AVR-Chat

On Feb 13, 2005, at 5:57 AM, Valerij Avrealij wrote:
>
>>  The problem is that your 7805 isn't protected against "back feeding."
>>  Its trying to regulate its output which is being driven higher than 
>> its
>>  unpowered input by your ISP cable.
>
>>  You need a different model of regulator, else insert a diode between
>>  the existing regulator and the ISP connector.
>
>>  Or you could power your circuit by normal means when using the ISP.
>>  This will keep your 7805 happy.
>
>
> Thanks for your help. How you think, the chip was damaged? Or it can
> be used?

Its hard to hurt a 7805. The problem is that your ISP was sourcing 
current as hard as it was able, only 200 mA. Should be thermal and 
current protection in the regulators and all should be fine once they 
cool down.

If my diagnosis is correct the AVR was not a factor in your problem.

Some regulators are safe in this situation, others are not. Some 
specifically state that current will not flow backwards thru the 
device. Others say something very similar with "protected against 
reverse battery connection" which is slightly different.

Curious as to whether the ISP was able to hold 5 volts on the AVR under 
these conditions? If so then you might "get away" with it as you should 
only need the ISP for short bursts.

--
David Kelly N4HHE, dkelly@HiWAAY.net
========================================================================
Whom computers would destroy, they must first drive mad.