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With this module, which costs less than seven euros, you can measure DC voltages up to 33.000 V, i.e. with a resolution of 1 mV and an accuracy of 0.3 ‰. You can power the module from the measured voltage or from a fixed supply voltage between 4.00 V and 30.0 V. |
Introduction to this 33.000 V panel meter
Brand- and type-free
With this module, the Chinese manufacturers and suppliers make it completely difficult to trace the origin of a product. The manufacturer is kept secret and the module does not even have a unique type number as identification. On the PCB, there is a reference to a Taobao shop. A further search on the number of this shop reveals the manufacturer to be Hangzhou Ruideng Technology. This manufacturer (www.ruidengkeji.com) also supplies the well-known digitally adjustable laboratory power supplies marketed under the name RIDEN.
In any case, you will find sufficient links to suppliers if you search for '5 Digit Mini LED Display Digital Voltmeter Voltage Detector Panel Meter High Accuracy 3 Wires'.
Prices of this digital panel meter
On AliExpress the module is offered for prices between € 3.43 and € 4.47 with € 1.68 shipping costs. At Banggood you can order such a meter for € 6.28 with € 1.93 shipping costs. The meter is available with a display in the colours red, yellow, green and blue. We bought the tested model at Banggood.
The appearance of the module
The meter is mounted in a kind of housing that is already known from the digital panel meters with only three or four digits. The meter fits in a rectangular cutout measuring 45 mm by 27 mm. You have very little tolerance on these dimensions. After all, the meter itself measures 48 mm by 29 mm. The seven-segment display is 9.5 mm high.
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| The appearance of the five digits voltmeter module. (© 2021 Jos Verstraten) |
The specifications
The specifications given by the manufacturer are:
- Supply voltage: 3.5 Vdc ~ 30.0 Vdc
- Measuring range at constant supply voltage: 0.0000 Vdc ~ 33.000 Vdc
- Measuring range, supplied from the measuring voltage: 4.000 Vdc ~ 33.000 Vdc
- Measurement resolution: 1 mV
- Measuring accuracy: 0.3 ‰ (± 2 digits)
- Temperature coefficient: less than 50 ppm/°C
- Operating temperature: -10 °C ~ +65 °C
- Display colours: red ~ yellow ~ green ~ blue
Connecting the module
You can connect the enclosed three-wire cable to the three-pole PCB connector. The three wires have the following coding:
- black: ground of the power supply and of the voltage to be measured
- red: positive connection of the external power supply
- yellow: positive connection of the voltage to be measured
- Supply voltage: 3.5 Vdc ~ 30.0 Vdc
- Measuring range at constant supply voltage: 0.0000 Vdc ~ 33.000 Vdc
- Measuring range, supplied from the measuring voltage: 4.000 Vdc ~ 33.000 Vdc
- Measurement resolution: 1 mV
- Measuring accuracy: 0.3 ‰ (± 2 digits)
- Temperature coefficient: less than 50 ppm/°C
- Operating temperature: -10 °C ~ +65 °C
- Display colours: red ~ yellow ~ green ~ blue
Connecting the module
You can connect the enclosed three-wire cable to the three-pole PCB connector. The three wires have the following coding:
- black: ground of the power supply and of the voltage to be measured
- red: positive connection of the external power supply
- yellow: positive connection of the voltage to be measured
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| Connecting the module. (© AliExpress) |
The electronics in the module
One side of the PCB
The size of the PCB is 41 mm by 21 mm. On one side only the display is present. This is a five digit display of type SD411336N by ARKLED with 2 x 7 pins. The connection details are summarised in the figure below.
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The connection details of the display. (© ARKLED) |
The other side of the PCB
On the other side of the PCB there are three chips of which only a voltage stabiliser can be clearly recognised. The two other chips are coded in an unrecognisable manner. There is room for a push button switch on the PCB. It is not possible to determine what this might be used for.
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| The two sides of the PCB. (© 2021 Jos Verstraten) |
The digital panel meter tested
Test with supply voltage from the measuring voltage
This module has a specified accuracy of 0.3 ‰ (0.03 %). Not so easy to test! However, thanks to our sponsors and donors, we now have a very accurate reference meter at our disposal, an 8842A from Fluke. We are therefore able to confirm or deny the manufacturer's claim.
We connected the red and yellow wires to the output voltage of our digitally adjustable power supply NPS-1601 and measured the output voltage with the module and with the Fluke. When calculating the percentage error, we took the Fluke's indication as 100 % reference. The results are summarised in the table below. The module requires a current of only 7 mA from the power supply.
The results show that the module measures DC voltages very accurately and meets the specifications.
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| The accuracy with supply voltage from the measuring voltage. (© 2021 Jos Verstraten) |
Test with a constant voltage supply
Although the results can hardly be improved, we were curious to know whether supplying the module with a constant voltage would yield any improvement. In this case, of course, you can measure from zero volts and the module can also be tested when measuring very low voltages. The module was powered from a stabilised 12 V voltage.
The results show that the module has a slightly larger error when measuring small DC voltages. However, when increasing the voltage to be measured, it quickly returns to the values in the previous table.
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| Accuracy when supplied from a constant voltage. (© 2021 Jos Verstraten) |
The internal resistance of the module
This is an important parameter that is not mentioned in the specifications. Measuring this parameter is very easy if you have an accurate 100 kΩ resistor at your disposal. Put this resistor in series with the yellow input wire of the module and connect that series connection to a stable DC voltage. Now note the reading on the meter. Short the resistor with a piece of wire and note again the reading on the meter. The difference between both readings is equal to the voltage drop over the 100 kΩ resistor. With a simple calculation, you can determine the input resistance of the module from this data. In our example this value is 337 kΩ.
The temperature coefficient
We mounted a thermocouple on top of the processor chip and slowly heated the temperature of the module to 80 °C with a hot air gun. The measurement results of a stable voltage of 30.00 V, summarised in the table below, show that the module has a positive temperature coefficient. A temperature increase of 50 °C gives an increase in the reading of 43 mV. Calculated per degree celsius, this gives a tempco of +860 μV/°C. So that is a lot more than the specified 50 ppm/°C!
This is an important parameter that is not mentioned in the specifications. Measuring this parameter is very easy if you have an accurate 100 kΩ resistor at your disposal. Put this resistor in series with the yellow input wire of the module and connect that series connection to a stable DC voltage. Now note the reading on the meter. Short the resistor with a piece of wire and note again the reading on the meter. The difference between both readings is equal to the voltage drop over the 100 kΩ resistor. With a simple calculation, you can determine the input resistance of the module from this data. In our example this value is 337 kΩ.
The temperature coefficient
We mounted a thermocouple on top of the processor chip and slowly heated the temperature of the module to 80 °C with a hot air gun. The measurement results of a stable voltage of 30.00 V, summarised in the table below, show that the module has a positive temperature coefficient. A temperature increase of 50 °C gives an increase in the reading of 43 mV. Calculated per degree celsius, this gives a tempco of +860 μV/°C. So that is a lot more than the specified 50 ppm/°C!
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| The temperature coefficient of the module. (© 2021 Jos Verstraten) |
Our conclusion about this 33.000 V module
It will be clear from the test results that there are really no points of criticism about this module. If you need to accurately measure a DC voltage somewhere, you can use this inexpensive module without any hesitation.
5-Digit-Mini-LED-Display-Digital-Voltmeter
