Fred,
I appreciate your input but think again:
1. A large (dimensional 12mm * 25mm) cap is not a problem. Electrically
1000uF reduces the effects on the supply and stiffens the supply for
regulating purpose. True it is electrically large and I have no problem if
you reduce its size.
2. The small unipolar cap of 10 nF to 100nF is absolutely needed to prevent
an extra unwanted radio transmitter beside the avionics.
3. The cap on the output is not harmful to the 7805 since it is completely
self protected. The cap helps to stabilize the relatively high inrush
current to the motor from the serve.
4. You said it by yourself. You have to monster filter already in the
system. The cap near the alternator and the battery. True, for the higher
frequencies all 3 "filters" do a lousy job. That's where the by NS
recommended 0.22uF to 0.33uF cap comes in handy because of its relatively
low impedance compared to the other 3 it has the ability to deal with the
higher frequency noise and therefore a good think to do.
5. The diode does not harm but does not buy you a real advantage
The overall verdict is:
1. Most values are in this circuit uncritical except the 10-100nF close to
pin 2 and 3
2. As an HF noise filter a cap of .22 to .33uF is recommended between pin 1
and 2. Thanks for reminding!
3. Input and output caps can be omitted but are beneficial. Rule here is
that the input cap should be 10 times larger as the output cap to have a
stable regulating result. Anything between 100uF and 1000uF for an input and
10 to 100uF for the output would be my recommendation.
Michael Grass
A266
----- Original Message -----
From: <n3eu@comcast.net>
Subject: Re: RE : Europa-List: roll trim tab and servo installation
>
> Michael Grass wrote:
> > ...
> > 1 Electrolytic capacitor of 1000 uF min.40V
>
> > 1 Voltage regulator from the 7805 family...
>
> It won't do any harm, but a big 1000uf/40V cap will not accomplish
anything, since you have a real monster filter capacitor already in the form
of the airplane's battery. However, National Semiconductor's application
notes for the 7805 say you _do_ need a .22 or .33uf cap here, where the
regulator is some distance from the input filter (here the battery), and for
the implied purpose, a small value cap does the better job of it than
1000uf.
>
> > 1 Electrolytic capacitor of 100 uF min.40V
>
> A cap that size on the output can be harmful per NS, but the unknown here
is the effect of high-freq alternator noise on the input (vs 60Hz AC) and
motor noise on the output. I'd thus eliminate this added cap on the output
and help protect the 7805 with a rectifier diode (1N400x) across
input/output, with the banded end toward the input.
>
> Regards,
> Fred F.
>
>
|