Digital voltmeter with automatic selection. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In various devices, specialized LSIs began to be used to implement the analog-to-digital conversion (ADC) function. One of the variants of a multimeter assembled on a similar LSI is known - KR572PV2, (K572PV2) [1]. Currently, the domestic industry produces another LSI of this series - KR572PV5. It has outputs for working with liquid crystal indicators (LCD) and can be operated from a 9 V single-ended power supply, which allows it to be used in small-sized and economical measuring instruments (multimeters). ADC KR572PV5 converts the input DC voltage (Uin.max. = ±199,9 mV) into a parallel seven-segment code that directly controls a 3,5-digit LCD.

The unipolar 9 V supply voltage is internally converted into a stabilized positive and an unregulated negative voltage (2,8 and -6,2 V) with respect to pin 32 (analogue common bus). These voltages are necessary to power the analog part of the KR572PV5. The digital part is also powered by an internal stabilized 5 V ADC source with pins 1 and 37 (digital common bus). The LSI clock generator is connected to the pin. 21 through a divider of 1:800 and at a generator frequency of 50 kHz on the pin. 21 received a rectangular signal with a frequency of 62,5 Hz, necessary for the operation of the LCD.

The principle of operation of KR572PV5 is similar to that described in [1] for KR572PV2 and is not considered in this article.

The measuring device offered to the attention of readers is designed to measure DC voltage and resistance.

Main technical characteristics:

• Upper measurement limits, V, kOhm....... 2, 20, 200, 2000
• Selection of the measurement limit.......automatic
• Settling time of readings, at a clock frequency of 50 kHz, s, not more than ....... 2,5
• Input resistance, MOhm, not less....... 9
• Consumed current, mA, no more.......1

The schematic diagram of the device is shown in fig. 1. It consists of a measurement mode switch SA1, analog switches DD2-DD6 with exemplary resistors R2-R5 and R7-R10, ADC DD1 with a source of exemplary voltage VT1, LCD HG1 and an automatic measurement limit selection device (UAVPI) on microcircuits DD7-DD11 . For the sake of simplicity, the diagram shows the connection of only those segments of the indicator that contain the necessary information for the operation of the UAVPI.

Fig.1 (click to enlarge)

The full numbering of LCD pins is shown in fig. 2.

Ris.2

The principle of operation of the UAVPI is based on the assessment of the state of the hundreds and thousands of digits of the 3,5-bit output parallel code KR572PV5 (segments a, b, g, f - hundreds and b, c - thousands). If the input voltage UBX of the ADC is greater than 199,9 mV in absolute value, then the overload mode sets in and the indicator will show 1 in the thousands digit, and there is no indication in the hundreds digit (and in other digits). Such a signal at the output of the LSI causes the meter to switch to the coarsest limit. On the other hand, if |UBX| <20 mV, then the display shows 0 or 1 in the hundreds digit, while there is no indication in the thousands digit. Such output code combinations give permission to move to a more sensitive limit.

The signal overload and "underload" ADC generates a decoder on the elements DD7, DD8, DD9.1. The signals from the decoder control the operation of the counter DD10.1 and the counter-decoder DD11. Serially connected counters DD10.1 and DD10.2 (the latter uses only one bit) divide the frequency of 62,5 Hz (pin 21 DD1) by 32.

The received frequency (about 2 Hz) is fed to the counting input DD11 and is clocked when switching the measurement limits. When the ADC is overloaded, the output DD8.4 has a level of 1, which resets the counter DD11 to zero, while the level 1 at the output of the least significant bit of this counter corresponds to the inclusion of the largest measurement limit. At the same time, level 0 at the output of DD8.3 disables the account DD10.1. When the ADC is "underloaded", the input of the SR DD10.1 will be 1, allowing the account, while the counter DD11 is also included in the work. At its output, with each counting cycle in the bit corresponding to the cycle number, there will be a high logic level.

The number of bits used DD11 is equal to the number of measurement limits. If the optimal measurement limit is reached, then 0 at the output of DD8.3 will stop the counter DD10.1, and with it DD10.2 and DD11. When the minimum limit is reached, DD10.1 is blocked via the R input, even if the ADC is still in the "underload" state. Switching the measurement limits of the volt-ohmmeter is carried out by analog keys DD2-DD5. Their state determines the output code DD11. The keys have a sufficiently high resistance in the conductive state (several hundred ohms), but they are connected in such a way that they practically do not introduce errors in any of the measurement limits.

The measured voltage is supplied to the DD1 input through the SA1 type of operation switch (upper position) and a divider, the upper arm of which is the resistor R1, the lower one is one of the resistors R2-R5, depending on the state of the keys DD2, DD3. The maximum voltage of the lower arm of the divider is limited by diodes VD1-VD4. The exemplary voltage source is made on a transistor VT1 operating at a thermostable point. An exemplary voltage of 100 mV from the resistor R16 is applied to the pin. 36 DD1 through one of the keys DD6.

The volt/ohmmeter uses an unconventional method for measuring resistance [2]. It is illustrated by the diagram in Fig. 3.

Ris.3

A certain current 06 flows through series-connected exemplary resistor R10P and measured resistor Rx under the action of voltage U0. Since the same current flows through the resistors R0gp and Rx, the ratio of voltage drops across them is equal to the ratio of their resistances. Thus,

Aind \uXNUMXd Ux / Uobr \uXNUMXd IoRx / IoRobr \uXNUMXd Rx / Robr

where: Aind - indicator readings.

The advantage of this resistance measurement method lies in the simplicity of its implementation and the independence of the measurement accuracy from the instability of the voltage U0. In the resistance measurement mode, switch SA1 is moved to the lower position. The positive voltage of the power supply is applied through VD7 and R6 to the keys DD4, DD5, which carry out the necessary switching of the exemplary resistors R7-R10, depending on the measurement limit of the selected UAVPI. The voltage on the reference and measured resistors is limited by diodes VD5 and VD6 to exclude the overload mode of the ADC integrator. For the same purpose, the lower (according to the scheme) key DD6 serves. With its help, the time constant of the integrator when measuring resistances is doubled. Transistor VT2 serves as an inverter signal that controls the keys DD6. The volt-ohmmeter is powered from a 9 V battery ("Krona VTs", "Korund") or from a 7D-0,115-U 1.1 battery. All microcircuits, except for DD6, are powered by the internal regulator DD1, since the current they consume is extremely small when operating at a low switching frequency.

The design is designed for trained radio amateurs, so the description of the circuit board and the design of the device is not given. It is only necessary to pay attention that the SA1 switch has reliable isolation between the groups of contacts, designed for the maximum measured voltage. The resistor R1, on which most of the measured voltage drops, must also be designed for the same voltage. It can be made up of several low-voltage resistors of suitable ratings. It should be noted that the accuracy of the device is limited practically only by the accuracy and stability of the reference voltage source and resistors R2-R5, R7-R10, which must be precise. In extreme cases, they can be selected from common resistors with a tolerance of at least 5%, but the temperature and time stability of these resistors will be low. As a resistor R16, you can use a non-wire multi-turn resistor SDR-37.

In the case of using a wire resistor of the SP5-2 type, its value must be reduced to 100 ... 150 Ohms and a constant resistor of 300 ... 360 Ohms must be connected in series with it, otherwise it will be difficult to accurately set the reference voltage due to the large discreteness of its resistance change when adjusting. Capacitors C4, C5 must be with a low coefficient of dielectric absorption - K71-5, K72-9, K73-16, etc. Before installing the transistor VT1 in the device circuit, you need to find its thermostable operating point. To do this, you need to collect a reference voltage source (VT1, R13, R16), connect a milliammeter with a maximum current of 16 mA in series with resistor R1 and apply a voltage of +1 V to the VT2,8 gate relative to the lower (according to the circuit) output of resistor R16 from any stabilized source voltage. Further, by changing the temperature of the transistor VT1 (for example, touching its body first with a hot, then with a cold metal object), achieve the smallest change in the drain current in the operating temperature range (0 ... 40 ° C) by selecting resistor R13. The value of this resistor may differ significantly from that indicated in the diagram.

A properly assembled volt/ohmmeter starts working immediately and only needs to be set by resistor R19 to the frequency of the KR572PV5 clock generator of 50 kHz and by resistor R16 of a reference voltage of 100 mV (in voltage measurement mode).

The volt-ohmmeter can also measure alternating voltages, for this it is necessary to provide for the inclusion of a detector of average rectified values ​​in the break of the wire going from SA1 to resistor R14. Due to the fact that the detector introduces with its filter an additional time constant (inertia) into the circuit of the system for automatically selecting the measurement limit, oscillations may occur in this circuit, as a result of which the voltmeter can “overshoot” the desired measurement limit. To eliminate this shortcoming, it is only necessary to reduce the filter capacitance, which is possible only up to a certain limit, or to reduce the clock frequency for switching the measurement limits. The last method is very easy to implement. It is enough to switch the input CN DD11 to the output of the next unused bit DD10.2 (pin 12) when switching to measuring AC voltage. As a result, limit switching will be twice as slow. This will increase the settling time to 5 s and ensure reliable operation of the UAVPI.

References:

1. Anufriev L. Multimeter on VIS. - Radio, 1906, No. 4, p. 34-39.

2. Oswald G. Widerstand-Messung mit DVM.- Funkschau, 1981, No. 8, S. 98.

3. Raatsch P. Bereichsautomatik fur C7136D.- Radio fernsehen elektronik, 1986, No. 10, S. 636-638.

Author: V.Tsibin

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