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SMPS wall wart failure.
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David Farber
Jun 6, 2015, 11:40:40 PM6/6/15
to
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
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
Jun 7, 2015, 3:04:57 AM6/7/15
to
situation? Perhaps the ovelay component character set does not allow for
VTS .
Gareth Magennis
Jun 7, 2015, 4:54:00 AM6/7/15
to
"N_Cook" wrote in message news:ml0qcb$rh8$1...@dont-email.me...
You mean TVS.
Gareth.
N_Cook
Jun 7, 2015, 5:09:16 AM6/7/15
to
flooding issues, standing for (Southampton) Vessel Traffic Services, and
also Voltage Transient sounds more appropriate than Transient Voltage
Phil Allison
Jun 7, 2015, 9:00:20 AM6/7/15
to
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.
> 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.
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
Arfa Daily
Jun 7, 2015, 10:22:00 AM6/7/15
to
"Phil Allison" <palli...@gmail.com> wrote in message
news:cb6f774e-22d4-4abb...@googlegroups.com...
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
Phil Allison
Jun 7, 2015, 10:54:31 AM6/7/15
to
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.
>
> 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 ... ??
>
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
M Philbrook
Jun 7, 2015, 11:02:21 AM6/7/15
to
In article <ml0ed9$vou$1...@dont-email.me>, farberbe...@aol.com
says...
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
says...
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
Jun 7, 2015, 11:47:39 AM6/7/15
to
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?
David Farber
Jun 7, 2015, 3:05:37 PM6/7/15
to
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.
John-Del
Jun 7, 2015, 5:29:14 PM6/7/15
to
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.
>
> 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
Jeff Liebermann
Jun 7, 2015, 5:37:24 PM6/7/15
to
On Sun, 7 Jun 2015 12:04:01 -0700, "David Farber"
<farberbe...@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
<farberbe...@aol.com> wrote:
>The zener diode is paralleled by the filter cap, not the power diode.
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 je...@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
Jun 7, 2015, 8:02:03 PM6/7/15
to
"John-Del" <ohg...@aol.com> wrote in message
news:22d3f51e-41b6-4f4c...@googlegroups.com...
Arfa
Arfa Daily
Jun 7, 2015, 8:09:55 PM6/7/15
to
Also, please see my corrections to my original post.
> The zener diode is paralleled by the filter cap, not the power diode.
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
Mark Zacharias
Jun 7, 2015, 9:23:05 PM6/7/15
to
"Arfa Daily" <arfa....@ntlworld.com> wrote in message
news:j35dx.855892$I97.1...@fx31.am4...
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.
news:j35dx.855892$I97.1...@fx31.am4...
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
Jun 7, 2015, 9:31:37 PM6/7/15
to
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?
Phil Allison
Jun 8, 2015, 12:52:06 AM6/8/15
to
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.
> 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?
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 Farber
Jun 8, 2015, 2:39:22 PM6/8/15
to
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.
David Farber
Jun 8, 2015, 2:43:25 PM6/8/15
to
Jeff Liebermann wrote:
> On Sun, 7 Jun 2015 12:04:01 -0700, "David Farber"
> <farberbe...@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,
> On Sun, 7 Jun 2015 12:04:01 -0700, "David Farber"
> <farberbe...@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.
As you suggested, I installed a 5.6v zener and it works fine now. The output
voltage is a solid 5.17v.
David Farber
Jun 8, 2015, 2:45:28 PM6/8/15
to
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.
Arfa Daily
Jun 8, 2015, 8:31:49 PM6/8/15
to
news:f5a1ba52-ed7d-4ef8...@googlegroups.com...
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
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
Phil Allison
Jun 8, 2015, 10:07:01 PM6/8/15
to
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
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
>
> > 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.
> >
>>
>
> >
> > 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.
> 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 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
Arfa Daily
Jun 9, 2015, 4:29:29 AM6/9/15
to
news:5942d2d7-2abb-4cd1...@googlegroups.com...
Seems like a good move ...
Arfa
Arfa
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