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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING ESR meter for oxide capacitors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology The equivalent series resistance (ESR or ESR) of a capacitor is its most important parameter and largely determines its filtering and smoothing properties. Often the reason for the inoperability of various devices is the increased value of the ESR of the capacitors used in them. This parameter is especially unstable for oxide capacitors. It can change significantly upwards over time or with changes in temperature. The proposed article describes another EPS meter. The peculiarity of the device is that it is assembled on the basis of a compact pointer multimeter Sanwa YX-1000A (Fig. 1). A case, a pointer device, as well as an ohmmeter scale of this device were used from it, which simplifies the manufacture of the entire structure.
The measurement interval is from 0 to 100 ohms. The power source is a galvanic cell with a voltage of 1,5 V, size AA, the current consumption is 5 ... 7 mA, the operability is maintained when the supply voltage drops to 1,3 V. supply voltage), so the meter allows you to check oxide capacitors without soldering them out of the repaired device. The scheme of the device is shown in fig. one.
On the transformer T1 and transistors VT1, VT2, a rectangular pulse generator with a repetition rate of about 116 kHz is assembled. Winding II provides positive feedback. Trimmer resistor R2 can change the duty cycle of the pulses, achieving their symmetry. This is important because the duty cycle affects the current drawn by the device. From winding III, rectangular pulses enter the measuring circuit, consisting of probes XP1, XP2, which are connected to the measured capacitor, and resistor R4, which acts as a current sensor. A synchronous rectifier is assembled on the VT3 transistor assembly, control pulses come to it from the collectors of transistors VT1 and VT2, resistors R5-R7 are current-limiting, capacitors C3, C4 smooth out the rectified voltage. Thanks to the use of a synchronous rectifier, it was possible to obtain high sensitivity and low losses of the rectified voltage, which, in turn, made it possible to use one galvanic cell as a power source. A pointer device RA1 is connected to the output of the rectifier, a variable resistor R8 is a calibration resistor. When connecting the probes to the tested capacitor, the voltage across the resistor R4 depends on the ESR of the capacitor - the larger the ESR, the lower the voltage and the smaller the deviation of the arrow of the RA1 device. If the tested capacitor was charged, the discharge current will limit the resistor R4, and the diodes VD1 and VD2 will protect the transistor assembly VT3. Since the resistance of the microammeter frame is several times greater than the input resistance of the resistor R8, and it is wound with a copper wire, when the ambient temperature changes, the current through it even at a constant voltage changes. Therefore, a calibration resistor R8 is introduced into the device, with the help of which, with the probes closed, the arrow of the device is set to "0" of the scale. Calibration is also necessary as the battery runs out. As a basis for the design of the meter, a SanwaYX-1000A pointer multimeter was used. A case and a pointer device were used - a microammeter, which has a frame resistance of 876 ohms, a current of maximum deflection of the pointer - 146 μA, and a voltage on it at a maximum current of 130 mV. The remaining parts are mounted on a printed circuit board, the drawing of which is shown in Fig. 3. It is made of one-sided foil fiberglass.
Fixed resistors C2-23 are used, trimmer - SPZ-3, variable - SP4-1, capacitor C2 - KT-2 with TKE is not worse than M75, since this capacitor affects the stability of the generated frequency, the rest - K10-17. KSA539 transistors can be replaced by transistors of the KT3107 series with indexes B, G and E, it is desirable to select them with similar current transfer coefficients. It is not recommended to replace the transistor assembly with individual transistors, since this will require their careful selection. The transformer is wound on a ring ferrite magnetic circuit with a permeability of 1000 with an outer diameter of 10, an inner diameter of 6 and a thickness of 5 mm. Before winding, the edges are smoothed with sandpaper or a file. Windings I and II are wound simultaneously with three PEV or PEL winding wires twisted together with a diameter of 0,1 mm. After winding 50 turns, two wires are connected in accordance with the diagram - this is how winding I is formed. Winding III is wound with PEV-2 wire with a diameter of 0,3 ... 0,4 mm and contains 5 turns. The phasing of this winding can be any and will only affect the polarity of connecting the PA1 microammeter (the polarity is shown conditionally in the diagram). All windings must be distributed evenly on the magnetic circuit. A piece of PVC tube is tightly inserted into the opening of the transformer, slightly longer than the thickness of the wound transformer. Two washers with a diameter of 1 ... 10 mm are cut out of thick (12 mm) soft plastic, between which the transformer is fixed with little effort on the board with an M3 screw, and the nut is fixed with hot glue. All parts were removed from the multimeter board, after which it was used as a stencil for the manufacture of a new printed circuit board. Resistor R8 and power switch SA1 are fixed on the side walls of the case with hot glue (Fig. 4).
The switch used is an imported small-sized slide switch and is installed in a slot in the housing, designed for the slide of the trimming resistor for setting the zero of the ohmmeter. A hole has been made for the R8 resistor slider. The switch for the measurement limits of the multimeter was removed, and the resulting hole was sealed with a rectangular plate of thin fiberglass. The wires for the probes were used from a computer power supply, two long pins with heads were soldered to their ends, and several millimeters of wire insulation were fastened to the pins with threads and impregnated with universal glue. As practice has shown, this design of the probes turned out to be quite convenient. The adjustment begins with setting the minimum current consumption in the power circuit. To do this, an ammeter is switched on in series with the battery (probes XP1 and XP2 should be open at the same time) and the minimum current consumption is set with a trimming resistor R2. Then, with the probes closed, the variable resistor R8 sets the arrow of the device to "0" of the scale (the rightmost position). By connecting resistors with a known resistance (from units to tens of ohms) to the probes, they check the compliance of the instrument readings and the resistance of the resistors. If necessary, select the resistor R4. If the instrument readings are higher, a resistor with a higher resistance is installed, and vice versa. Due to the fact that the standard scale of the multimeter is used, the accuracy in its various sections will be different, so you need to choose which of the readings should be the most accurate. Based on this, a resistor with such a resistance is connected to the probes and a selection of resistor R4 sets the arrow of the device to a mark corresponding to this resistance. According to the author, such resistance can be 5 ... 6 ohms. During the operation of the device, one effect appeared related to the design of the pointer device. A charge of static electricity accumulates on its protective glass, which can stop the arrow in an arbitrary place, thereby making the further operation of the device almost impossible. To eliminate this effect, a refinement was carried out. If the scale is fixed unevenly and there are bulges, it is removed, straightened and firmly glued into place with a minimum amount of glue. The arrow is carefully bent so that it moves at a minimum distance from the scale and, therefore, at a maximum distance from the protective glass. It is also useful to install arrow travel stops made of enameled copper wire 0,2 ... 0,4 mm thick, which are fixed on both sides under the scale mounting screws. Attention! When measuring the ESR of capacitors, some care must be taken, as there is a possibility of electric shock to a charged capacitor! Author: A. Mulyndin, Alma-Ata, Kazakhstan
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