LM7812 semiconductor has zero volts in but still measures 12.12 volts out?

[AliTonto]

This LM7812 is in a TV power supply without any raster or sound.
When I power on and test the voltage input at pin 1 I measure no
voltage there but on checking pin 2 (output) I get 12.12volts.
My question is: how can that be? where is the voltage coming from when
the input is not there?
Thanks to anyone who might be able to eplain this one.
AliT

 

[Mark D. Zacharias]

My guess is that somehow you made bad contact measuring, that there was
indeed voltage at the input. Remove the device and see if there's still
voltage at pin 3.

Another possibility: the device is rectifying (AC at it's input).

Mark Z.

 

[Mark D. Zacharias]

P.S.

I once had this happen on a NAD receiver. A shorted diode fed AC to the
regulator, which happily put out 12v DC. The standby transformer, however,
wasn't so happy. Opened the thermal fuse before I found out what was
happening.


Mark Z.

 

[Asimov]

"AliTonto" bravely wrote to "All" (07 Jun 04 21:03:18)
--- on the heady topic of "LM7812 semiconductor has zero volts in but still measures 12.12 volts o"

Al> From: [email protected] (AliTonto)

Al> This LM7812 is in a TV power supply without any raster or sound.
Al> When I power on and test the voltage input at pin 1 I measure no
Al> voltage there but on checking pin 2 (output) I get 12.12volts.
Al> My question is: how can that be? where is the voltage coming from when
Al> the input is not there?
Al> Thanks to anyone who might be able to eplain this one.
Al> AliT

Spatial anomaly?

A*s*i*m*o*v

.... This message transmitted on 100% recycled photons.

 

[Jacques Carrier]

I would connect a scope to the input pin of the regulator to see what is going on
really.

Jacques

 

[Michael Black]

This LM7812 is in a TV power supply without any raster or sound.
When I power on and test the voltage input at pin 1 I measure no
voltage there but on checking pin 2 (output) I get 12.12volts.
My question is: how can that be? where is the voltage coming from when
the input is not there?
Thanks to anyone who might be able to eplain this one.
AliT

Assuming this is a to-220 package, it's input, ground, and output.
I would never refer to the output as "pin 2". Given that, I'd make
sure you had the pinout right, before looking for some obscure matter.

Michael

 

[Sofie]

Michael Black:
Yes indeed, the output is pin 2.
Read carefully........
This can really be confusing for the un-initiated if they just look at pin
numbers. For example, if you look at the National Semiconductor Linear IC
data book for the LM78xx series and the LM340-xx 3 terminal POSITIVE
regulators in the TO-220 package, the pins are numbered from the front and
top, left to right are 1, 3, 2 .... whereas
pin1 (left) is the input (+)
pin 3 (center pin) is ground or common (-)
pin 2 (right) is the output.(+)

I was stung on this way back in the early 70's when I first started working
with these when they were a new thing.

To confuse thing even further, Motorola, in their Linear IC reference book
shows the very same packages but with pin numbering that actually makes
sense...... i.e. 1, 2, 3
pin1 (left) is the input (+)
pin2 (center pin) is ground or common (-)
pin3 (right) is the output (+)

So, as you can see, the National Semi or Motorola chips are directly
interchangeable with each other with no rewiring or cutting of traces....
inputs, common, outputs all are the same and in the same physical location
BUT the actual pin numbers are different..... the pins identified as 2 & 3
are reversed.

And to make things even more confusing for the un-initiated, the NEGATIVE 3
terminal regulators in the T0-220 & other SIP packages have the
ground/common and input/output pins all changed around.
National:
pin1 (left) is the ground/common (+)
pin3 (center pin) is the input (-)
pin2 (right) is the output (-)

Motorola:
pin1 (left)is the ground/common (+)
pin2 (center pin) is the input (-)
pin3 (right) is the output (-)

Look twice, design and install once.

 

[Michael Black]

Michael Black:
Yes indeed, the output is pin 2.
Read carefully........

You're right! It just goes to show how long it's been since I looked
carefully at that pintout. I just remember input/ground/output.

In my favor, I just checked a Motoral databook (merely out of curiosity
about this curiosity) and they show the TO-220 78XX regulator as
I defined it.

I wouldn't even talk in terms of pin numbers on three terminals, it
seems less than useful. You can just as easily speak of input/ground/output


Michael

 

[[email protected]]

"AliTonto" bravely wrote to "All" (07 Jun 04 21:03:18)
--- on the heady topic of "LM7812 semiconductor has zero volts in but still measures 12.12 volts o"

Al> From: [email protected] (AliTonto)

Al> This LM7812 is in a TV power supply without any raster or sound.
Al> When I power on and test the voltage input at pin 1 I measure no
Al> voltage there but on checking pin 2 (output) I get 12.12volts.
Al> My question is: how can that be? where is the voltage coming from when
Al> the input is not there?
Al> Thanks to anyone who might be able to eplain this one.
Al> AliT

Spatial anomaly?

A*s*i*m*o*v

... This message transmitted on 100% recycled photons.

What case and what are you using as a ground reference? For a TO220
case, it'll be pin 2 tied to ground. Sure it's not a LM 7912 which
has pin 1 grounded?

-Chris

 

[Artur Butryn]

This LM7812 is in a TV power supply without any raster or sound.
When I power on and test the voltage input at pin 1 I measure no
voltage there but on checking pin 2 (output) I get 12.12volts.
My question is: how can that be? where is the voltage coming from when
the input is not there?
Thanks to anyone who might be able to eplain this one.
AliT

Where do you get the ground from for your multimeter?
Try checking the voltage between pins 1-2 (this is the input voltage)
and pins 2-3 (output voltage)

Greetz
Artur
..

 

[JURB6006]

Some TVs use a standby supply that runs all the time and simply feeds the 5V
and enough 12V to kick the relay in. Once turned on these supplies are
"overridden" by a run supply, sometimes from the horizontal.

If you've actually got the 12V, but no input, something else that is supposed
to run from the source that feeds that regulator is down, and probably causing
your symptom.

Usually they use blocking diodes though, and a shorted one of those was
probably the original cause of the problem. Best I can do with the info
provided.

Good Luck.

JURB

 

[AliTonto]

Some TVs use a standby supply that runs all the time and simply feeds the 5V
and enough 12V to kick the relay in. Once turned on these supplies are
"overridden" by a run supply, sometimes from the horizontal.

If you've actually got the 12V, but no input, something else that is supposed
to run from the source that feeds that regulator is down, and probably causing
your symptom.

Usually they use blocking diodes though, and a shorted one of those was
probably the original cause of the problem. Best I can do with the info
provided.

Good Luck.

JURB

Thank you all, I now have plenty to proceed with;
I shall reply once job is completed.
Jim