Design Ideas: January 20, 1994

The expanded voltmeter in Fig 1 doesn't require trimpots to obtain usable accuracy or require the tedious selection of Zener diodes. This circuit consists of only two parts and a 5V meter. The circuit is completely linear and requires no calibration except the rare mechanical zeroing of the meter. By using a split-voltage reference system with floating output, the zero point of the voltage supplied to the meter equals the absolute sum of the two references. With the regulators in the figure, the 0 to 5V meter reads 10 to 15V inputs. If the voltage into the system falls below this level, the output to the meter changes polarity. This feature allows use of the mechanical zeroing of the meter, although this zeroing should never be necessary because most regulators yield outputs more accurate than the meter can read.

The pinouts of these TO-92 devices are perfect for this application. The small size allows installation of the circuit within most meter cases. Keep in mind that this circuit will present a load of 3 to 5 mA, and connections should minimize any voltage drops between meter and battery.

For use with other voltages, select regulators for sums equaling the lowest voltage to be displayed. As an example, a 78L15 and a 79L05 will output a zero voltage at 20V. A 5V meter will then read from 20 to 25V. You can make the circuit adjustable by substituting a 317L adjustable regulator and the necessary resistors in place of the 78L05.