I made myself a symmetric power supply for my lab projects using several 78xx and 79xx components. The PCB was ready and I was testing it. I saw that 7905 is giving -12V output.
(See full image.)
(You see no connection to the middle pins of 79xx elements. That is why -Vmax=-20V is given through the heat-sink.)
I didn't check it twice or thrice; I checked it ten times to make sure that I'm not making a measurement mistake. I measured the voltage on C-5, on the connector pin and on the 3rd pin of 7905. Each time I made sure that I'm taking reference from a -0V point (this power supply consists of internally isolated two symmetric parts, there is also a +0V, that's why I call "-0V" instead of only "0V"). Each time I read a value near -11.96V (there is also a -12 node in the circuit; I'm sure that I didn't accidentally measured it).
There are also other 78xx and 79xx ICs on this circuit. All of them are working perfectly. The only problem is with this 7905 IC. By the way, it is brand new. I hadn't used it anywhere else.
This is my first time seeing a malfunction in 79xx/78xx series. I was very surprised, I thought I could be something I did, so I decided to ask this question. Is this erroneous voltage reading due to something I did? Did this 7905 get broken because of a problem in my design? Or it is just that the 7905 was broken from the beginning (I didn't test it before using). Could it even be that a 7912 accidentally mixed into 7905 basket in the production factory, and then they mislabeled it as 7905?
EDIT:
It came out to be that the 7905 was broken.
I tested the broken 7905 under different input voltage levels to analyze its behavior:
Best Answer
The 78/79xx series has OTP and OCP but not OVP. The only explanation for a good part is you have an external load that is connected to 12V. The 7905 uses single sided regulation ie. it relies on the load to ground to pull, the internal transistor will push to Vin. (-24v here)