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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Ammeter voltmeter for laboratory power supply. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology This device is designed to work with a power supply, the description of which is published in [1], however, it can also be connected to another similar unit. It not only shows the output voltage and load current of the unit, but also performs several additional functions that make the laboratory power supply more reliable and facilitate practical work with it. The main function of the proposed ampervoltmeter (hereinafter AVM) - measuring the output voltage and load current of the power supply - is supplemented by the ability to indicate the set threshold for the operation of the current protection of the unit, assembled according to the description in [1]. This eliminates the need to load the unit with a given maximum current during the process of setting this threshold, and then carefully "catch" the desired position of the control knob. The microcontroller available in the AVM easily calculates the current threshold value from the voltage measured by it on the engine of the variable resistor R5 (see Fig. 1 in [1]) and the resistance of the current sensor resistor R13 (ibid.). The calculated value is displayed on the LCD.
Based on the results of measuring the voltage at the input and output of the unit and the load current, the values of the load power and the power dissipated by the control transistor of the unit are calculated and displayed. In addition, the temperature of the heat sink of this transistor is controlled. According to the results of its measurement, the fan blowing the heat sink is automatically turned on and off. And in case of significant overheating, the power supply is disconnected from the network. An additional function of the AVM is to limit the surge of the charging current of the smoothing capacitors of the rectifier supplying the unit, which occurs when it is connected to the network. In addition, the AVM provides a self-calibration mode. The dimensions of the device only slightly exceed the dimensions of the LCD used in it. Depending on the selected display mode, the output voltage, V, and the load current, A, are displayed on its screen (Fig. 1); load power, W (Fig. 2); current protection threshold, A (Fig. 3); temperature of the heat sink of the regulating transistor, оC, power dissipated by it, W (Fig. 4). If during operation any of the parameters that are not currently displayed on the screen has changed, its value appears on it, and after a while the previous display mode is restored.
The AVM scheme is shown in fig. 5. Its main components are input voltage dividers and noise suppression filters, a DD1 microcontroller containing an ADC and performing all the necessary calculations, as well as a ten-bit LCD HG1.
AVM is controlled using two buttons. The SB1 button switches the display modes around the ring in the one shown in fig. 1-4 sequences. The SB2 button is designed to turn on and off the power supply with which the AVM works. Since the ADC built into the microcontroller is capable of measuring only a voltage that does not exceed the voltage of its supply, voltage dividers are installed at two ADC inputs. The first, consisting of resistors R1 and R3, reduces the output voltage of the power supply by ten times. The second divider consists of resistors R2 and R10 and has a division factor of 20. It reduces the voltage supplied to the power supply from the rectifier to an acceptable value for the ADC. The measurement of this voltage is necessary to calculate the power dissipated in the control transistor. Dividers are not needed in the circuits for measuring the load current and the current protection threshold, since the voltage at the current sensor R13 [1] and the variable resistor R5 [1] does not exceed the value allowed for the ADC. The measured voltages are applied to all used ADC inputs of the microcontroller through a low-pass filter with a cutoff frequency of about 7 Hz. This is R4C1 in the output voltage measurement channel (UO), R5C2 in the load current measurement channel (Iн), R6C3 in the channel for measuring the current protection threshold (IMax), R7C4 in the temperature measurement channel and R9C5 in the rectified voltage measurement channel Uvypr needed to reduce the error associated with the ripple of the measured voltage. The results of the ADC operation processed by the program are displayed on the HG1 indicator, which is connected to the microcontroller via the I interface2C. Since, according to specification I2C, interface signal outputs must be open-collector (drain), the program configures the PB0 and PB2 lines of the microcontroller accordingly. Load for them are two resistor assembly DR1. Two more resistors of the same assembly maintain a high level at the inputs PB1 and PB3 when the buttons SB1 and SB2 connected to them are not pressed. Pressing any of them sets the corresponding input low. A high level at the input of the microcontroller resetting is supported by resistor R10. The microcontroller pins used to load the program into its memory are routed to the X3 connector, which, if necessary, is connected to the programmer. Transistor VT1, by signals from the microcontroller, controls the backlight of the LCD screen HG1. The measured signals are supplied by a flexible cable, on which the socket X1 is installed. The signals for controlling the fan, turning on the power supply, as well as controlling the current limiting circuit for charging the smoothing capacitors of the rectifier are output to pin block X2. A supply voltage of 5 V is applied to pins 5 and 15 of the microcontroller. Since the built-in ADC is powered from pin 15, an L1C9 filter is included in the circuit of this pin to eliminate interference with its operation. Through the capacitor C7, the pulse component of the current consumed by the microcontroller is closed. AVM is mounted on a double-sided printed circuit board (Fig. 6). Before installation, you need to "ring" it and remove the detected unetched jumpers between the conductors. It is recommended to install a panel for the microcontroller on the board, since in case of programming errors of microcontrollers of the AVR family, there are often cases of disruption of their connection with a conventional serial programmer. AVM.
Since it is difficult to metallize the board holes at home, the leads of the parts must be soldered on both sides of it. In this case, the panel for the microcontroller must be a collet, otherwise it will not be possible to solder its conclusions from the part installation side. Through the holes shown in Fig. 6 filled, in the absence of metallization, it is necessary to insert and solder short pieces of bare wire on both sides. Metallization can also be performed using hollow copper rivets (percussion caps), inserting them into the holes of the board and expanding them on both sides. Sets of such pistons are sold, for example, under the trademarks LPKF EasyContac and BG9.S rivets, but they are quite expensive. The board has holes for mounting it and places for installing buttons SB1 and SB2, as well as another button not shown in the diagram (it is designated SB3 and can be used as an SB1 button in [1] through an intermediate relay) and the HL1 LED [1]. The contacts of the SB3 button and the outputs of the LED are connected to the X5 connector, which is also not shown in the diagram. If necessary, the dimensions of the board can be reduced to 65x42 mm by cutting it according to the one in fig. 6 dashed line. In this case, the buttons SB1 and SB2 are located in any convenient place and connected to the X4 connector with a wire harness or a piece of flat cable. Voltage divider resistors (R1-R3, R10) - C2-23 with a tolerance of ±1%. If the resistor R2 with a nominal value of 191 kOhm cannot be found, it can be made up of two values of 180 and 10 kOhm. The remaining resistors are C1-4-0,125. NTC thermistor RK1 - B57703. The 5A332J resistor assembly can be replaced by the domestic HP-1-4-4M from resistors with a nominal value of 3,3 kOhm. Capacitors - ceramic K10-17 or imported. Choke L1 - EC-24 100 uH. The AVM uses connectors BLD-6 (X1), PLD-6 (X2), PLD-10 (X3), PLS-4(X4, X5). Buttons - any clock with a suitable pusher length, for example TS-A6PS. Indicator - MT-10T11 [2] with any alphabetic and digital indices, except 3V0. Indicators with this index are designed for a supply voltage of 3 V and will not work at 5 V. The MT-10T12 indicator will also work, but it is twice the size. The 2N7000 field effect transistor can be replaced with any other n-channel insulated gate transistor with a threshold voltage of no more than 3 V. Even an npn bipolar transistor can be used, but this will lead to more power dissipated on it and lower backlight brightness. You can try to replace the ATtiny26-16PU microcontroller with the ATtiny26L-PU, but its operation is guaranteed at a quartz resonator frequency of no more than 8 MHz. The microcontroller program was developed in the Atmel AVR Studio environment and written in assembly language. You can load it into the microcontroller memory using the proprietary AVR ISP mk II programmer directly from the development environment, or use the AVReAl program [3] and the Altera ByteBlaster adapter [4]. The pin assignment of the X3 connector corresponds to this particular adapter. It is not excluded the use of other programmers for microcontrollers of the AVR family. The codes from the avm.hex file are entered into the FLASH memory of the microcontroller, and from the avm.eep file into its EEPROM. The configuration of the microcontroller must correspond to fig. 7.
The program operation algorithm consists in cyclic polling of five measurement channels with a frequency of 50 Hz. When measuring in the voltage and current channels, the reference voltage of the ADC is 2,56 V and is supplied from a source built into the microcontroller. When measuring temperature, the microcontroller supply voltage (5 V) is exemplary. The results of the ADC operation are added to the ring buffer, which contains 25 readings, each of which occupies two bytes (the ADC of the microcontroller is ten-bit). In fact, a history of the last five readings is stored for each channel. To reduce the fluctuation of readings in each channel, the average of the last five readings is calculated [5]. After processing, the values of current and voltage are represented by integers lying in the range 0-255, and the value of the least significant digit of voltage is 0,1 V, and current is 0,01 A. Therefore, the measurement limits for voltage and current are, respectively, 25,5 V and 2,55 A. The value of the rectified voltage at the input of the power supply [1] is not displayed on the indicator, but is used to calculate the power dissipated by this power supply. Correction coefficients for each channel (except for the temperature channel), taking into account the spread of the ADC parameters and voltage divider resistors, are stored in the EEPROM of the microcontroller. By default, they are all equal to 1, but as a result of the self-calibration procedure, they can take values from 0 to 2-1/64 in increments of 1/64. The temperature can take a value from -55 to +125 ° C and is displayed on the LCD in whole degrees Celsius. To calculate it, a table transformation of the result of the ADC operation is used. If the measured temperature value is greater than 45 оC, a command to turn on the fan is generated if it is less than 40 оC, the fan is turned off. If the temperature exceeds 90 оWith the emergency shutdown of the power supply, and the LCD displays the inscription "Overheat". To start the self-calibration mode, it is necessary to use the SB2 button to signal the power supply off (AVM remains on), then press the SB1 button and, while holding it, press SB2 again. After that, the following exemplary voltages are applied to the X1 AVM connector: to the input Uvypr (pin 6) - 40 V, input UO (cont. 1) - 20 V, to inputs Iн(cont. 2) and IMax (pin 5) - 0,5 V, which corresponds to the voltage drop on the current sensor (R13 in [1]) at In = 2 A. Voltage 7 IN. During calibration, the channels are indicated on the indicator by letters in the leftmost familiarity: U - output voltage, I - load current, L - protection operation current, t - temperature, r - rectifier voltage. For example, before calibrating the output voltage channel, the inscription shown in fig. 8.
The channels for calibration are selected one by one by pressing the SB1 button, and with the help of SB2 the calibration process of the selected channel is started. The inscription "Saved" will inform you about its completion and writing the result to EEPROM, and after another 2 s you can see the value of the corresponding parameter calculated using the selected coefficient on the indicator. After that, you can go to the next channel by pressing the SB1 button or repeat the calibration of the previous one by pressing SB2. By displaying the value of the output voltage on the indicator, the AVM takes into account the voltage drop across the current sensor, subtracting it from the measurement result. Therefore, upon completion of the calibration, while the reference voltages from the AVM inputs are removed, 19,5 V (0,5 V less than the reference voltage of 20 V) and 2 A (corresponding to a drop in voltage 0,5 V on the current sensor). AVM is connected to the power supply unit [1] according to the scheme shown in fig. 9. Resistor R13, according to the description of the block, is made up of three one-watt resistors with a nominal value of 1 ohm, connected in parallel, and has a resistance of 0,33 ohms. You need to add one more resistor of the same to them, reducing the total resistance to 0,25 ohms. This simplifies the calculations performed by the AVM microcontroller.
The same diagram shows a rectifier serving as an input voltage source of the power supply on transformer T1 and diodes VD1-VD4, equipped with a current limiting unit for charging the smoothing capacitor after switching on. For its operation simultaneously with the signal that opens the transistor VT1, which leads to the operation of the relay K1 and the supply of mains voltage to the mains winding of the transformer, the microcontroller also sends a signal that opens the phototransistor of the optocoupler U1. As a result, the transistor VT2 remains closed after the unit is turned on, and the charging current of the smoothing capacitors of the rectifier flows through the resistor R5 that limits it. The AVM microcontroller program monitors the rate of voltage change across these capacitors. As soon as it decreases sufficiently (this means that the capacitors are almost completely charged), the signal that opens the phototransistor of the optocoupler U1 will be removed. As a result, the gate-source voltage of the transistor VT2 will increase. Its drain-source channel will open. Since the resistance of the open channel is only 0,018 ohms, any noticeable current through the resistor R5 no longer flows and does not affect the further operation of the device. Transformer T1 - TTP-60 2x12 V. Schottky diodes 90SQ045, from which the bridge rectifier is assembled, can be replaced by 1N5822. The AVM itself is powered by a separate source U2 with a voltage of 5 V, the main requirement for which is a minimum of ripples. The microcontroller consumes no more than 20 mA, the indicator backlight consumes about 100 mA, another 100 mA is needed for relay K1 (TRIL-5VDC-SD-2CM). The AVM printed circuit board file in Sprint Layout 5.0 format and its microcontroller program can be downloaded from ftp://ftp.radio.ru/pub/2014/02/avm.zip. Literature
Author: V. Rybakov
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