APPLICATION NOTE 3536

Moving-Coil Meter Measures Low-Level Currents

By: Kevin Bilke

Abstract: The MAX4172 IC substitutes a current amplifier for the shunt normally associated with a moving-coil meter. This circuit allows use of the moving-coil meter, even when the meter current is a large fraction of the current being measured.

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.
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, IOUT, equals VSENSE/100Ω, where VSENSE is the voltage across RSENSE1. IOUT is boosted by the op amp and transistor, and the meter's full-scale current is easily changed by adjusting the value of RSENSE2.

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.