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

"Phil Allison" wrote in message
...
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

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

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.

"John-Del" wrote in message
...
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