Microfaradometer. Scheme, description

Free technical library

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Free library / Schemes of radio-electronic and electrical devices

Microfaradometer. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Measuring technology

Comments on the article

The article describes a capacitance meter for non-polar and oxide capacitors, based on the PIC16F876A microcontroller. Capacitance measurement range - 1...999 10³ uF - divided into two sub-ranges. Measurement results are indicated by a three-digit LED digital indicator with automatic decimal point setting. Some effect of the equivalent series resistance on the measurement accuracy at a higher limit is compensated for by calibrating the instrument.

In amateur radio practice, the need to measure large values of electrical capacitance is obvious. Many modern multimeters have the function of measuring the capacitance of a capacitor, their upper limit does not exceed 20-100 μF, and when the range is beyond the limit, the measurement accuracy is significantly reduced [1]. Professional RLC meters measure capacitance up to 1 F or more [2], but due to their high cost, they are not widely available for most radio amateurs. The magazine "Radio" describes several devices for measuring the capacitance of oxide capacitors [3,4]; they are usually designed in the form of prefixes and are based on indirect methods of measurement.

At the same time, using the modern elemental base and basic physical relationships, it is possible to build a simple device with sufficiently high metrological characteristics. The proposed device uses the principle of proportionality of the charge Q of the electric capacitance C at a fixed voltage value U: C = Q/U; where Q = It. In turn, at a given charging current, the charge of the capacitor is proportional to the time of flow of the charging current [5].

Technical specifications

Measurement range, µF .. .1...999 10³
Error in the entire range, %, no more than ............... ± 3
Measurement time, s, no more than ..........................2,5
Selection of measurement limits ... .automatic
Number of display digits .........3
Mains voltage, V 220
Power consumption,
Tue, no more ............... 12
Overall dimensions of the capacitance meter - 127x72x25 mm, weight with power supply - no more than 0,8 kg.

The device is based on the PIC16F876A microcontroller [6], which performs all the main functions: controlling the measurement process, calculating its results, and displaying the obtained value of the measured capacitance on the indicator.

Microfaradometer
Fig. 1

The schematic diagram of the device is shown in fig. 1. The DD1 microcontroller works according to the program, the codes of which are given in the table. After turning on the power and initializing the microcontroller, the device operates in automatic mode. The output RA0 is configured as the input of the comparator, RA3 is the input of the reference voltage of the comparator, RCO, RC1 are the outputs for controlling the charging current sources, RC2 is the output for switching on the discharge of the measured capacitor.

The measurement cycle begins with the discharge of the capacitor through the transistor VT2 and resistor R5. Then the source of the charging current is turned on, equal to 1 mA, on the transistor VT3 [5]. The voltage across the capacitor begins to increase. When it reaches a value of approximately 1 V, equal to the reference voltage at the RA3 input, the DD1 microcontroller stops the charging process and fixes its duration.

If the voltage on the measured capacitor does not reach the exemplary one within 1,2 s, the transition to the highest measurement limit occurs: the current source is turned on, equal to 1 A, on the transistor VT1, the indication "x1000" and the measurement is repeated. Next, the microcontroller calculates the value of the measured capacitance from the charging time, charging current and capacitor voltage, taking into account the measurement limit and the corresponding calibration coefficient. The measurement cycle is periodically repeated.

Dynamic indication of results is organized on a three-digit LED indicator HG1-HG3, transistors VT5-VT7 and microcontroller ports RC3-RC5, RBO-RB7 according to the classical scheme.

Buttons SB1-SB3, connected to ports RA1, RA2, RA5, are used to enter calibration coefficients when setting up and checking the device. "Mode" button - enter the calibration mode, select the coefficient, switch to the measurement mode.

Buttons "+" and "-" - setting the value of the selected coefficient in the range from 1 to 255. The calibration coefficient for the range "uF" is displayed without decimal points, for "uFx1000" - with a comma in the units place. The set values are automatically recorded in the memory of the microcontroller, stored there after the power is turned off and read when the device is turned on.

The source code of the control program is written in C in the MPLAB IDE programming environment version 6.5 [7] equipped with the PICC compiler version 8.05PL1 [8].

Microfaradometer

Structurally, the device is designed in a case from the M838 multimeter (see photo in Fig. 2). An external rectifier (in a mains plug) is used for power supply, providing an output voltage of 9 ... 12 V at a current of up to 1 A. Among those available for sale, for example, BP7N-12-1000 is suitable. The voltage regulator DA1 is installed on the board of the device. It is necessary to solder the leads of the oxide capacitor C1 with a capacity of at least 2 microfarads for a voltage of 1 V to the contact pads X1000, X16. It will take place in the battery compartment of the instrument case.

Microfaradometer
Fig. 3

Microfaradometer
Fig. 4

Microfaradometer
Fig. 5

Printed circuit board of the meter - with double-sided printed wiring and double-sided arrangement of parts; its main dimensions are shown in fig. 3. A drawing of the printed circuit board from the installation side of the indicators is shown in fig. 4, and from the side of the installation of the microcircuit and transistors - in fig. 5. To form vias in the board, holes with a diameter of 0,5 mm were drilled, into which segments of leads from MLT-0,25 resistors were riveted and soldered. The DD1 microcontroller must be installed on the device board in a panel with spring clips. The appearance of the mounted board is shown in the photo fig. 6, 7.

Microfaradometer

The device uses MLT resistors or similar; resistor R5 - from a manganin wire with a diameter of 1 mm and a length of 15 mm, you can use a current sensor from an M838 multimeter. Most capacitors are KM, K10-17 series, oxide - K53-4, K53-14, K52-1, and C1 (1000 uF) - K50-35. Quartz resonator - at a frequency of 10 ... 12 MHz in the NS-49 package. Buttons - small-sized clock SWT2, TS-A1PS-130. TR319 LED indicators can be replaced by any others with the same pinout, such as SA05-11HWA. Transistor VT2 is a powerful field transistor with a drain current of at least 10 A and a drain-source resistance of not more than 0,1 Ohm. Terminals ХЗ, Х4 are similar to those used in the M838 multimeter. Stabilizer DA1 and transistor VT1 are installed on plate heat sinks with an area of 12 and 5 cm2, respectively.

Microfaradometer

The device setup begins before the microcontroller is installed in the panel on the board. Turn on the power with switch SA1 and check the presence and correctness of the supply voltage of 5 V to the contacts of the microcontroller panel. The voltage on pins 1-3, 7 should be approximately equal to the supply voltage, on pins 14-16 about 4 V, and on pins 21-28 the voltage is close to zero. Then they check the operability of the buttons SB1-SB3: by pressing them, they control the appearance of a low level at the inputs RA1, RA2, RA5. The dynamic indication circuits are checked by serially connecting a common wire to the corresponding terminals of the RBO-RB7 and RC3-RC5 ports: in this case, the glow of the specified segments in the selected digit is observed. The current sources are turned on in turn by applying a low level to contacts 11, 12, while the ammeter must be connected to the X4, X0 sockets instead of the measured capacitor. When switched on through the RC0,5 circuit, the current must be in the range of 1 ... 1 mA; and through the RC0,5 circuit - 1 ... 1 A. The discharge circuit is checked with a 5 A current source turned on by applying a voltage of +13 V to pin 4. The readings of the voltmeter connected to the XXNUMX, XXNUMX sockets should drop to zero.

Further, after turning off the power, insert the programmed microcontroller into the panel and turn on the device. The display should show readings close to zero, the "Cycle" indicator (HL1) is lit intermittently, and the "x1000" indicator (HL2) is not lit. Now you can make trial measurements to assess the performance of the device as a whole.

The results obtained may differ significantly from the true ones due to the large spread in the parameters of the current sources, the error in setting the reference voltage, the comparator error, the frequency of the installed quartz resonator, and a number of other less noticeable factors. Instrument calibration required.

To calibrate the meter, you need to have four reference capacitors of different ratings: two - for the "μF" range with a capacity of 100 ... 900 μF, two - for the "μF x1000" range with a capacity of more than 10000 μF. To accurately determine their capacity, it is advisable to use a verified industrial meter or some indirect method. By taking measurements and changing the calibration coefficients according to the readings of the device, the true value of the capacitance of the calibration capacitors and the readings of the device are matched. After calibration, the instrument is ready for use.

At the highest measurement limit, the instrument readings depend to some extent on the equivalent series resistance (ESR) of the measured capacitor; this is expressed in an underestimation of the true capacitance value. To ensure that the error of the device does not exceed the specified value, the EPS should not exceed 0,1 ohm. For serviceable oxide capacitors with a capacity of more than 1000 μF, the average statistical value of the ESR is within these limits [9], its effect is compensated during the calibration of the device. For a more objective assessment of the performance of oxide capacitors, a joint measurement of capacitance and ESR is necessary - this is the topic of the next development.

Experience with the described meter showed its good consumer characteristics: accuracy, long-term stability of readings, ease of use. It allows you to carry out the necessary measurements that arise during the development, manufacture and repair of electronic equipment.

The microcontroller program can be downloaded hence.

Literature

Zagorulko A. Expanding the measurement limits of the multimeter M830G. - Radio, 2004, No. 9, p. 27.
RLC meters WayneKerr4265,4276. - Radio, 2005, No. 11, p. 73.
Dereguz A. Capacitance meter of oxide capacitors. - Radio, 2001, No. 12, p. 27.
Savosin A. Microfaradometer. - Radio, 2003, No. 5, p. 22, 23.
Horowitz P., Hill W. The art of circuitry. - M.: Mir, 2001.
Microchip Technology Inc. DS39582B. PlC16F87xA. data sheet. 28/40/44 - Pin Enhanced Flash Microcontrollers. microchip.com.
Microchip Technology Inc. DS1281C. MPLAB IDE v6xx. Quick start guide. microchip.com.
HI-TECH Software. PICC Manual.-htsoft.com.
Peak Electronic Design Limited. Equivalent Series Resistance Analyzer. Model ESR 60. Users Guide.- peakelec.co.uk.

Author: A. Topnikov, Uglich, Yaroslavl region; Publication: radioradar.net

See other articles Section Measuring technology.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Machine for thinning flowers in gardens 02.05.2024

In modern agriculture, technological progress is developing aimed at increasing the efficiency of plant care processes. The innovative Florix flower thinning machine was presented in Italy, designed to optimize the harvesting stage. This tool is equipped with mobile arms, allowing it to be easily adapted to the needs of the garden. The operator can adjust the speed of the thin wires by controlling them from the tractor cab using a joystick. This approach significantly increases the efficiency of the flower thinning process, providing the possibility of individual adjustment to the specific conditions of the garden, as well as the variety and type of fruit grown in it. After testing the Florix machine for two years on various types of fruit, the results were very encouraging. Farmers such as Filiberto Montanari, who has used a Florix machine for several years, have reported a significant reduction in the time and labor required to thin flowers. ... >>

Advanced Infrared Microscope 02.05.2024

Microscopes play an important role in scientific research, allowing scientists to delve into structures and processes invisible to the eye. However, various microscopy methods have their limitations, and among them was the limitation of resolution when using the infrared range. But the latest achievements of Japanese researchers from the University of Tokyo open up new prospects for studying the microworld. Scientists from the University of Tokyo have unveiled a new microscope that will revolutionize the capabilities of infrared microscopy. This advanced instrument allows you to see the internal structures of living bacteria with amazing clarity on the nanometer scale. Typically, mid-infrared microscopes are limited by low resolution, but the latest development from Japanese researchers overcomes these limitations. According to scientists, the developed microscope allows creating images with a resolution of up to 120 nanometers, which is 30 times higher than the resolution of traditional microscopes. ... >>

Air trap for insects 01.05.2024

Agriculture is one of the key sectors of the economy, and pest control is an integral part of this process. A team of scientists from the Indian Council of Agricultural Research-Central Potato Research Institute (ICAR-CPRI), Shimla, has come up with an innovative solution to this problem - a wind-powered insect air trap. This device addresses the shortcomings of traditional pest control methods by providing real-time insect population data. The trap is powered entirely by wind energy, making it an environmentally friendly solution that requires no power. Its unique design allows monitoring of both harmful and beneficial insects, providing a complete overview of the population in any agricultural area. “By assessing target pests at the right time, we can take necessary measures to control both pests and diseases,” says Kapil ... >>

Random news from the Archive

Link between zinc deficiency and hypertension 26.01.2019

Zinc deficiency can cause high blood pressure (hypertension) by changing the way the kidneys regulate sodium.

Zinc deficiency is common in people with chronic conditions such as type 2 diabetes and chronic kidney disease. People with low zinc levels are also at an increased risk of hypertension. The way the kidneys either excrete sodium into the urine or reabsorb it into the body—especially through a pathway called the sodium chloride cotransporter (SCH)—also plays a role in blood pressure control. Less sodium in the urine usually corresponds to higher blood pressure. Recent studies have shown that zinc may help regulate proteins that in turn regulate CCH, but a direct link between zinc deficiency-induced hypertension has not been investigated.

The researchers compared zinc-deficient male mice with healthy mice with normal zinc levels. Mice deficient in zinc developed high blood pressure and a corresponding decrease in urinary sodium excretion. The control group did not experience the same changes. A small group of zinc-deficient mice received a zinc-rich diet throughout the study. Once the zinc levels in the animals reached adequate levels, blood pressure began to fall and urine sodium levels rose.

Other interesting news:

▪ Samsung Gear VR Innovator Edition - virtual reality glasses for smartphones

▪ Water unmanned taxi

▪ TLV73333P New 300mA Capacitorless LDO Regulator

▪ Accumulators of the third millennium

▪ Diamond nanowires are more efficient than Li-Ion batteries

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ radio section of the website. Article selection

▪ article Put your fingers in the ulcers. Popular expression

▪ article In which country are deceased relatives regularly removed from the crypt and changed clothes? Detailed answer

▪ article The role of first aid to victims