Moving-Coil Meter Measures Low-Level Currents

The display of choice for certain applications remains the large moving-coil meter. While a moving-coil meter may lack the accuracy of a digital panel meter, the perceived extra information derived from the needle's rate of change cannot be matched by the digital alternative.

It is not always possible to attach a current shunt to analog meters, when the meter current is a large fraction of the current being measured. Although a disadvantage, it can be overcome by driving the meter from a separate supply voltage (Figure 1). The circuit shown for the MAX4172 drives a large 8-inch meter with full scale of 15mA.


Figure 1. This circuit allows use of a moving-coil meter in applications for which the meter current is a substantial fraction of the current being measured.

IC1 was chosen from the many current-sense amplifiers available because it provides a separate supply-voltage terminal for the internal circuitry. (Other devices take power from the current being measured.) IC1's output current, I_OUT, equals V_SENSE/100Ω, where V_SENSE is the voltage across R_SENSE1. I_OUT is boosted by the op amp and transistor, and the meter's full-scale current is easily changed by adjusting the value of R_SENSE2.

This circuit also allows the meter display to be remote from the point of measurement. Note that the passive components are chosen for convenience rather than high accuracy, because moving-coil meters are not intended for applications that require precision measurement.

A similar version of this article appeared as a Design Idea in the March 3, 2005 issue of EDN magazine.