The ubiquitous, and now inexpensive, 1999-digit digital panel meter or digital voltage meter (DVM) module is a very useful addition to the range of test and display gear currently available. It's accurate, robust, and easily scaled for ranges from 199.9 mV to 199.9 V, and more. But it loses much of its accuracy if you want to measure, say, 25 V when the only scale available is to 200 V. Of course a 3999-digit digital panel meter is the simple solution, but it's not inexpensive.
A standard 19.99-V digital panel meter can have its scale doubled, while maintaining the same level of accuracy over the entire range (see the figure). Resistor R5, together with a correctly adjusted VR2, scales the input of the DVM so that it reads up to 19.99 V. The appropriate decimal point needs to be linked across the module.
Variable resistor VR1 adjusts the voltage output from pin 2 of the LM317LZ variable voltage regulator until, at the test point, it's exactly 20 V. This is connected directly to pin 3 of IC2, a standard op amp configured as a voltage comparator. The voltage to be measured is applied to common ground and pin 2 of IC2.
When this voltage to be measured is under 20 V, the output of IC2 at pin 6 is high. Consequently, the LED is unlit. But the MOSFET, Q1, is full-on, which effectively joins the "north" end of R5 to ground. The DVM then directly reads the voltage to be measured.
However, when this voltage is greater than 20 V, the output of IC2 is low and Q1 is off. Therefore, the "north" end of R5 will be at a 20-V potential via low-value resistor, R4. Then, the reading on the DVM will represent the difference between the voltage being measured and 20 V. Also, the LED will be lit as a reminder that 20 V must be added to the reading.
The supply to the circuit should always be at least 4 V greater than the highest voltage to be measured. An MC33171 op amp is recommended because it allows single-ended supply voltages of up to 44 V. If more modest voltages are to be measured, a 741 op amp will serve well, provided that its supply doesn't exceed 36 V. The ZVN4306A MOSFET is the recommended type for Q1 because of its very low ON resistance.
In this circuit, the error caused by the voltage drop across Q1 when it's on is no greater than 0.5 mV. This is beyond the accuracy of the digital panel meter.
Some may think that the figure shows the DVM incorrectly connected, with its low-level input connected to the positive side of the voltage to be measured. But the panel meter will display voltages with either polarity. Connecting it the other way around results in less stable readings due to the potential for hum pickup in R5.