Hi Everyone,

I have a friend's DMM that wouldn't power up. I opened the case and
found that there was a power to ground short. The short is being caused
by a SMT diode that is across the 9v battery wires on the PCB. I lifted
one side of the diode and the diode is shorted and burned pretty badly.
I powered up the DMM briefly and everything seems to be working ok. I
want to know if anyone has any suggestions on what type of diode should
I use to replace the shorted diode. It doesn't have to be a SMT diode.
I am guessing that is a protection diode and not a zener since most of
the ICs run off of 9v according to their datasheets. Any help would be
greatly appreciated. Thanks in advance.

Jason

me wrote:

Hi Everyone,

I have a friend's DMM that wouldn't power up. I opened the case and
found that there was a power to ground short. The short is being caused
by a SMT diode that is across the 9v battery wires on the PCB. I lifted
one side of the diode and the diode is shorted and burned pretty badly.
I powered up the DMM briefly and everything seems to be working ok. I
want to know if anyone has any suggestions on what type of diode should
I use to replace the shorted diode. It doesn't have to be a SMT diode.
I am guessing that is a protection diode and not a zener since most of
the ICs run off of 9v according to their datasheets. Any help would be
greatly appreciated. Thanks in advance.

Jason

Hi Jason...

Can't think of any reason you'd need one at all, unless you plan
on somehow installing the battery backwards

Take care.

Ken

Ken Weitzel wrote in
newshRBg.332437$Mn5.252184@pd7tw3no:



me wrote:

Hi Everyone,

I have a friend's DMM that wouldn't power up. I opened the case and
found that there was a power to ground short. The short is being
caused by a SMT diode that is across the 9v battery wires on the PCB.
I lifted one side of the diode and the diode is shorted and burned
pretty badly.
I powered up the DMM briefly and everything seems to be working ok.
I
want to know if anyone has any suggestions on what type of diode
should I use to replace the shorted diode. It doesn't have to be a
SMT diode.
I am guessing that is a protection diode and not a zener since most
of
the ICs run off of 9v according to their datasheets. Any help would
be greatly appreciated. Thanks in advance.


Can't think of any reason you'd need one at all, unless you plan
on somehow installing the battery backwards

Sounds like someone momentarily touched the battery terminals backward
and the diode forward-conducted and blew out. Conclusion: a surface-
mount diode doesn't work for backwards-connection protection, it just
self-destructs. Unsolder the diode and leave it out of the circuit.

"Jim Land" wrote in message
. 3.44...
Ken Weitzel wrote in
newshRBg.332437$Mn5.252184@pd7tw3no:



me wrote:

Hi Everyone,

I have a friend's DMM that wouldn't power up. I opened the case and
found that there was a power to ground short. The short is being
caused by a SMT diode that is across the 9v battery wires on the PCB.
I lifted one side of the diode and the diode is shorted and burned
pretty badly.
I powered up the DMM briefly and everything seems to be working ok.
I
want to know if anyone has any suggestions on what type of diode
should I use to replace the shorted diode. It doesn't have to be a
SMT diode.
I am guessing that is a protection diode and not a zener since most
of
the ICs run off of 9v according to their datasheets. Any help would
be greatly appreciated. Thanks in advance.


Can't think of any reason you'd need one at all, unless you plan
on somehow installing the battery backwards

Sounds like someone momentarily touched the battery terminals backward
and the diode forward-conducted and blew out. Conclusion: a surface-
mount diode doesn't work for backwards-connection protection, it just
self-destructs. Unsolder the diode and leave it out of the circuit.



Most reverse protection diodes do fail short-circuit. It's unusual to find
one that survives when it does its basic job. The battery must have been
connected backwards for the original to have failed, and as the rest of the
meter seems to work ok, then IMHO, it has done the job it was designed-in
for. For the minimal cost, and the protection afforded, I really can't see
any reason NOT to replace it ... ?? Any old diode will do from a 1N4148 up.

Arfa

"me" wrote in message
...
Hi Everyone,

I have a friend's DMM that wouldn't power up. I opened the case and found
that there was a power to ground short. The short is being caused by a
SMT diode that is across the 9v battery wires on the PCB. I lifted one
side of the diode and the diode is shorted and burned pretty badly. I
powered up the DMM briefly and everything seems to be working ok. I want
to know if anyone has any suggestions on what type of diode should I use
to replace the shorted diode. It doesn't have to be a SMT diode. I am
guessing that is a protection diode and not a zener since most of the ICs
run off of 9v according to their datasheets. Any help would be greatly
appreciated. Thanks in advance.

Jason

I'd look for a convenient point on the PCB to solder in something more
substantial like a 1N4001 or similar. Across the solder pads where the
battery leads are soldered to the PCB would be fine, just make sure you get
it round the right way, ie the white band to the positive terminal.

If your friend's a bit accident prone or absent minded I certainly wouldn't
give him it back with no diode- the first time he touches the battery to the
terminals the wrong way round he'll have an ex-DMM!

Dave

In article ,
Arfa Daily wrote:
Most reverse protection diodes do fail short-circuit. It's unusual to
find one that survives when it does its basic job.

Eh? The usual way is a series diode and that won't fail if the battery is
reverse connected. Be a bit silly if it did...

--
*I almost had a psychic girlfriend but she left me before we met *

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

"Dave Plowman (News)" wrote in message
...
In article ,
Arfa Daily wrote:
Most reverse protection diodes do fail short-circuit. It's unusual to
find one that survives when it does its basic job.

Eh? The usual way is a series diode and that won't fail if the battery is
reverse connected.

But not in this case as the OP said it is in parallel. I've seen loads of
battery equipment with parallel reverse polarity protection diodes. I think
the designers baulk at the losses involved in using a series diode, small
though they may be in the case of a DVM.

Dave

Arfa Daily wrote in message
news

"Jim Land" wrote in message
. 3.44...
Ken Weitzel wrote in
newshRBg.332437$Mn5.252184@pd7tw3no:



me wrote:

Hi Everyone,

I have a friend's DMM that wouldn't power up. I opened the case and
found that there was a power to ground short. The short is being
caused by a SMT diode that is across the 9v battery wires on the PCB.
I lifted one side of the diode and the diode is shorted and burned
pretty badly.
I powered up the DMM briefly and everything seems to be working ok.
I
want to know if anyone has any suggestions on what type of diode
should I use to replace the shorted diode. It doesn't have to be a
SMT diode.
I am guessing that is a protection diode and not a zener since most
of
the ICs run off of 9v according to their datasheets. Any help would
be greatly appreciated. Thanks in advance.


Can't think of any reason you'd need one at all, unless you plan
on somehow installing the battery backwards

Sounds like someone momentarily touched the battery terminals backward
and the diode forward-conducted and blew out. Conclusion: a surface-
mount diode doesn't work for backwards-connection protection, it just
self-destructs. Unsolder the diode and leave it out of the circuit.



Most reverse protection diodes do fail short-circuit. It's unusual to find
one that survives when it does its basic job. The battery must have been
connected backwards for the original to have failed, and as the rest of
the
meter seems to work ok, then IMHO, it has done the job it was designed-in
for. For the minimal cost, and the protection afforded, I really can't see
any reason NOT to replace it ... ?? Any old diode will do from a 1N4148
up.

Arfa



I've asked this before here , but again.
Does anyone know of series of diodes that are guaranteed to go open circuit
on excess forward current or guaranteed go short circuit on reverse voltage
overload.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

"Dave Plowman (News)" wrote in message
...
In article ,
Arfa Daily wrote:
Most reverse protection diodes do fail short-circuit. It's unusual to
find one that survives when it does its basic job.

Eh? The usual way is a series diode and that won't fail if the battery is
reverse connected. Be a bit silly if it did...


Sorry to disagree with you on this one Dave, but that's not the case. All
car radios have a diode connected in shunt with the supply. When you connect
the radio backwards, the diode conducts heavily and blows the fuse. Usually,
the diode itself fails also. The same applies to CB radios, ham radios and
PMR radios. Likewise, a couple of pieces of portable audio equipment whose
schematics I just pulled at random, also have a shunt diode. A series diode
is seldom found because apart from anything else, it would have to be rated
to carry the maximum continuous current of the equipment, and would also
cause a voltage drop, which is wasteful of power, and not desirable for
battery powered equipment. In any case, the principles are not in question
here. This meter *did have* a shunt protection diode, which had failed as I
would expect it to. The OP said so.

Arfa

"Dave Plowman (News)" wrote:

In article ,
Arfa Daily wrote:
Most reverse protection diodes do fail short-circuit. It's unusual to
find one that survives when it does its basic job.

Eh? The usual way is a series diode and that won't fail if the battery is
reverse connected. Be a bit silly if it did...


The forward drop may be enough to keep the meter from working.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

On Tue, 08 Aug 2006 13:14:49 GMT, Arfa Daily wrote:

"Dave Plowman (News)" wrote in message
...
In article ,
Arfa Daily wrote:
Most reverse protection diodes do fail short-circuit. It's unusual to
find one that survives when it does its basic job.

Eh? The usual way is a series diode and that won't fail if the battery is
reverse connected. Be a bit silly if it did...


Sorry to disagree with you on this one Dave, but that's not the case. All
car radios have a diode connected in shunt with the supply. When you connect
the radio backwards, the diode conducts heavily and blows the fuse. Usually,
the diode itself fails also. The same applies to CB radios, ham radios and
PMR radios. Likewise, a couple of pieces of portable audio equipment whose
schematics I just pulled at random, also have a shunt diode. A series diode
is seldom found because apart from anything else, it would have to be rated
to carry the maximum continuous current of the equipment, and would also
cause a voltage drop, which is wasteful of power, and not desirable for
battery powered equipment. In any case, the principles are not in question
here. This meter *did have* a shunt protection diode, which had failed as I
would expect it to. The OP said so.

What the OP did not mention was whether or not the DVM could also run
off of a wall wart -- or, at least, make use of a wall wart-style
charger. *That* could well be why the diode was toasted....

Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
38.24N 104.55W | @ config.com | Jonesy | OS/2
*** Killfiling google posts: http//jonz.net/ng.htm

In article ,
Arfa Daily wrote:
Eh? The usual way is a series diode and that won't fail if the battery
is reverse connected. Be a bit silly if it did...


Sorry to disagree with you on this one Dave, but that's not the case.
All car radios have a diode connected in shunt with the supply. When
you connect the radio backwards, the diode conducts heavily and blows
the fuse. Usually, the diode itself fails also. The same applies to CB
radios, ham radios and PMR radios. Likewise, a couple of pieces of
portable audio equipment whose schematics I just pulled at random, also
have a shunt diode. A series diode is seldom found because apart from
anything else, it would have to be rated to carry the maximum
continuous current of the equipment, and would also cause a voltage
drop, which is wasteful of power, and not desirable for battery powered
equipment.

I'd agree with that if a high power consumer, but on something like a DVM?

In any case, the principles are not in question here. This
meter *did have* a shunt protection diode, which had failed as I would
expect it to. The OP said so.

Indeed. But only of use if also protected by a fuse?

--
*Income tax service - We‘ve got what it takes to take what you've got.

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

"Allodoxaphobia" wrote in message
...
On Tue, 08 Aug 2006 13:14:49 GMT, Arfa Daily wrote:

"Dave Plowman (News)" wrote in message
...
In article ,
Arfa Daily wrote:
Most reverse protection diodes do fail short-circuit. It's unusual to
find one that survives when it does its basic job.

Eh? The usual way is a series diode and that won't fail if the battery
is
reverse connected. Be a bit silly if it did...


Sorry to disagree with you on this one Dave, but that's not the case. All
car radios have a diode connected in shunt with the supply. When you
connect
the radio backwards, the diode conducts heavily and blows the fuse.
Usually,
the diode itself fails also. The same applies to CB radios, ham radios
and
PMR radios. Likewise, a couple of pieces of portable audio equipment
whose
schematics I just pulled at random, also have a shunt diode. A series
diode
is seldom found because apart from anything else, it would have to be
rated
to carry the maximum continuous current of the equipment, and would also
cause a voltage drop, which is wasteful of power, and not desirable for
battery powered equipment. In any case, the principles are not in
question
here. This meter *did have* a shunt protection diode, which had failed as
I
would expect it to. The OP said so.

What the OP did not mention was whether or not the DVM could also run
off of a wall wart -- or, at least, make use of a wall wart-style
charger. *That* could well be why the diode was toasted....


Jonesy

Absolutely agreed, and that, if for no other reason, is as good as any to
make the case for fitting a replacement diode.

Arfa

Morse wrote:
"me" wrote in message
...
Hi Everyone,

I have a friend's DMM that wouldn't power up. I opened the case and found
that there was a power to ground short. The short is being caused by a
SMT diode that is across the 9v battery wires on the PCB. I lifted one
side of the diode and the diode is shorted and burned pretty badly. I
powered up the DMM briefly and everything seems to be working ok. I want
to know if anyone has any suggestions on what type of diode should I use
to replace the shorted diode. It doesn't have to be a SMT diode. I am
guessing that is a protection diode and not a zener since most of the ICs
run off of 9v according to their datasheets. Any help would be greatly
appreciated. Thanks in advance.

Jason

I'd look for a convenient point on the PCB to solder in something more
substantial like a 1N4001 or similar. Across the solder pads where the
battery leads are soldered to the PCB would be fine, just make sure you get
it round the right way, ie the white band to the positive terminal.

If your friend's a bit accident prone or absent minded I certainly wouldn't
give him it back with no diode- the first time he touches the battery to the
terminals the wrong way round he'll have an ex-DMM!

Dave




Thanks to everyone for their insights. My friend told me that he just
turned on the DMM and it started smoking. The 1 amp fuse for current
measurement was blown as well. Don't know if the blown fuse is related
to the shorted diode. It is only powered by a 9v battery, no AC jack on
the meter. It is a Mac Tools EM720 DMM. I will solder in a replacement
diode. Thanks again, everyone.

me wrote:

Morse wrote:

"me" wrote in message
...

Hi Everyone,

I have a friend's DMM that wouldn't power up. I opened the case and
found that there was a power to ground short. The short is being
caused by a SMT diode that is across the 9v battery wires on the
PCB. I lifted one side of the diode and the diode is shorted and
burned pretty badly. I powered up the DMM briefly and everything
seems to be working ok. I want to know if anyone has any suggestions
on what type of diode should I use to replace the shorted diode. It
doesn't have to be a SMT diode. I am guessing that is a protection
diode and not a zener since most of the ICs run off of 9v according
to their datasheets. Any help would be greatly appreciated. Thanks
in advance.

Jason


I'd look for a convenient point on the PCB to solder in something more
substantial like a 1N4001 or similar. Across the solder pads where the
battery leads are soldered to the PCB would be fine, just make sure
you get it round the right way, ie the white band to the positive
terminal.

If your friend's a bit accident prone or absent minded I certainly
wouldn't give him it back with no diode- the first time he touches the
battery to the terminals the wrong way round he'll have an ex-DMM!

Dave




Thanks to everyone for their insights. My friend told me that he just
turned on the DMM and it started smoking. The 1 amp fuse for current
measurement was blown as well. Don't know if the blown fuse is related
to the shorted diode. It is only powered by a 9v battery, no AC jack on
the meter. It is a Mac Tools EM720 DMM. I will solder in a replacement
diode. Thanks again, everyone.

Hi Me...

Something a little wrong with that; can't imagine how you could
get anything to smoke using a 9 volt battery (unless for some
reason he was using an NiMh in it - even then not much)

Betcha a dollar to a doughnut he did the same thing that I did
when I was a kid. Tried to measure the resistance of the AC line

Or, perhaps how much current was available from the AC line

Take care.

Ken