Moving-coil meter measures low-level currents

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 8-in. moving-coil meter that requires 15 mA for full-scale 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, I_OUT, equals V_SENSE/100V, where V_SENSE is the voltage across R_SENSE1.

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 R_SENSE1, which IC₁ converts to a maximum output current of 10 mA 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 15 mA develops 1V across 66Ω resistor R_SENSE2. 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.

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