrote:

In article <ml0ed9$vou$1@dont-email.me>, farberbear.unspam@aol.com
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

 

[Jeff Liebermann]

On Sun, 7 Jun 2015 12:04:01 -0700, "David Farber"
<farberbear.unspam@aol.com> 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 jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Arfa Daily]

"John-Del" <ohger1s@aol.com> wrote in message
news:22d3f51e-41b6-4f4c-a893-c2199ff79e6d@googlegroups.com...

On Sunday, June 7, 2015 at 11:47:39 AM UTC-4, David Farber wrote:


Apologies folks. Looks like I misread the pc traces. The anode of the
power
diode is only in common with the secondary of the transformer. The
cathode
connects to one end of the zener diode, the filter cap (which I
replaced),
then goes to a choke, then to the output wires.

Checking further, I've discovered that the shorted zener diode was in
parallel with the filter cap. That explains the low, in circuit ESR
reading.
The negative side of the filter cap and zener are also connected back to
the
secondary of the transformer. I imagine the zener is used to protect the
cap
and output circuit from voltage spikes?

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

Basic crowbar zener. Meant to protect the device powered by the supply in
case the supply runs away.

That would be my feeling for the purpose of it, too ...

Arfa

 

[Arfa Daily]

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

 

[Phil Allison]

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

 

[Mark Zacharias]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:j35dx.855892$I97.187416@fx31.am4...

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.

 

[David Farber]

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

 

[David Farber]

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

 

[David Farber]

Jeff Liebermann wrote:

On Sun, 7 Jun 2015 12:04:01 -0700, "David Farber"
farberbear.unspam@aol.com> 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

 

[David Farber]

Mark Zacharias wrote:

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:j35dx.855892$I97.187416@fx31.am4...

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