Home |
Search |
Today's Posts |
|
Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
Reply |
|
LinkBack | Thread Tools | Display Modes |
|
#1
|
|||
|
|||
LM7812 semiconductor has zero volts in but still measures 12.12 volts out?
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 |
#2
|
|||
|
|||
LM7812 semiconductor has zero volts in but still measures 12.12 volts out?
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. -- Please reply only to Group. I regret this is necessary. Viruses and spam have rendered my regular e-mail address useless. "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 |
#3
|
|||
|
|||
LM7812 semiconductor has zero volts in but still measures 12.12 volts out?
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. -- Please reply only to Group. I regret this is necessary. Viruses and spam have rendered my regular e-mail address useless. "Mark D. Zacharias" wrote in message ... 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. -- Please reply only to Group. I regret this is necessary. Viruses and spam have rendered my regular e-mail address useless. "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 |
#4
|
|||
|
|||
LM7812 semiconductor has zero volts in but still measures 12.12 volts out?
I would connect a scope to the input pin of the regulator to see what is going on
really. Jacques |
#5
|
|||
|
|||
LM7812 semiconductor has zero volts in but still measures 12.12 volts o
"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: (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. |
#6
|
|||
|
|||
LM7812 semiconductor has zero volts in but still measures 12.12 volts o
On Tuesday, 08 Jun 2004 08:18:06 -500, "Asimov"
wrote: "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: (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 |
#7
|
|||
|
|||
LM7812 semiconductor has zero volts in but still measures 12.12 volts out?
AliTonto ) writes:
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 |
#8
|
|||
|
|||
LM7812 semiconductor has zero volts in but still measures 12.12 volts out?
Uzytkownik "AliTonto" napisal w wiadomosci 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 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 .. |
#9
|
|||
|
|||
LM7812 semiconductor has zero volts in but still measures 12.12 volts out?
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 |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Zenith Model SY3288DT 32" TV WILL NOT TURN ON | Electronics Repair |