At 09:42 AM 3/18/2004 -0700, you wrote:
>Speaking of the trusty 7805, I'm wondering how you decide how much filtering
>is required on the input.  I've always put a couple of 2200 uFs in for my
>own use, but these are the most expensive part of the board, so if I could
>get away with one, I'd do it.
>
>I realize that the needed capacitance goes up with less headroom, and with
>increased current draw.  I'm thinking 9V DC in, up to 1A out.

Ripple p-p =  I/(C * 120)  (for full-wave and 60Hz)
Eg. 1A, you need 1V of ripple, C = I*1V/120 = *1/120)F = 8,300uF.

Why do you need 1A? That's an awful lot. At 100mA you might be able to
use a cheap and small 1000uF part.

>You might want to ask how much ripple is acceptable, and that's another
>thing I don't know, but there must be a standard number for this, for PICs
>as well as HCT/HC logic?


You must ensure that the troughs of the ripple, at minimum
line voltage and maximum current draw, at minimum ambient temperature and
capacitor tolerance, *always* exceed the regulator drop-out voltage for the
current if you want regulated 5V to be there. Easy math, but it has to be
done if you want a solid result.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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