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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Probe of oxide capacitors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology When repairing modern household appliances, one of the most difficult defectological processes is determining the health of capacitors. And they "age" much faster than other radioelements. This article is devoted to the problem of fast and reliable identification of a faulty element during repair. The reliability of semiconductor devices in modern equipment has increased so much that oxide-electrolytic capacitors have taken the first place in terms of the number of defects [1]. This is due to the presence of an electrolyte in them. Exposure to elevated temperature, dissipation of power losses in the capacitor, depressurization in the housing seals lead to the drying of the electrolyte. An ideal capacitor, when operating in an alternating current circuit, has only reactive (capacitive) resistance. The real capacitor, for the case considered below, can be represented as an ideal capacitor and a resistor connected in series with it. This resistor is called the equivalent series resistance of the capacitor (hereinafter referred to as ESR, in the English literature you can find a similar term with the abbreviation ESR - Equivalent Series Resistance). At the initial stage of the occurrence of defects in oxide capacitors, the ESR of the capacitor is overestimated. Because of this, the power loss increases, heating the capacitor from the inside. This power is directly proportional to the ESR of the capacitor and the square of its recharge current. In the future, the process progresses rapidly, up to a complete loss of capacitance by the capacitor. The appearance of defects in products where oxide capacitors are used can be at different stages of this process. It all depends on the operating conditions of the capacitor, including its electrical modes and the features of the device itself. The difficulty in diagnosing such defects is that capacitance measurements with conventional instruments in most cases do not give results, since the capacitance is within the normal range or only slightly underestimated. Especially demanding on the quality of oxide capacitors are power supplies with high-frequency converters, where such capacitors are used as filters, and in switching circuits of power elements at frequencies up to 100 kHz. The ability to measure ESR would make it possible to both identify failed capacitors (except for short circuits and leaks), and early diagnosis of device defects that have not yet manifested themselves. To do this, you can measure its complex resistance at a sufficiently high frequency, at which the capacitance is much lower than the allowable ESR. For example, at a frequency of 100 kHz, a 10 microfarad capacitor has a capacitance of about 0,16 ohms, which is already a fairly small value. If a signal of such a frequency is applied through a current-setting resistor to a controlled capacitor, the voltage across the latter will be proportional to the modulus of its complex resistance. The signal source can be any suitable generator, and the shape of the signal does not play a special role, and the output impedance of the generator can serve as a resistor. An oscilloscope or an AC millivoltmeter can be used to measure the voltage across a capacitor. So, with a generator output signal level of 0,6 V, a 600 Ohm resistor on a capacitor with an ESR equal to 1 Ohm, the measured voltage will be about 1 mV, and with a 50 Ohm resistor resistance - 12 mV. The practice of diagnosing defects in oxide capacitors by measuring ESR has shown that in the vast majority of cases in defective capacitors with a capacity of 10 to 100 μF, it significantly exceeds 1 Ohm. This criterion is not strict and depends on several factors. It is generally accepted that good capacitors have an ESR in the range of 0,3 ... 6 Ohm, depending on the capacitance and operating voltage [2]. The measurement accuracy for determining defective capacitors does not play a special role. An error of up to 1,5 ... 2 times can be considered quite acceptable. These data were used in the development of the device described below. In addition, it is very important to be able to measure without removing the capacitors from the device. To do this, it is necessary that the controlled capacitor is not shunted by elements with a resistance close to the measured ESR values, which is done in most cases. Semiconductor devices do not affect the measurement results, since the measuring voltage on the capacitor is units and tens of millivolts. It is also desirable to limit the maximum voltage on the probes of the device to 1...2 V and the current through them to 3...5 mA, so as not to disable other elements of the device. As for the design of the device, obviously, it should be self-powered and small in size. Connecting conductors and clamps for connection to the tested capacitors are undesirable. When working with them, both hands are busy, you need a place to place the device itself and you have to constantly look from the measurement points to the indicator of the device. These requirements are met by a small probe with pointed probes. Main Specifications
Additionally, the probe can be used to assess the capacitance of electrolytic capacitors - in the author's version, from 15 to 90 microfarads. The schematic diagram of the probe is shown in fig. one.
On the element DD1.1 of the digital microcircuit, a generator of rectangular pulses (frequency-setting elements R2, C2) is made. The outputs of the remaining elements are combined to increase the load capacity. Resistors R3, R4 and the internal resistance of the elements set the current through the tested capacitor Cx, from which a signal with a level proportional to the ESR of the controlled capacitor is fed to the input of the preamplifier on the transistor VT1. The zener diode VD1 limits the voltage pulses when the probes of the device are connected to undischarged capacitors. Residual voltages on them no more than 25 ... 50 V are not dangerous for the device. The DA1 chip has a five-step LED level indicator, such a chip is used in some VCRs. The microcircuit includes an input signal amplifier, a linear detector, comparators with current stabilizers at the outputs. The ratios of the input signal levels at which the next comparator turns on correspond to -10; -5; 0; 3; 6 dB. Thus, the entire display range covers 16 dB. To ignite all the LEDs, a signal with a level of about 1 mV must be applied to the input of the DA8 chip (pin 170). The RC circuit connected to pin 7 determines the time constant of its detector. Resistor R12 limits the current consumed by the LEDs. Criteria for choosing its value: the required brightness of the LEDs on the one hand and the current consumed from the power source on the other. Elements R6, C6 and R11, C7 are filters in the power circuits of the corresponding nodes. The possibility of using the chip at frequencies up to 100 kHz was determined experimentally. The minimum passport value of the supply voltage of the microcircuit is 3,5 V, however, checking several copies showed their performance up to a voltage of 2,7 V, with a further decrease in it, the LEDs stop glowing. The device indicates the controlled value of the EPS according to the principle: the lower the resistance, the lower the number of lit LEDs. When the contacts of the switch SA1 are closed, capacitor C2 is also connected in parallel with the capacitor C1. In this case, the generator frequency will be reduced to approximately 1200 Hz, so the signal level at the terminals of the tested capacitor will depend mainly on its capacitance. The higher the capacitance, the lower the number of LEDs lit. Chip resistors and capacitors are used in the device, but other small sizes can be used. Capacitors C3-C5, C8, C10 - imported ceramic small-sized. Their capacity is not critical. VD2-VD6 LEDs are micro-consuming, they glow quite brightly already at a current of 0,5 ... 1 mA. You can use other red LEDs that meet the specified requirement, for example, KIPD-05A. Switch SA1 - small-sized sliding, SB1 - push-button, without fixing in the pressed position. Transistor VT1 can be replaced by KT315, KT3102 (with any letter indices) with a current transfer coefficient of more than 100. The probe is powered by two alkaline elements LR44 (357, G13) with a size of 11,6x5,4 mm. The operating frequency of the generator is controlled by resistor R3. It should be within 60 ... 80 kHz. If necessary, it is installed by selecting elements R2 or C2. The voltage at the collector of the transistor VT1 should be within 1,0 ... 1,7 V, it is set by selecting the resistor R8. The probe is calibrated by connecting non-inductive (non-wire) resistors to the probes in the ESR measurement mode and selecting resistor R3. The required capacitance control range in the closed position of the SA1 switch contacts is set by selecting the capacitor C1, connecting capacitors with a known capacitance to the probes.
The appearance of the probe is shown in fig. 2.
The probes are made of hard steel wire with a diameter of 1 mm, the ends are slightly curved and pointed. The distance between the probes is 4 mm, which allows, taking into account the dimensions of the contact pads on the printed circuit board, to check capacitors with a distance between the leads from 2,5 to 7,5 mm. The apparent inconvenience associated with the orientation of the position of the device relative to the terminals of the capacitors disappears after a few days of using it. During measurements, the product under test must be de-energized, the capacitors, on which dangerous voltages can be stored, must be discharged. The probe probes must be pressed against the contact pads of the board, to which the tested capacitor is soldered, and the power button must be pressed. Due to transients, all LEDs flash for a short time, after which the state of the capacitor can be estimated by the number of LEDs lit. Thus, the turn-on time of the probe to test one capacitor does not exceed 1 s. For good capacitors with a capacity of 10 uF and higher for operating voltages up to 100 V, all LEDs should go out. Capacitors of a smaller capacity and for a higher operating voltage have a higher ESR, so 1-2 LEDs can light up. The criteria for evaluating the suitability of oxide capacitors depend on the functions they perform in the units of the apparatus, electrical modes, and operating conditions. The most critical nodes: the key transistor control circuit in power supplies with high-frequency conversion, filters in such sources, including those powered by a horizontal scan transformer of TVs and monitors, a filter in the power supply circuit of the "buildup" of the horizontal scan transistor, etc. The higher operating frequency and recharging currents, the better the capacitors used should be. In the above circuits, capacitors with a temperature range up to 105 °C should be used, which have a significantly lower ESR and higher reliability at elevated temperatures. In the absence of such elements at hand, it is desirable to shunt oxide capacitors with ceramic capacitors with a capacity of 0,33-1 μF. Sometimes such capacitors are installed by the manufacturer of the device. They can distort the readings of the probe in the ESR measurement mode (the capacitance of the capacitor is 1 μF at a frequency of 80 kHz - about 2 ohms). It happens that defective capacitors, after soldering them out of the board, can be identified as serviceable by the device when dialing. Apparently, this is due to the effect of high temperature during dismantling. There is no point in installing such capacitors back into the device - the defect will reappear sooner or later. This is another argument in favor of testing capacitors without dismantling them. The device was created as a "workhorse", which is convenient to use in almost any conditions, has no frills and is intended not so much for measurements as for determining according to the "good - bad" principle. Therefore, in doubtful and especially critical cases, it is necessary to additionally check the capacitors using available methods or replace them with known good ones. The operation of the probe in a TV repair shop for 6 months showed the optimality of its metrological parameters and the selected type of indication. The performance in diagnostics has sharply increased, especially in devices that have worked for more than 5-7 years, it has become possible to early diagnose defects associated with a gradual deterioration in the state of oxide capacitors. The batteries of the probe did not have to be changed during this period. The range of controlled values of the ESR of the probe can be extended towards lower resistances by increasing the current through the tested capacitor. To do this, you need to replace the DD1 chip with the KR1554TLZ, which will increase the output current of the generator by reducing the resistance of the resistor R3. It is enough to use only one element of the microcircuit in the generator by connecting its output to the left, according to the scheme, output of the resistor R3. Connect the inputs of unused elements (pins 4, 5, 9, 10, 12, 13) to a common wire. The current consumed by the device will increase. In this way, you can reduce the lower limit of EPS control to 0,5 ... 1 Ohm. To cover the recommended range of ESR values, a limit switch will have to be introduced using two switchable resistors instead of one resistor R3. You can add another capacitance measurement range by using the SA1 three-position switch and adding another capacitor similar to C1. Recommended ranges: 7...40 and 40...220 uF (generator frequency - approximately 2400 and 550 Hz). In the capacitance measurement mode, an audio frequency signal is present on the probes of the device. It can be used to test acoustic transducers or to check the signal flow in 3H amplifiers. Literature
Author: R. Khafizov, Sarapul, Udmurtia
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