Features of replacing radio components in circuits. reference Information

Features of replacing radio components in circuits

Background

When assembling a circuit you like or repairing radio devices, sometimes it may be difficult to purchase a particular part. What can replace it? To answer this question, you need to know the main features of the parts and have a good understanding of the principle of operation of the circuit in which this part is used, which will allow you to evaluate the limiting modes for a particular node.

Most of the parts can be easily replaced with similar ones, similar in parameters, without losing the quality characteristics of the device. This is often explained by the fact that the designer of the scheme, when choosing a particular type of element, often focuses on the list of parts that are easily accessible to him.

The easiest way is to replace resistors and capacitors. For fixed resistors the main parameters are: resistance rating (typically ±20% tolerance, unless otherwise specified), power dissipation and temperature coefficient. When replacing resistors, you can install more power than indicated in the diagram, but they are usually larger in size. The temperature coefficient is taken into account in precision measuring instruments or devices designed to operate over a wide temperature range.

Variable resistors in addition to the parameters listed above, they have one more - the form of the dependence of the change in resistance on the angle of rotation of the engine (usually indicated as a letter, see figure). The smoothness of parameter adjustment depends on this parameter. The letter A is a linear relationship, and the most common non-linear relationships - logarithmic (B) and inverse logarithmic (C) - are used to adjust the volume and timbre of the sound, the brightness of the indicators, etc., to compensate for the non-linearity of our perception.

Features of replacing radio components in circuits. Marking variable resistors

Permanent Capacitors in addition to the nominal capacity and the maximum permissible operating voltage, they have another important parameter - the temperature coefficient of change in capacitance (TKE). This parameter must be taken into account in the circuits of highly stable generators, oscillatory circuits, timers.

Usually, TKE is indicated in high-frequency circuits, but if it is not specified, then it is advisable to use capacitors with a small change in capacitance with temperature, for example, with codes MPO, PZZ, MZZ, M47.

Capacitors with the H90 code have the worst TKE (their capacitance can vary up to -90% when the temperature changes from -60 ° C to + 85 ° C), but they are usually used in power filtering circuits or as separators between cascades, where TKE does not matter for the operation of the circuit.

Most often, capacitors of any type can be used when replacing, taking into account only the rated capacitance and operating voltage, which should not be less than the one actually operating in the circuit.

Features of replacing radio components in circuits

Electrolytic polar capacitors it is permissible to replace non-polar ones, but they are usually larger in size, and the reverse replacement is unacceptable (out of two polar ones (see figure above), one can be made non-polar by connecting them in series plus to plus, while the capacitance of the capacitors should be twice as large, than shown in the diagram). Among the available electrolytic capacitors, the best are tantalum and oxide-semiconductor, for example, types K52-1A, K53-28 and the like - they can replace other types of polar capacitors. In power filter circuits, it is permissible to use larger capacitors than indicated in the diagram.

For diodes the main parameters are the maximum allowable forward current and reverse voltage, and in some device nodes, when replacing, it is also necessary to take into account the reverse current (leakage of the diode when it is locked) and forward voltage drop. In low-power germanium diodes, the reverse current is much greater than that of silicon, and it also depends to a greater extent on temperature. For this reason, it is better to use silicon diodes in digital circuits, for example KD521, KD522, KD509 and others. The forward voltage drop of most germanium diodes is about half that of similar silicon diodes. Therefore, in circuits where this voltage is used to stabilize the operating mode of the circuit, for example, in some final sound amplifiers, replacing diodes with a different type of conductivity is unacceptable.

For rectifiers in power supplies, the main parameters are the maximum permissible forward current and reverse voltage. For example, at currents up to 10 A, diodes D242 ... D247, KD213 can be used; for a current of 1 ... 5 A, diodes of the KD202, KD213 series are suitable; at a current of 0.5 ... 1 A, diodes KD212, KD237 or diode bridges KTs402 ... KTs405, and at lower currents, diodes KD105, KD102, diode assemblies KTs407A and many others, with the corresponding letter index, which indicates the permissible operating voltage.

Switching power supplies often use special Schottky diodes (KD222, KD2998, etc.). They are designed to operate at higher frequencies (10 ... 200 kHz) than conventional diodes and, due to the low internal resistance in the open state, have lower losses. Ordinary diodes in such a circuit will work with strong overheating and not for long.

Transistors when replacing, they must be selected from the same class (low-power, medium-power, powerful, high-frequency, etc.) and with parameters no worse than those used in the circuit. The main parameters of transistors that are taken into account when replacing: the maximum allowable emitter-collector voltage, collector current, collector power dissipation, and gain.

The parameters of silicon transistors are more stable with temperature changes than those of germanium. Outdated types of germanium transistors (for example, MP37, MP42) that have been discontinued can be replaced with silicon ones (KT315, KT361 or better for KT3102, KT3107, etc.) of a similar structure (p-p-p or p-p-p).

In devices where transistors are used in key applications, such as logic circuits and relay control stages, the choice of transistor does not matter much if it is of similar power and has close speed and gain.

So, for example, KT838A transistors used in switching power supplies for TVs can be replaced with KT839A or KT846V.

Features of replacing radio components in circuits

High-gain transistors KT829A can be replaced with a composite circuit of two transistors (see figure above). And the failed KT848A transistor in the electronic ignition unit of cars is replaced by the circuit shown in the figure above (this will increase the reliability of the device).

Microcircuits can be divided into three conditional groups - logic, analog and specialized. Specialized microcircuits (for example, DAC 594PA1) cannot be replaced with another type, since this will require changing the circuit design. Logic chips series 155 (133) are everywhere replaced by more modern and economical series 555 (1533) - they consume 5 ... 10 times less current with the same basic parameters. At the same time, it is desirable that all surrounding digital microcircuits be from the same series (this will save the device from malfunctions due to different speeds of logic elements).

The difference between the 555 and 1533 series is only in the package design, the pin numbering is retained.

The most widely used microcircuits of the 561st series can be replaced by the 1561 series (or the 564th series, but it has a different package design - "planar pins", and you will need to make an adapter block to install them or change the board topology).

The K544SAZ comparator is often used in circuits. It can be replaced with a similar K521SAZ (in a plastic case 201.14-1) or K521CA301 (in a plastic case 3101.8-1), it is also possible to use 521SAZ (in a case 301.8-2), but the numbering of connected outputs changes.

Features of replacing radio components in circuits

When replacement is needed, the choice of analog microcircuits from the series of operational amplifiers (op-amps) is quite wide, but different parameters must be taken into account, depending on the specific circuit in which they are used. Here you need to find the microcircuit closest in parameters to the reference book, and even better if you can consult with a specialist with experience in circuit design, since some op-amps require the use of external correction circuits for stable operation or have other application features, as a rule, not reflected in household reference books.

Publication: radioman.ru

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