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.
Basic parameters of resistors.
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.
Color coding table.
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.
Most often, resistor malfunctions are associated with burnout of the conductive layer or poor contact between it and the clamp. For all cases of defects there is a simple test. Let's figure out how to test a resistor with a multimeter.
Types of Multimeters
The device can be pointer or digital. The first one does not require a power source. It works as a microammeter with switching of shunts and voltage dividers into specified measurement modes.
The digital multimeter shows on the screen the results of a comparison of the difference between the reference and measured parameters. It requires something that affects the accuracy of measurements as it discharges. It is used to test radio components.
Types of faults
A resistor is an electronic component with a specific or variable value of electrical resistance. Before checking the resistor with a multimeter, it is inspected, visually checking its serviceability. First of all, the integrity of the body is determined by the absence of cracks and chips on the surface. The terminals must be securely fastened.
A faulty resistor often has a completely burnt surface or partially in the form of rings. If the coating has darkened a little, this does not yet indicate the presence of a malfunction, but only indicates its heating, when the power released on the element at some point exceeded the permissible value.
The part may look like new even if the contact inside breaks. Many people have problems here. How to check the resistor with a multimeter in this case? It is necessary to have a circuit diagram according to which voltage measurements are made at certain points. To facilitate troubleshooting in the electrical circuits of household appliances, control points are identified with the value of this parameter indicated on them.
Checking resistors is done as a last resort, when there is no doubt about the following:
- semiconductor parts and capacitors are in good condition;
- there are no burnt tracks on the printed circuit boards;
- there are no breaks in the connecting wires;
- The connector connections are secure.
All of the above defects appear with a much higher probability than resistor failure.
Resistor characteristics
Resistance values are standardized in series and cannot take any value. For them, permissible deviations from the nominal value are specified, depending on the manufacturing accuracy, ambient temperature and other factors. The cheaper the resistor, the greater the tolerance. If, during measurement, the resistance value goes beyond its limits, the element is considered faulty.
Another important parameter is the power of the resistor. One of the reasons for premature failure of a part is its incorrect selection according to this parameter. Power is measured in watts. It is chosen the one for which it is designed. In the symbol diagram, the power of the resistor is determined by the signs:
- 0.125 W - double slash;
- 0.5 W - straight longitudinal line;
- Roman numeral - power value, W.
The replacement resistor is selected according to the same parameters as the faulty one.
Checking resistors for compliance with ratings
To check, you need to find the resistance values. They can be seen by the serial number of the element on the diagram or in the specification.
Measuring resistance is the most common way to test a resistor. In this case, compliance with the rating and tolerance is determined.
The resistance value must be within the range that is set by the switch on the multimeter. The probes are connected to the COM and VΩmA sockets. Before checking a resistor with a tester, the serviceability of its wires is first determined. They are connected to each other, and the device should show a resistance value equal to zero or slightly more. When measuring small resistances, this value is subtracted from the instrument readings.
If the energy of the batteries is insufficient, a resistance other than zero is usually obtained. In this case, the batteries should be replaced as the measurement accuracy will be low.
Beginners, not knowing how to test a resistor for functionality with a multimeter, often touch the probes of the device with their hands. When quantities are measured in kilo-ohms, this is unacceptable, since distorted results are obtained. Here you should know that the body also has a certain resistance.
When the device registers a resistance value equal to infinity, this indicates the presence of a break ("1" lights up on the screen). It is rare to see a breakdown of a resistor when its resistance is zero.
After measurement, the resulting value is compared with the nominal value. In this case, tolerance is taken into account. If the data matches, the resistor is OK.
When doubts arise about the correctness of the instrument readings, you should measure the resistance value of a working resistor with the same rating and compare the readings.
How to measure resistance when the value is unknown?
Setting the maximum threshold when measuring resistance is not necessary. In ohmmeter mode, you can set any range. The multimeter will not fail because of this. If the device shows "1", which means infinity, the threshold should be increased until the result appears on the screen.
Dialing function
How can you check the resistor with a multimeter for serviceability? A common method is dialing. The switch position for this mode is indicated by a diode icon with a signal. The sign of the signal can be different, the upper limit of its response does not exceed 50-70 Ohms. Therefore, it makes no sense to ring resistors whose values exceed the threshold. The signal will be weak and may not be heard.
When the circuit resistance values are below the limit value, the device emits a squeak through the built-in speaker. Continuity testing is done by creating a voltage between the points of the circuit selected using probes. For this mode to work, you need suitable power sources.
Checking the serviceability of the resistor on the board
Resistance is measured when the element is not connected to the others in the circuit. To do this, you need to release one of the legs. How to check a resistor with a multimeter without desoldering it from the circuit? This is done only in special cases. Here it is necessary to analyze the connection diagram for the presence of shunt circuits. Semiconductor parts especially affect the readings of the device.
Conclusion
When deciding how to test a resistor with a multimeter, you need to understand how electrical resistance is measured and what limits are set. The device is intended for manual use and you should remember all the techniques for using the probes and switch.
A variable resistor (rheostat) is an electrical device invented by Johann Christian Poggendorff, which serves to adjust and obtain the required resistance value... Well, blah blah blah... In a nutshell, we will talk about how to make a full-fledged variable from a tuning multi-turn resistor with the ability mounts on the instrument panel.
In amateur radio, there are three types of variable resistors - single-turn, multi-turn and slider. Not a word more will be said about sliders here. As is clear from the definition, the former can rotate by only one revolution, or more precisely, somewhere around 270°, while the latter can rotate by more than 1 revolution. Single-turn resistors are used where the value of resistance from the position of the resistor shaft is not particularly critical; multi-turn resistors are used, naturally, where it is critical. Let's look at examples.
Example 1."Classic" adjustable voltage source on LM317. Let's take the adjustment range to be 1.25 - 12 V. The angle of deflection of the resistor shaft from zero resistance to maximum is 270°, this is an approximate value, the exact value is indicated in the reference sheet for a specific resistor. We also assume that the voltage at the output of the power source will change according to a linear law depending on the resistance of our experimental subject, F(y)=x. We divide our output voltage range, 10.75 V, by the deflection angle of 270° and get approximately 0.04 V per 1° of resistor rotation. This is enough to accurately set the output voltage in 0.1 V steps. Conclusion: a single-turn resistor will suit us.
Example 2. Analog-to-digital conversion (ADC) on a microcontroller (MC). Let's assume that the variable resistor works as a voltage divider, the microcontroller converts the voltage entering it into digital information of 10 bits, and to make it clearer, it converts the voltage in the range, let it be 0 - 5 V, into a numerical value of 0 - 1023. Let's carry out the calculation as in example 1. Let's divide the number of our ADC values 1024 (0 in the digital world is always taken into account, so we have 1024 values) by 270°. We got about 3.8. That is, when the resistor shaft is rotated by 1°, the ADC value changes by almost 4 values! All your accuracy of the 10 bit ADC is reduced to NO. This is where we conclude that a single-turn variable resistor is not suitable for us. Let’s take, for example, a 10-turn resistor and see. 10 revolutions is 3600°, divide 1024 by 3600 and get 0.28. That is, when the resistor shaft is rotated by 1°, the ADC value will change by 0.28 (for reference, the ADC value is always an integer value and there will never be any hundredths or tenths, this is just an example). To change the ADC value by one unit, you need to rotate the shaft 3.5°.
I think that from the above examples it is clear which resistor should be used in this situation. If everything is clear with the numbers, then you can ask Uncle Google for the words ADC and LM317.
There is another classification of resistors with variable parameters: actual variable and tuning. I’ll immediately make a reservation that in the future in this article I will consider a multi-turn resistor. So, the tuning resistor is small and is installed, in most cases, on the board. The adjustment is made with a screwdriver and during normal operation of the device no one touches it.
The picture above shows the most common multi-turn trimmer. Its price is approximately $0.3. But pure-blooded variable resistors differ in size, have fasteners for mounting on a panel, and in most cases are connected to the board with wires.
Such resistors, compared to trimmers, have more power and service life and have the ability to install a rotary knob. And the price is approximately $2.8. To constantly change parameters, it is necessary to use a variable resistor. But if I don’t have it at hand, the price bothers me, and I won’t turn it often... You can make a variable out of a trimmer.
Such homemade products found on the Internet did not impress me. And soldering to the brass shaft of the trimmer completely killed me, so you can reduce the resource to zero from the start. I decided to do it my way. I used a small domestically produced single-turn variable as a basis:
I took it apart:
I was very lucky, the shaft and slider were not riveted together. Of all this, I only need a mounting flange with threads and a shaft. First, we cut off the excess at the flange, making two parallel edges:
The shaft shank needs to be ground down a little and the spline in the trimmer needs to be expanded at the same time. We adjust one to the other so that the shaft fits into the spline without wedging. We make the “housing” of the new resistor from a strip of tin. We put the strip on the flange, first drilling a hole in it, and then tighten the nut and bend the protruding petals. We find the alignment position of the trimmer with the shaft and place pieces of tin in the right places. We drill two holes to install screws that will tighten the sidewalls, thereby holding the trimmer in place. It should work out like this.
(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.
1.1 Non-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.
1.2. Wire.
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.
2. Basic parameters of variable resistors.
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.
2.1. Nominal resistance.
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.
2.2. Form of functional characteristics.
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.
3. Designation of variable resistors on diagrams.
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.
4. Trimmer resistors.
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.
5. Inclusion of variable resistors in an electrical circuit.
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 handle 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
An electrical circuit is impossible without the presence of resistance in it, which is confirmed by Ohm's law. That is why the resistor is rightfully considered the most common radio component. This state of affairs suggests that knowledge of testing such elements can always be useful when repairing electrical equipment. Let's consider the key issues related to how to check a regular resistor for serviceability using a tester or multimeter.
Main stages of testing
Despite the variety of resistors, conventional elements of this class have a linear current-voltage characteristic, which greatly simplifies the test, reducing it to three stages:
- visual inspection;
- the radio component is tested for breakage;
- Compliance with the nominal value is checked.
If everything is clear with the first and second points, then with the last there are nuances, namely, you need to find out the nominal resistance. Having a schematic diagram, this will not be difficult to do, but the trouble is that modern household appliances are rarely equipped with technical documentation. You can get out of this situation by determining the denomination from the markings. We'll briefly tell you how to do this.
Types of markings
On components produced during the Soviet Union, it was customary to indicate the denomination on the body of the part (see Fig. 1). This option did not require decoding, but if the integrity of the structure was damaged or the paint burned out, problems with text recognition could arise. In such cases, you could always turn to the circuit diagram that supplied all household appliances.
Figure 1. “ULI” resistor, the part rating and tolerance are visible on the bodyColor designation
Now color marking has been adopted, representing from three to six rings of different colors (see Fig. 2). There is no need to see this as the machinations of enemies, since this method allows you to set the denomination even on a heavily damaged part. And this is a significant factor, given that modern household electrical appliances are not equipped with circuit diagrams.
Rice. 2. Example of color marking Information on decoding this designation on components is easy to find on the Internet, so it makes no sense to present it within the framework of this article. There are also many calculator programs (including online) that allow you to obtain the necessary information.
Marking of SMD elements
Surface-mounted components (for example, SMD resistor, diode, capacitor, etc.) began to be marked with numbers, but due to the small size of the parts, this information needed to be encrypted. For resistances, in most cases, a designation of three numbers is accepted, where the first two are the value, and the last is the multiplier (see Fig. 3).
Rice. 3. An example of decoding the value of an SMD resistor Visual inspection
Violation of the normal operating mode causes overheating of the part, therefore, in most cases, the problematic element can be identified by its appearance. This can be either a change in the color of the case or its complete or partial destruction. In such cases, it is necessary to replace the burnt element.
Figure 4. A clear example of how a resistor can burn out Notice in the photo above, the component marked "1" clearly needs to be replaced, while the adjacent parts "2" and "3" may be working, but need to be checked.
Checking for a break
Actions are performed in the following order:
If the model of the device you are using differs from the one shown in the figure, read the instructions that came with the multimeter.
- We touch the pins of the problematic element on the board with the probes. If the part “does not ring” (the multimeter will show the number 1, that is, an infinitely large resistance), we can state that the test showed a break in the resistor.
Please note that this testing can be carried out without desoldering the element from the board, but this does not guarantee a 100% result, since the tester can show communication through other components of the circuit.
Validation check
If the part is soldered, then this stage will guarantee its functionality. For testing we need to know the denomination. How to identify it by markings was written above.
The algorithm of our actions is as follows:
What is clearance and how important is it?
This value shows the possible deviation of a given series from the specified nominal value. A correctly calculated circuit must take this indicator into account, or appropriate adjustments are made after assembly. As you understand, our friends from the Celestial Empire do not bother themselves with this, which has a positive effect on the cost of their goods.
The result of such a policy was shown in Figure 4; the part works for some time until the limit of its safety margin is reached.
- We make a decision by comparing the readings of the multimeter with the nominal value; if the discrepancy goes beyond the error limits, the part definitely needs to be replaced.
How to test a variable resistor?
The principle of operation in this case is not very different; we will describe them using the example of the part shown in Figure 7.
Rice. 7. Trimmer resistor (internal circuit marked with red circle) The algorithm is as follows:
- We take a measurement between legs “1” and “3” (see Fig. 7) and compare the resulting value with the nominal value.
- We connect the probes to terminals “2” and any of the remaining ones (“1” or “3”, it doesn’t matter).
- We rotate the adjustment knob and observe the readings of the device; they should change in the range from 0 to the value obtained in step 1.
How to check a resistor with a multimeter without desoldering on the board?
This testing option is only permissible with low-resistance elements. Above 80-100 ohms, it is likely that other components will interfere with the measurement. The final answer can only be given by carefully studying the circuit diagram.
