I'm troubleshooting the 6 volt side of a power supply for a BPM tube
mic. I have one failed (shorted) diode left to replace which goes to
ground, in parallel with a filter cap and .002 cermic cap. It comes
between the rectifier output and pin one of the voltage regulator
(7806). I assume the power supply should work without this diode there
at all, and I can't figure out why it's there. The rest looks pretty
much like a textbook power supply cicuit. Can anyone explain what the
purpose of this diode might be and what might be a replacement?

apa wrote:

I'm troubleshooting the 6 volt side of a power supply for a BPM tube
mic. I have one failed (shorted) diode left to replace which goes to
ground, in parallel with a filter cap and .002 cermic cap. It comes
between the rectifier output and pin one of the voltage regulator
(7806). I assume the power supply should work without this diode there
at all, and I can't figure out why it's there. The rest looks pretty
much like a textbook power supply cicuit. Can anyone explain what the
purpose of this diode might be and what might be a replacement?

Are you sure it goes to ground?

You sometimes see a diode between the output and input of a 78 series
regulator to protect the part from its output going more positive then its
input (which can happen if there is lots of capacitance on the output and
some other load takes the input down quickly).

The 78 series do not like this and tend to suffer....

Just a thought.

Regards, Dan.

Dan Mills wrote:
apa wrote:

I'm troubleshooting the 6 volt side of a power supply for a BPM
tube
mic. I have one failed (shorted) diode left to replace which goes
to
ground, in parallel with a filter cap and .002 cermic cap. It comes
between the rectifier output and pin one of the voltage regulator
(7806). I assume the power supply should work without this diode
there
at all, and I can't figure out why it's there. The rest looks
pretty
much like a textbook power supply cicuit. Can anyone explain what
the
purpose of this diode might be and what might be a replacement?

Are you sure it goes to ground?

You sometimes see a diode between the output and input of a 78 series
regulator to protect the part from its output going more positive
then its
input (which can happen if there is lots of capacitance on the output
and
some other load takes the input down quickly).

The 78 series do not like this and tend to suffer....

Just a thought.

Regards, Dan.
There's one there too, but this one definitely goes to ground.

Dan Mills wrote:
apa wrote:

I'm troubleshooting the 6 volt side of a power supply for a BPM
tube
mic. I have one failed (shorted) diode left to replace which goes
to
ground, in parallel with a filter cap and .002 cermic cap. It comes
between the rectifier output and pin one of the voltage regulator
(7806). I assume the power supply should work without this diode
there
at all, and I can't figure out why it's there. The rest looks
pretty
much like a textbook power supply cicuit. Can anyone explain what
the
purpose of this diode might be and what might be a replacement?

Are you sure it goes to ground?

You sometimes see a diode between the output and input of a 78 series
regulator to protect the part from its output going more positive
then its
input (which can happen if there is lots of capacitance on the output
and
some other load takes the input down quickly).

The 78 series do not like this and tend to suffer....

Just a thought.

Regards, Dan.
There's one there too, but this one definitely goes to ground.

Dan Mills wrote:
apa wrote:

I'm troubleshooting the 6 volt side of a power supply for a BPM
tube
mic. I have one failed (shorted) diode left to replace which goes
to
ground, in parallel with a filter cap and .002 cermic cap. It comes
between the rectifier output and pin one of the voltage regulator
(7806). I assume the power supply should work without this diode
there
at all, and I can't figure out why it's there. The rest looks
pretty
much like a textbook power supply cicuit. Can anyone explain what
the
purpose of this diode might be and what might be a replacement?

Are you sure it goes to ground?

You sometimes see a diode between the output and input of a 78 series
regulator to protect the part from its output going more positive
then its
input (which can happen if there is lots of capacitance on the output
and
some other load takes the input down quickly).

The 78 series do not like this and tend to suffer....

Just a thought.

Regards, Dan.
There's one there too, but this one definitely goes to ground.

"apa" wrote in message
ups.com...
I'm troubleshooting the 6 volt side of a power supply for a BPM tube
mic. I have one failed (shorted) diode left to replace which goes to
ground, in parallel with a filter cap and .002 cermic cap. It comes
between the rectifier output and pin one of the voltage regulator
(7806). I assume the power supply should work without this diode there
at all, and I can't figure out why it's there. The rest looks pretty
much like a textbook power supply cicuit. Can anyone explain what the
purpose of this diode might be and what might be a replacement?


Is the filter cap a tantalum? If so this would be an attempt at reverse bias
protection. In any case it is probably for some sort of back bias protection
to avoid destruction of some other component.

bobs

Bob Smith
BS Studios
we organize chaos
http://www.bsstudios.com

apa wrote:

Dan Mills wrote:
apa wrote:

I'm troubleshooting the 6 volt side of a power supply for a BPM
tube
mic. I have one failed (shorted) diode left to replace which goes
to
ground, in parallel with a filter cap and .002 cermic cap. It comes
between the rectifier output and pin one of the voltage regulator
(7806). I assume the power supply should work without this diode
there
at all, and I can't figure out why it's there. The rest looks
pretty
much like a textbook power supply cicuit. Can anyone explain what
the
purpose of this diode might be and what might be a replacement?

Are you sure it goes to ground?

You sometimes see a diode between the output and input of a 78 series
regulator to protect the part from its output going more positive
then its
input (which can happen if there is lots of capacitance on the output
and
some other load takes the input down quickly).

The 78 series do not like this and tend to suffer....

Just a thought.

Regards, Dan.
There's one there too, but this one definitely goes to ground.

Output to ground ?

Someone was just being very fussy. I've seen this on some ppl's schematics
before. Usually instead of the diode 'across' the regulator.

Graham

apa wrote:
I'm troubleshooting the 6 volt side of a power supply for a BPM tube
mic. I have one failed (shorted) diode left to replace which goes to
ground, in parallel with a filter cap and .002 cermic cap. It comes
between the rectifier output and pin one of the voltage regulator
(7806). I assume the power supply should work without this diode there
at all, and I can't figure out why it's there. The rest looks pretty
much like a textbook power supply cicuit. Can anyone explain what the
purpose of this diode might be and what might be a replacement?

Could be protection from reverse-polarity spikes caused by something else
sharing the supply. Could just be there because they needed a .6V drop
to reduce the power dissipated by the regulator.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."