Fred,
please contribute to the forum and do not confuse people.
Look at my answers in the body and believe them or forget them. This will
be my last contribution(confusion) to this topic. If you won't to be helpful
to this forum, do not state what is bad, rather come up with a good design.
Keep in mind that many designs are possible. My approach is: Keep it simple
and robust.
Michael Grass
----- Original Message -----
From: <n3eu@comcast.net>
Subject: Re: RE : Europa-List: roll trim tab and servo installation
>
> Michael Grass wrote:
>
> > 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.
>
> Not my input ultimately, but National Semiconductor. ;-)
(MG) not true, the stated 0.22uf comes later after the stabilized source.
>
> The larger the cap the bigger the inductor, and they best filter 60Hz AC
line hum. At high freqenies (alternator) and fast-rise spikes (Rotax
regulator), a large cap will do nothing if not make things worse. In
looking at various avionics schematics, I suspect this is one reason one
often finds a small value cap on the 14V input, as little as .01uF, some a
hundred. Some use small caps and a choke, the effective way if needed to
condition high-freq, noisy input.
>
(MG) Fred this is exactly right. this is where the .22uF comes into the game
not the 0.01. Where dou you get the 60HZ from??? The rotax produces between
aprox. 6kHz to 36Hz fundumental plus much higher harmonics. Thats a 2 pole
motor on a DB3 rectifier means rpm times 6. There are no 60Hz unless you
are in an american garage or in europe it would be 50Hz induced vthrough the
air.
When 14V power is removed from the circuit, and if the input lead goes to
ground (a master relay can briefly do that), the 7805 regulator can be
destroyed. Plus maybe pop a fuse. The diode I suggested prevents that and
is recommended by National Semi if any input grounding can happen.
>
(MG) The master relais will not do that but the other loads of course will
discharge the larger capacitor. So will do the servo tester and the servo
itself. NS states if a cap of 10uF is used a diode can be omitted. This
stands for no load connected. The diode will not hurt but you don't gain
much.
> > The [output] cap helps to stabilize the relatively high inrush
> > current to the motor from the servo.
>
> It sure will, but the inrush current (caused by a large output cap) when
you merely switch on the circuit is many times the current jolt to the
regulator than motor startup should cause. IOW, a net loss in abuse to the
regulator IC, the more microfarads the worse.
> (MG) my advise is, do not switch the unit of and on several times in a
second. Usualy when I go flying, I switch he master on, fly for at least
30min or much much longer and then switch it of. Gusty winds, means
disturbance force on the tab, means current spikes in the servo , happends
more frequent then me toggeling the avionics master.
> > 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.
>
> Actually my texts cite no 10/1 rule, but rather National says the output
cap is mandatory and should be selected (value and ESR) for phase shift
relative to input noise frequency. Else it can oscillate, and that you
could hear in the avionics (plus a DC servo wouldn't work too well)! As
practical matter, the noise products on the input should be small enough,
due to what the battery does, so this shouldn't happen. Further effective
is a tantalum cap, due to its low ESR, they say. 1 uF typical.
> (MG) Interesting, before you did state a cap on the output would be
harmfull. Hmm no more comment.
> Regards,
> Fred F.
>
>
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