You really don't need a cap on the middle pin - the middle pin is tied to
ground (for ground reference for the chip), and if you put a cap to it, what
would you put it between, connecting to positive supply is the same as input
pin to ground, and regulated is the same as output to ground...

The places you need caps are on the input, and on the output. Take a look at
this datasheet...

http://www.national.com/ds/LM/LM340.pdf

It has a typical config for a fixed output regulator.

Personally, I like a decent sized cap on the input to limit input
fluctuations, and a not as big one on the output of the regulator.  I'd say
your first cap (100uF) on input, second (10uF) on output, and save the 100nF
for use as a decoupling cap somewhere in what uses the power supply :)  I
would also consider a diode for a discharge path (see later in the data
sheet)


-Tony

----- Original Message -----
From: "Marc Nicholas" <marc@GEEKYTHINGS.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Saturday, May 10, 2003 4:43 PM
Subject: [EE]: simple 7805 breadboard power supplies


> I must admit I'm a neophyte when it comes to analog electronics...so
please
> bear with me ;-)
>
> I'm building a 7805-based breadboard power supply...to be fed by one of
> those selectable "wall wart" 250mA transformers.
>
> I'm thinking 9vdc in to a 1A 7805, and then a 100uF (35v electro), 10uF
(15v
> electro), and 100nF (100v ceramic -- all I could find) capacitor off pins
> 1-3 respectively. Does this sound reasonable?
>
> Obviously I'll be careful about things like polarity and over heating, and
> the circuit will be semi-permanent (ha! oxy moron!), so I'm not too
worried.
>
> I probably have some 1N4001s kicking around and numerous LEDs if folk
really
> suggest a use 'em.
>
> More importantly, I'm after feedback about what's acceptable in the way of
> capacitor values (and why) and general "gotchas" with power.
>
> TIA :-)
>
>
> -marc
>
> --------------------------------------------------
> Marc Nicholas Geekythings Inc. C/416.543.4896
> UNIX, Database, Security and Networking Consulting
>
> --
> http://www.piclist.com#nomail Going offline? Don't AutoReply us!
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