Re: 431 shunt regulator help


Jim Thompson wrote:

On Wed, 18 Feb 2009 08:34:01 -0800, John Larkin
<jjSNIPlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Wed, 18 Feb 2009 01:20:46 -0800 (PST), bill.sloman@xxxxxxxx wrote:

On 18 feb, 05:16, Jim <jims...@xxxxxxxxx> wrote:
trying to drop 12 vdc to 5vdc for an extended scale A/D, I would like
to use a TI431 to sense anything above 9 or 10 volts, feed it into my
uC for display on a digital meter. I am a software guy with limited HW
experience so here are my questions, they all pertain to a simple
shunt regulator circuit, eg; current resistor on Vin. Ref tapped into
voltage divider (R1, R2)

with a regulated input voltage that of 12v and a uC A/D pin that can
accept 5v, what resistors do I need for the current resistor and the
voltage dividers in order to capture the top 5 volts (lets say 9v and
above).

If you would rather "teach me to fish" I will accept a formula that is
written in 8th grade terms ; )

The only useful help we can offer is the suggestion to go for a less
ambitious project.

If you want to drop the 12V to 5V to get a regulated 5V supply for
your A/D converter power supply, the TL431 is a poor choice of
regulator. The LM6805 will do a perfectly adequate job and leaves you
less room to screw up.

You can scale down the 0-12V input range to 0V to 5V with a simple
voltage divider. If you were to tell us which A/D converter you were
planning on using (or identify the processor of which it forms a part)
we could suggest specific resistor values.

If you want to expand the range 9V to 12V to fill the 0V to 5V range
of your A/D converter, life gets a little more complicated. If you
told us quite a lot more about what you think you are trying to do we
could certainly come up with something - but granting the expertise
you are exhibiting here, it is less than certain that you would be
able to get it to work.


Ah, the ever helpful and sociable Sloman.

Hasn't himself designed anything useful in decades, but professes
enough expertise to insult people who ask sincere questions.

Speaking of "work"...

John


As in "piece of" ?:-)


As in, 'not capabale of'.


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