Although an analog moving coil meter may lack the resolution and accuracy that a digital readout provides, a meter remains the display of choice for certain applications. A digital readout simply cannot provide information about a measurement’s rate of change, and tracking a reading’s trend is easier on an analog meter.
Large moving-coil meters may require significant amounts of current for full-scale deflection, and using a shunt resistor may prove impractical when the meter current is larger than the current you are measuring. You can solve the problem by driving the meter from a separate power supply (
Figure 1).
In this example, an 8in. moving-coil meter that requires 15mA for fullscale deflection displays a current range of 0 to 1A dc. This technique can also simplify specifying or fabricating shunt resistors for custom current ranges. Unlike other current sense amplifiers that derive operating power from the current you are measuring, IC1 provides a separate supply-voltage terminal for its internal circuitry. In operation, IC1’s output current, IOUT, equals VSENSE/100V, where VSENSE is the voltage across RSENSE1.
This Design Idea uses IC1 rather than the many current-sense amplifiers available because it provides a separate supply-voltage terminal for the internal circuitry, whereas other devices take power from the current you are measuring.
In this application, a full-scale current of 1A develops 1V across RSENSE1, which IC1 converts to a maximum output current of 10mA that produces a maximum voltage of 1V across R1. Operational amplifier IC2 and transistor Q1 form a voltage-controlled current sink that draws current through meter M1. A full-scale reading of 15mA develops 1V across 66Ω resistor RSENSE2. You can adjust the resistor’s value to calibrate the meter or to alter the full scale current range.
This circuit also allows separation of the measurement point and meter location. Moving-coil meters are not intended for applications that require precision measurement, and you can use relaxed-accuracy passive components. Bypass the instrument supply voltage with decoupling capacitors that the electrical-noise environment requires.
