"But what about the small capacitor(s)?"

It has to do with killing high frequency 'gain'
of a servo-loop sufficiently at (and above) the
frequency where total feedback effects in the loop
reach 180 degrees (this is that magical point
where an 'amplifier' turns into an 'oscillator').

To actually determine this point (and the
required caps for a particular circuit layout
and a particular chip) is what they pay real
EEs to use HP -er- Agilent phase/gain meters
to do WHEN the situation demands it - the rest
of us just plunk down a good pair .1 uF caps
on the input and output and get on with our
lives ...

Jim


----- Original Message -----
From: "Pic Dude" <picdude@AVN-TECH.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, April 03, 2002 7:55 PM
Subject: Re: [EE]: 7805 heating up...


> Allow me to shift the discussion a bit...
>
> How are the capacitor values for a 7805 regulator design calculated?
> I've seen a ton of 7805 circuits, but never a good discussion of how
> the capacitor values are calculated.  Obviously the (larger) electrolytic
> is for stabilizing the input voltage, but I'm not sure how critical that
is
> in an automobile (vs. say a rectified AC source).
>
> But what about the small capacitor(s)?  Anyone have links to a doc
> that explains this?
>
>
>
> ----- Original Message -----
> From: "Larry G. Nelson Sr." <nr@MA.ULTRANET.COM>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Wednesday, April 03, 2002 4:11 PM
> Subject: Re: [EE]: 7805 heating up...
>
>
> > The .1 cap is critical. I had experience with a design that worked fine
> > with a National regulator but failed with a Motorola regulator. The
design
> > had no .1 caps and adding them corrected the problem. They need to be as
> > close as possible to the regulator to be effective.
> > Larry
> >
> > At 03:15 PM 4/3/02 -0600, you wrote:
> > >That's pretty much what I have -- circuit diagram attached.
> > >
> > >So why would it oscillate?  The 12V supply should be fairly
> > >stable (at least any changes should not be high enough
> > >frequency to be called "oscillation"), and the output has a
> > >33uF cap on it.  So my guess is that the oscillation is being
> > >produced internally (within the 7805)?
> > >
> > >
> > >----- Original Message -----
> > >From: "michael brown" <n5qmg@AMSAT.ORG>
> > >To: <PICLIST@MITVMA.MIT.EDU>
> > >Sent: Wednesday, April 03, 2002 2:41 PM
> > >Subject: Re: [EE]: 7805 heating up...
> > >
> > >
> > > > > but it gets hot enough that I cannot leave my finger on it for
more
> > > > > than 5-10 seconds.
> > > >
> > > > > Should it get this hot?
> > > >
> > > > Only when it's transmitting!  Sounds like it is oscillating to me.
> Put a
> > > > .1uF cap on the input, near the pins on the regulator.  Others here
> can
> > > > offer much better suggestions I'm sure.
> > > >
> > > > michael brown
> > > >
> > > > --
> > > > http://www.piclist.com hint: The list server can filter out
subtopics
> > > > (like ads or off topics) for you. See http://www.piclist.com/#topics
> > > >
> > > >
> > > >
> >
> > Larry G. Nelson Sr.
> > mailto:L.Nelson@ieee.org
> > http://www.ultranet.com/~nr
> >
> > --
> > http://www.piclist.com hint: The list server can filter out subtopics
> > (like ads or off topics) for you. See http://www.piclist.com/#topics
> >
> >
> >

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