Hi Carey, there are number of ways to design meter shunts. A method used by
many is to install a low value resistor in series with the circuit and
measuring the voltage drop across the resistor with the meter.
Meter over current protection is easily added by shunting the low value
resistor with opposite connected diodes. That clamps the voltage to +/- .7
volts or multiples of that with diodes in series.
Starting with a 1 ohm resistor and a target of 500 mA full scale for
example, the voltage would be I X R. At 500 mA, there would be .5 volts. Your
meter being 1 mA, needs a resistor in series to limit the current to 1 mA when
there is .5 volts present. Normally that would be .5 / .001 = 500 ohms.
But, the meter has resistance that has to be included in the 500 ohms.
Personally, I determine the series resistor experimentally while measuring the
voltage drop with a known good DVM.
The 1 ohm resistor needs to handle full current reliably and for a long,
stable life should be rated for several times the current expected. At 500 mA
that would be .5 X .5 X 1 or .25 watts. 2 to 5 watt resistors would give
plenty of head room and are small enough to fit most anywhere.
At a lower current, like 10 mA for example, the low value resistor is not
exactly low value. If you want to use the diode protection scheme, then a
value of about 50 ohms will have a voltage drop of .01 X 50 = .5 volts. The
meter resistor then would be .5 / .001 = 500 ohms less the meter resistance.
The meter and it's resistor are in parallel with the low value resistor.
That means the 1 mA of current flowing through the meter is not flowing
through the low value resistor. So, the 500 mA example would actually be 499
mA full scale and the 10 mA example would be 9 mA full scale. To compensate,
the meter resistor would need to be adjusted to a slightly lower value for
accurate measurements.
All of this is just using Ohm's Law; no rocket science at all!
73,
Gerald K5GW
In a message dated 8/24/2011 10:47:28 P.M. Central Daylight Time,
kc5gtt@gmail.com writes:
i have a 1ma meter that i would like to rescale and use with a little gi7
2 meter amplifier. what size shunt should i use or what is the calculation
i should use?
Carey, kc5gtt
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