How do I know a 7812 is broken?

[ericpeters]

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,

[VS]

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...

[bobo1on1]

It could also be shorted, since this regulator has a short-circuit protection.

Can't you just lift the output pin and measure it?

[ericpeters]

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

[nyman]

it's probably TZM15V

[Tube_Dude]

Hi Eric

The regulator is dead.

Change the regulator...

PS: Atention!!....The positive and negative regulators have different pin outs.

[ericpeters]

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.

[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.

[johnnyx]

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.

[Nisbeth]

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.