On Fri, 22 Sep 2017 14:13:13 -0700 (PDT), whit3rd
wrote:

On Thursday, September 21, 2017 at 10:51:33 AM UTC-7, T i m wrote:
Hi All,

Not a 'repair' question as such but I was thinking of using an old
skool 250uA FSD moving coil panel meter to read the 10 to 15V DC range
(a lead acid battery charge / discharge indicator), next to a 0-30A
Ammeter (inc shunt etc).

I was thinking of using either a voltage reference such as
LM4040DIZ-10.0 or could I get away with a simple zener (BZX55C?) with
associated resistor(s) please?

You can use any zener under 10V to do a 10V zero offset (just divide down the
input voltage until that 10V input matches the zener knee). So, with an
arbitrary zener (I'd use a TL431 as a 2.5V reference, they're convenient
and accurate) and a trimmer acting as voltage divider, it just remains
to make a range-setting resistor in series with the meter.

Hmm, that's crafty. ;-)

I'm trying to picture how the voltage rise would still be linear but I
think I can see how it would be.

The -ve of the meter is held at some voltage threshold low enough to
ensude a low change in difference between the voltage measured and the
ground. A variable (to start with at least) resistor set as a pd then
sets the upper voltage to give FSD at 15V.

The important thing, is to doodle up a really nifty scale for your meter
that indicates 10V to 15V, with clear markings, along the pointer's arc,
maybe with a bunch of subdivision marks.

Yeah, that sounds like a interesting project for a Mk2 meter. ;-)

took
a plotter and a bit of custom software to draw the arcs and labels,

I saw the link to some free software for doing such but not had chance
to try it yet.

then some fiddling with the faceplate of the meter to
affix it (warning: you need to worry about laser-print ink, paper, glue compatibility).

Ok. ;-)

Cheers, T i m