Do-it-yourself

Precision Voltage Reference

for Multimeter Calibration






created by Frank Weithöner




Precision voltage reference for multimeter calibration
Recently I wanted to adjust the end-of-charge voltage of a solar charge controller. I failed. Each time I checked the state of charge the result was different from my previous adjustments. After several days I found out that the problem was not the charge controller or the battery but just the fact that I used two different multimeters. The multimeters simply showed different voltages.
Becoming curious I collected all multimeters I could get and applied a fixed voltage to all of them. Here the result:



Unbelievable the differences between the multimeters. All are connected to the same power supply. But which one shows the right voltage? And is my 10 V really 10.00 V?

The case is clear: All digital multimeters have to be checked and if necessary, calibrated. Therefore a reliable reference is needed, either a calibrated voltmeter or a sufficiently accurate reference voltage. A calibrated multimeter is expensive but a precision voltage stabilizer is not. For less than 10 € you can build a reference voltage source with a precision of 0.3 % or less. That correspond to the standard of a normal DMM (e.g. the popular UNI-T61A,B,C,D has ±0.5 %). And even better, you can turn a cheap multimeter into a precision measurement equipment.


Precision stabilizer
A precision voltage reference is nothing else than a power supply with a precision zener diode or better a precision voltage stabilizer. There are some precision stabilizers on the market. They all differ in accuracy and price. The output voltage is mostly 5.000 V or 10.00 V.
Here are some precision stabilizers (there are probably more):

Type Output Accuracy Price Data sheet
LT1431
5.000 V
0.4 %
2.50 €
download
AD581J
10.00 V
0.3 %
9.00 €
download
AD587J
10.00 V
0.2 %
5.00 €
download
LM4040
10.00 V
0.1 %
1.00 €
download
LT1021C
10.00 V
0.05 %
6.00 €
download
LT1236
10.00 V
0.05 %
7.00 €
download

When choosing a stabilizer look at the accuracy of the multimeter you want to calibrate and at the accuracy of the stabilizer. The precision of the multimeter after your calibration can not be better than the precision of the stabilizer itself.
The disadvantage of precision stabilizer is, that they are very special parts and not always and everywhere available.


Assembly
The circuit diagram is simple. I have chosen a LT1236 stabilizer just because I could get it easily. The input voltage is not critical, as long it is between 15 V and 30 V. The capacitors are against any oscillating. That's it.



A board is not needed. All parts are mounted between the output sockets and the switch.

In order to get a second voltage of 1.000 V I added a voltage divider. But that is not really essential. The value of the resistor combination has to be 9:1. I have chosen 18 KΩ and 2.0 KΩ but any other combination is fine, as long as the current does not exceed the maximum current of the stabilizer.
But the main problem is the quality of the resistors. They of course also have to have precision quality. Standard metal film resistors of 1 % or even 2 % are not good enough - in principle. But a good possibility is to select a pair out of a pile of metal film resistors with your ohmmeter. Be very careful and critical. Compromises here are out of place.
Another possibility is the following (this is what I did): Instead of taking one 18 KΩ resistor, I took 10 of 180 KΩ in parallel (plus 10 of 20 KΩ). The idea is that the overall tolerance gets smaller, because the tolerances compensate each other the more resistors are used. I tested the method and the result is the following: All 1 % resistors actually had a tolerance of only 0.25 % (each one). Putting all in parallel the tolerance of the overall resistance dropped to 0.04 %.
A suitable enclosure I could not get here in Tanzania, so I mounted everything into my workshop multimeter. There was just enough space inside and also at the front for the sockets. An other advantage was, that I could use the internal power supply.



The reference voltage source build in my workshop multimeter (the blue and the black socket under the rotation switch). The red knob in between is the change-over switch for 10 V to 1 V.


Perfect. Even after 15 years my DMM is still in a good shape...

Now we come to the adjustment.


Inside a DMM
The heart of all digital multimeters is a highly integrated IC, the A/D converter with the LCD or LED display driver. The IC processes and displays a DC voltage in a range of 0-200 mV. Different voltage dividers selected by the rotation switch (or by an automatic control) extend this mili-voltmeter to a practical voltmeter. The converters for currents and resistances we ignore here.
When we now do our calibration we only adjust the reference voltage of this A/D converter, that means the 200 mV range. The resistors of the dividers are fixed and can not be adjusted. That makes the work easy.


Just one trimmer


That's easy-peasy - there is only one trimmer.
Switch to the voltage range you mostly use (e.g. 200 V), connect the reference voltage and set the display to 10.00 V. That's it.


Making a precise measurement equipment out of cheap multimeter.

How to find the right trimmer
It is easy when you have a simple DMM. There is only one. But some multimeters have several trimmers. Please do NOT turn the trimmers to find out which is the right one. You will mess up the measurement ranges of other modes like AC or current. It is much better to identify the A/D converter and look it up in the data sheet. There you find where the adjustment trimmer is positioned.
The most common IC is the ICL7106. The IC comes in a 40 pin DIL package or as a square SMD package. Another common IC is the ES51922 which is used for example in the popular UNI-T T61 models.

IC Connection Pin Data sheet
ICL7106 40 pin DIL
REF HI, REF LO
35, 36
download
ICL7106 44 pin SMD
REF HI, REF LO
43, 44
download
ES51922
VR, VRH
37, 38
download



The common ICL7106. Follow the marked pins and get to the right trimmer.

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The M-890G. An example for a DMM with the ICL7106.

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The circuit diagram of the popular Uni-t T61. The calibration trimmer is marked red.

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This clamp multimeter has many trimmers. Identify the A/D converter and follow the conducting tracks according data sheet.

Do not turn the trimmers to find out the right one! You will distort the other modes.
By the way, it is always a good idea to bring the equipment first to operating temperature before making any adjustments. Switch on both, multimeter and reference voltage half an hour before doing the calibration.



The cheapest meter. This 5 €-
multimeter from a local market somewhere in the heart of Tanzania has not even a trimmer. But it can be easily added.


The exception. Here is nothing to adjust. Obviously the calibration is done with a special software via the data port (on the right side in the centre). Fortunately there was no need for a calibration.

Addendum
I wish I had had this idea before. It has always bothered me that for developing aid projects a lot of money is spent on top-class equipment. Why must every electrician have at least a FLUKE 179? I have seen so many expensive tools and measurement equipment which were cracked or filthy dirty (or simply have disappeared), so that I basically only buy inexpensive (but reasonable) equipment that I find in local shops. In future everybody will also get an inexpensive multimeter from me which the technician together with me will calibrate. By doing so he gets to know his DMM better and hopefully also appreciate the value of his instrument, especially when he realizes that his DMM after his tuning is at least as precise as a 10-times more expensive FLUKE 179...


Links and sources
For manuals or circuit diagrams try Workshop equipment manuals.
More information about Multimeters at wikipedia.
Source for electronic components and prices: Reichelt