DIYbanter - View Single Post - Analogue moving coil meter range extension?

mike[_22_]

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.

You could stack an AA-cell or two on top of your battery system
and alleviate that problem.

yuck. 2 resistors beats 2 cells any day.
Can't argue with that assertion, but, if you can
provide a circuit that is precisely linear and accurate over the WHOLE
range using
two resistors, I'd like to see it. I'm betting it's more than two
resistors.

I maintain that a digital voltmeter is the best option.
They're dirt cheap on EBAY.

but noncompliant with the basic spec
Yep, that's a problem with any customer expectation.
I try to adjust expectations to meet the ACTUAL requirements
before delving into complex solutions to implement a nonsense
requirement.
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.
That way, the characteristics of the zener knee are important
over a 5:1 current range instead of an infinite ratio.
I'm far too lazy to do the math on how nonlinear it will be.

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

NT