Digital mini-voltmeter with LCD. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The author of this article investigated the performance of a number of ADC microcircuits of the ICL71x6 family (including KR572PV5) and its analogues at a reduced supply voltage. Using the results of these studies, he developed the original design of a miniature digital voltmeter.

Based on the ADC MAX130 microcircuit, belonging to the ICL71x6 microcircuit family, a miniature voltmeter with four measurement limits has been developed: 200 mV, 2, 20 and 200 V. The main circuit of the device is shown in fig. 1.

(click to enlarge)

The appearance of the voltmeter is shown in fig. 2.

The choice of this series of microcircuits is explained by the fact that its operability limit at a reduced supply voltage approximately coincides with the threshold voltage of a cheap and affordable power-down detector - the KR1171SP42 microcircuit. The use of this detector makes it possible to avoid erroneous measurements when the supply voltage drops below a certain level. Significantly lower current consumption in comparison with ICL7106 (KR572PV5) made it possible to increase the duration of the voltmeter operation even when using a small-sized battery.

At a quartz frequency of 32,768 kHz, the voltmeter gives about 2 (32768/16000) readings per second. The frequency of the VR signal is approximately 40 Hz (32768/800).

For the instance of the MAX130CPL microcircuit used in the mini-voltmeter, the minimum operating voltage at which the measurement accuracy is still maintained turned out to be 4,27 V. Therefore, out of the ten available instances of the 1171SP42 detectors (and they have a significant spread in the response voltage), an instance with the response voltage was chosen Us = 4,3 V and hysteresis 60 mV (off voltage - approx. 4,36 V).

The accuracy of an ADC is largely related to the quality of the capacitors used in the ADC. In company documentation and articles on the use of the ICL71xx ADC, it is recommended to use capacitors with a low absorption coefficient in the dielectric. If a ceramic capacitor is used as C6 (integrating circuit capacitance), the conversion linearity error will be on the order of 0,1%, and with a polystyrene and polypropylene dielectric, 0,01% and 0,001%, respectively. Of the domestic capacitors, K71-4, K71-5, K72P-6, K72-9, K73P-7, K73-16, K73-17 can be recommended. Capacitors C4 in the zero correction node and C2 with a reference voltage can be used with a polyethylene terephthalate dielectric. For reference: capacitors of groups K70-xx and K71-xx - with a polystyrene dielectric, groups K72-xx - fluoroplastic, K73-xx - polyethylene terephthalate, K78-xx - polypropylene.

The voltmeter is assembled on a breadboard printed circuit board, the dimensions of which coincide with the dimensions of the indicator. The ADC chip is located under the indicator, and the rest of the elements are placed on the reverse side of the breadboard. All resistors, except for the input divider, are 0805 chip resistors. The installation is done with a thin wire.

The voltmeter board is installed in a tin case with dimensions of 80x35x15 mm. Opposite the indicator, there is a window in the case, into which a transparent plastic plate is glued (from the lid of the CD box). Battery compartment dimensions - 35x15x15 mm.

To obtain a reference voltage of 100 mV, elements R1, VD1, R3, R5, R6 are used. The performance of the AD1580ART integrated zener diode (Ist min = 50 µA!) is slightly better than that of the RER1004 (LM385), and it is available in a miniature SOT-23 package. An exemplary voltage of 100 mV is set across the resistor R5 by adjusting the tuning resistor R6.

A voltmeter for four voltage measurement limits was made without a special switch: an external voltage divider built into the measuring probe connector is connected to the XP1 connector. The input divider circuit is shown in fig. 3, and in fig. 4 shows its design. The transition to another measurement limit can be carried out by disconnecting the probe with a divider from the XP1 connector of the voltmeter, turning the probe 90 degrees, and again connecting both nodes.

The input impedance of the voltmeter with a connected voltage divider is 11,1 MΩ (without a divider - about 100 MΩ). It is best to use resistors C2-29V 0,062 or 0,125 W with a tolerance of 0,1% in the input voltage divider. The resistances of the divider resistors are chosen as multiples of 10, which facilitates their selection. The resistance of the lower arm of the divider in this case should be 11,11 kOhm; such a rating exists for resistors C2-29V (row E192). You can use the advice from [1] and connect in parallel two resistors with ratings from the E24 series of 12 kOhm and 150 kOhm (it is important to choose a value of 11,11 kOhm). When installing resistors with a tolerance of 0,1% in the divider, no additional selection is required. Unfortunately, it is impossible to find an accurate small resistor with a nominal value of 10 MΩ. Therefore, I had to make a home-made resistor from 0805 chip resistors. The circuit and design of such a resistor are shown in fig. 5a and fig. 5 B.

Resistors R1 '= 8,2 MΩ and R1 "= 1,8 MΩ should be selected with a negative tolerance. If the deviation is about 0,1% (this is quite a possible value for chip resistors with a tolerance of 1%), then the other two resistors must have the ratings indicated in Fig. 5, a. Resistor R1 * is soldered from above to resistor R1 "'. The size of the structure approximately corresponds to the resistors C2-23 0,125 or 0,25 watts. The maximum allowable voltage at the input of the divider is determined by the maximum allowable voltage of the 0805 R1 'chip resistor and, according to the passport data, is 300 V. The operating voltage of the chip resistors should not exceed 150 V. Given that the resistance of the resistor R1' is 8,2 MΩ / 11,1 MΩ \u73,8d 150% of the impedance of the divider, then the operating voltage of the divider is 0,73 V / 203 \u10d 1,999 V, which corresponds to the maximum measurement limit of the mini-voltmeter. You can adjust the 200 MΩ resistor either according to the readings of an accurate ohmmeter, or by measuring the calibrated voltage of 10 V with an accurate voltmeter on the assembled divider. Naturally, the mini-voltmeter itself must be set to a limit of 71 mV. In principle, the first ohmmeter tuning option cannot give a good result, since there is always an input current that creates a noticeable voltage drop across the 31 MΩ resistor. But the input currents of the ICL2xx family of microcircuits are so small that the results for a XNUMX/XNUMX-digit indication are quite satisfactory.

It is not recommended to adjust the divider by connecting a multimeter to its steps, even if it is an order of magnitude more accurate, since some error may occur due to the internal voltage divider, which is not switched off even at the lower limit.

Before filling the probe structure with epoxy resin, it is recommended to thoroughly rinse the 10 MΩ resistor and the entire divider with an alcohol-gasoline mixture, water and shampoo and dry thoroughly. For this purpose, it is convenient to use a hair dryer at an air temperature at its outlet in the range of 70 ... 90 ° C. Care must be taken to ensure that the epoxy does not flow into the body of the PBD-8 male connectors.

The second input probe of the voltmeter with a crocodile clip at the end is connected to the KhRS connector. Since the 200 mV input is also connected to the same connector, it can be used to connect a voltmeter to various external attachments or simply as an indicator with a large input resistance and a 200 mV scale. The voltage divider is disconnected from the XP1 connector, so the position of the decimal point can be set on the XP1 connector with a conventional jumper.

The metal case of the voltmeter has no connections with the electrical part of the device, but there is a "case" contact on the KhRZ connector. You can use it depending on the situation (for example, with a high level of interference). The scheme and design of the negative probe are shown in fig. 6.

The voltmeter uses an LCD indicator ITS-0803 from INTECH [2]. Its dimensions are 51x30,5 mm. The image appears already at a voltage of 2 ... 2,1 V, and the indicator reaches the maximum contrast of the sign in relation to the background (in IZhTs5-4/8 the contrast is worse) at a supply voltage of 3 ... 3.3 V. This type of indicator can be called standard , since its full counterparts are produced by a number of companies (Standish, Epson). The purpose and location of pins and segments are shown in fig. 7.

The most common indicator, PLI5-4/8, can be used but not recommended. The image of the segments appears, starting with a voltage of 2,5 ... 2,7 V on it, but the maximum contrast is achieved only at a voltage value of 4,3 ... 4,8 V. In addition, this indicator does not have a segment "Low battery". There is an analogue of this LCD with the name "Sobol", the characteristics of which are better, and the price is higher.

A good indicator is IZHTS14-4/7 from the Reflector plant. It works fine at 3V, but has 50 pins, lots of extra segments, and is therefore larger than the ITS-0803. There is a character-segment "LB". Information about these components can also be found on the site [2].

If there is no "LOW BATTERY" segment on the LCD, an LED can be used in the low battery indication unit; the voltage control circuit for this case is shown in fig. 8. It is desirable to choose an LED with a minimum operating current. The L-934SRC LED shown in the diagram is red, very bright (King Bright). It works fine with a direct current of 200 ... 300 μA!

Transistor VT2 inverts the BP signal and is used to turn on the decimal points on the indicator. The point required at the current limit is switched on by a jumper on the XS2 connector of the external voltage divider. The same inverted BP signal (denoted as DP0 in the diagram) can be applied to the LCD segment "Low Battery" through a switch on the transistor VT1. The state of the key (on/off) is controlled by the undervoltage detector DA1. Resistors R4, R7, R10, R11 (with a resistance of 5 ... 10 MΩ) are installed to exclude the "backlight" of unused segments. In most cases, the indicator works well without these resistors.

The dimensions of the power compartment, linked to the width of the indicator used, allow you to install batteries 4LR44, A544, V34PX, PX28A, 4SR44 with a voltage of 6 V into the device. 13 V. They are quite common and are used in laser pointers and the children's toy "Tomagotchi".

A sufficiently long spring is installed in the compartment, which allows the use of three or four disc elements. With four elements, the final discharge voltage is 4,3 V / 4 \u1,07d 1,43 V. When only three elements are turned on, their capacity is not fully used (the minimum allowable voltage is about 131 V). Low current consumption (especially for MAX5) allows you to install a set of "fresh" and partially discharged cells for a more complete use of their capacity. With some effort, even 0,9 pieces of completely discharged cells (5 V x 4,5 \uXNUMXd XNUMX V) can be placed in the compartment, and the voltmeter will work fine.

The capacity of alkaline galvanic cells and batteries of these types is approximately 100 mAh, and silver-zinc - 160 mAh. This means that the estimated operating time of the voltmeter from one battery at an average current consumption of about 220 μA (MAX130) will be about 500 for alkaline and 800 hours for silver-zinc batteries. For other types of microcircuits of this ADC family, you can estimate the operating time based on their current consumption.

The error of the voltmeter at the limit of 200 mV corresponds to the capacity of the LCD indicator, i.e. all the numbers on the indicator are accurate (tested using a DT41F + 2/930-digit voltmeter, accuracy class 0,05), which, of course, is not the merit of the author, but the high quality of the manufacture of the MAXIM ADC and an Analog Devices reference voltage source. Since the voltage of the ION measured in the tuned voltmeter was slightly lower than the calculated value (about 99,98 mV), the non-linearity of the device that inevitably occurs with this method of correcting absorption in the capacitor is beyond 3 1/2 digits of the indicator. The absence of a noticeable difference in readings at the end points of the scale (less than 1/2 of the least significant digit of the indicator) in this case is just a pleasant accident. On the remaining (three) limits, the error will be determined by the quality of manufacture of the voltage divider.

The author is aware of some ambiguity in the material presented in this article. On the one hand, the use of an ADC chip in the design of a device is considered, which cannot be considered otherwise than as an amateur radio, since the power supply mode of the microcircuit does not comply with the manufacturer's recommendations. On the other hand, the author has at his disposal the results of a rather deep study of the sources of error, which was not included in this article. At the same time, the author claims that two dozen copies of microcircuits of different types from the ICL71x6 family work normally in the proposed switching circuit. However, the author cannot give any generalizing conclusions about the performance of all microcircuits from this group of ADCs in this article.

For himself, the author nevertheless made a certain conclusion. If in an amateur or industrial design it is necessary to connect a microcircuit of the ICL71x6 family to a device with a supply voltage of 5 ... 6 V, you can use the ADC without polarity converters, taking into account the supply voltage margin.

Literature

1. Biryukov S. Portable digital multimeter: Sat: "To help the radio amateur", vol. 100, p. 70-90. - M.: DOSAAF. 1998.
2. chipindustry.ru/shop/

Author: O. Fedorov, Moscow

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