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Home > Archive > Alternative Power sources > December 2006 > Vol & Amp Meter add ons
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Vol & Amp Meter add ons
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| 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.
Cordially,
west
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| Jan Rasmussen 2006-12-18, 1:25 pm |
| "west" <restccq2@verizon.net> skrev i en meddelelse news:Dgphh.214$pr5.58@trnddc01...
>I have a 40 amp, 13.8v supply and would like to add a voltage (0-15v) meter
> and an amp (0-50amp) meter. The meters look frail and I can't imagine
> running all that current through those flimsy connectors and tiny meter
> coil. I have no paperwork on the meters and was wondering if I need a shunt
> resistor or something to take all that current. Any ideas? Thank you.
>
> Cordially,
> west
>
Try measuring the resistance between the too connectors on the amp meter,
anything other than 0,0xx ohms, i guess will means that it needs a shunt.
Jan Rasmussen
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| Gordon 2006-12-19, 3:25 am |
| "west" <restccq2@verizon.net> wrote in news:Dgphh.214$pr5.58@trnddc01:
> 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.
>
> Cordially,
> west
>
>
The markings on the meter face may be just that, markings.
Look cosly at the meter and see if it has any specs printed or
embossed on it anywhere. What you need to know is the full scale
current rating. It's usualy in the micro amp range.
With that information you can use Ohm's law to figure out the
series resister you need for the volt meter.
The amp meter needs a shunt resistor of a very low value, like
0.01ohms or less. I cant remember the formulas for figuring
a shunt resistor. But you can experiment. Use a controled
environment to do your experimenting.
1) Place a 0.01 Ohm resistor in parallal with the meter.
2) Place a 10,000 Ohm resistor in series with the meter.
3) Apply 1 Volt DC to the meter and resistor combo. That is:
One leg of the power supply to the free end of the 10,000
ohm resistor the other to the free leg of the meter.
4) Observe the meter and see if there is any detectable movement.
5) If the meter shows limited movement, change the series
resistor to a lower value, IE: 1,000 Ohms.
6) Adfjust the voltage to get a full scale deflection of the meter.
7) Use Ohm's law (I=E/R) to determine the full scale reading
of the meter and shunt combination.
8) Make adjustments to the value of the shunt to get the meter
to read in the 0-50 amp range.
Having said that. I doubt that you will be able to find a
bench top power supply that can deliver 50 amps. So you will
probably be working down near the 10% end of the meter range.
That is: you will be trying to get an accurate reading of
5 amps at 10% needle deflection. Once you get that, you can
install it in your final application and then tweak the value
of the shunt to tweek the meter to spot on accuracy.
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