piclist 2002\12\29\120228a >
Thread: power reg questions
picon face BY : Spehro Pefhany email (remove spam text)

At 09:09 AM 12/29/02 -0600, you wrote:
>a couple of quick questions...
>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?

Have you considered reading the data sheet to see what
the tolerance is?  That's well within specs, FWIW.

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

It certainly might. But not for most digital circuits.

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

There's something like that, but I suggest an LM317 and
two resistors. You can easily tweak that one if you like.

Or if it's just a reference, a cheap part is the TL431,
an adjustable (>=2.5V) shunt regulator.  See the data
sheet for the connection and resistor values. It needs
1mA or so to regulate, and make sure any bypass
capacitor is in the "guaranteed stable" range.

>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,

Ok for some purposes, but could be noisy (from the
digital circuits) as well the tolerance issues.

>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 ??

No, there are references available from various mfrs,
as good as 0.1% tolerance or better, and with better
temperature drift specifications. Obviously the cost
goes up with the performance. I'm designing in one that
is guaranteed not to drift more than 0.0003%/degree C.
Sounds great, but that's 3 LS bits of the DAC per degree
of temperature change!

Best regarsd,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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