The work with lithium cells called for voltages to be measured with an accuracy of at least 50 mV, for instance to prevent overcharging of the cells.
Over the years, a number of multimeters have accumulated in my shack – with quite different quality. Now the question was: which meter should I trust? For some of the devices, not even information about their accuracy was available any more.
Therefore I had the idea of designing a reference voltage source. There are many instructions on the web. A quite simple circuit, which outputs a voltage sufficiently accurate for 3 1/2 digit multimeters, uses the LT1021C-10. The datasheet can be found under this Link.
The circuitry is very simple: At the input (pin 2) the LT1021 is supplied with 15V. To protect the input from reverse polarity I added a diode.
At the output (pin 6) the LT1021 provides a voltage of 10,000 V +/- 5 mV.
In order to suppress oscillation tendency, it added 150nF capacitors against Gnd (pin 4) at the input and output. The other pins remain not connected.
The accuracy of the circuit could be further increased by connecting the Trim input (pin 5). But this requires an really precise meter, which I don’t have at my disposal.
The whole thing is packed into a small plastic case.
My old digital multimeters were 70mV to 80mV off. Re-calibration is needed. The old meter with moving coil is also a bit off but not too badly. The green multimeter appears to be the most accurate one.