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Damaged 7805 regulator?

M

Mike

Jan 1, 1970
0
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
 
P

petrus bitbyter

Jan 1, 1970
0
Mike said:
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

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
 
A

Anthony Fremont

Jan 1, 1970
0
Mike said:
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.

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.
 
M

Michael Heydon

Jan 1, 1970
0
Hi,

I've got a small PIC project with a 7805 regulator.

...

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

Regards,

Michael


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
 
M

Mike

Jan 1, 1970
0
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

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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