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PICList Thread
'power reg questions'
2002\12\29@104136 by Russell McMahon

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> 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)

{Quote hidden}

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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2002\12\29@110317 by Olin Lathrop

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> 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.
>
> ... or is this
> as accurate as these things get?

This is clearly answered in the spec sheet, Duh.  RTFM!

> I don't really care,

Then there is no reason I should either.

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

I don't know.  Does it matter if I use a 10K resistor in my circuit?

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

This is getting rediculous.  READ THE DATA SHEET before asking stupid
questions.  Yes, I know the answer to this question and the one above.  No,
I'm not going to tell you because I'm training you to look it up yourself,
and I don't want to reward your laziness.

> 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,

Voltage divider from what?  The +5V supply?  In that case the 2V reference
will decrease your accuracy, decrease the range, but increase resolution
within that range.  Also, the divider will draw 10mA.  This is not outright
wrong, but wastes a lot of power unnecessarily.  Compute the impedence of
the divider output.  Now compare that to the Vref+ input impedence.

> 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 ??

Unlikely.  A 7805 is generally not considered "precision", but of course
that is subjective and dependent on the application.  I've done a bunch of
designs where the accuracy of a basic 5V regulator was good enough for the
PIC A/D for the particular purpose.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2002\12\29@141608 by Chris Hunter

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----- Original Message -----
From: "Russell McMahon" <spam_OUTapptechTakeThisOuTspamPARADISE.NET.NZ>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
>
> References are usually MUCH more prcise than a regulator. A regulator can
be
> used as a low precision reference if you are desperate.

.... or you can use an LED as a slightly better reference.

Chris

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2002\12\29@143253 by Spehro Pefhany

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At 07:15 PM 12/29/02 +0000, you wrote:
>----- Original Message -----
>From: "Russell McMahon" <apptechspamKILLspamPARADISE.NET.NZ>
>To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
> >
> > References are usually MUCH more prcise than a regulator. A regulator can
>be
> > used as a low precision reference if you are desperate.
>
>.... or you can use an LED as a slightly better reference.

How do you figure that?

Best regards,

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

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