[OZAPRS] LM7805 and caps

[Darryl Smith]

Most of the time when I find the DC is really bad is when I do dumb
things... Like...
	* Set pins as output high and then ground them
	* Set pins as oscillating, and then connect them to a supply
	* mis-wire the MAX232

If there is noise getting into radio equipment then changing the caps on
the
main oscillator helps. This was notorious with some of the TNC-2
implemementations, where there was a birdie on 145.01 MHz. It could be
moved
with a trimcap. I can remember helping a friend out when he had spent days
looking for the problem. I asked for a ceramic capacitor - any value. And
it
worked. He did not believe that I did not care about the value. Just
enough
to change the frequency.

Darryl

-----Original Message-----
From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] On
Behalf Of Ray Wells
Sent: Monday, 27 August 2007 1:38 PM
To: Ian ZL1VFO; VK / ZL APRS Users
Subject: Re: [OZAPRS] LM7805 and caps

Ian ZL1VFO wrote:

>>Message: 4
>>From: "Andrew Rich" <vk4tec at people.net.au>
>>Subject: [OZAPRS] LM7805 and caps
>>    
>>
>
>  
>
>>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 ?
>>    
>>
>
>The little beggers sometimes take off and oscillate without any C
>around them.
>
>I usually put 10-100uF & a 0.1uF ceramic across the input to gnd, and
>several uF and another 0.1uF ceramic across the output. Might be a bit
>of an overkill, but I'd prefer these things didn't take off and cause
>wierd effects in uP or other logic circuits...
>
>-Ian ZL1VFO
>
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>  
>
My Kenwood TS430 developed severe (S9+) interference at around 455khz. 
Turned out to be a dried out electro associated with a 78L08 regulator. 
Difficult find, easy fix.

Ray vk2tv
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