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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Measurement of capacitance and ESR of capacitors with a combined instrument. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology The author offers radio amateurs who have assembled the device [1] an attachment to it, with which you can measure the capacitance and ESR of capacitors. Knowing these parameters, especially EPS, is required quite often today, for example, in the manufacture of various impulse devices. During the modernization of the combined instrument [1], I decided, by creating small attachments to it, to introduce new relatively rarely used functions into the device that cannot be implemented only in software. This makes it possible not to change anything in it, except for the microcontroller program. The implementation of this method of modernization is ensured by the presence of a connector in the device, to which four information lines of its microcontroller and the supply voltage are connected. Attachments are connected to this connector. The first step in this direction was the creation of an attachment for measuring inductance, described in [2]. The new attachment is designed to collect capacitors that are only supposed to be installed in some device, and not to measure their parameters without desoldering from the device. Based on this, I considered it possible to increase the voltage on the measured capacitor, which made it possible to reduce the measurement error. With the proposed attachment, the device in the capacitance and ESR measurement mode has the following Features:
The measurement of capacitance and EPS is based on the principle of charging the measured capacitor with a stable current and fixing the moments when the voltage on it reaches two control levels (thresholds). This principle is used in many other devices, for example [3]. Structurally, the prefix under consideration repeats the measuring part of this device.
The attachment scheme is shown in fig. 1. Compared with [3], the following changes have been made to it: - removed diodes, which should protect the elements of the device from damage when a charged high-capacity capacitor is connected to it. There are two reasons. First, according to the author, they perform their protective function very limitedly. For example, from a capacitor accidentally connected to the device with a capacity of several thousand microfarads, charged to a voltage of 50 V or more, they still will not save. Secondly, diodes do not allow to make the voltage on the measured capacitor greater than the level of their opening. If diodes are abandoned, the protective function within the same limits can be implemented using the VT3 transistor with proper control by the microcontroller. And from the point of view of the safety of working with the device, it will be correct before connecting a large-capacity capacitor (especially high-voltage) to the device, be sure to discharge it; - the set-top box uses only one stable current generator (GST), which provides measurements in the entire capacitance range indicated above. It differs from the original in a higher stability of the output current. This is achieved through the use of a parallel integrated voltage regulator of increased accuracy and a transistor with a high base current transfer coefficient. In addition, the output current of the GTS has been increased, which has reduced the measurement error (especially ESR) associated with the capacitor leakage current. The operation of the set-top box, the processing of the signals coming from it and the necessary calculations are performed by the microcontroller of the combined instrument. Time intervals are counted by its 32-bit timers, clocked at 32 MHz, which provides not only high measurement accuracy, but also a large theoretical upper limit of the measured capacitance (several farads). However, the achievement of such a limit in practice is difficult because the rate of voltage rise on the measured capacitor becomes very small with an increase in its capacitance, as a result of which the error in determining the moment the threshold is reached by the comparator increases. Therefore, the maximum measurable capacitance is software-limited to 99999 uF, which is quite enough for most practical purposes. After connecting the attachment to the device and switching it to the capacitance and EPS measurement mode, the microcontroller opens the VT3 transistor and closes the VT1 transistor, which turns off the GTS. The inverting inputs of the comparators of the DA2 microcircuit are supplied with exemplary voltages from the divider R4-R6, which set the thresholds for their operation (U1≈0,25 V; U2≈0,5 V). Logically low voltage levels are set at the outputs of both comparators in the initial state. Next measured capacitor Cx connect to the X1 connector of the set-top box and by pressing the corresponding key on the device start the measurement process. During the first three seconds after starting, the program keeps the VT3 transistor in the open state in order to remove a possible residual charge of the measured capacitor, after which it closes this transistor and opens the VT1 transistor, including the HTS. From this moment, the output current of the HTS IArt starts charging capacitor Cx. The input current of the comparators can be ignored, since compared to IArtit is extremely small. During charging, the voltage across the capacitor increases linearly. Simultaneously with turning on the GTS, the program starts two 32-bit timers of the microcontroller to determine the duration of the voltage rise on the capacitor to the thresholds of the comparators. At the moment of operation of each comparator, the voltage level at its output becomes high. Having fixed this, the program stops the corresponding timer. After the operation of both comparators, the measurement process ends, the program closes the transistor VT1, turning off the GTS with this, and opens VT3, discharging the measured capacitor through its open channel in order to prepare the prefix for the next measurement cycle. It then performs capacitance and ESR calculations and displays the results on the combined instrument's LCD screen. Capacity calculation formula: C=IArt (t2 - T1)/(U2 - OR1) where t1, T2 - the moments of reaching the voltage on the measured capacitor, respectively, the first and second threshold levels; U1, SHE IS2 - voltages of the first and second threshold levels. After calculating the capacitance, the program calculates the EPS. The method of its calculation is illustrated by the graphs in Figs. 2. The red line on it is the charging graph of the real measured capacitor. Due to the presence of EPS, the voltage on it at the moment charging starts abruptly increases to UR - voltage drop on the ESR of the capacitor when the charging current Icr flows through it. Threshold U1 and U2 the voltage on the capacitor reaches, respectively, at the moments t1 and t2. The blue line shows the charging curve of an ideal capacitor of the same capacitance (recall that the capacitance has already been measured). Since the ESR of an ideal capacitor is zero, the voltage across the capacitor begins to increase linearly from zero. The blue line runs parallel to the red line because the charging current IArt stabilized and does not depend on EPS. The voltage across an ideal capacitor would reach U2 at time t3, which can be determined by the formula t3 = U2 Cx/IArt. Now consider two triangles ABC and A'B'C. They are similar, therefore, you can make a proportion: B'C/BC = A'C/AC
From fig. 2 it follows that: BC=t2; AC=U2 - ORR; B'C = t3; A'C = U2. Substituting these values into the above proportion, we get t3 /t2 = U2 /(U2 - ORR). Given the formula for calculating t3 after simple transformations, it is easy to determine that the voltage drop across the EPS is equal to UR = U2 - TheArt (t2/Cx). And finally, we obtain the desired value of EPS by dividing by IArt the left and right sides of the previous formula: R = (U2/IArt) - (t2/Cx). This calculation can also be carried out on the first threshold, replacing the variables U2 and t2 respectively on U1 and t1. The found capacitance and ESR values of the measured capacitor are displayed by the program on the LCD screen of the combined instrument. The prefix is assembled on a printed circuit board with dimensions of 30x60 mm, the drawing of which is shown in fig. 3. It is designed for installation of surface mount components.
All resistors and capacitors are of size 1206. The prefix is connected to the XS1 connector of the device [1] with a flat cable with an X2 plug (PLS8). +2 V voltage from the instrument's internal power supply must be connected to pin 1 of connector XS5. Instead of the VS857C transistor, you can use another low-power transistor of the p-n-p structure with a base current transfer coefficient of at least 250, and instead of the VS847C transistor, any low-power transistor of the npn structure can be used. Both transistors must be in the SOT23 package, otherwise the PCB will need to be reworked. Replacing the transistor IRLL024Z - field-effect with an insulated gate and n-channel. It must be designed to control logic voltage levels, have an open channel resistance of no more than 50 ... 80 mOhm, a gate capacitance of no more than 500 ... /P can be replaced with LM850. The board is placed in any convenient case. It is convenient to use spring clamps as connector X1 for connecting the measured capacitor to the attachment. Setting up such devices is usually the most difficult stage in their manufacture. All devices for measuring capacitance and ESR, the descriptions of which I met, require an accurate selection of several parts, and some (for example, [3]) also perform a number of calculations and modify the microcontroller program for a specific instance of the manufactured device. This is a rather laborious process, therefore, when designing the set-top box in question, I replaced the hardware adjustment by measuring the values of the determining parameters and entering them into the operating device for further use. In other words, the part picking process has been replaced by a software calibration operation. Calibration results are stored in the EEPROM of the combined instrument panel's microcontroller, so it only needs to be done once. Calibration requires a multimeter capable of measuring DC 5...20 mA with an accuracy of at least two decimal places after the decimal point and a DC voltage of 0...2 V with an accuracy of at least three decimal places after the decimal point. These requirements are well met by most inexpensive digital multimeters. The program of version 2.05 attached to the article must be loaded into the microcontroller of the device. Connect the set-top box, to the X1 connector of which nothing is connected, to the device and apply power to it. The LCD screen will display the main menu shown in Fig. 4. Then let the device warm up for two to three minutes to establish thermal conditions. The capacitance and EPS measurement mode is entered by the third pressing of the "GN" key. This is not very fast and convenient, but there are no free keys on the device keyboard for a long time.
When you first switch to the capacitance and ESR measurement mode, the microcontroller program, not finding calibration coefficient values in its EEPROM that can be correctly interpreted, will automatically call the calibration subroutine. If this does not happen, call it by pressing the "2" key. The LCD screen will take the form shown in fig. 5.
The program will ask you to enter the values of four parameters in turn: the GTS current, the voltages of the first and second thresholds and the connection resistance, accompanying the requests with a detailed interactive menu. The exact value of each requested parameter should be measured with a multimeter and typed on the keyboard of the device. GTS current (IArt) is measured by connecting a multimeter in the current measurement mode to the X1 connector of the set-top box. It should lie within 10 ... 25 mA. Voltage U1 measured at pin 6 of the DA2 chip. Permissible limits - 0,2 ... 0,32 V. Voltage U2 measured at pin 2 of the same chip. Permissible limits - 0,42 ... 0,55 V. Set the connection resistance value to zero for now. This is the resistance of the connecting wires and connector contacts, with which the measured capacitor is connected to the attachment. Often it is comparable to the ESR of this capacitor. But we will talk about its accounting later. After entering all the required parameters, the inscription "CALIBRATED" will appear on the screen for 2 s and the device will switch to the capacitance and ESR measurement mode. The view of the LCD screen after switching to this mode is shown in fig. 6, and after the measurement - in fig. 7. If the measured ESR value is less than 0,01 ohm, then it is displayed as zero.
Now the device is operational and allows you to perform the last step of the calibration - determination of the connection resistance. To do this, connect a capacitor with a capacity of 1 ... 3300 μF to the X4700 connector and, by pressing the "D" button, start measuring its capacitance and ESR. Having remembered the measured ESR value, you should repeat the operation by connecting the same capacitor directly to the contact pads for the mentioned connector on the set-top box printed circuit board. The difference between the two obtained EPS values will be the value of the connection resistance. Now it remains to transfer the device to the calibration mode by pressing the "2" button and enter the obtained value into the program. The device is ready to work. The execution time of one measurement lies in the range of 3...6 s. It cannot be less than 3 s, since that is how much time is allotted in the program for discharging the measured capacitor. The measurement process itself takes no more than 3 s. In the course of measurements, messages can be displayed on the device screen about the measured capacitance value exceeding the upper or lower allowable limit, as well as about a malfunction of the attachment. The latter indicates a malfunction of the microcontroller interrupt system, which can occur during any manipulations with a working set-top box using devices that have mains power. To restore normal operation, the combined instrument panel must be switched off and on again. The described prefix makes it possible to measure low active resistance in the range of 0,01 ... 0,2 Ohm, which simple multimeters do poorly with. To do this, the measured resistor should be connected to connector X1 in series with the capacitor, the ESR of which was measured in advance. After measuring the ESR of such a circuit, the value of the ESR of the capacitor is subtracted from the result. The remainder is the resistance of the measured resistor. The device is transferred to other operating modes by pressing the "OS", "LA" or "GN" buttons. If the user has a capacitor at his disposal, the parameters of which are known in advance with high accuracy, it is advisable to measure them using a manufactured attachment in order to assess the correctness of its operation. If significant differences between the measured parameters and the known ones are found, their causes should be sought. These may be defective parts or errors in measuring and entering parameters into the program during calibration. The presence of defective parts either radically distorts the measurement results by several times, or leads to their significant jumps from measurement to measurement. The latter is typical for unstable comparators. With errors in measurement and input of calibration parameters, the results are stable, but not true. It is these errors that are the main sources of instrument error. Erroneous threshold values have a particularly strong influence on the result. Here, an error of 2 ... 3 mV leads to a change in the measured ESR value by several ohms. Without an accurate multimeter, but with a reference capacitor, the error can be eliminated experimentally by changing the input calibration parameters within small limits. The microcontroller program version 2.05 and the PCB file in Sprint Layout 5.0 format can be downloaded from ftp://ftp.radio.ru/pub/2017/02/2-05.zip. Literature
Author: A. Savchenko
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