Features of the device
Anyone who regularly repairs electronic equipment knows what percentage of malfunctions are caused by defective electrolytic capacitors. Moreover, if a significant loss of capacity can be diagnosed using a conventional multimeter, then such a very characteristic defect as an increase in equivalent series resistance (ESR) is fundamentally impossible to detect without special devices.
For a long time, when carrying out repair work, I managed to do without specialized instruments for checking capacitors by substituting known good ones in parallel with the “suspected” capacitors; in audio equipment, use checking the signal path by ear using headphones, and also use indirect defect detection methods based on personal experience , accumulated statistics and professional intuition. When we had to join the mass repair of computer equipment, in which electrolytic capacitors account for a good half of all malfunctions, the need to control their ESR became, without exaggeration, a strategic task. Another significant circumstance was the fact that during the repair process, faulty capacitors very often have to be replaced not with new ones, but with dismantled ones from other devices, and their serviceability is not at all guaranteed. Therefore, the moment inevitably came when I had to seriously think about solving this problem by finally acquiring an EPS meter. Since purchasing such a device was obviously out of the question for a number of reasons, the only obvious solution was to assemble it yourself.
An analysis of circuit solutions for constructing EPS meters available on the Internet has shown that the range of such devices is extremely wide. They differ in functionality, supply voltage, used element base, frequency of generated signals, presence/absence of winding elements, form of displaying measurement results, etc.
The main criteria for choosing a circuit were its simplicity, low supply voltage and a minimum number of winding units.
Taking into account the whole set of factors, it was decided to repeat Yu. Kurakin’s scheme, published in an article from the magazine “Radio” (2008, No. 7, pp. 26-27). It is distinguished by a number of positive features: extreme simplicity, absence of high-frequency transformers, low current consumption, the ability to be powered by a single galvanic cell, low frequency of generator operation.
Details and design. The device, assembled on a prototype, worked immediately and after several days of practical experiments with the circuit, a decision was made on its final design: the device should be extremely compact and be something like a tester, allowing the measurement results to be displayed as clearly as possible.
For this purpose, a dial indicator of the M68501 type from the Sirius-324 Pano radio with a total deviation current of 250 μA and an original scale calibrated in decibels, which was at hand, was used as a measuring head. Later, I discovered similar solutions on the Internet using tape level indicators made by other authors, which confirmed the correctness of the decision made. As the body of the device, we used the case from a faulty LG DSA-0421S-12 laptop charger, which is ideal in size and has, unlike many of its counterparts, an easily disassembled case held together with screws.
The device uses exclusively publicly available and widespread radio elements available in the household of any radio amateur. The final circuit is completely identical to the author's, with the only exception being the values of some resistors. The resistance of resistor R2 should ideally be 470 kOhm (in the author’s version - 1 MOhm, although approximately half of the engine stroke is still not used), but I did not find a resistor of this value that has the required dimensions. However, this fact made it possible to modify resistor R2 in such a way that it simultaneously acts as a power switch when its axis is rotated to one of the extreme positions. To do this, it is enough to scrape off with the tip of a knife part of the resistive layer at one of the outer contacts of the resistor “horseshoe”, along which its middle contact slides, over a section of approximately 3...4 mm in length.
The value of resistor R5 is selected based on the total deflection current of the indicator used in such a way that even with a deep discharge of the battery, the EPS meter remains operational.
The type of diodes and transistors used in the circuit is absolutely uncritical, so preference was given to elements with minimal dimensions. The type of capacitors used is much more important - they should be as thermally stable as possible. As C1...C3, imported capacitors were used, which were found in the board from a faulty computer UPS, which have a very small TKE and have much smaller dimensions in comparison with domestic K73-17.
The inductor L1 is made on a ferrite ring with a magnetic permeability of 2000 Nm, having dimensions of 10 × 6 × 4.6 mm. For a generation frequency of 16 kHz, 42 turns of PEV-2 wire with a diameter of 0.5 mm are required (the length of the winding conductor is 70 cm) with an inductance of 2.3 mH. Of course, you can use any other inductor with an inductance of 2...3.5 mH, which will correspond to the frequency range of 16...12 kHz, recommended by the author of the design. When making the inductor, I had the opportunity to use an oscilloscope and an inductance meter, so I selected the required number of turns experimentally solely for reasons of bringing the generator exactly to a frequency of 16 kHz, although, of course, there was no practical need for this.
The probes of the EPS meter are made non-removable - the absence of detachable connections not only simplifies the design, but also makes it more reliable, eliminating the potential for broken contacts in the low-impedance measuring circuit.
The printed circuit board of the device has dimensions of 27x28 mm, its drawing in .LAY6 format can be downloaded from the link https://yadi.sk/d/CceJc_CG3FC6wg. The grid pitch is 1.27 mm.
The layout of the elements inside the finished device is shown in the photo.
Test results. A distinctive feature of the indicator used in the device was that the ESR measurement range was from 0 to 5 Ohms. When testing capacitors of significant capacity (100 μF or more), most typical for filters in power supply circuits of motherboards, power supplies for computers and TVs, laptop chargers, network equipment converters (switches, routers, access points) and their remote adapters, this range is extremely convenient , since the instrument scale is maximally stretched. Based on the averaged experimental data for the ESR of electrolytic capacitors of various capacities shown in the table, the display of measurement results turns out to be very clear: the capacitor can be considered serviceable only if the indicator needle during measurement is located in the red sector of the scale, corresponding to positive decibel values. If the arrow is located to the left (in the black sector), the capacitor from the above capacitance range is faulty.
Of course, the device can also test small capacitors (from about 2.2 μF), and the device readings will be within the black sector of the scale, corresponding to negative decibel values. I got approximately the following correspondence between the ESR of known-good capacitors from a standard series of capacitances and the calibration of the instrument scale in decibels:
First of all, this design should be recommended to novice radio amateurs who do not yet have sufficient experience in designing radio equipment, but are mastering the basics of repairing electronic equipment. The low price and high repeatability of this EPS meter distinguish it from more expensive industrial devices for similar purposes.
The main advantages of the ESR meter can be considered the following:
— extreme simplicity of the circuit and availability of the element base for its practical implementation while maintaining sufficient functionality of the device and its compactness, no need for a highly sensitive recording device;
— no need for adjustments that require special measuring instruments (oscilloscope, frequency meter);
- low supply voltage and, accordingly, low cost of its source (no expensive and low-capacity “Krona” is required). The device remains operational when the source is discharged even to 50% of its rated voltage, that is, it is possible to use elements to power it that are no longer capable of functioning normally in other devices (remote controls, watches, cameras, calculators, etc.);
- low current consumption - about 380 µA at the time of measurement (depending on the measuring head used) and 125 µA in standby mode, which significantly extends the life of the power source;
- minimal quantity and extreme simplicity of winding products - any suitable choke can be used as L1 or you can easily make it yourself from scrap materials;
— a relatively low frequency of generator operation and the ability to manually set zero, allowing the use of probes with wires of almost any reasonable length and arbitrary cross-section. This advantage is undeniable in comparison with universal digital element testers that use a ZIF panel with deep contacts to connect the capacitors being tested;
— visual clarity of the display of test results, allowing you to quickly assess the suitability of the capacitor for further use without the need for an accurate numerical assessment of the ESR value and its correlation with a table of values;
— ease of use — the ability to perform continuous measurements (unlike digital ESR testers, which require pressing the measurement button and pausing after connecting each capacitor being tested), which significantly speeds up the work;
— it is not necessary to pre-discharge the capacitor before measuring ESR.
The disadvantages of the device include:
- limited functionality in comparison with digital ESR testers (lack of ability to measure capacitance of the capacitor and the percentage of its leakage);
— lack of exact numerical values of measurement results in ohms;
- relatively narrow range of measured resistances.
Recently, in amateur radio and professional literature, a lot of attention has been paid to such devices as electrolytic capacitors. And it’s not surprising, because frequencies and powers are growing “before our eyes,” and these capacitors bear a huge responsibility for the performance of both individual components and the circuit as a whole.
I would like to warn you right away that most of the components and circuit solutions were gleaned from forums and magazines, so I do not claim any authorship on my part; on the contrary, I want to help novice repairmen figure out the endless circuits and variations of meters and probes. All the diagrams provided here have been assembled and tested more than once, and appropriate conclusions have been drawn regarding the operation of this or that design.
So, the first scheme, which has become almost a classic for beginner ESR Metrobuilders “Manfred” - this is how forum users kindly call it, after its creator, Manfred Ludens ludens.cl/Electron/esr/esr.html
It was repeated by hundreds, and maybe thousands of radio amateurs, and were mostly satisfied with the result. Its main advantage is a sequential measurement circuit, due to which the minimum ESR corresponds to the maximum voltage on the shunt resistor R6, which, in turn, has a beneficial effect on the operation of the detector diodes.
I did not repeat this scheme myself, but came to a similar one through trial and error. Among the disadvantages, we can note the “walking” of zero on temperature, and the dependence of the scale on the parameters of the diodes and op-amp. Increased supply voltage required for device operation. The sensitivity of the device can be easily increased by reducing resistors R5 and R6 to 1-2 ohms and, accordingly, increasing the gain of the op-amp; you may have to replace it with 2 higher speed ones.
My first EPS sampler, which still works well to this day.
The circuit has not been preserved, and one might say that it never existed; I collected from all over the world, bit by bit, what suited me from the circuit design, however, the following circuit from a radio magazine was taken as a basis:
The following changes have been made:
1. Powered by mobile phone lithium battery
2. The stabilizer is excluded, since the operating voltage limits of the Lithium Battery are quite narrow
3. Transformers TV1 TV2 are shunted with 10 and 100 Ohm resistors to reduce emissions when measuring small capacities
4. The output of 561ln2 was buffered by 2 complementary transistors.
In general, the device turned out like this:
After assembling and calibrating this device, 5 Meredian digital telephone sets, which had been lying in a box labeled “hopeless” for 6 years, were immediately repaired. Everyone in the department started making similar samples for themselves :).
For greater versatility, I added additional functions:
1. infrared radiation receiver, for visual and auditory testing of remote controls (a very popular function for TV repairs)
2. illumination of the place where the probes touch the capacitors
3. “vibrick” from a mobile phone, helps to localize bad soldering and microphone effects in details.
Remote control video
And recently on the “radiokot.ru” forum, Mr. Simurg posted an article dedicated to a similar device. In it, he used a low-voltage supply, a bridge measurement circuit, which made it possible to measure capacitors with ultra-low ESR levels.
His colleague RL55, taking the Simurg circuit as a basis, extremely simplified the device, according to his statements, without deteriorating the parameters. His diagram looks like this:
The device below, I had to assemble hastily, as they say, “out of necessity.” I was visiting relatives, and the TV there was broken and no one could repair it. Or rather, it was possible to repair it, but for no more than a week, the horizontal transistor was on all the time, there was no TV circuit. Then I remembered that I had seen a simple test kit on the forums, I remembered the circuit by heart, a relative was also a little involved in amateur radio, he “riveted” audio amplifiers, so all the parts were quickly found. A couple of hours of puffing with a soldering iron, and this little device was born:
In 5 minutes, 4 dried electrolytics were localized and replaced, which were determined by a multimeter to be normal, and a certain amount of the noble drink was drunk for success. After repair, the TV has been working properly for 4 years.
A device of this type has become like a panacea in difficult times when you don’t have a normal tester with you. It is assembled quickly, repairs are made, and finally it is solemnly presented to the owner as a souvenir, and “in case something happens.” After such a ceremony, the soul of the payer usually opens twice, or even three times wider :)
I wanted something synchronous, I started thinking about the implementation scheme, and now in the magazine “Radio 1 2011”, as if by magic, an article was published, I didn’t even have to think. I decided to check what kind of animal it was. I assembled it and it turned out like this:
The product did not cause any particular delight, it works almost like all the previous ones, there is, of course, a difference in the readings of 1-2 divisions, in certain cases. Maybe its readings are more reliable, but a probe is a probe, and this has almost no effect on the quality of defect detection. I also equipped it with an LED so that I could see “where are you putting it?”
In general, you can do repairs for the sake of your soul. And for accurate measurements, you need to look for a more solid ESR meter circuit.
Well, lastly, on the website monitor.net, member buratino posted a simple project on how you can make an ESR probe from an ordinary cheap digital multimeter. The project intrigued me so much that I decided to try it, and this is what came out of it.
The case is adapted from a marker. The weakest point in any radio circuit is the electrolytic capacitors, which are subject to constant drying out. And the greater the currents pass through them, the faster this process. It is impossible to determine a bad capacitor with a regular ohmmeter, so you need a special device - an esr meter.
Electrical circuit for esr capacitor meter
Printed circuit boards - drawing
In a typical circuit, there may be 10 or even 100 capacitors. Desoldering each one for testing is very tedious and there is a high risk of damaging the board. This tester uses low voltage (250 mV) high frequency (150 kHz) and is capable of measuring the ESR of capacitors directly in the circuit. The voltage is chosen low enough so that other surrounding radio elements of the circuit do not affect the measurement results. And if you accidentally happen to test a charged capacitor, it doesn’t matter. This meter can withstand up to 400V charge on the capacitor. Experience has shown that the ESR meter identifies about 95% of capacitors with potential problems.
Features of the device
- Electrolytic capacitor test > 1 µF.
- Polarity is not important for testing.
- Transfers capacitor charge up to 400V.
- Low current consumption from the battery - about 25 mA.
- Easy to read analog meter data.
- Measures ESR in the range from 0-75 Ohms on an extended scale using an ohmmeter.
How to use an ESR meter
Turn on the device. Make sure that the circuit being tested is not energized. Discharge the capacitor before testing - the ESR meter does not do this automatically. Short-circuit the terminals of the capacitor and hold them there for a few seconds. Use a voltmeter to make sure the capacitor is completely discharged. The voltmeter should show zero reading. Touch the ESR meter probes to the capacitor. Determine ESR resistance. We find out whether the ESR value is acceptable by comparing the measured ESR with the reference data. View this table
It's been about a year and a half since I started doing electronics repairs regularly. As it turned out, this matter is no less interesting than the design of electronic structures. Little by little people appeared who wanted, some from time to time and some regularly, to collaborate with me as a master. Due to the fact that the profitability of most of the repairs carried out does not allow renting premises, otherwise rent eats up most of the profit, I work mainly at home or go with tools to familiar individual entrepreneurs who buy consumer electronics and have a workshop.
These are absolutely any circuits using stabilizers, DC-DC power converters, switching power supplies for any equipment, from computers to mobile chargers.
Swollen capacitor
Without this device, a significant part of the repairs I performed either could not have been performed at all, or was still performed, but with great inconvenience in the form of constant soldering and soldering back electrolytic capacitors of small value, in order to measure the equivalent series resistance using a transistor tester. My device allows you to measure this parameter without desoldering the part, simply by touching the terminals of the capacitor with tweezers.
These capacitors with a nominal value of 0.33-22 uF, as is known, very rarely have notches in the upper part of the case, along which capacitors of a higher nominal value swell and open like a rose, for example, the familiar capacitors on motherboards and power supplies. The fact is that a capacitor that does not have these notches to release the excess pressure generated is visually, without measuring with a device, even for an experienced electronics engineer, in no way distinguishable from a fully working one.
Of course, if a home craftsman needs a one-time repair, for example, an ATX computer power supply, there is no point in assembling this device; it is easier to immediately replace all the small capacitors with new ones, but if you repair at least five power supplies every six months, this device is already desirable for you assembly. What alternatives are there to assembling this meter? A purchased device that costs about 2000 rubles, ESR micro.
ESR micro - photo
Of the differences and advantages of a purchased device, I can only name that its readings are displayed immediately in milliOhms, while my device needs to be converted from milliVolts to milliOhms. Which, however, does not cause any difficulties, it is enough to calibrate the device using the values of low-resistance precision resistors and create a table for yourself. After working with the device for a couple of months, visually, without any tables, just by looking at the display of the multimeter you can already see the normal value of the ESR capacitor - on the verge or replacement is already necessary. The diagram of my device, by the way, was once taken from Radio magazine.
Schematic diagram of the device
Initially, the device was assembled with homemade probes - tweezers with wide jaws, inconvenient when measuring on boards, with tight installation. Then I looked at express probes on Ali - tweezers for measuring SMD, connected to a multimeter. Having ordered tweezers, the wire was mercilessly shortened so that the accuracy would not be greatly affected during the measurement due to the length of the probe wires. Don't forget, the count is in milliOhms.
At first, my device was connected with probes to a multimeter and was made in the form of an attachment, but gradually I got tired of turning the multimeter knob every time, thereby exhausting the switching life. It was then that a friend gave me a multimeter, due to the fact that I temporarily burned mine on an undischarged electrolytic capacitor. Subsequently, the device was restored, the resistors were re-soldered, and this multimeter, its connectors for connecting the probes on the board were broken off, and jumpers were thrown by someone, but the accuracy of the measurements was no longer the same.
But for my purposes, an error of 1-2 percent did not solve anything, and I decided to make the device completely autonomous. To do this, I fastened the case of the multimeter and the case of the ESR meter with screws, and for greater convenience, switched the simultaneous switching of the built-in multimeter and the ESR meter using a switch into two groups of contacts. Connections between the multimeter and the ESR meter, previously made using probes, were made by wires inside connected housings.
Capacitor tester - appearance
As practice has shown, the time required to bring the device into combat readiness, and then, after taking measurements, to turn it off, began to take significantly less time, and the ease of use has accordingly increased. Among the further improvements planned for this device is to switch it to battery power, from a Li-ion battery from a phone, with the ability to recharge from a charge adapter board via a built-in Mini USB socket, from any charger from a smartphone with the ability to connect a USB cable.
As practice has shown, I had previously converted it to battery power using a similar method, which, like the ESR meter, also has high consumption due to the graphic display installed in it. Feelings from the remodel were only positive. I only charged it once in six months. The device was equipped with a step-up DC-DC converter that converts 3.7 volts at the battery output into 9 volts, necessary for the operation of the device.
In this case, my device will have a double voltage conversion: first from 3.7 volts to 9 volts, although I may also set the minimum voltage allowed for the input of the 7805 CV stabilizer to 7.5 volts; the device circuit is now powered from this stabilizer. The device itself, as can be seen in the photo, is initially powered by a Krohn battery, which, as is known, has a relatively small capacity.
The supply voltage of this microcircuit allows it to be powered directly from 9 volts, but the fact is that as the battery discharges, I noticed that the measurement readings begin to slowly float away. To combat this, a 7805 stabilizer was installed, which, as you know, produces a stable 5 volt output.
Also, due to the fact that you often have to carry the device with you in a briefcase, for repairs on the road, and there have already been cases of spontaneous switching on of the switch, and accordingly the Krona battery was drained to zero, which now, when switching with this switch, 2 power lines, a multimeter and the device itself would be no longer desirable, since in this case, you would have to buy two crowns, costing 45 rubles.
It was decided to simply glue two self-tapping screws from the cooler mount in the computer power supply onto the edges of the switch. The microcircuit used in the device is widespread and quite cheap; I purchased it for only about 15-20 rubles.
The whole device cost me, taking into account the free multimeter, probes - tweezers, costing 100 rubles, and the cost of parts for assembling the device, and the Krona battery, in total it took about 150 rubles, in total everything needed cost a ridiculous amount of 250 rubles.
Tweezers for measuring capacitors on a board
Which has already paid off with the use of the device in repairs for a long time and many times. Of course, someone who has the opportunity and desire to purchase an ESR micro can now say why I need these inconveniences, every time converting from milliVolts to milliOhms, although this is not required, as I wrote above, if I can immediately see on the purchased device , ready-made values.
ESR Value Table
The fact is that such devices incorporate a microcontroller, and during measurement they are connected directly, so to speak, by the “port” of the microcontroller to the capacitor being measured. What is extremely undesirable, it is enough not to discharge the capacitor once after de-energizing the circuit before measurement, by shorting its terminals with a metal object, for example a screwdriver, as we risk getting a non-working device.
First version of the probes
Which, given its rather high cost, you will agree, is not the best option. In my device, a 100 Ohm resistor is connected in parallel to the capacitor being measured, which means that if the capacitor is nevertheless charged, it will begin to discharge when the probes are connected. In the worst case scenario, if the microcircuit used in my device burns out, to make repairs you will only need to remove the microcircuit from the DIP socket and plug in a new one.
Device upgrade
That's it, the repair of the device is completed, you can take measurements again. And given the low cost of the microcircuit, this does not become a problem; it is enough to just purchase one or two microcircuits in reserve when purchasing parts for assembling this EPS meter.
Final version
In general, the device turned out to be simply chic and very convenient, and even if the parts for its assembly cost 2 times more, I would still safely recommend this EPS meter for assembly to all novice craftsmen who have a modest budget, or who want to save money and don't overpay. Happy repairs everyone! AKV.
In a simplified form, an electrolytic (oxide) capacitor consists of two aluminum strip plates, separated by a gasket made of porous material impregnated with a special composition - electrolyte. The dielectric in such capacitors is a very thin oxide film that forms on the surface of aluminum foil when a voltage of a certain polarity is applied to the plates. Wire leads are attached to these tape covers. The tapes are rolled into a roll, and the whole thing is placed in a sealed housing. Due to the very small thickness of the dielectric and the large area of the plates, oxide capacitors have a large capacity despite their small dimensions.
During operation, electrochemical processes occur inside the capacitor, destroying the junction of the terminal with the plates. The contact is broken, and as a result, the so-called. transition resistance, sometimes reaching tens of ohms. This is equivalent to connecting a resistor in series with the capacitor, the latter being located in the capacitor itself. Charging and discharging currents cause this “resistor” to heat up, which further aggravates the destructive process.
Other cause of electrolytic capacitor failure- this is a “drying out” known to radio amateurs, when due to poor sealing the electrolyte evaporates. In this case, the capacitive reactance (Xc) of the capacitor increases, because the capacity of the latter decreases. The presence of series resistance negatively affects the operation of the device, disrupting the logic of the capacitor in the circuit. (If you connect, for example, a resistor with a resistance of 10 - 20 Ohms in series with the rectifier filter capacitor, the ripple of the rectified voltage at the output of the latter will sharply increase). The increased value of the Equivalent Series Resistance (ESR) of capacitors (up to only 3 - 5 Ohms) has a particularly strong effect on the operation of switching power supplies, causing failure of expensive transistors or microcircuits.
The operating principle of the described equivalent series resistance meters is based on measuring the capacitance of a capacitor, i.e. In essence, it is an ohmmeter operating on alternating current. From the course of radio engineering it is known that
X c = 1/ 2ПfC (1), where X c is capacitance. Ohm; f - frequency, Hz; C - capacity, F
Checking the capacitor. Average ESR values in milliohms for new capacitors depending on voltage
The pulse generator generates pulses with a repetition rate of 120 kHz, built on logic elements 1 and 2. The generator frequency is set by an RC circuit on the radio components R1 and C1.
To coordinate logical levels, the third logical element DD1.3 is used. To amplify the pulses, DD1.4-DD1.6 were added to the circuit. Then the signal, following through a voltage divider across resistances R2 and R3, is supplied to an unknown capacitor Cx. The AC voltage meter unit consists of diodes VD1 and VD2 and a multimeter. The latter needs to be switched to constant voltage measurement mode. The device for testing capacitors is adjusted by changing the value of resistor R2.
Structurally, the device is placed in the same housing with the battery. Probe X1 is attached to the body of the device, probe X2 is a regular wire no more than 10 centimeters at the end of which there is a needle or crocodile. Testing the capacitors under study is possible directly on the board, without removing them from the circuit, which significantly speeds up the repair time of any radio equipment.
Upon completion of assembling the device for testing electrolytic capacitors, it is advisable to measure the frequency on probes X1 and X2 with an oscilloscope. It should be in the range of 120-180 kHz. Otherwise, you will need to select the value of resistor R1.
Then using resistors of the following values: 1, 5, 10, 15, 25, 30, 40, 60, 70 and 80 ohms. We connect a resistance of 1 ohm to pins X1 and X2 and adjust R2 so that the multimeter shows a value of 1 mV. Then we take the next 5 Ohm resistor and, without changing the resistance R2, record the multimeter reading. The same continues with the remaining resistances. As a result, we will obtain a table of values from which we can find out the reactance.
Let's consider the operation of a simple ESR meter circuit for testing oxide capacitors. We should immediately make a reservation that the essence of the electrical processes occurring in the circuit is given in a somewhat simplified form to facilitate understanding.
Checking a capacitor circuit diagram using a microammeter head
The DD1 chip contains a rectangular pulse generator (elements D1.1, D1.2) and a buffer amplifier (elements D1.3, D1.4). The generation frequency is determined by elements C2 and R1 and is approximately equal to 100 kHz. Rectangular pulses are fed through a separating capacitor SZ and resistor R2 to the primary winding of step-up transformer T1. A microammeter PA1 is connected to the secondary winding after the rectifier on diode VD1, on the scale of which the ESR value is measured. Capacitor C4 smoothes out the ripples of the rectified voltage. When the power is turned on, the microammeter needle deviates to the final scale mark (this is achieved by selecting resistor R2), this position corresponds to an infinite ESR value.
If you now connect a working oxide capacitor Cx in parallel with winding I of transformer T1, then due to the low capacitance (remember, when C = 10 µF, X c = 0.16 Ohm at 100 kHz), the capacitor will bypass the winding, and the meter needle will drop to almost zero. If any of the defects described above are present in the measured capacitor, the ESR value in it increases. Some of the alternating current will flow through the winding, and the arrow will deviate by a certain angle.
The higher the ESR, the more current will flow through the winding and the less through the capacitor, and the closer to the “infinity” position the needle will deviate. The scale of the device is nonlinear and resembles the ohmmeter scale of a conventional tester. Any microammeter for current up to 500 µA can be used as a measuring head; heads from tape recorder recording level indicators are well suited. It is not necessary to calibrate the scale; it is enough to note where the arrow will be by connecting calibration resistors.
But we'll talk about this a little later. Thanks to the isolating step-up transformer, the voltage on the measuring probes of the device does not exceed 0.05 - 0.1 V, at which the junctions of semiconductor devices do not yet open. This makes it possible to test capacitors without removing them from the circuit!
It is easy to notice that if a faulty capacitor with a dielectric breakdown is connected to the circuit, the instrument needle, just as in the case of checking a working capacitor, will drop to zero. To eliminate this drawback, switch S1 was introduced into the circuit. In the upper position of the contacts (as shown in the diagram), the device works as an ESR meter, and the arrow of the measuring head is deflected under the influence of the rectified voltage of the generator. In the lower position of the contacts of switch S1, the meter needle is deflected under the influence of constant voltage from the power source, and the capacitor being measured is connected in parallel to the head. The measurement procedure looks like this: connect the probes to the capacitor being measured and observe the arrow. Let's say the needle drops to zero; as far as ESR is concerned, the capacitor is working. Switch S1 to the lower position. If the capacitor is working properly, the meter needle should return to the “infinity” position, because capacitors do not conduct (or rather: should not conduct) direct current. A broken capacitor will bypass the head, and the meter needle will remain in the zero position. Deviations of the needle to the end mark of the scale at direct current (in the lower position of S1) are achieved by selecting resistor R3.
To protect the measuring head from mechanical damage by a pulse of discharge current (when measuring probes are accidentally connected to a charged capacitor), diodes VD2, VD3 are used. The charged capacitor will be discharged through winding I of transformer T1.
The presence of switch S1 makes it possible to “ring” the conductors of the printed circuit board, allowing you to detect breaks, microcracks or accidental short circuits between tracks. This cannot be done on alternating current, because, for example, due to the presence of a blocking capacitor in the circuit, the device will show a short circuit between the common wire and the power conductor.
There are other areas of application of the device. With its help, thanks to the presence of a pulse generator, you can check the serviceability of the RF and IF paths of radios and televisions, as well as video amplifiers, pulse shapers, etc. The harmonic spectrum of a square wave generator operating at a frequency of 100 kHz extends up to hundreds of megahertz. The TV reacts to connecting the device’s probes even to the UHF antenna input. In the MB range, horizontal stripes are clearly visible on the TV screen.
In order to be able to check the AF paths, another switch (S2) is introduced into the device circuit, with the help of which the frequency of the pulse generator is reduced to 1 kHz. In addition, measurements showed that the current consumed by the device does not exceed 3-5 mA. The device can be powered from a Krona battery through a low-power 5-volt stabilizer. Switch S3 turns on the power of the device.
Long-term work with the device made it possible to identify another “hidden reserve” - with its help you can check inductors (transformer windings) for the presence of short-circuited turns. In this case, the device measures the same reactance, only this time it is inductive (XL). Inductive reactance can be calculated using the formula:
X L = 2ПfL (2), where X L is inductive reactance, Ohm; t - frequency, Hz; L - inductance, H.
For example, a coil with an inductance of 100 microhenry (µH) at a frequency of 100 kHz will have an inductive reactance of XL = 62.8 ohms (assuming a sinusoidal current waveform). If such a coil is connected to our device, the meter needle will practically remain in the “infinity” position, the deviation will be barely noticeable. The presence of a short-circuited turn(s) in the coil winding will lead to a sharp decrease in inductive reactance to units of Ohms, and the instrument needle in this case will show some small resistance. The inductance of coils used in radio engineering devices can be in a very wide range - from units of microhenry in RF chokes to tens of henry in power transformers, so testing coils with high inductance at a frequency of 100 kHz can be difficult. To test such coils (for example, the primary windings of power transformers), the generator frequency must be set to 1 kHz (switch S2).
Transformer T1 is wound on a ferrite ring with an outer diameter of 10-15 mm and a magnetic permeability of 600-2000 (the values are not critical). The primary winding contains 10 turns of PEV-2 wire with a diameter of 0.4-0.5 mm, the secondary winding contains 200 turns of PEV-2 wire with a diameter of 0.1-0.15 mm. An MGTF-0.5 mounting wire is excellent as a primary wire. Diode VD1 must be germanium, for example, types D9, D310, D311, GD507. Silicon diodes have a high threshold opening voltage (0.5-0.7 V), which will lead to strong nonlinearity of the device scale in the area of measuring low resistances. Germanium diodes begin to conduct current at a forward voltage of 0.1-0.2 V. A correctly assembled device begins to work immediately, you just need to select the resistance of the resistors, as indicated above. To make adjustments easier, trimming resistors can be used as resistors R2 and R3.
The master oscillator can be assembled using a different scheme; it is only important that the frequency of the generator signal is about 100 kHz. You can do without an internal generator altogether, using an existing stationary generator and a dial avometer, and design the device as an attachment to them.
The device for testing electrolytic capacitors is calibrated using several constant resistors with a resistance of 1 Ohm. Having closed the probes, we notice where the zero mark of the scale will be. Due to the presence of resistance in the connecting wires, it may not coincide with the arrow position when the power is turned off. Therefore, the wires going to the probes should be as short as possible, with a cross-section of 0.75-1 mm2. Next, we connect two parallel-connected 1 Ohm resistors and note the position of the arrow corresponding to the measured resistance of 0.5 Ohm. Then we connect resistors of 1, 2, 3, 5 and 10 ohms and note the positions of the arrow when measuring these resistances. We can stop here, since electrolytic capacitors with a capacity of more than 4.7 μF with an ESR of more than 10 ohms, although they can work, for example, as isolation capacitors in ULF, however, raise serious doubts about their durability.
The ESR value of new serviceable capacitors depends on the manufacturer, type, properties of the materials used in the manufacture, etc. Most capacitors with a capacity of 1-4.7 μF for a voltage of 50-400 V, as well as low-voltage ones, have an increased (up to 3-6 ohms) ESR ultra-small capacitors. A proven capacitor, for example, with a capacity of 1000 uF at 16 V, having an ESR of 5 Ohms, is clearly “not good” and must be replaced. As noted above, in particularly critical components of radio equipment, for example, in switching power supplies, scanning circuits for televisions, high-quality capacitors with an ESR of no more than 0.5-1 Ohm should be used. For interstage capacitors of low-frequency circuits, these requirements may not be so stringent. (It is in the ULF, assembled a couple of years ago, that the miniature “electrolytics” mentioned above work safely).
To test the device’s ability to detect short-circuited turns, conduct the following experiment: connect the device to a working inductor, for example, DM - 0.1 with an inductance of 20-100 μH, at a frequency of 100 kHz. The arrow will deviate slightly in the direction of decreasing the measured resistance. Then wind a couple of turns of stripped mounting wire over the choke and twist the ends together. Connect the device again: this time the needle should deviate at a much larger angle, indicating a resistance of several ohms. In any case, the coil check function is optional.
The probe is assembled on a microassembly. If the capacitor being tested is broken, the LED goes out. If the capacitance is broken, the LED is constantly lit. If the monitored capacitor is working properly, then the LED blinks, and the blinking frequency of the light sequences changes depending on the resistance of the variable resistor.
