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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Pocket multimeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology A feature of this multimeter is an electronic switch of measurement limits and an original unit for matching the outputs of the BIS KR572PV2 with a liquid crystal indicator. The device is powered by one "Korund" battery, the energy of which is enough for 50 hours of continuous operation. The minimum voltage at which the multimeter still works is 6,5 V. At this value, the LCD "goes out", although the electronic part of the avometer functions properly when the supply voltage drops to 5,6 V. The measuring unit of the device is made on the operational amplifier (op-amp) DA2 (see schematic diagram) and on the LSI DD6. This LSI structure CMOS operates on the principle of double integration to 3,5 decimal places with outputs for driving seven-segment LED indicators. The maximum current consumed by the KR572PV2 series LSI from both power sources is no more than 1,8 mA, the output current of the most significant order is at least 10 mA, the rest is at least 5 mA. The ADC common-mode signal attenuation coefficient reaches 100 dB, the conversion error does not exceed 1, 3 and 5 count units, respectively for KR572PV2A, KR572PV2B and KR572PV2V. The specified parameters are guaranteed at a temperature of 25±10°С and supply voltages of +5 V (Upit1.) and -5 V (Upit2.) with ±1% instability. Supply voltage Upit1. can be in the range from +4,5 to +5,5 V, Upit.2, from -8 to -4,5 V. The input and reference voltages should not exceed the voltage of the power sources. So that the LSI does not fail, it is first connected to a common wire (pins 21 and 32), and then voltages are applied in series: power (pins 1 and 26), exemplary (pins 35 and 36) and, finally, input (pins 30 and 31). Relieve tension in reverse order. When converting the input signal, measured relative to the common wire, the conclusions 30, 32 and 35 MC are connected to the common wire. In this multimeter, the inclusion of the BIS KR572PV2 differs from the typical one. Feature - in work on a liquid crystal indicator, the outputs of the segments of which are connected through diodes VD14-VD36 to the LSI outputs and through resistors R34-R59 to the common LCD wire. A pulsed voltage with a frequency of 1 kHz is applied to it. Such an inclusion of the KR572PV2 microcircuit allows it to work with the LCD, however, in this case, the constant component of the voltage on the indicator segments somewhat exceeds the permissible value. The pulse repetition rate of the clock generator, which is part of the LSI, is determined by the elements R71, C20 and is equal to 40 kHz. The measured voltage through the switch SA1 is supplied to the electronic attenuator formed by the multiplexer DD2 and op-amp DA1. The selected measurement scale corresponds to a certain binary code at terminals 9 and 10 of the multiplexer, which introduces the corresponding resistance formed by resistors R25, R27, R29, R33 into the feedback circuit of the op-amp. These resistors, depending on the control code of the multiplexer, provide the following transfer ratios from the X1 input terminal to the DA1 output: 1, 0,1, 0,01; 0,001. The input resistance of the multimeter when measuring voltages is determined by the resistor R8. Together with diodes VD4 and VD5, it provides protection for the DA1 chip from overloads if the input voltage accidentally exceeds the limit value of the selected scale. When measuring current within 1, 10, 100 mA, the transfer coefficient of the electronic attenuator takes on the following values: 100; 10; 1. Current measurement from 0,1 to 1A is made through socket X4. In this case, the gain of the input circuit is 1. In the mode of measuring constant voltage or current, the signal to the input of the LSI ADC comes from the output of the DA1 microcircuit. When measuring variables, a bipolar signal from the output of DA1 is converted into a unipolar rectifier assembled on the op-amp DA2, and fed through a smoothing filter to the input of the LSI. In a bipolar rectifier, the stability of the output zero voltage is provided by the resistor R62. Negative feedback on the alternating voltage is carried out by the capacitor C 15. When measuring resistance, a current flows through the input jacks of the multimeter and the resistor connected to them, the value of which does not depend on the value of the measured resistor. It is generated by a stable current generator, assembled on transistors VT2-VT4. Multiplexer DD1, depending on the selected limit, connects one of the resistors R12 - R15, setting the required value of stable current. The measurement limit selection device is based on DD4, DD5 microcircuits and contains two RS flip-flops (DD4.1 and DD4.2), a single vibrator (DD4.3), and a reversible counter (DD5). The logic levels at the outputs Q1 and Q2 of the counter DD5 control the operation of the multiplexers DD1 and DD2 and thus determine the selected measurement limit. Code 00 corresponds to measurement limit 2; 01-20; 10-200, 11-2000 V (mA, kOhm). When the multimeter is turned on, the code 5 is set at the output of the counter DD01 and the limit of 20 V (mA, kOhm) is turned on. Select the required measurement limit by pressing the SB1 or SB2 button. In the first case (+1) there is a transition to a larger measurement limit, in the second (-1) - to a smaller one. On the LCD, this is indicated by moving the decimal point to the right or left. Let's see how this happens. When SB1 (SB2) is pressed, the charge current of the capacitor C5 (C6) causes a positive pulse to appear at the input of the RS flip-flop DD4.1 (DD4 2), and it fires. The voltage drop at the output of DD4.1 (DD4.2) starts the DD4.3 one-shot, the output pulse of which is fed to the counting input of the DD1 counter and changes its state to 5. The DD1 4 one-shot through the R3C32 chain affects the RS flip-flops DD11 and DD4.1. 4.2, returning them to their original state after 100 µs. The operating mode of the counter DD5 sets the logical level at the input ± 1: if there is a logical 1, then the summation occurs, if the logical 0 is subtracted. In the multimeter, the zero level is present at the ±1 input of the DD5 chip all the time, but when you press the SB1 (+1) button, a logical unit appears at this input, it disappears after 1 is written to the counter. The duration of the positive pulse at the input is ±1 of the counter DD5 is about 101 µs. The described method of switching measurement limits was chosen with the prospect of creating a multimeter based on this device with automatic selection of limits. The supply voltages of +5,5 V and -4,7 V are generated by a power unit consisting of a stabilizer and a polarity converter. A positive voltage forms a stabilizer assembled on transistors VT1, VT5, VT6. Such a stabilizer has a voltage stabilization factor of at least 500 and short circuit protection. When the multimeter is turned on, the trigger circuit, consisting of elements C1, VD1, R6, brings the stabilizer to operating mode. The voltage drop across the regulator transistor VT1 of the stabilizer is only 0,05-0,1 V.
DA1, DA2 K544UD1A, DD1, DD2 K564KP1, DD3 K564LA7, DD4 K564TR2, DD6 KR572PV2B, VT1, VT7 K.T361B; VT2 - VT4 KT3107B, VT5, VT6, VT8 KT315B, VD1, VD6. VD7, VD10, VD11, VD14 - VD36 KD103A, VD4, VD5 KD503B, VD8, VD9. VD12. VD13 D9D The main technical parameters of the multimeter:
A negative polarity voltage of -4,7 V is obtained in a converter that contains a generator, an output transistor stage and a capacitive voltage multiplier. The output voltage of the generator assembled on a DD3 chip is a sequence of pulses with a frequency of about 1 kHz. These pulses are fed to the bases of transistors VT7 and VT8 of the output stage and alternately open and close them. When the transistor VT7 is open, the capacitor SU is charged through it and the diode VD8, and when VT8 - the capacitor C 10 is discharged through it and the diode VD9, charging the capacitor C9, where a negative voltage of -4,7 V is set. Since a stabilized voltage is applied to the output stage of the converter and the -4,7 V circuit load is unchanged, then the negative voltage is stable. The amplitude of the negative voltage ripple under load does not exceed 10 mV. The current consumed by the power supply unit without load is 1,5-2 mA. The multimeter mainly uses MLT resistors with a tolerance of ± 5% and only a resistor R4 brand C1 8 has a tolerance of ± 1% select on a digital ohmmeter with an accuracy of no worse than ± 3% This operation will significantly reduce the multimeter setup time. Trimmer resistors-SPZ-4. The device uses oxide capacitors K8-9 and K25-27, constant capacitors C29 - C33, C0,1 - C18 brand KM. The buttons for selecting the limits are microswitches of the MP-53 or MP-1 type, the power-on toggle switch-MT-53, the switches SA19-PG4-ZP-ZN, SA8 is made of two toggle switches MP 11. The elements of the multimeter are placed on a printed circuit board made of double-sided foil fiberglass 20 mm thick (see pic) Fixed resistors and MS DD9 are installed vertically on the board. Elements R12, R1, R1, R2 and FU3 are mounted on switch SA12. To reduce the size of the device, the LCD is placed above the LSI. In the figure of the board, it is necessary to connect points A - A, B - C, G - G and 1-7, respectively, and connect point D to terminal 3 DD4. Resistors R8 - R9 and diodes VD2-VD1 are not marked. Establishing a multimeter begins with checking the power node. Its proper operation is evidenced by the presence of both output voltages and the current consumption in the absence of load is not more than 2 mA. The output voltage in the range of 5,2-5,5 V is set using the Zener diode VD3. In the DC current measurement mode, with the output sockets not connected, the number 000 or -000 should appear on the LCD display, which is replaced by the reading -1888, if +5 V is applied to pin 37 of the BIS DD6. By pressing the SB1 and SB2 buttons, the operation of the measurement limit selection node is checked, controlling the correct display of commas. If necessary, check on the oscilloscope RS-triggers DD4.1 and DD4 2, single vibrator DD4.3 and counter DD5. Then proceed to the test of the electronic attenuator. To do this, in the voltage measurement mode, a signal of 1 V and a frequency of 1 kHz is fed to the input of the multimeter. The output signal is controlled at pin 6 of the DA1 chip. The transfer coefficient of the input device depends on the selected measurement limit and should be 1, respectively; 0,1; 0,01; 0,001 within 2, 20; 200; 2000 V (mA, k0m). If the electronic divider is working properly, the input jacks are shorted and the trimming resistor R28 is set to 0 at the output of the op-amp DA1. Next, connect the oscilloscope to the output of the DA2 chip (pin 6) and balance it with a variable resistor R53. In both cases, the zero setting accuracy is ±0,1 mV. To set the sensitivity, the multimeter is switched to the DC voltage measurement mode at a limit of 2 V. Having applied a calibrated DC voltage of 1000 mV to the input, the reading "69" is set on the display with a trimming resistor R1.000. In the AC voltage measurement mode, a signal of 1000 mV, frequency 1 kHz and trimming resistor R65 set the number "1.000" on the display. The values of resistors R12-R15 are selected with exemplary resistors connected to the input with a resistance of 100 Ohm 10, 100 kOhm and 1 MΩ. Authors: E. Velik, V. Efremov
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