When assembling any device, even the simplest one, radio amateurs often have problems with radio components; it happens that they cannot get some kind of resistor of a certain value, a capacitor or a transistor... in this article I want to talk about replacing radio components in circuits, which radio elements can be replaced with what and which ones are not allowed, how they differ, what types of elements are used in which nodes, and much more. Most radio components can be replaced with similar ones with similar parameters.
Let's start with resistors.
So, you probably already know that resistors are the most basic elements of any circuit. Without them, no circuit can be built, but what to do if you do not have the necessary resistances for your circuit? Let's look at a specific example, take for example the LED flasher circuit, here it is in front of you:
In order to understand which resistors here can be changed within what limits, we need to understand what they generally affect. Let's start with resistors R2 and R3 - they influence (together with capacitors) the blinking frequency of the LEDs, i.e. You can guess that by changing the resistance up or down, we will change the blinking frequency of the LEDs. Therefore, these resistors in this circuit can be replaced with similar ones in value if you do not have those indicated on the circuit. To be more precise, in this circuit you can use resistors, say, from 10 kOhm to 50 kOhm. As for resistors R1 and R4, to some extent the operating frequency of the generator also depends on them; in this circuit they can be set from 250 to 470 Ohms. There is one more point here, LEDs come in different voltages, if this circuit uses LEDs with a voltage of 1.5 volts, and we put an LED with a higher voltage there - they will burn very dimly, therefore, we need resistors R1 and R4 will put on less resistance. As you can see, the resistors in this circuit can be replaced with other, similar values. Generally speaking, this applies not only to this circuit, but also to many others; if, say, when assembling the circuit, you did not have a 100 kOhm resistor, you can replace it with 90 or 110 kOhm, the smaller the difference, the better it is not to use 10 kOhm instead of 100 kOhm , otherwise the circuit will not work correctly or even some element may fail. By the way, do not forget that resistors have a permissible nominal deviation. Before changing the resistor to another, carefully read the description and operating principle of the circuit. In precision measuring instruments, you should not deviate from the nominal values specified in the diagram.
Now, as for the power, the more powerful the resistor, the thicker it is, there is no way to install a 0.125 watt resistor instead of a powerful 5 watt one; at best it will get very hot, at worst it will simply burn out.
And you are always welcome to replace a low-power resistor with a more powerful one, nothing will come of it, only powerful resistors are larger, you will need more space on the board, or you will have to place it vertically.
Do not forget about parallel and series connection of resistors, if you need a 30 kOhm resistor, you can make it from two 15 kOhm resistors, connected in series.
In the circuit that I gave above, there is a trimming resistor. Of course, it can be replaced with a variable, there is no difference, the only thing is that the trimmer will have to be turned with a screwdriver. Is it possible to change trimmer and variable resistors in circuits to ones that are close in value? In general, yes, in our circuit it can be set to almost any value, at least 10 kOhm, at least 100 kOhm - the regulation limits will simply change, if we set it to 10 kOhm, by rotating it we will quickly change the blinking frequency of the LEDs, and if we set it to 100 kOhm, the blinking frequency will be adjusted produced more smoothly and “longer” than with 10k. In other words, at 100 kOhm the adjustment range will be wider than at 10 kOhm.
But replacing variable resistors with cheaper trimmers is not worth it. Their motor is rougher and with frequent use the conductive layer is severely scratched, after which, when the motor rotates, the resistance of the resistor can change abruptly. An example of this is wheezing in the speakers when changing the volume.
You can read more about the types and types of resistors.
Now let's talk about capacitors, they come in different types, types and, of course, capacities. All capacitors differ in such basic parameters as rated capacity, operating voltage and tolerance. There are two types of capacitors used in radio electronics: polar and non-polar. The difference between polar capacitors and non-polar ones is that polar capacitors must be included in the circuit while strictly observing the polarity. Capacitors are shaped like radial, axial (the terminals of such capacitors are on the side), with threaded terminals (usually these are high-capacity or high-voltage capacitors), flat, and so on. There are pulse capacitors, noise suppression capacitors, power capacitors, audio capacitors, general capacitors, etc.
Where are which capacitors used?
In power supply filters, ordinary electrolytic ones are used, sometimes ceramics are also used (they serve to filter and smooth the rectified voltage), high-frequency electrolytes are used in switching power supply filters, ceramics are used in power circuits, and ceramics are also used in non-critical circuits.
On a note!
Electrolytic capacitors usually have a high leakage current, and the capacitance error can be 30-40%, i.e. The capacity indicated on the can may vary greatly in reality. The nominal capacity of such capacitors decreases as they age. The most common defect of old electrolytic capacitors is loss of capacity and increased leakage; such capacitors should not be used further.
Let's return to our multivibrator (flasher) circuit, as you can see there are two electrolytic polar capacitors, they also affect the blinking frequency of the LEDs, the larger the capacitance, the slower they will blink, the smaller the capacitance, the faster they will blink.
In many devices and instruments, you cannot “play” with capacitor capacities in this way, for example, if the circuit has 470 μF, then you should try to put 470 μF, or 2 220 μF capacitors in parallel. But again, it depends on which node the capacitor is located in and what role it plays.
Let's look at an example using a low frequency amplifier:
As you can see, there are three capacitors in the circuit, two of which are non-polar. Let's start with capacitors C1 and C2, they are at the input of the amplifier, a sound source passes/is supplied through these capacitors. What will happen if instead of 0.22 µF we put 0.01 µF? Firstly, the sound quality will deteriorate slightly, and secondly, the sound in the speakers will become noticeably quieter. And if instead of 0.22 µF we set 1 µF, then at high volumes we will experience wheezing in the speakers, the amplifier will overload, it will heat up more, and the sound quality may deteriorate again. If you look at the circuit diagram of some other amplifier, you may notice that the input capacitor can be 1 µF or even 10 µF. It all depends on each specific case. But in our case, 0.22 µF capacitors can be replaced with similar ones, for example 0.15 µF or better 0.33 µF.
So, we have reached the third capacitor, it is polar, it has a plus and a minus, you cannot confuse the polarity when connecting such capacitors, otherwise they will heat up, or, even worse, explode. And they bang very, very loudly, it can cause your ears to become blocked. We have a capacitor C3 with a capacity of 470 uF in the power circuit; if you don’t know yet, then I will say that in such circuits, and for example in power supplies, the larger the capacitance, the better.
Nowadays every home has computer speakers, maybe you have noticed that if you listen to music loudly, the speakers wheeze, and the LED in the speaker blinks. This usually just means that the capacitor capacity in the power supply filter circuit is small (+ the transformers are weak, but I won’t talk about that). Now let's return to our amplifier, if instead of 470 uF we put 10 uF - this is almost the same as not installing a capacitor at all. As I already said, in such circuits, the larger the capacitance, the better; to be honest, in this circuit, 470 μF is very little, you can put all 2000 μF.
It is impossible to put a capacitor at a lower voltage than it is in the circuit, this will cause it to heat up and explode; if the circuit operates from 12 volts, then you need to install the capacitor at 16 volts; if the circuit operates from 15-16 volts, then it is better to place the capacitor at 25 volts.
What to do if the circuit you are assembling contains a non-polar capacitor? A non-polar capacitor can be replaced with two polar ones by connecting them in series in the circuit, the pluses are connected together, and the capacitance of the capacitors should be twice as large as indicated on the circuit.
Never discharge capacitors by shorting their terminals! You should always discharge through a high-resistance resistor, but do not touch the terminals of the capacitor, especially if it is high-voltage.
Almost all polar electrolytic capacitors have a cross pressed into them on the top; this is a kind of protective notch (often called a valve). If alternating voltage is applied to such a capacitor or the permissible voltage is exceeded, the capacitor will begin to get very hot, and the liquid electrolyte inside it will begin to expand, after which the capacitor will burst. This often prevents the capacitor from exploding, causing the electrolyte to leak out.
In this regard, I would like to give a little advice: if after repairing any equipment, after replacing capacitors, you turn it on for the first time (for example, in old amplifiers all electrolytic capacitors are replaced), close the lid and keep your distance, God forbid that something goes wrong.
Now the final question: is it possible to connect a 230-volt non-polar capacitor to a 220-volt network? And at 240? Just please, don’t immediately grab such a capacitor and plug it into a socket!
For diodes, the main parameters are the permissible forward current, reverse voltage and forward voltage drop; sometimes you also need to pay attention to the reverse current. Such parameters of replacement diodes must be no less than those of the ones being replaced.
Low-power germanium diodes have a much higher reverse current than silicon diodes. The forward voltage drop of most germanium diodes is approximately 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 audio amplifiers, replacing diodes with a different type of conductivity is not permissible.
For rectifiers in power supplies, the main parameters are reverse voltage and maximum permissible current. For example, for currents of 10A you can use diodes D242...D247 and similar ones; for a current of 1 ampere you can use KD202, KD213; among imported ones, these are diodes of the 1N4xxx series. Of course, you can’t install a 1-amp diode instead of a 5-amp diode; on the contrary, it’s possible.
In some circuits, for example, in switching power supplies, Schottky diodes are often used; they operate at higher frequencies than conventional diodes; these should not be replaced with conventional diodes, they will quickly fail.
In many simple circuits, any other diode can be used as a replacement; the only thing is, don’t confuse the output; you should treat this with caution, because diodes can also burst or smoke (in the same power supplies) if the anode is confused with the cathode.
Is it possible to connect diodes (including Schottky diodes) in parallel? Yes, it is possible, if two diodes are connected in parallel, the current flowing through them can be increased, the resistance, voltage drop across the open diode and power dissipation are reduced, therefore, the diodes will heat up less. Diodes can only be paralleled with the same parameters, from the same box or batch. For low-power diodes, I recommend installing a so-called “current equalizing” resistor.
Transistors are divided into low-power, medium-power, high-power, low-frequency, high-frequency, etc. When replacing, you need to take into account the maximum permissible emitter-collector voltage, collector current, power dissipation, and, of course, the gain.
The replacement transistor, firstly, must belong to the same group as the one being replaced. For example, low low frequency power or high medium frequency power. Then a transistor of the same structure is selected: p-p-p or p-p-p, a field-effect transistor with a p-channel or n-channel. Next, the values of the limiting parameters are checked; the replacement transistor must have them no less than the one being replaced.
It is recommended to replace silicon transistors only with silicon ones, germanium ones with germanium ones, bipolar ones with bipolar ones, etc.
Let's return to the circuit of our flasher, it uses two n-p-n structure transistors, namely KT315, these transistors can easily be replaced with KT3102, or even with an old MP37, suddenly someone has a lot of transistors lying around that can work in this circuit.
Do you think KT361 transistors will work in this circuit? Of course not, KT361 transistors have a different structure, p-n-p. By the way, an analogue of the KT361 transistor is KT3107.
In devices where transistors are used in key modes, for example, in control stages of relays, LEDs, in logic circuits, etc... the choice of transistor does not matter much, choose a similar power and similar parameters.
In some circuits, you can replace each other, for example, KT814, KT816, KT818 or KT837. Let's take a transistor amplifier as an example, its diagram is below.
The output stage is built on KT837 transistors, they can be replaced with KT818, but the KT816 is no longer worth replacing, it will get very hot and will quickly fail. In addition, the amplifier's output power will decrease. Transistor KT315, as you probably already guessed, changes to KT3102, and KT361 to KT3107.
A high-power transistor can be replaced by two low-power transistors of the same type; they are connected in parallel. When connected in parallel, transistors should be used with similar gain values; it is recommended to install equalizing resistors in the emitter circuit of each, depending on the current: from tenths of an ohm at high currents, to units of ohms at low currents and powers. In field-effect transistors, such resistors are usually not installed, because they have a positive TKS channel.
I think we’ll finish here, in conclusion I want to say that you can always ask Google for help, it will always tell you, give you tables for replacing radio components with analogues. Good luck!
Continuation of the article about starting to study electronics. For those who decided to start. A story about details.
Amateur radio is still one of the most common hobbies and hobbies. If at the beginning of its glorious journey, amateur radio mainly affected the design of receivers and transmitters, then with the development of electronic technology, the range of electronic devices and the range of amateur radio interests expanded.
Of course, even the most qualified radio amateur will not assemble such complex devices as, for example, a VCR, CD player, TV or home theater at home. But many radio amateurs are engaged in repairing industrial equipment, and quite successfully.
Another direction is the design of electronic circuits or the modification of industrial devices to the “luxury class”.
The range in this case is quite large. These are devices for creating a “smart home”, 12…220V converters for powering TVs or sound-reproducing devices from a car battery, various thermostats. Also very popular, and much more.
Transmitters and receivers have faded into the background, and all equipment is now simply called electronics. And now, perhaps, we should call radio amateurs something else. But historically, they simply couldn’t come up with another name. Therefore, let there be radio amateurs.
Electronic circuit components
With all the variety of electronic devices, they consist of radio components. All components of electronic circuits can be divided into two classes: active and passive elements.
Radio components that have the property of amplifying electrical signals are considered active, i.e. having a gain factor. It is not difficult to guess that these are transistors and everything that is made from them: operational amplifiers, logic chips, and much more.
In a word, all those elements in which a low-power input signal controls a fairly powerful output signal. In such cases, they say that their gain (Kus) is greater than one.
Passive parts include parts such as resistors, etc. In a word, all those radioelements that have a Kus within 0...1! One can also be considered a strengthening: “However, it does not weaken.” Let's look at the passive elements first.
Resistors
They are the simplest passive elements. Their main purpose is to limit the current in an electrical circuit. The simplest example is turning on an LED, shown in Figure 1. Using resistors, the operating mode of the amplifier stages is also selected at different .
Figure 1. LED connection circuits
Properties of resistors
Previously, resistors were called resistances, this is precisely their physical property. In order not to confuse the part with its resistance property, it was renamed resistors.
Resistance, as a property, is inherent in all conductors and is characterized by the resistivity and linear dimensions of the conductor. Well, about the same as in mechanics, specific gravity and volume.
Formula for calculating conductor resistance: R = ρ*L/S, where ρ is the resistivity of the material, L is the length in meters, S is the cross-sectional area in mm2. It is easy to see that the longer and thinner the wire, the greater the resistance.
You might think that resistance is not the best property of conductors, but simply prevents the passage of current. But in some cases this very obstacle is useful. The fact is that when current passes through a conductor, thermal power P = I 2 * R is released on it. Here P, I, R are power, current and resistance, respectively. This power is used in various heating devices and incandescent lamps.
Resistors on circuits
All details on electrical diagrams are shown using UGO (symbolic graphic symbols). UGO resistors are shown in Figure 2.
Figure 2. UGO resistors
The dashes inside the UGO indicate the power dissipation of the resistor. It should be said right away that if the power is less than required, the resistor will heat up and eventually burn out. To calculate power, they usually use a formula, or rather even three: P = U * I, P = I 2 * R, P = U 2 / R.
The first formula says that the power released in a section of an electrical circuit is directly proportional to the product of the voltage drop in this section and the current through this section. If the voltage is expressed in Volts, the current in Amps, then the power will be in watts. These are the requirements of the SI system.
Next to the UGO, the nominal value of the resistor resistance and its serial number on the diagram are indicated: R1 1, R2 1K, R3 1.2K, R4 1K2, R5 5M1. R1 has a nominal resistance of 1 Ohm, R2 1KOhm, R3 and R4 1.2KOhm (the letter K or M can be placed instead of a comma), R5 - 5.1MOhm.
Modern marking of resistors
Currently, resistors are marked using colored stripes. The most interesting thing is that color marking was mentioned in the first post-war magazine Radio, published in January 1946. It was also said there that this is the new American marking. A table explaining the principle of “striped” markings is shown in Figure 3.
Figure 3. Resistor markings
Figure 4 shows SMD surface mount resistors, also called "chip resistor". For amateur purposes, resistors of size 1206 are most suitable. They are quite large and have decent power, as much as 0.25 W.
The same figure indicates that the maximum voltage for chip resistors is 200V. Resistors for conventional installation have the same maximum. Therefore, when a voltage is expected, for example 500V, it is better to install two resistors connected in series.
Figure 4. Surface Mount SMD Resistors
Chip resistors of the smallest sizes are produced without markings, since there is simply nowhere to put them. Starting from size 0805, a three-digit marking is placed on the “back” of the resistor. The first two represent the denomination, and the third is a multiplier, in the form of an exponent of the number 10. Therefore, if, for example, 100 is written, then it will be 10 * 1 Ohm = 10 Ohm, since any number to the zero power is equal to one, the first two digits must be multiplied exactly by one .
If the resistor says 103, then it turns out 10 * 1000 = 10 KOhm, and the inscription 474 says that we have a resistor 47 * 10,000 Ohm = 470 KOhm. Chip resistors with a tolerance of 1% are marked with a combination of letters and numbers, and the value can only be determined using a table that can be found on the Internet.
Depending on the resistance tolerance, resistor values are divided into three rows, E6, E12, E24. The values of the denominations correspond to the figures in the table shown in Figure 5.
Figure 5.
The table shows that the smaller the resistance tolerance, the more ratings in the corresponding row. If the E6 series has a tolerance of 20%, then it has only 6 denominations, while the E24 series has 24 positions. But these are all resistors for general use. There are resistors with a tolerance of one percent or less, so any value can be found among them.
In addition to power and nominal resistance, resistors have several more parameters, but we won’t talk about them for now.
Connection of resistors
Despite the fact that there are quite a lot of resistor values, sometimes you have to connect them to get the required value. There are several reasons for this: precise selection when setting up the circuit or simply the lack of the required nominal value. Basically, two resistor connection schemes are used: series and parallel. Connection diagrams are shown in Figure 6. Formulas for calculating the total resistance are also given there.
Figure 6. Resistor connection diagrams and formulas for calculating total resistance
In the case of a series connection, the total resistance is simply the sum of the two resistances. It is as shown in the picture. In fact, there may be more resistors. Such inclusion occurs in . Naturally, the total resistance will be greater than the largest one. If these are 1KOhm and 10Ohm, then the total resistance will be 1.01KOhm.
With a parallel connection, everything is just the opposite: the total resistance of two (or more resistors) will be less than the smaller one. If both resistors have the same value, then their total resistance will be equal to half of this value. You can connect a dozen resistors this way, then the total resistance will be just a tenth of the nominal value. For example, ten 100 ohm resistors are connected in parallel, then the total resistance is 100 / 10 = 10 ohms.
It should be noted that in a parallel connection, according to Kirchhoff's law, the current will be divided into ten resistors. Therefore, the power required for each of them is ten times lower than for one resistor.
Continue reading in the next article.
A resistor serves to limit the current in an electrical circuit, create voltage drops in its individual sections, etc. There are a lot of applications, it’s impossible to count them all.
Another name for a resistor is resistance. In fact, this is just a play on words, since translated from English resistance– is resistance (to electric current).
When it comes to electronics, you can sometimes come across phrases like: “Replace the resistance”, “Two resistances have burned out”. Depending on the context, resistance may refer specifically to an electronic part.
In the diagrams, a resistor is indicated by a rectangle with two terminals. On foreign diagrams it is depicted a little differently. The “body” of the resistor is indicated by a broken line - a kind of stylization of the first examples of resistors, the design of which was a coil wound with a high-resistance wire on an insulating frame.
Next to the symbol the element type is indicated ( R) and its serial number in the circuit (R 1 ). Its nominal resistance is also indicated here. If only a figure or number is indicated, then this resistance is in Ohms. Sometimes, Ω is written next to the number - so the Greek capital letter “Omega” stands for ohms. Well, if so, - 10 To, then this resistor has a resistance of 10 kilo Ohm (10 kOhm – 10,000 Ohm). You can talk about multipliers and prefixes “kilo” and “mega”.
Do not forget about variable and tuning resistors, which are becoming increasingly rare, but are still found in modern electronics. I have already talked about their structure and parameters on the pages of the site.
Nominal resistance.
This is the factory resistance value of a particular device; this value is measured in Ohms (derivatives kiloohm– 1000 Ohm, megaohm– 1000000 Ohm). The resistance range extends from fractions of an Ohm (0.01 - 0.1 Ohm) to hundreds and thousands of kiloOhms (100 kOhm - 1 MOhm). Each electronic circuit requires its own sets of resistance values. That is why the spread of nominal resistance values is so large.
Power dissipation.
I have already written in more detail about resistor power.
When electric current passes through a resistor, it heats up. If a current exceeding a specified value is passed through it, the conductive coating will heat up so much that the resistor burns out. Therefore, there is a division of resistors according to power dissipation.
On the graphical designation of a resistor inside a rectangle, power is indicated by an inclined, vertical or horizontal line. The figure shows the correspondence between the graphic designation and the power of the resistor indicated on the diagram.
For example, if a current of 0.1A (100 mA) flows through a resistor, and its nominal resistance is 100 Ohms, then a resistor with a power of at least 1 W is needed. If you use a 0.5 W resistor instead, it will soon fail. Powerful resistors are used in high-current circuits, for example, in power supplies or welding inverters.
If a resistor with a power of more than 2 W (5 W or more) is needed, then a Roman numeral is written inside the rectangle on the symbol. For example, V – 5 W, X – 10 W, XII – 12 W.
Tolerance
When manufacturing resistors, it is not possible to achieve absolute accuracy of the nominal resistance. If the resistor says 10 ohms, then its actual resistance will be around 10 ohms, but not exactly 10. It can be 9.88 or 10.5 ohms. In order to somehow indicate the error limits in the nominal resistance of resistors, they are divided into groups and assigned a tolerance. The tolerance is specified as a percentage.
If you bought a 100 Ohm resistor with a tolerance of ±10%, then its actual resistance can be from 90 Ohms to 110 Ohms. You can find out the exact resistance of this resistor only using an ohmmeter or multimeter by taking the appropriate measurement. But one thing is certain. The resistance of this resistor will not be less than 90 or more than 110 ohms.
Strict accuracy of resistance values in conventional equipment is not always important. For example, in consumer electronics it is allowed to replace resistors with a tolerance of ±20% of the value required in the circuit. This comes in handy in cases where it is necessary to replace a faulty resistor (for example, with a 10 Ohm one). If there is no suitable element with the required rating, then you can install a resistor with a nominal resistance from 8 Ohms (10-2 Ohms) to 12 Ohms (10+2 Ohms). It is calculated as follows (10 Ohm/100%) * 20% = 2 Ohm. The tolerance is -2 ohms in the direction of decrease, +2 ohms in the direction of increase.
There is equipment where such a trick will not work - this is precision equipment. This includes medical equipment, measuring instruments, electronic components of high-precision systems, for example, military ones. In critical electronics, high-precision resistors are used, their tolerance is tenths and hundredths of a percent (0.1-0.01%). Sometimes such resistors can be found in consumer electronics.
It is worth noting that currently on sale you can find resistors with a tolerance of no more than 10% (usually 1%, 5% and less often 10%). High-precision resistors have a tolerance of 0.25...0.05%.
Temperature coefficient of resistance (TCR).
Under the influence of external temperature or self-heating due to flowing current, the resistance of the resistor changes. Sometimes within limits that are undesirable for the operation of the circuit. To evaluate the change in resistance due to temperature, that is, the thermal stability of the resistor, a parameter such as TCR (Temperature Coefficient of Resistance) is used. Abbreviated as T.C.R.
As a rule, the TCR value is not indicated in the resistor markings. For us, it is necessary to know that the lower the TCR, the better the resistor, since it has better thermal stability. I talked in more detail about such a parameter as TKS.
The first three parameters are basic, you need to know them!
Let's list them again:
Nominal resistance (marked as 100 Ohm, 10kOhm, 1MOhm...)
Power dissipation (measured in Watts: 1 W, 0.5 W, 5 W...)
Tolerance (expressed as a percentage: 5%, 10%, 0.1%, 20%).
It is also worth noting the design of the resistors. Nowadays you can find both microminiature surface-mount resistors (SMD resistors), which do not have leads, and powerful ones in ceramic cases. There are also non-flammable, explosive and so on. The list could go on for a very long time, but their basic parameters are the same: rated resistance, power dissipation And admission.
Currently, the nominal resistance of resistors and their tolerance are marked with colored stripes on the body of the element itself. As a rule, such marking is used for low-power resistors that have small dimensions and a power of less than 2...3 watts. Each manufacturer establishes its own labeling system, which creates some confusion. But basically there is one established labeling system.
For newcomers to electronics, I would also like to tell you that in addition to resistors, miniature capacitors in cylindrical cases are also marked with color stripes. This sometimes causes confusion because such capacitors are falsely mistaken for resistors.
Resistance is calculated using color stripes as follows. For example, the first three stripes are red, the last fourth is golden. Then the resistor resistance is 2.2 kOhm = 2200 Ohm.
The first two numbers according to the red color are 22, the third red stripe is the multiplier. Therefore, according to the table, the multiplier for the red stripe is 100. You need to multiply the number 22 by the multiplier. Then, 22 * 100 = 2200 Ohms. The golden stripe represents a 5% tolerance. This means that the actual resistance can be in the range from 2090 Ohms (2.09 kOhms) to 2310 Ohms (2.31 kOhms). The dissipation power depends on the size and design of the housing.
In practice, resistors with a tolerance of 5 and 10% are widely used. Therefore, gold and silver stripes are responsible for admission. It is clear that in this case, the first stripe is on the opposite side of the element. This is where you need to start reading the denomination.
But what if the resistor has a small tolerance, for example 1 or 2%? Which side should you read the denomination on if there are stripes of red and brown on both sides?
This case was provided for and the first strip is placed closer to one of the edges of the resistor. This can be seen in the table figure. The stripes indicating tolerance are located further from the edge of the element.
Of course, there are cases when it is not possible to read the color markings of a resistor (forgot the table, the marking itself is erased/damaged, incorrect stripes, etc.).
In this case, you can only find out the exact resistance of the resistor by measuring its resistance with a multimeter or ohmmeter. In this case, you will 100% know its real value. Also, when assembling electronic devices, it is recommended to check resistors with a multimeter in order to eliminate possible defects.
(fixed resistors), and in this part of the article we’ll talk about, or variable resistors.
Variable resistance resistors, or variable resistors are radio components whose resistance can be change from zero to nominal value. They are used as gain controls, volume and tone controls in sound-reproducing radio equipment, are used for precise and smooth adjustment of various voltages and are divided into potentiometers And tuning resistors.
Potentiometers are used as smooth gain controls, volume and tone controls, serve for smooth adjustment of various voltages, and are also used in tracking systems, in computing and measuring devices, etc.
Potentiometer called an adjustable resistor having two permanent terminals and one movable. The permanent terminals are located at the edges of the resistor and are connected to the beginning and end of the resistive element, forming the total resistance of the potentiometer. The middle terminal is connected to a movable contact, which moves along the surface of the resistive element and allows you to change the resistance value between the middle and any extreme terminal.
The potentiometer is a cylindrical or rectangular body, inside of which there is a resistive element made in the form of an open ring, and a protruding metal axis, which is the handle of the potentiometer. At the end of the axis there is a current collector plate (contact brush) that has reliable contact with the resistive element. Reliable contact of the brush with the surface of the resistive layer is ensured by the pressure of a slider made of spring materials, for example, bronze or steel.
When the knob is rotated, the slider moves along the surface of the resistive element, as a result of which the resistance changes between the middle and extreme terminals. And if voltage is applied to the extreme terminals, then an output voltage is obtained between them and the middle terminal.
The potentiometer can be schematically represented as shown in the figure below: the outer terminals are designated by numbers 1 and 3, the middle one is designated by number 2.
Depending on the resistive element, potentiometers are divided into non-wire And wire.
In non-wire potentiometers, the resistive element is made in the form horseshoe-shaped or rectangular plates made of insulating material, on the surface of which a resistive layer is applied, which has a certain ohmic resistance.
Resistors with horseshoe-shaped resistive element has a round shape and rotational movement of the slider with a rotation angle of 230 - 270°, and resistors with rectangular the resistive element has a rectangular shape and the translational movement of the slider. The most popular resistors are the types SP, OSB, SPE and SP3. The figure below shows a SP3-4 type potentiometer with a horseshoe-shaped resistive element.
The domestic industry produced potentiometers of the SPO type, in which the resistive element is pressed into an arcuate groove. The body of such a resistor is made of ceramic, and to protect against dust, moisture and mechanical damage, as well as for electrical shielding purposes, the entire resistor is covered with a metal cap.
Potentiometers of the SPO type have high wear resistance, are insensitive to overloads and are small in size, but they have a drawback - the difficulty of obtaining nonlinear functional characteristics. These resistors can still be found in old domestic radio equipment.
IN wire In potentiometers, the resistance is created by a high-resistance wire wound in one layer on a ring-shaped frame, along the edge of which a moving contact moves. To obtain reliable contact between the brush and the winding, the contact track is cleaned, polished, or ground to a depth of 0.25d.
The structure and material of the frame is determined based on the accuracy class and the law of change in resistance of the resistor (the law of change in resistance will be discussed below). The frames are made of a plate, which, after winding the wires, is rolled into a ring, or a finished ring is taken, on which the winding is laid.
For resistors with an accuracy not exceeding 10 - 15%, the frames are made of a plate, which, after winding the wires, is rolled into a ring. The material for the frame is insulating materials such as getinax, textolite, fiberglass, or metal - aluminum, brass, etc. Such frames are easy to manufacture, but do not provide precise geometric dimensions.
Frames from the finished ring are manufactured with high precision and are mainly used for the manufacture of potentiometers. The material for them is plastic, ceramics or metal, but the disadvantage of such frames is the difficulty of winding, since special equipment is required to wind it.
The winding is made of wires made of alloys with high electrical resistivity, for example, constantan, nichrome or manganin in enamel insulation. For potentiometers, wires made of special alloys based on noble metals are used, which have reduced oxidation and high wear resistance. The diameter of the wire is determined based on the permissible current density.
The main parameters of resistors are: total (nominal) resistance, form of functional characteristics, minimum resistance, rated power, rotational noise level, wear resistance, parameters characterizing the behavior of the resistor under climatic influences, as well as dimensions, cost, etc. However, when choosing resistors, attention is most often paid to the nominal resistance and less often to the functional characteristics.
Nominal resistance resistor is indicated on its body. According to GOST 10318-74, the preferred numbers are 1,0 ; 2,2 ; 3,3 ; 4,7 Ohm, kiloohm or megaohm.
For foreign resistors, the preferred numbers are 1,0 ; 2,0 ; 3,0 ; 5.0 Ohm, kiloohm and megaohm.
Permissible deviations of resistances from the nominal value are set within ±30%.
The total resistance of the resistor is the resistance between the outer terminals 1 and 3.
Potentiometers of the same type may differ in their functional characteristics, which determine by what law the resistance of the resistor changes between the extreme and middle terminals when the resistor knob is turned. According to the form of functional characteristics, potentiometers are divided into linear And nonlinear: for linear ones, the resistance value changes in proportion to the movement of the current collector, for nonlinear ones it changes according to a certain law.
There are three basic laws: A— Linear, B– Logarithmic, IN— Reverse Logarithmic (Exponential). So, for example, to regulate the volume in sound-reproducing equipment, it is necessary that the resistance between the middle and extreme terminals of the resistive element varies according to inverse logarithmic law (B). Only in this case is our ear able to perceive a uniform increase or decrease in volume.
Or in measuring instruments, for example, audio frequency generators, where variable resistors are used as frequency-setting elements, it is also required that their resistance varies according to logarithmic(B) or inverse logarithmic law. And if this condition is not met, then the generator scale will be uneven, which will make it difficult to accurately set the frequency.
Resistors with linear characteristic (A) are used mainly in voltage dividers as adjustment or trimmers.
The dependence of the change in resistance on the angle of rotation of the resistor handle for each law is shown in the graph below.
To obtain the desired functional characteristics, major changes are not made to the design of potentiometers. For example, in wirewound resistors, the wires are wound with varying pitches or the frame itself is made of varying width. In non-wire potentiometers, the thickness or composition of the resistive layer is changed.
Unfortunately, adjustable resistors have relatively low reliability and limited service life. Often, owners of audio equipment that has been in use for a long time hear rustling and crackling sounds from the speaker when turning the volume control. The reason for this unpleasant moment is a violation of the contact of the brush with the conductive layer of the resistive element or wear of the latter. The sliding contact is the most unreliable and vulnerable point of a variable resistor and is one of the main reasons for part failure.
On circuit diagrams, variable resistors are designated in the same way as constant ones, only an arrow directed to the middle of the case is added to the main symbol. The arrow indicates regulation and at the same time indicates that this is the middle output.
Sometimes situations arise when requirements for reliability and service life are imposed on a variable resistor. In this case, smooth control is replaced by step control, and a variable resistor is built on the basis of a switch with several positions. Constant resistance resistors are connected to the switch contacts, which will be included in the circuit when the switch knob is turned. And in order not to clutter the diagram with the image of a switch with a set of resistors, only the symbol of a variable resistor with a sign is indicated step regulation. And if there is a need, then the number of steps is additionally indicated.
To control volume and timbre, recording level in stereo sound-reproducing equipment, to control frequency in signal generators, etc. apply dual potentiometers, the resistance of which changes simultaneously when turning general axis (engine). In the diagrams, the symbols of the resistors included in them are placed as close to each other as possible, and the mechanical connection that ensures the simultaneous movement of the sliders is shown either with two solid lines or with one dotted line.
The belonging of resistors to one double block is indicated according to their positional designation in the electrical diagram, where R1.1 is the first resistor of the dual variable resistor R1 in the circuit, and R1.2- second. If the resistor symbols are at a great distance from each other, then the mechanical connection is indicated by segments of a dotted line.
The industry produces dual variable resistors, in which each resistor can be controlled separately, because the axis of one passes inside the tubular axis of the other. For such resistors, there is no mechanical connection that ensures simultaneous movement, therefore it is not shown on the diagrams, and membership of a dual resistor is indicated according to the positional designation in the electrical diagram.
Portable household audio equipment, such as receivers, players, etc., often use variable resistors with a built-in switch, the contacts of which are used to supply power to the device circuit. For such resistors, the switching mechanism is combined with the axis (handle) of the variable resistor and, when the handle reaches the extreme position, it affects the contacts.
As a rule, in the diagrams, the contacts of the switch are located near the power source in the break of the supply wire, and the connection between the switch and the resistor is indicated by a dotted line and a dot, which is located at one of the sides of the rectangle. This means that the contacts close when moving from a point, and open when moving towards it.
Trimmer resistors are a type of variables and are used for one-time and precise adjustment of electronic equipment during its installation, adjustment or repair. As trimmers, both variable resistors of the usual type with a linear functional characteristic, the axis of which is made “under a slot” and equipped with a locking device, and resistors of a special design with increased accuracy of setting the resistance value, are used.
For the most part, specially designed tuning resistors are made in a rectangular shape with flat or circular resistive element. Resistors with a flat resistive element ( A) have a translational movement of the contact brush, carried out by a micrometric screw. For resistors with a ring resistive element ( b) the contact brush is moved by a worm gear.
For heavy loads, open cylindrical resistor designs are used, for example, PEVR.
In circuit diagrams, tuning resistors are designated in the same way as variables, only instead of the control sign, the tuning control sign is used.
In electrical circuits, variable resistors can be used as rheostat(adjustable resistor) or as potentiometer(voltage divider). If it is necessary to regulate the current in an electrical circuit, then the resistor is turned on with a rheostat; if there is voltage, then it is turned on with a potentiometer.
When the resistor is turned on rheostat the middle and one extreme output are used. However, such inclusion is not always preferable, since during the regulation process, the middle terminal may accidentally lose contact with the resistive element, which will entail an unwanted break in the electrical circuit and, as a consequence, possible failure of the part or the electronic device as a whole.
To prevent accidental breakage of the circuit, the free terminal of the resistive element is connected to a moving contact, so that if the contact is broken, the electrical circuit always remains closed.
In practice, turning on a rheostat is used when they want to use a variable resistor as an additional or current-limiting resistance.
When the resistor is turned on potentiometer All three pins are used, which allows it to be used as a voltage divider. Let's take, for example, a variable resistor R1 with such a nominal resistance that it will extinguish almost all of the power source voltage coming to the HL1 lamp. When the resistor knob is twisted to the highest position in the diagram, the resistance of the resistor between the upper and middle terminals is minimal and the entire voltage of the power source is supplied to the lamp, and it glows at full heat.
As you move the resistor knob down, the resistance between the upper and middle terminals will increase, and the voltage on the lamp will gradually decrease, causing it to not glow at full intensity. And when the resistor reaches its maximum value, the voltage on the lamp will drop to almost zero and it will go out. It is by this principle that volume control in sound-reproducing equipment occurs.
The same voltage divider circuit can be depicted a little differently, where the variable resistor is replaced by two constant resistors R1 and R2.
Well, that’s basically all I wanted to say about variable resistance resistors. In the final part, we will consider a special type of resistors, the resistance of which changes under the influence of external electrical and non-electrical factors -.
Good luck!
Literature:
V. A. Volgov - “Parts and components of radio-electronic equipment”, 1977
V. V. Frolov - “The language of radio circuits”, 1988
M. A. Zgut - “Symbols and radio circuits”, 1964
