1.5 AMP - Shunt Ammeter Circuit
Shunt Ammeter Schematic
WHAT IT IS
The above schematic is of a circuit that will convert a 0-15Volt
voltmeter to a 1.5Amp ammeter.
The Circuit is for a 'Shunt' type ammeter that allows a meter
movement with a small current rating to be used to measure a much larger
current. In this example 1 milliamp through the meter coil will represent
1.5 amps by shunting most of the current through the 0.1 Ohm resistor.
The meter used in this example is a meter from Radio Shack Part
number 270-1754. This meter is scaled from 0 to 15 volts and has a coil
resistance of 85 Ohms. A current of 1 milliamp will drive the needle full
scale.
By adding two components and a little art work to the meter face
you can have a 1.5 amp meter that would perfectly match with a voltmeter
made with the same unit.
HOW IT WORKS
In this example when a current of 1.5 amps is flowing through the
circuit, 1.499 amps will be flowing through the 0.1 ohm shunt resistor
(RS), and 0.001 amps will flow through the meter and the calibrating resistance
(RC).
Under these conditions the voltage across the shunt will be,
1.499 Amps times 0.1 Ohms equals 0.1499 Volts.
Therefore; for a current of 1 milliamp to flow through the meter the resistance
of the meter circuit must be as follows,
0.1499 Volts divided by 0.001 Amps equals 149.99 Ohms (150 ohms).
If the resistance of the meter itself is 85 ohms then the resistance
of the trimmer would be,
150 Ohms minus 85 Ohms equals 65 Ohms.
To calibrate the circuit, another ammeter is connected in series
with the new meter and a load applied. The trimmer (RC) is then adjusted
so that the shunt meter and the test meter agree.
A D.P.D.T. switch and the original voltmeter calibrating resistor
(Rv) could be added to the circuit allowing one meter to be used for both
amps and volts. See the schematic below.
Volts and Amps Schematic
Finding The Coil Resistance Of A Meter
The following diagram shows a method for finding the coil resistance
of a meter. Sometimes the coil resistance is marked on the meter face below
the clear area of the meter's window.
Volts and Amps Schematic
Notes
This type of circuit can be used for any current range from
one milliamp to thousands of amps. As long as the ratio between the shunt
and meter leg resistances is correct the reading will be accurate.
This circuit can also be used with the many small, battery powered
LCD millivolt displays that are now available. These meters usually come
with instructions for this same type of circuit. For example, see:
http://www.electronics123.com/amazon/catalogue/c3-3-4.htm,
Model CPS34 for $19.95)