On Monday 15 October 2001 08:23, you wrote:
> Dennis sorry for this crude ( but true ) answer:
> You have killed another PIC because you're not carefully.
> Once you have a short anywhere on the board you'll not rush to insert
> another PIC on the board before you solve the short circuit problems.
> I've guess you have a reverse voltage on one pair of powering pins.
> Vasile

Hi Vasile,

Actually my first prototype was on a veroboard, and I had lots of tracks
shorting (though soldering did not cover the whole width of a track, I still
don't know why it's so fragile :-)) which I had to fix until finally the
5V-GND went and everything went dead :)

I didn't rush out to stick a new PIC on the board, I reassembled the circuit
more carefully on a breadboard, and everything was going fine until the power
supply part (the 05 regulator, the ONLY thing on a veroboard! Ohh the irony!
:) had to some more work due to an external device and the tracks shorted out.

From memory (dont have the board in front of me :-) there would be have been
caps going to ground (this is to answer some other people's answers) at CLKIN
and CLKOUT with the 4 MHz crystal. Otherwise a bunch of LED's to test the
bootloader/serial programming modes, and not yet any real other circuitery.

The regulator had caps (though small, like 0.1 uF) between the main +12V and
GND, and another cap between the GND and +5V. Then another cap on the
breadboard where the +5V and GND start.

So a good idea would be to stick a bigger cap there to filter the supply
lines at least for the PIC (100uF?) and maybe diodes to protect against
reverse polarity? (hey just in case :)  Maybe a resistor going to the VDD
which would limit any current to less than the max allowed by the PIC?

Thanks :)
Dennis




> On Sun, 14 Oct 2001, Dennis Noordsij wrote:
> > Heips,
> >
> > This is the second time the following happened, it's getting kinda
> > frustrating!
> >
> > I have a circuit on a breadboard, a PIC(16F877), a MAX232 and support
> > components, some LED's, etc.
> >
> > The power comes from a 7805 regulator with a source of 12 volts, this is
> > the only component on a veroboard rather than a breadboard. The regulator
> > can go up to 1.5 amps if properly heatsinked.
> >
> > For some reason (just my luck I guess!) every veroboard I use manages to
> > attract mysterious particles from the air and create a short between 2
> > tracks, even though the solder doesn't even take up the whole width of
> > the track. I end up butchering the groove until it is wider. (thats why I
> > switched to breadboards, saved me lots of time! :)
> >
> > Anyways, in this case it created a short between the 5V and GND tracks.
> > The regulator is heatsinked (well isolated though) and got hot quite fast
> > :-) It only took a second before I had disconnected it again, but at that
> > point the PIC was burning hot at both VDD-VSS pin locations, and
> > consequently the resistance when measured between VDD and VSS pins is
> > ZERO. I might as well use a paper clip in the circuit rather than the PIC
> > after this "incident" :(
> >
> > But I'm wondering what it is exactly that kills the PIC in this case.
> > Shouldn't the current simply go through the short (which is at the
> > beginning of the circuit, way before the power lines reach the other
> > components)? Why did the LED's and MAX232 survive fine but the PIC died?
> >
> > And more importantly, what is the easiest way to build protection against
> > this into the circuit? (other than fuses which I guess would do the trick
> > :)
> >
> > Just wondering :)
> > Regards,
> > Dennis
> >
> > --
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