On Thu, 21 Sep 2017 18:11:32 -0500, Foxs Mercantile
wrote:

How things work:
Amp meters, by definition are suppose to be low resistance items.
When you're measuring current through a circuit, you don't want
to waste any of the available voltage across the meter.

As an example of this, current shunts for meters are usually rated
at X-amps for 50-100 mV across the shunt.

On the other hand, volt meters are put in parallel with a load, and
to be accurate, they should be many time higher than the load
resistance so they don't pull the supply voltage down that you're
trying to measure.

Back in the "good old days" for example, they would specify taking
a voltage reading with a "minimum 20K ohms/volt" meter.
If your trying to measure 100 volts, if you have a 150 volt range
on the meter, this would be a load resistance of 3 megohms across
the circuit you're measuring.

In the example given for a 250 uA meter, that's only 4K/volt.
But for measuring 10-15 volts, most supplies are good for anywhere
from 1-25 amps, so the extra 250 uA is hardly noticeable.

But still, ohms law applies and with just a series resistor to limit
the current for FSD, zero on the meter (re-marked to display 10V)
won't reflect the required 10V.

For the meter to read zero there would have to be zero volts across it
and with just your series resistor that would only happen when the
supply voltage = zero.

So, a simple series resistor won't work for a meter reading from 10
(not 0) to 15V (but I'll be happy for you to explain it otherwise).
;-)

So, you need something to hold the -ve of the meter at 10V (and the
+ve of the meter to supply via a ~20k resistor) to allow the meter to
work in the range of 0 to 5V, marked to show 10 - 15V.

I hope that makes the requirement clearer. ;-)

Cheers, T i m