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.

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"AliTonto" wrote in message
om...
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