Damaged 7805 regulator?

May 17, 2007 4 Replies
M

Hi,



I've got a small PIC project with a 7805 regulator. It was working fine until I added ICSP (I soldered in the ICSP header and reprogrammed the uC without checking that it still worked in between :/ ) I have checked all my new connections and can't see any bridges.



After reprogramming via ICSP the regulator doesn't appear to function correctly. With input around 8V I get about 3V out, with input around 12V I get almost 4V out.



Is it possible that the 5V that is fed into the circuit by the programmer could have damaged the regulator (the regulator was not connected to a power source at the time)?



The project works ok if I connect 5V from an external source.



Regards,



Michael


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P

"Mike" schreef in bericht news: snipped-for-privacy@mheydon.net...

As from your description I'm almost (99.99%) sure the 7805 has been gone. They do not survive a voltage on the their output that's more than 1-2V above the voltage on their input. (AFAIK it's not in the specifications, and values may vary.) When you replace the 7805, add a backward diode between the input and the output. For small supplies a 1N400x will do.

petrus bitbyter

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A

Reverse biasing the regulator can indeed damage it. That's why the output shouldn't have a huge cap if the input doesn't, and why you sometimes see a diode across the regulator in/out to make sure the output pin voltage never excedes the input pin voltage by more than .6V.

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M

Thanks for your help guys. I've re-read my programmer manual and apparently it is able to steal power from the circuit being programmed. I assume that is to prevent exactly this situation.

I'll replace the reg and be a bit more careful next time.

Thanks

Michael

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M

In the end I don't think it was the regulator (I let the smoke out while I was debugging so I'm not 100% sure). New regulators showed similar output to my "broken" one, turns out my bench power supply is a bit dodgy, I put a bigger cap across the input and all is good now.

Michael

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