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[aprssig] LM7805 and caps

Jason Rausch jason at ke4nyv.com
Sun Aug 26 21:12:17 UTC 2007


I have experienced the same on my work bench.

Back when I got started with the PIC16F84, running at
10MHz, it seemed to be much more forgiving.  I was
routinely running with a 7805, without any caps.

When I moved to the PIC18F2XX parts, I noticed that I
was unable to get anything to run unless I added at
least a couple of 0.1uF caps to the input and output
of the 7805.

Not amateur related: Over the last few months I have
been developing a PIC based turnout (switch track)
controller for my N-scale model railroad.  I had it
breadboarded and working just fine.  Once day, I
powered it up and all I had was a single LED
flickering.  I noticed that one of the caps had come
loose from the breadboard.  I plugged it back in and
the damn thing ran fine.  It took just that little bit
to completely throw it off.

Checkin' it twice 73's
Jason KE4NYV
RPC Electronics
www.rpc-electronics.com

--- Andrew Rich <vk4tec at people.net.au> wrote:

> Can someone explain to me why this is so:
> 
> I have been building some projects now that use
> LM7805 voltage regulators.
> 
> This is now the second time I have had issues with a
> PIC chip not running.
> 
> It is only when I add caps to the input and the
> output that it settles down.
> 
> Looking on the CRO, the DC looked noisy.
> 
> I thought that the caps where there to trap AC.
> 
> Do caps on an LM7805 player a larger role ? Is there
> something inside the
> reg that needs external C ?
> 
> Curious
> 
> 
> 
>
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> Andrew Rich VK4TEC
> vk4tec at people.net.au
> http://www.tech-software.net
> 
> 
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