I have a 40 amp, 13.8v supply and would like to add a voltage (0-15v) meter
and an amp (0-50amp) meter. The meters look frail and I can't imagine
running all that current through those flimsy connectors and tiny meter
coil. I have no paperwork on the meters and was wondering if I need a shunt
resistor or something to take all that current. Any ideas? Thank you.
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The measurements derived from PSU fitted operational meters do not need
to be very accurate.
To make the correct series resistor for the voltmeter and the shunt for
the current meter you need a digital volt/mA meter ,preferably with a
10 Ampere- DC range included.

For both measurements a moving coil meter in the 0.1 -1 mA range will be
fine . For the current meter a say 10 mA full scale meter will be OK.

Try to find moving coil meters with a 0-15/0-20 and 0-50 scale.

You probably will find these at a fleamarket/junksale.

To find the respective full scale current of the respective (mystery)
meters ,take a 1.5 V battery and put the DVM and the meter in series
with a 22 KOhm linear potmeter , adjust potm. until mystery meter reads
full scale and read current on DVM.
Remove DVM from circuit and adjust current again to full scale.
Measure voltage (probably in mV range) across meter.
You now know everything of meter you need incl the meter resistance.

Voltmeter: Calculate series resistor (probably odd value) and use a
fixed resistor or resistors or 1 resistor in series with a suitable
(10 turn) trim pot.
Apply say 10 Volts across system (measured with DVM) and check meter
deflection . If not to your satisfaction ,change fixed resistor or in
case of using a trim pot ,adjust the trimpot.

Ampere meter:
Take approx 1 metre of thick copper wire capable of carrying 50 Ampere
, meaning that at that current its temperature does not get noticeably
above ambient .

Make a current of say 5 Amperes to flow through the wire .
With the DVM determine the length of the wire over which a voltage
developes which is 20-30 % (not critical) higher than 10% of the full
scale voltage across the designated Ampere meter ,assuming its scale is
0-50.
You can now reduce the overall length of the wire shunt accordingly.
Connect the designated meter in series with a (preferably 10 turn) trim
pot ,with a value of the same order of magnitude as the meter
resistance, across the shunt and adjust the trimpot such that with a
current of 5 Amperes (as measured by the DVM) the now Ampere meter reads
10% of its scale.
Increase the current to say 20 Amperes (as shown by the DVM or any other
Ampere meter) The now Ampere meter should read 40% of scale.

I have applied the above with 'junk box' meters to measure voltage and
current from 2 solar panels (0-10 A) and a wind generator (0-20 A).
Voltmeter reads 0-15V.
The method described is adequate for 'operational' meters . Their
uncertainty (accuracy)is better than 3%.

If the meters you have don't have the wanted scale ,you can change the
scale by (carefully)glueing a paper scale on top of the existing one .
There are a number of free meter scale programs available from the www

Frank KN6WH / GM0CSZ