lm7812 problem

[jpp412]

I have a problem with a 7812 in a power supply such that the output voltage (measured with a dvm) is only 9.5v. 

I at first thought the 7812 was bad so I put a new one in and still getting the same results.

I have a 7805 and 7905 in the same supply that work fine.

Any thoughts?  Is is possible that I got a batch of 7812's that are no good? (or perhaps mismarked 7810's)

Thanks in advance.
Joe

 

[Kevin Weddle]

I would suggest you determine the pin out to be correct. I did that with some voltage regulators. It turns out that I could not get the package pinout of the IC over the internet. So I just guessed until I go the correct voltage.  In the process I think I destroyed the regulator.

 

[jpp412]

I don't think that's the problem - at least based on what I had ordered the 7812 was from TI and I used the pinout on their datasheet.  However is there someway I could check it? - short of removing it from the circuit and using separate test circuits?

 

[audioguru2]

Hi Joe,
Is the input voltage at least 15V at its lowest points during ripple from the rectifier's filter cap?

 

[jpp412]

Hey Audio,
Yes it is about 18v according to the DVM.

 

[Kevin Weddle]

Ah. And then the regulator was bad.

 

[audioguru2]

How much more than 1A of output current? Is it hot?

 

[jpp412]

I haven't even connected any load to it yet - the regulators are cool.

 

[ante1]

Hi jjp,

Is it possible to measure the current going through the earth pin?

 

[Thomas1]

Hi jpp,

Did you probably forget the filter caps of 100n and 330n near the regulator?

 

[ante1]

Hi Thomas,

If you get a problem because you forget the cap at the input the problem is usually a very hot regulator due to self-resonance (swing). The cap at the output will not affect the regulator at this stage since there are no load variations without any load.

It would be interesting to see the schematic!