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The question was about a 7805, which is a very, very old design. The
datasheet I pointed to was published in 1976 and I cited it as a
reaction to Carl's post.<br>
<br>
But I just ran a modern 7805 backwards and at seven volts (notice
that for *this chip* TI said the hazard is for voltages "greater
than approximately seven volts") it draws very little current. At
30 volts it draws a fraction of an amp but a quick test showed it
still worked when connected normally before I tossed it into the
trash can. So even if the design hasn't been updated this thing is
not fragile. And no amount of capacitive load is going to cause more
than five volts to go through the output to ground if the source
current is turned off. <br>
<br>
I didn't actually care about Brian's question. I wanted to make sure
folks realized the implication of Carl's suggestion.<br>
<br>
But I used the thread as an excuse for the sermon about reverse
biasing with MCU GPIO pins, which is a very serious issue and the
mechanism that some folks are inadvertently using to get "dead pins"
or completely dead chips. But that had nothing to do with Brian's
question and looking back I see that the example of "VCC+.3v" in the
context of an MCU pin was a really awful confusion factor to throw
in. It was a terrible mistake to mention this sooner than later in
my posting.<br>
<br>
-Pete<br>
PS If anybody is interested in a step by step explanation of the MCU
hazard I piggy-backed <a
href="https://wisense.wordpress.com/2014/11/28/interfacing-safely-with-the-msp430/">here's
a good starting point</a> (written about a TI MCU family, but
*depending on the data sheet* applicable to many other MCUs having
very vaguely defined diode current limits. The takeaway is to avoid
accidentally powering your application via current through a GPIO
pin!) And yes, this hazard applies to other kinds of chips, not just
MCUs.<br>
<br>
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