Expanded-scale indicator revisited

Abel Raynus, Armatron International Inc, Melrose, MA -- 8/8/2002

A note to readers: the author of this Design Idea has found several problems with its performance. We will try to provide a corrected version soon.

The visualization aid that a previous Design Idea describes allows only the expansion of the upper end of the scale (Reference 1). But what can you do if, according to your project requirements, you need to expand the middle region of the scale? Figure 1a illustrates the challenge. A voltmeter comprises a 100-µA dc meter and a series resistor. The voltage under test, V_TEST, ranges from 0 to 5V. The voltage changes between 2 and 3V (the "green zone") are of interest. But at the same time, you cannot ignore the voltages from 0 to 2V and from 3 to 5V, and you need to be able to observe these voltages. With a linear scale, the green zone consumes only 20% of the full-scale range. Your objective is to expand this zone to 80%, leaving 10% at the lower end and 10% at the upper end of the scale (Figure 1b). The circuit in Figure 2 solves the problem. The window comparator, IC₁, controls the variable impedance of the voltmeter. Analog switches S₁ and S₂ provide a contact-logic AND function and put resistor R₂ in parallel with R₁ only upon closure of both switches. This closure occurs when V_TEST is between the threshold voltages V_T1 and V_T2 (Figure 3). You can calculate the resistor values as follows:







You can calculate resistors R₃, R₄, and R₅ from the equations for the threshold voltages:





In this case, V_CC=5V, V_T1=2V, V_T2=3V; hence, R₃=R₅=200 kΩ, and R₄=100 kΩ.

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Reference

Raynus, Abel, "Indicator features expanded scale," EDN, Feb 21, 2002, pg 86.

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