Undead 78L05.

[JTechno]

I have a NTC-based temperature module which I connected in reverse causing its death. I thought that what was death was its 78L05 voltage regulator so I connected it again, correct polarity this time, and measured the voltage at the 78L05 output pin, nothing as expected.

Before replacing it I decided to inject 5v directly to that pin, without disconnecting the 12v, to check that the rest of the circuit still worked. It still worked but to my surprise it continued working after disconnecting the 5v supply.

Now it works normally and my question is: What caused this behavior?

[bartm]

The classic 7805 is a integrated circuit that contains around 20 transistors. See this 'tear-down':
http://www.righto.com/2014/09/reverse-engineering-counterfeit-7805.html
One or more internal components could be damaged, shorten or open-circuit, who knows.
It's hard to predict it's behavior now, I guess.
You could try to mock around with an ltspice simulation model of it :-). it would be a nice learning opportunity.

Best thing to do is to replace it tough.