Measuring device for the power supply. Scheme, description

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Measuring device for the power supply. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The proposed device is designed to be built into a laboratory power supply. It is assembled on a microcontroller, LED seven-element indicators and is designed to measure the output voltage and current.

The device is a simplified version of a similar device, the description of which was published in the journal "Radio", 2007, No. 7, p. 26-28 (Zaets N. "Advanced digital protection device with a measurement function"). Compared with the prototype, a more accessible microcontroller is used and the circuit is simplified by eliminating overcurrent and overvoltage protection. It is assumed that such protection is implemented in the power supply itself. In this regard, the control program of the microcontroller was developed anew. Voltage measurement intervals - 0 ... 25.5 V with a resolution of 0,1 V, current - 0 ... 1,55 A with a resolution of 0,01 A.

Rice. 1 (click to enlarge)

The device diagram is shown in the figure. The measured voltage is supplied to the RA0 port line (configured as an ADC input) of MK DD1 through a resistive divider R1R5R6. The signal from the current sensor - resistor R4 is fed to the voltage amplifier on the op-amp DA2.1 and after amplification from the output of the voltage follower on DA2 2 - to the port line RA1, also configured as an ADC input. The clock frequency of the MK is set by a quartz resonator ZQ1. Since its frequency is not critical, you can use resonators from old VCRs at frequencies of 3,57. 4,43 or 4,5 MHz. The device uses four-digit seven-element LED indicators with a common cathode. In this case, the symbols of the measured parameter "U" (HG1) and "I" (HG2) are displayed in the extreme right digit in each of them.

The microcontroller and the op amp receive a stabilized +5 V power supply from the integrated voltage regulator DA1. This voltage is simultaneously used as a reference for the ADC. The current consumed by the device is about 100 mA, depending on the number of included elements and the type of indicators used. The supply voltage is unstabilized - 8. .25 V, but at a voltage of more than 12 ... 15 V, you will need to install the DA1 stabilizer on a heat sink with an area of 10 ... 20 cm2. The microcontroller is installed in the panel.

It should be noted that the current sensor (resistor R4) is connected in series with the load, so the voltmeter readings will be overestimated by the value of the voltage drop across this sensor. The error is proportional to the load current and at 1A it is 0.12 V. This drawback of the device can be eliminated by correcting the MC program.

Most of the elements are mounted on a breadboard printed circuit board. Fixed resistors for surface mounting are used, except for R4 - it is wire-wound 0 12 Ohm 5 W or home-made, trimmers - SPZ-19, oxide capacitors - imported, the rest - for surface mounting, for example K10-17v Indicators are connected with pieces of insulated mounting wires and after verification and adjustments are mounted on the front panel of the power supply. The voltage stabilizer is installed on a heat sink with an area of 15 ... 25 cm2.

It should be noted that the microcontroller program contains an interval of the measured current of 0 ... 2,55 And the measurement error increases (underestimation). If the power supply in which the device is built in provides a current of up to 1.5 A, you should use an op-amp of the KR1,7 series, for example, KR2,55UD1446A KR1446UD1A.

Any programmer with the appropriate software is suitable for programming the MK. The author used a self-made ExtraPic with a 1C-Prog shell. The programming procedure has been repeatedly described in the literature.

The assembled device (without a microcontroller) is temporarily connected to the appropriate circuits of the power supply. The voltage at terminals 1 and 20 of the MK panel should be +5 V at terminals 6 and 7 of the op-amp DA2.2 - close to zero.

Turn off the power supply and install the microcontroller. After the supply voltage is applied, the indicators should turn on. By connecting a load to the output of the power supply, check the operability of the device. If the brightness of the indicators is low, it can be increased by selecting resistors R8-R15 (downwards). But their resistance should not be less than 33 ohms to avoid current overload of the lines of the RB0-RB7 ports. When overloaded, the indication of individual elements randomly disappears.

Calibration of the device is carried out using a digital voltmeter and ammeter. The first is connected to the output of the power supply, the second - in series with the load. By setting the maximum voltage at the output of the power supply, re (resistor R4) is connected in series with the load, so the voltmeter readings will be overestimated by the value of the voltage drop across this sensor. The error is proportional to the load current and at 1A it is 0.12 V. This drawback of the device can be eliminated by correcting the MK program.

Resistor R1 equalizes the readings of the voltage indicator of the device and the voltmeter. By changing the load resistance, set the current to about 1 A and resistor R3 equalize the readings of the current indicator and the ammeter. uF

The program for the microcontroller can be downloaded from ftp://ftp.radio.ru/pub/2012/05/vamper.zip.

Author: P. Chubarov

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