On Sun, 24 Sep 2017 17:52:51 -0700, mike wrote:

On 9/24/2017 6:31 AM, wrote:
On Sunday, 24 September 2017 12:06:45 UTC+1, mike wrote:

A series zener needs to draw ZERO current at 10V and still
be 10V at MUCH higher current ratio. You will have to live
with some amount of nonlinearity.

In fact you could re-zero the meter so it reads 10v spot on. Add an extra mark for where it points when unpowered so you can see if it ever decalibrates.

Yes, you can , but it's still nonlinear.
It's a matter of how much nonlinearity you can tolerate.
It's my assertion that the measured quantity is so crude that you don't
need to worry much about anything. Just learn what it reads and calibrate
your expectations to match.

(I've been a bit preoccupied elsewhere but still following the thread
with interest and will reply individually when I get the chance)

That was part I had already considered in that:
The meter is fairly small. The meter is glued shut. The meter is only
marked in whole volts.

So, as you say, afa this experiment is concerned it was a cheap
toe-dipping into the concept of meter range enhancement, partly
because I think analogue display still have some merit and partly for
the S&G's. ;-)


snip

I try to adjust expectations to meet the ACTUAL requirements
before delving into complex solutions to implement a nonsense
requirement.

In this case the actual requirements were fairly simplistic as it was
an experiment. A feasibility study if you will. As you say, a DMM can
be bought for next to nothing (and I have several and one I'm already
using) but that wasn't the (entire) point.

Yes, the customer is always right. My job is to alert him
to the consequences of his demands.

;-)



Another non-complient solution is to use the two resistors
and a zener
to make a 9V-15V meter and hope that most of the nonlinearity
of the two resistor solution happens in the 9-10V range.

With a very small buck converter you could get a higher voltage supply
to ensure a voltage reference of some sort had sufficient overhead to
work correctly when close to the 10V threshold?

That way, the characteristics of the zener knee are important
over a 5:1 current range instead of an infinite ratio.

ITRW, as long as it's starting to work reasonably by 10.5V it
shouldn't really be an issue.

I'm far too lazy to do the math on how nonlinear it will be.

I do like the idea of printing my own scale and if this basic one
works, I might go for a bigger / better meter that I can open etc.

There's also an elephant in the room.
Unless you start with a meter that happens to have scale
markings ranging from 10 to 15, you're gonna need a new scale
no matter what you do with the electronics.

Unless it's already marked 0, 1, 2, 3, 4 and 5 'Volts' and you know
just have to add 10 to all of them. I would imaging the needle
pointing up the middle to 12.x would be a sufficient comforter. ;-)

If you're not gonna
do that, might as well just mark calibrations on the face with a
sharpie.

Only if I can get the thing open and it wouldn't be worth damaging it
to try.

Makes no sense to go to great lengths in one part of the project
only to create something that still doesn't do what you want.

My only hope is that it works 'better' than a similarly sized and
accuracy meter that is marked 0 to 15 and using it over a tiny
proportion of the range.

As has been suggested, if you aren't happy to try to KISS then you are
then going to be worrying about thermal stability and scale linearity
and I'd like to leave that for the Mk2. ;-)

Cheers, T i m