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Andrew,<br>
If the input is an ac power source, then without the filter caps, the
output would drop with the input. The capacitors are necessary for
filtering the ac noise out. For the input, a bridge diode circuit and
input capacitor keeps the input voltage up as the ac voltage drops
through the 0V level. For the output, it helps in the feedback to
stabalize the regulator circuit.<br>
Derek<br>
KE6JTP<br>
<br>
<br>
Andrew Rich wrote:
<blockquote cite="mid:LGEKIKIAKJPNFGILGJBBMECCDKAA.vk4tec@people.net.au"
type="cite">
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<div><span class="343033420-26082007"><font face="Arial" size="2">Can
someone explain to me why this is so:</font></span></div>
<div><span class="343033420-26082007"></span> </div>
<div><span class="343033420-26082007"><font face="Arial" size="2">I
have been building some projects now that use LM7805 voltage regulators.</font></span></div>
<div><span class="343033420-26082007"></span> </div>
<div><span class="343033420-26082007"><font face="Arial" size="2">This
is now the second time I have had issues with a PIC chip not running.</font></span></div>
<div><span class="343033420-26082007"></span> </div>
<div><span class="343033420-26082007"><font face="Arial" size="2">It
is only when I add caps to the input and the output that it settles
down.</font></span></div>
<div><span class="343033420-26082007"></span> </div>
<div><span class="343033420-26082007"><font face="Arial" size="2">Looking
on the CRO, the DC looked noisy.</font></span></div>
<div><span class="343033420-26082007"></span> </div>
<div><span class="343033420-26082007"><font face="Arial" size="2">I
thought that the caps where there to trap AC.</font></span></div>
<div><span class="343033420-26082007"></span> </div>
<div><span class="343033420-26082007"><font face="Arial" size="2">Do
caps on an LM7805 player a larger role ? Is there something inside the
reg that needs external C ?</font></span></div>
<div><span class="343033420-26082007"></span> </div>
<div><span class="343033420-26082007"><font face="Arial" size="2">Curious</font></span></div>
<div><span class="343033420-26082007"></span> </div>
<div><span class="343033420-26082007"></span> </div>
<div> </div>
<div><font face="Arial" size="2">----------------------------------------------------------------------------</font></div>
<div><font face="Arial" size="2">Andrew Rich VK4TEC</font></div>
<div><font face="Arial" size="2"><a moz-do-not-send="true"
href="mailto:vk4tec@people.net.au">vk4tec@people.net.au</a></font></div>
<div><font face="Arial" size="2"><a moz-do-not-send="true"
href="http://www.tech-software.net/">http://www.tech-software.net</a></font></div>
<div> </div>
<div> </div>
<div> </div>
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<pre class="moz-signature" cols="72">--
Derek Koonce
DDK Interactive Consulting Services
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Providing quality consulting services for electronic design development, and e-commerce profit.</pre>
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