Use a Few Resistors to Improve the Accuracy of the Multimeter
My most commonly used multimeter is a Ulead UT33-A, which has been used for over ten years. Its main advantages in the multimeter below 100 yuan are: small, automatic range, 3 3 / 4 readings, with 2 7 batteries. After replacing my first watch, DT9205, I have no doubt about its accuracy, but I have not compared it with high-end instruments.
At the end of last year, I bought a new ZT111 multimeter to play. When comparing, I found that the two meters measured the voltage difference of the 1st 5th battery. A few words can be understood, and there are problems with it. In general, the new watch is more credible, so I flipped out the previously purchased 10V voltage reference and calibrated the UT33-A. On the back of the PCB of this multimeter, there are two variable resistors, and VR1 is used to calibrate the voltage reference.
Unexpectedly, after the 10V calibration, the reading of the voltage of one cell is still quite different from the reading of the two meters, and the UT33-A has a significantly higher reading. If I adjust the VR1 of the UT33-A to make the measured battery voltage consistent, the UT33A reading is lower when the 10V reference is measured. I took out the UT61A four-and-a-half watch to compare, three hits and two wins, and judged that UT33-A is not allowed. According to the DC accuracy of 0.5% in the specification, the error of this watch is also exceeded, and it is unqualified.
I don't know if it was like this when I bought it. It has not been verified. The ordinary automatic watch uses the input resistor to divide the basic voltage. For example, if I have a UT33-A with a minimum range of 400mV, then I can measure 4V with a 10:1 partial pressure and 40V with a 100:1 partial pressure. Because the input impedance of the meter is 10M ohms, it is speculated that the voltage divider network uses a common The 10M resistors, the respective resistors below become:
Looking for the UT33-A PCB, I found these resistors with similar values:
It’s like to take it apart and check it out (I decided to replace them before the split):
The 10M resistor is obtained by connecting two 5M series, and the green ring indicates the accuracy of 0.5%.
1.11M resistor is used in 4V gear, the accuracy is unknown, there is no color ring mark.
101k resistor for 40V, blue for accuracy 0.25%
10k resistor is used in 400V, the same accuracy is 0.25%
It seems that it is not surprising that my UT33-A voltage file error exceeds 0.5%, as long as the resistance error is at their upper limit (the luck is not so good, or the resistance itself ages after many years). Although I have bought a better performance (9999 maximum display) ZT111, it is a pity that UT33A will be eliminated. As a learning, I decided to "upgrade" these resistors to improve the accuracy of this watch.
Unfortunately, except for 10k, these resistors are special resistance values, and high precision is not easy to buy at once. Retreat to the next, then use the concatenation of commonly used values to get it. I can buy a 0.1% error 25ppm 0805 precision chip resistor from Lichuang Mall, I will buy some. Considering the value of the purchased resistance, the alternative is as follows:
|5M||1M+1M+1M+1M+1M||Direct impact accuracy|
|1.1111M||1.1M+10k+1k+110||1k error 1% affects 10ppm||Cancel 110 affects 90ppm|
|101.01k||100k+1k+10||Cancel 10 impact 100ppm|
|10.010k||10k+10||10 error 1% affects 10ppm||Cancel 10 impact 0.1%|
According to the sensitivity analysis, the 110 ohm and 10 ohm resistors in the above combination can be used normally. The UT33-A is only a 3 3/4 position meter, and it is not enough for daily use with a 0.1% resistor.
Today, I started to replace the resistor with a chip precision resistor. I used the "studden" method for direct lap welding:
After the replacement, the voltage file is calibrated with the 10V reference, and then the voltage of one battery is measured. The reading is consistent with my ZT111 and UT61A. Sure enough, the source of the error was found and the upgrade was successful.
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