MOTM

What issues need to be considered when conditioning power rails. 
Usually there are 2 ferrite beads (1 if only one rail utilized or 3 if
there is +5V also supplied externally) and 2 caps.  On a lot of the
schematics, there are also a lot more caps.  How is the placement and
size determined?

Thanks!
Shemp

__________________________________________________
Do you Yahoo!?
Yahoo! Tax Center - File online, calculators, forms, and more
http://tax.yahoo.com

The usual practice is to have an electrolytic cap and ferrite for 
each power supply "rail" as close as possible to where the power 
supply voltages enter the circuit board.  For MOTM stuff the 
electrolytic cap of choice seems to be a 10uF 50V.  If your circuit 
draws a lot of power you might consider a larger value such as 33uF 
or 50uF.  In addition it is a good practice to liberally sprinkle 
bypass caps throughout the circuit board.  The MOTM choice is 
a .01uF axial cap.  There should generally be at least 1 bypass cap 
for each power supply voltage every other IC chip, located as close 
as possible to the chip.  Some people prefer to use more some less.  
The idea is just to keep noise off the power supply lines, and 
possibly to prevent parasitic oscillation as well.  There are even 
some IC sockets available with the bypass caps built in but when 
using these it is important to make sure the power supply pins on 
your IC chips match the location of the cap built into the socket.  
If your circuit includes additional voltage regulators you would 
want to have an electrolytic cap and ferrite on the output of each 
regulator as well.

>If your circuit draws a lot of power you might consider a larger value such
>as 33uF or 50uF.

Yet don't make it too big.  Consider two caps -- one big, one small.

>In addition it is a good practice to liberally sprinkle
>bypass caps throughout the circuit board.  The MOTM choice is
>a .01uF axial cap.

They are also ceramic, a radial lead ceramic might work even better.

>There should generally be at least 1 bypass cap
>for each power supply voltage every other IC chip, located as close
>as possible to the chip.

Yes, it is very important to place it as close as possible.

>If your circuit includes additional voltage regulators you would
>want to have an electrolytic cap and ferrite on the output of each
>regulator as well.

I agree about the cap, but I don't see the point in adding an 
inductor.  Ferrite beads are useful keeping noise from coming down 
the pike into each PCB, but what is the advantage when used with a 
local regulator, such as a low-current three-legged regulator 
providing 5V to a few logic chips??