Voltmeter for laboratory power supplies. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Voltmeter for laboratory power supplies. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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This voltmeter is designed to be built into regulated laboratory power supplies. It measures and indicates on a four-digit seven-element digital LED voltage indicator within 0 ... 99.9 V. This interval is divided into two subranges: 0 ... 9,99 V and 10,0 ... 99,9 V, and their switching takes place automatically. The indicator does not use decimal points, so the division of units and tens of volts from tenths and hundredths of a volt is carried out by one "extinguished" digit of the indicator. The scheme of the device is shown in fig. 1. Its basis is the DD1 microcontroller, which works according to the program, which you can take from the link at the end of the article.

Rice. 1 (click to enlarge)

Voltage measurement and conversion to a digital code is carried out by a 1-bit ADC built into the DD10 microcontroller. Diode VD1 protects its input from negative polarity voltage, and diode VD2 limits the voltage on it at 3,1 ... 3,3 V. Inductor L1 together with capacitor C3 form a power filter for the analog part of the microcontroller DD1. Capacitor C1 reduces the noise level at the input of the ADC, and capacitor C4 - at the output PA3 of the microcontroller, which has a reference voltage for the ADC (2,56 V), set by software.

While the input voltage is less than 9,99 V, the value in the ADC data registers is less than the set threshold and the output PA0 of the microcontroller is low. Therefore, the transistor VT1 is closed and the resistors R1-R3 form a voltage divider with a transfer coefficient of 0,25. In this case, the first and second digits of the HG2 indicator "shine", which indicate hundredths and tenths of a volt, respectively. The third digit is extinguished, since it is separating, the first digit of the HG1 indicator also “lights up”, which in this case is the third digit of the entire voltmeter indicator, it displays units of volts. If the input voltage reaches a value of 10 V or more, a high level is set at the output PA0 of the microcontroller, the transistor VT1 opens and, in parallel with the resistor R3, resistor R4 will be connected through the low drain-source resistance of the open transistor, reducing the transfer coefficient of the resistive voltage divider R1-R4 ten times - 0,025.

In this case, the first (tenths of a volt) and the third (units of volts) digits of the HG2 indicator "shine" (the second is separating and extinguished), as well as the first digit (tens of volts) of the HG1 indicator. Most of the parts, except for LED indicators, are mounted on a board made of one-sided foil fiberglass, the drawing of which is shown in Fig. 2.

Voltmeter for laboratory power supplies. Printed circuit board
Fig. 2

The device uses oxide capacitors K50-35 or imported, resistors - MLT, C2-23, BSS88 transistors are interchangeable with BS170P, KP504A. You can use one four-digit, two two-digit or four single-digit common cathode seven-element LED indicators. Inductor L1 - DM-0,1 or imported EC24, it is installed on the board between pins 5 and 15 of the microcontroller from the side of the printed conductors. It is necessary to power the device from a stabilized voltage source, for example, an integral stabilizer 78L05, by connecting it to the output of the power supply rectifier. But it should be remembered that the maximum input voltage of the 78L05 stabilizer is 30 V. The average current consumed by the device is about 12 mA. Establishment comes down to a selection of resistors R1 and R4. First, by applying a voltage of about 5 V to the input and controlling it with an exemplary voltmeter, the required value is set on the indicator by selecting the resistor R1. Then the input voltage is increased to 15 ... 20 V and the required value is also set on the indicator by selecting the resistor R4.

Program for microcontroller firmware

Author: M. Ozolin, p. Krasny Yar, Tomsk region; Publication: cxem.net

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