Having decided to replace a capacitor on a printed circuit board, the first step is to select a replacement capacitor. As a rule, we are talking about an electrolytic capacitor, which, due to the exhaustion of its working life, began to create an abnormal mode for your electronic device, or the capacitor burst due to overheating, or maybe you just decided to install a newer or better one.
Selecting a suitable replacement capacitor
The parameters of the replacement capacitor must certainly be suitable: its rated voltage should in no case be lower than that of the capacitor being replaced, and the capacitance should not be lower, or maybe 5-10 percent higher (if this is permissible according to the rules known to you). circuit diagram of this device) than it was originally.
Finally, make sure the new capacitor will fit into the space that its predecessor will leave. If it turns out to be a little smaller in diameter and height, it’s not a big deal, but if the diameter or height is larger, components located nearby on the same board may interfere or it will rest against the elements of the case. It is important to take these nuances into account. So, the replacement capacitor has been selected, it suits you, now you can start dismantling the old capacitor.
Getting ready for the process
Now it will be necessary to remove the faulty capacitor from the board and prepare a place for installing a new one here. To do this, you will need, of course, and it is also convenient for this action to prepare a piece of copper braid for removing solder. As a rule, a soldering iron power within 40 W will be quite sufficient even if refractory solder was initially used on the board.
As for copper braiding to eliminate solder, if you don’t have one, it’s very easy to make it yourself: take a piece of not very thick copper wire consisting of thin copper strands, remove the insulation from it, lightly (you can use simple pine rosin) - Now these flux-impregnated veins will easily, like a sponge, absorb solder from the legs of the soldered capacitor.
Soldering the old capacitor
First, look at the polarity of the soldered capacitor on the board: which way is it facing minus, so that when you solder in a new one, you will not make a mistake with the polarity. Usually the negative leg is marked with a stripe. So, when the braid for desoldering is prepared, and the soldering iron is already warm enough, first lean the braid against the base of the capacitor legs that you decided to free from solder first.
Carefully melt the solder on the leg directly through the braid so that the braid also heats up and quickly draws the solder from the board. If there is too much solder on the leg, move the braid as it fills with solder, collecting all the solder from the leg onto it so that the leg ends up free of solder. Do the same with the second leg of the capacitor. Now the capacitor can be easily pulled out by hand or tweezers.
Soldering in a new capacitor
The new capacitor must be installed in compliance with the polarity, that is, the negative leg is in the same place where the negative leg of the soldered one was. Usually the minus leg is indicated by a stripe, and the plus leg is longer than the minus leg. Treat the capacitor legs with flux.
Insert the capacitor into the holes. There is no need to shorten the legs in advance. Bend the legs slightly in different directions so that the capacitor stays in place well and does not fall out.
Now, warming up the leg near the board itself with the tip of the soldering iron tip, poke the solder towards the leg so that the leg is enveloped, moistened, and surrounded by solder. Do the same with the second leg. When the solder has cooled, all you have to do is shorten the legs of the capacitor with wire cutters (to the same length as the adjacent parts on your board).
The next no less common parts, widely used in pocket receivers, are permanent capacitors of various capacities. In high-frequency circuits where low capacitance is required, it is advisable to use special miniature capacitors such as KDM and KTM, produced by industry with nominal values from 1 to 1500 pF and from 1 to 3000 pF, respectively. These capacitors are relatively scarce, but there is a replacement for them, namely: widespread capacitors of the KTK-1 type with nominal values from 2 to 180 pF, KSO-1 from 21 to 750 pF and KSO-2 from 100 to 2400 pF. The latter type of capacitors are somewhat larger than the first two, but they can be “miniaturized.” The protective plastic molding must be removed from the capacitor and replaced with impregnation with nitro varnish or BF-2 glue. This way it is possible to obtain a very miniature part.
As isolation and blocking capacitors in high-frequency circuits of receivers, capacitors of significantly larger capacitance than indicated above are used. Capacitors of the KDS type with a capacity of 1000, 3000 and 6800 pF, KLS and KM with a capacity of 0.01, 0.033 and 0.047 μF, well known to radio amateurs, are suitable here. True, the last two types of capacitors are relatively scarce, but they can be successfully replaced with capacitors of somewhat larger dimensions, for example, the MBM type for 160 V.
When selecting capacitors of the required capacity, one should not forget about the possibility of connecting them in series and in parallel. Regarding tolerance, the following must be taken into account. The nominal values of capacitors used in high-frequency circuits must be close to the recommended ones and within a tolerance of ±5-10%. Capacitors used for blocking can have a tolerance of up to ±20%. There is no need to talk about the operating voltage of the types of capacitors discussed above, since it is many times higher than what will be applied to them in transistor receiver circuits. |
In addition to capacitors of relatively small capacity, transistor circuits use decoupling and blocking capacitors with a capacity of 0.5 to 100.0 microfarads, and sometimes more. Common types of high-capacity capacitors are domestic miniature electrolytic capacitors of the EM and EM-M types, produced by industry with nominal values from 0.5 to 50.0 μF, which can be replaced with Tesla capacitors, which are periodically supplied to our radio stores.
When installing electrolytic capacitors in a circuit, in order to avoid possible failure, it is necessary to strictly observe the indicated polarity of connection. Determine the polarity of the capacitors. high-quality production is easy by the corresponding inscription (+) made on the case on the side of the output, isolated from it and connected to the plate connected to the plus of the power source; the opposite terminal, connected to the capacitor body, must be connected to the minus (Fig. 1, /). For capacitors manufactured by Tesla, the terminal isolated from the housing is positive (Fig. 1, 2).
In addition to the switching polarity, the operating voltage of electrolytic capacitors should also be taken into account, which in no case should be less than that recommended in the description of a particular receiver and, as a rule, indicated on the circuit diagram together with the nominal value of the capacitance.
The capacitance of the isolation capacitors can have a tolerance of up to +50%, and of the blocking capacitors up to +100-500%, which in some cases will only contribute to more stable operation of the circuit.
In addition to constant capacitors, almost all pocket receiver circuits contain variable capacitors: single ones in direct amplification receivers and combined into dual blocks in superheterodyne type receivers. Of the ready-made single capacitors, a ceramic tuning capacitor of the KPK-2 type with a capacity of 25-150 pf has become widespread. Besides him, in pro-
Fig. 1 External lead of common parts and pin locations: J – EM type capacitors. EM M, 2– b “dei! sators of the Tesla company, 3 ¦ tra.pistors type P13, GSh. P15. P16, P8. P9. PYU PI; – transistors type "pi m P40E P403A-5 circuit for determining the reverse current of the collector; (5 – diagram for determining
for transistor gain ¦ 7 – diodes of the D2 series; 8 – diodes of the D1 and D9 series; “low-frequency transformer /v – circuit diagram of the windings of the matching transformer: P – circuit diagram of the output transformer windings; 12 – capsule type DEMSH-1a: 13 – diagram of the windings of the capsule type DEMSH-1a.
There are even special single miniature capacitors with a solid dielectric, produced by our industry with a minimum capacity of 5 pf and a maximum of 350 pf, as well as Tesla capacitors with similar parameters.
Of the ready-made dual condenser blocks, you can use those used in portable receivers, for example, “Neva”, “Neva-2”, “Gauja”, “Selga”, “Start”, “Topaz”, “Sokol”, etc. Their maximum The capacitance ranges from 180 to 240 pf. In addition to them, a dual block of Tesla variable capacitors with a maximum capacity of 360-380 pF is also available for sale. The industrial tolerance for capacitance of the listed capacitors does not exceed ±10%. When selecting the required tuning capacitor, a novice radio amateur must adhere to the recommendations given in the description of a particular circuit he is assembling. A significant deviation of the capacitor capacitance from the required value, exceeding ±10%, will require recalculation of the winding data of the high-frequency coils of the oscillating circuits. Otherwise, the circuit settings will change and the receiver may become unusable. This remark is especially true for superheterodynes.
In cases where the maximum capacitance of the capacitor is significantly greater than the recommended value, recalculation of the loop coil data can be avoided if an additional mating capacitor is introduced into the circuit, connected in series with the main one. The capacitance of the mating capacitor is selected so that the total maximum capacitance is equal to that recommended in the description.
In direct amplification receivers, you can avoid recalculating the loop coil data when using a tuning capacitor with a smaller capacitance than required, but you should remember that the operating range of the receiver will change.
A few words should be said about trimmer capacitors with a small maximum capacitance. They are typically used to precisely couple the input and local oscillator circuits of superheterodyne receivers. Most industrial dual units have their own tuning capacitors KPE built into the housing. If they are not available, then you can use standard KPKM type trimmers with a maximum capacity of 15-30 pF or any others that are suitable in size.
Starting and running capacitors are used to start and operate electric motors operating in a single-phase 220 V network.
That's why they are also called phase shifters.
Installation location - between the power line and the starting winding of the electric motor.
Symbol for capacitors in diagrams
The graphic designation on the diagram is shown in the figure, the letter designation is C and the serial number according to the diagram.
Capacitor capacity- characterizes the energy that a capacitor is capable of accumulating, as well as the current that it is capable of passing through itself. Measured in Farads with a multiplying prefix (nano, micro, etc.).
The most commonly used values for run and start capacitors range from 1 μF to 100 μF.
Nominal voltage of the capacitor - voltage at which the capacitor is able to operate reliably and for a long time, maintaining its parameters.
Well-known capacitor manufacturers indicate on its body the voltage and the corresponding guaranteed operating time in hours, for example:
You can check the capacitor using a capacitor capacitance meter; such devices are produced both separately and as part of a multimeter, a universal device that can measure many parameters. Let's consider checking with a multimeter.
All devices have different designations for the capacitor measurement mode; the main types are shown below in the pictures.
In this multimeter, the mode is selected by a switch; it must be set to Fcx mode. The probes must be inserted into the sockets marked Cx.
Switching the capacitance measurement limit is manual. Maximum value 100 µF.
This measuring device has an automatic mode, you just need to select it, as shown in the picture.
The Mastech measuring tweezers also automatically measure capacitance, you just need to select the mode with the FUNC button, pressing it until the F indication appears.
To check the capacitance, we read its value on the capacitor body and set a deliberately larger measurement limit on the device. (If it's not automatic)
For example, the nominal value is 2.5 μF (μF), on the device we set 20 μF (μF).
After connecting the probes to the terminals of the capacitor, we wait for the readings on the screen, for example, the time to measure a capacitance of 40 μF with the first device is less than one second, with the second one more than one minute, so you should wait.
If the rating does not correspond to that indicated on the capacitor body, then it must be replaced and, if necessary, an analogue must be selected.
If you have an original capacitor, then it is clear that you simply need to put it in place of the old one and that’s it. Polarity does not matter, that is, the terminals of the capacitor do not have the designations plus “+” and minus “-” and they can be connected in any way.
It is strictly forbidden to use electrolytic capacitors (you can recognize them by their smaller sizes, with the same capacity, and the plus and minus markings on the case). As a consequence of application - thermal destruction. For these purposes, manufacturers specially produce non-polar capacitors for operation in an alternating current circuit, which have convenient mounting and flat terminals for quick installation.
If the required denomination is not available, you can obtain it parallel connection of capacitors. The total capacitance will be equal to the sum of the two capacitors:
C total = C 1 + C 2 +...C p
That is, if we connect two 35 μF capacitors, we get a total capacity of 70 μF, the voltage at which they can operate will correspond to their rated voltage.
Such a replacement is absolutely equivalent to one capacitor of larger capacity.
To start powerful compressor engines, oil-filled non-polar capacitors are used.
The housing is filled with oil inside for good heat transfer to the surface of the housing. The body is usually metal or aluminum.
The most affordable capacitors of this type CBB65.
To start less powerful loads, such as fan motors, dry capacitors are used, the housing of which is usually plastic.
The most common capacitors of this type CBB60, CBB61.
The terminals are double or quadruple for ease of connection.
The most common breakdown of modern electronics is the malfunction of electrolytic capacitors. If, after disassembling the case of an electronic device, you notice that there are capacitors on the printed circuit board with a deformed, swollen body, from which toxic electrolyte is oozing, then it’s time to figure out how to recognize a breakdown or defect in the capacitor and select an adequate replacement. Having professional soldering flux, solder, a soldering station, and a set of new capacitors, you can easily “revive” any electronic device with your own hands.
In essence, a capacitor is a radio-electronic component, the main purpose of which is the accumulation and release of electricity for the purpose of filtering, smoothing and generating alternating electrical oscillations. Any capacitor has two most important electrical parameters: capacitance and the maximum direct voltage that can be applied to the capacitor without breakdown or destruction. Capacitance, as a rule, determines how much electrical energy a capacitor can absorb if a constant voltage not exceeding a given limit is applied to its plates. Capacitance is measured in Farads. The most widely used capacitors are those whose capacitance is calculated in microfarads (μF), picofarads (pF) and nanofarads (nF). In many cases, it is recommended to replace a faulty capacitor with a serviceable one that has similar capacitance characteristics. However, in repair practice there is an opinion that in power supply circuits it is possible to install a capacitor with a capacity slightly higher than the factory parameters. For example, if we want to replace a ruptured electrolyte with 100 µF 12 Volt in the power supply, which is designed to smooth out fluctuations after the diode rectifier bridge, we can safely set the capacitance even to 470 µF 25 V. Firstly, an increased capacitor capacity will only reduce ripple, which in itself is not bad for a power supply. Secondly, an increased voltage limit will only increase the overall reliability of the circuit. The main thing is that the space allocated for installing the capacitor is suitable.
The most common reason why an electrolytic capacitor explodes is excess voltage between the capacitor plates. It is no secret that in many Chinese-made devices, the maximum voltage parameter exactly corresponds to the applied voltage. According to their idea, capacitor manufacturers did not foresee that when a capacitor is normally included in an electrical circuit, the maximum voltage will be supplied to its contacts. For example, if the capacitor says 16V 100uF, then you should not connect it to a circuit where 15 or 16V will be constantly supplied to it. Of course, he will withstand such abuse for some time, but the safety margin will be practically zero. It is much better to install such capacitors in a circuit with a voltage of 10–12V, so that there is some voltage reserve.
Electrolytic capacitors have negative and positive electrodes. As a rule, the negative electrode is identified by the markings on the body (a white longitudinal strip behind the “-” signs), and the positive plate is not marked in any way. The exception is domestic capacitors, where, on the contrary, the positive terminal is marked with a “+” sign. When replacing capacitors, it is necessary to compare and check whether the polarity of the capacitor connection corresponds to the markings on the printed circuit board (the circle where there is a shaded segment). By matching the negative strip with the shaded segment, you will correctly insert the capacitor. All that remains is to cut off the legs of the capacitor, process the soldering points and solder them properly. If you accidentally reverse the polarity of the connection, then even a completely new and completely serviceable capacitor will simply rupture, simultaneously smearing all adjacent components and the printed circuit board with conductive electrolyte.
It is no secret that replacing low-voltage capacitors can only be harmful to health if the polarity connection is incorrect. The first time you turn it on, the capacitor will explode. The second danger that can be expected from capacitors is the voltage between its plates. If you've ever taken apart computer power supplies, you've probably noticed the huge 200V electrolytes. It is in these capacitors that dangerous high voltages remain that can seriously injure you. Before replacing the capacitors of the power supply, we recommend completely discharging it either with a resistor or a 220V neon lamp.
Helpful advice: such capacitors do not like to be discharged through a short circuit, so do not short-circuit their terminals with a screwdriver to discharge them.
