How do I know a 7812 is broken? - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 8th January 2005, 09:17 PM   #1
diyAudio Member
 
ericpeters's Avatar
 
Join Date: Apr 2004
Location: Deep South
Unhappy How do I know a 7812 is broken?

I'm been using an evalution amplifier board.
While testing I had a shortcircuit (2 capacitor solderpoints hit metal) and after that the board fails.

All voltages are still correct except the output of a 7812 regulator.
Input is seems to be correct (over 12 VDC)but the output only has 0,14 volt. Now my first tought is that this regulator is broken. But it is surface mounted and I'm not so good in soldering these so I want to be sure before I take it out.

Could there be another reason why the regulator only has 0.14 v at the output or can I be sure that it is defect.

Thanks,
  Reply With Quote
Old 8th January 2005, 09:22 PM   #2
VS is offline VS  Portugal
diyAudio Member
 
Join Date: Dec 2004
Location: Lisboa
Hi,

Those class of linear regulators should be supplied
with an input voltage at least 3V above the output.
In your case at least 15V at the input for proper operation.
Measure the input voltage, if correct it is probably a
dead IC...
  Reply With Quote
Old 8th January 2005, 10:00 PM   #3
diyAudio Member
 
Join Date: Dec 2003
Location: Tulips, windmills and wooden shoes.
It could also be shorted, since this regulator has a short-circuit protection.

Can't you just lift the output pin and measure it?
  Reply With Quote
Old 9th January 2005, 01:20 AM   #4
diyAudio Member
 
ericpeters's Avatar
 
Join Date: Apr 2004
Location: Deep South
Default So shorted it is!

I lifted pin 3 and the voltage was back again.

Now the question remains, what is shorted, what would be most likely:

On the plus line I got:
one 25V / 1000u Elna (This one is still O.K. i took it out and measured it)
one 50V/1u elco (different brand)
one 100n SMD capacitor
one TZM15V
I also disconnected the main AMP modules (B&O ICE500) so there is nothing wrong with that.

The only thing left is the power supply connection of a HEF4013B dual Flip flop, Fortunately the +V connection on this tiny device is patched with a small cable. So I disconnected this cable as well but the voltage problem remained. So my guess is that one of the above mentioned devices is defect.

I've included a PDF file with schematic on page 4. The low power supply + rail is the problem.

thanks,

Eric
Attached Images
File Type: jpg lowvolpsu.jpg (55.4 KB, 193 views)
  Reply With Quote
Old 9th January 2005, 09:10 PM   #5
nyman is offline nyman  Finland
diyAudio Member
 
Join Date: Dec 2004
Location: southern finland
it's probably TZM15V
  Reply With Quote
Old 9th January 2005, 09:16 PM   #6
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
Hi Eric

The regulator is dead.

Change the regulator...

PS: Atention!!....The positive and negative regulators have different pin outs.
__________________
Jorge
  Reply With Quote
Old 9th January 2005, 09:27 PM   #7
diyAudio Member
 
ericpeters's Avatar
 
Join Date: Apr 2004
Location: Deep South
No The regulator is ok, I lifted the output leg and the voltage was back.
When I reconnect it, the voltage is gone again and the regulator gets very very hot. The -regulator does never get hot.

There is defitely a short circuit somewhere, every thing disconnected I measure zero ohm between + and ground.

As you see in the schematic. The capacity on the + rials is a lot higher than on the - rails. This is probably because of the start stop circuit on the board.

nyman: I tried to figure out why the zener would be dead.

I had a 80V (high current from bigger capcitors) peak on the GND-A so a lot of current would flow through the Zener and through the bypass diode, correct?

I didn't know this would break a zener diode and als didn't know it would become a short circuit.

If anyone else agrees, I will try to take out the zener first, measure is and find someone to replace it.
  Reply With Quote
Old 9th January 2005, 09:32 PM   #8
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
I don't see any reason for that zener to be in your circuit...it is not regulating anything , as the voltage of the zener is greater than the output voltage from the regulator.

Disconnect the Zenner and see if everything works properly.
__________________
Jorge
  Reply With Quote
Old 9th January 2005, 10:12 PM   #9
johnnyx is offline johnnyx  United Kingdom
diyAudio Member
 
Join Date: Jan 2004
Location: manchester
SMD ceramic caps go short cct sometimes, so I would check that too. And check for solder bridges, If it's "hit metal" then the solder pad can deform, splaying out to touch an adjacent one., or vaporised copper can bridge the tracks. Clean the board, (scrub with a nail brush) to remove solder blobs.
  Reply With Quote
Old 9th January 2005, 10:57 PM   #10
Nisbeth is offline Nisbeth  Denmark
diyAudio Member
 
Nisbeth's Avatar
 
Join Date: Jan 2001
Location: Denmark
Quote:
Originally posted by Tube_Dude
I don't see any reason for that zener to be in your circuit...it is not regulating anything , as the voltage of the zener is greater than the output voltage from the regulator.

Disconnect the Zenner and see if everything works properly.
The Zener is there to protect the amplifier module if the regulator fails. Leave it be!


/U.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
7812 vs 7912 KU40 Solid State 4 13th June 2006 04:30 PM
Broken CD HELP! Bassivus Digital Source 8 29th September 2004 10:46 PM
7812/7912 regulator questions Solid Snake Solid State 48 1st December 2003 06:42 PM
7812 TO220 regulator little help please strulon Tubes / Valves 19 23rd May 2003 03:54 PM
can I parallel 7812 Chips PaulHilgeman Parts 16 6th May 2003 10:05 PM


New To Site? Need Help?

All times are GMT. The time now is 02:34 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2