> I use the 7805 to regulate my 9vdc wall supply to
> +5vdc, but it's off a little. +5.05vdc
>
> I have a 10uf ceramic cap between the +5vdc output
> and ground.  This brings the out put up to +5.05 from
> 4.90 vdc.
>
> The question is how do I tweak this to get +5vdc ?
> --just change the value of this capacitor, or are there
> other little things that are normally done? or is this
> as accurate as these things get?

It's as accurate as they get, more or less.
Actual value also depends on accuracy of your meter :-)

Capacitor adds smoothing and stability - doesn't change actual voltage - may
change what your meter reads either because it changes ripple voltage (which
should be utterly minimal) or removes some instability (possibly a marginal
case of) which was causing output to vary in a manner which your meter
couldn't see. I have seen regulator outputs that read close to actual value
on a DC meter but which were found to be in full oscillation when looked at
with  'scope.

> I don't really care, and I don't think this matters does it?

Not for most purposes.

> But, secondly, I want to know if there is a 780x (2)
> for giving you +2vdc?

Look up LM317 datasheets on web.
This is 1.25 volt output but DESIGNED to be scaled up to any voltagegreater
than that using just 2 resistors to set value. Datasheet will show you how
and explain how it works.

    R1 ref to ground
    R2 output to ref

    Vout = (R1 + R2)/R1 x 1.25v     APPROXIMATELY

    Read data sheet
    Note Iadj current affect on actual voltage
    Don't make R2 too large (see datasheet)

> I have finally gotten back to my AtoD setup using,
> Vss--Vref- and +2vdc--Vref+
>
> it is working fine, using a 200 ohm and 300 ohm
> volatge divider, and that what I will use for now,
>
> I wanted to ask these questions before I forgot, thanks
>
> when talking before about this Vref+ for A/D someone
> mentioned precision voltage sources, is this refering to
> something like the 7805 for +5vdc ??

References are usually MUCH more prcise than a regulator. A regulator can be
used as a low precision reference if you are desperate.


        Russell McMahon

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