What is a tuning resistor: description of the device and its scope

Potentiometers are adjustable voltage dividers for adjusting it when supplied to a powered device at constant current. Other names are variable or tuning resistor, but it is necessary to distinguish between the rheostat mode. The resistance of the part, and, accordingly, the voltage power can be changed, thereby adjusting a specific function of the device being serviced. It is potentiometers that regulate the light intensity of lamps (dimmers), LEDs, sound (simple selectors or equalizers), the rotation speed of fans, and small electric motors. The controls for these radio components are known to everyone: rotating knobs or sliders (on old stereos, TVs, tape recorders), software solutions can be used (via microcircuits), there is also an automatic, automatically controlled potentiometer.

Potentiometer concept

In this article we will use the following abbreviations:

• PT and RS - potentiometer, rheostat;
• I - current;
• U is voltage;
• R - resistance (resistance).

Other names for potentiometer:

• resistor: trimmer or variable (trimmer, variable). These are two different types of the same part in question, they must be distinguished, but sometimes these names are used as a general name. Here we mean these parts in potentiometer mode, not rheostat mode. By default, we will assume them in this inclusion option;
• voltage divider. The name most correctly reflects the concept of what a potentiometer is.

A voltage divider is the same resistor; the part is created according to its type and according to a similar principle. But an ordinary such element (“constant” “fixed”) provides a fixed amount of resistance that blocks the current in the circuit, resisting it, thereby lowering it according to Ohm’s law.

A variable/tuning resistor has a plate (like a car wiper, scraper) or similar element that runs with a constant (sliding) contact along the resistive (sensitive) part. In this way, the resistance is changed, and most importantly, the voltage is divided, that is, it is adjusted, decreased/increased. Therefore, such radio components are called “voltage dividers.” Below in Fig. Western standard part designation:

The name DC potentiometer is a combination of the phrases “potential difference” and “meter” - I measure, the term appeared at the dawn of the development of electronics and implies a measuring device. When setting up dimensional resistive coils with wire windings, the property of the part was used to measure the potential difference values, which classified it as a measuring type. So it is, it’s just that now these qualities are adapted to voltage regulation. This aspect of the application quickly became the main one and 95% of such elements, or even more, are manufactured specifically for controlling electrical parameters.

Types of Potentiometers

Variable potentiometers are an analog device consisting of two main mechanical parts.

1. A fixed or stationary resistive element, track or wire coil that defines its resistance value, such as 1 kOhm, 10 kOhm, etc.

2. The mechanical part that allows the contact to move along the entire length of the resistive track changes its resistance value as it moves. There are many different ways to move a contact across a resistive track, either mechanically or electrically.

But along with the resistive track and wiper, potentiometers also contain a housing, a shaft, a slide block, and a bushing or bearing. The sliding contact movement itself can be a rotary (angular) action or a linear (direct) action. There are four main groups of variable potentiometer.

Rotary potentiometer

A rotary potentiometer (the most common type) changes its resistance value as a result of angular movement. Rotation of a knob or dial attached to the shaft causes the internal contact to move around the curved resistive element. The most common use of a rotary potentiometer is as a volume control.

Carbon rotary potentiometers are designed to be mounted on the front panel of a housing, enclosure, or printed circuit board (PCB) using a ring nut and lock washer. They can also have one single resistive track or multiple tracks known as a group potentiometer, in which all rotate together using one single rod. For example, a pot with two gangs for simultaneously adjusting the left and right volume levels of a radio or stereo amplifier. Some rotating pots have switches.

Rotary potentiometers can produce linear or logarithmic output with tolerances typically ranging from 10 to 20 percent. Because they are mechanically controlled, they can be used to measure shaft rotation, but a single-turn rotary potentiometer typically offers less than 300 degrees of angular movement from minimum to maximum resistance. However, there are multi-turn potentiometers called trimmers that provide a higher degree of rotational accuracy.

Multi-turn potentiometers allow the shaft to rotate through more than 360 degrees of mechanical movement from one end of the resistive track to the other. Multi-turn pots are more expensive but very stable with high precision, used mainly for trimming and fine adjustment. The two most common multi-turn potentiometers are 3-turn (1080 o) and 10-turn (3600 o), but 5-turn, 20-turn, and higher 25-turn pots are available with a variety of ohmic values.

Features of electrical parameters adjustment

To understand the operation of variable resistors, you need to know that always - in rheostat or potentiometer mode - both voltage and current change (U depends proportionally on I). Both operating algorithms are based on a change in resistance (R), which remains independent of the specified values. But it is precisely its adjustment on the variable that reduces/increases U and I.

Voltage dividers are resistors with a fixed resistance value. (ohm number) on it, and with a variable one set by an additional lever (scraper). This is a common element of electrical circuits, electronics, and household appliances. The controls are familiar to everyone - small round knobs, sliders, sliders, selectors.

Generally accepted classification of resistors

A wirewound variable resistor is considered the main and main element in any electronic equipment. It is applied as a discrete component or component to an integrated circuit. It is classified according to basic parameters, such as method of protection, installation, nature of resistance changes or production technology.

Classification by general use:

• General purpose.
• Special purpose. They are high-resistance, high-voltage, high-frequency or precision.

Depending on the nature of the change in resistance, the following resistors can be distinguished:

1. Permanent.
2. Variables, adjustable.
3. Adjusted variables.

If we take into account the method of protecting resistors, we can distinguish the following designs:

• With insulation.
• No insulation.
• Vacuum.
• Sealed.

Where and for what are voltage dividers used?

PTs normalize voltage; more often they are used to adjust application parameters (serviced equipment) within the normal values ​​for which it is designed, when such a function is built into it, for example, sound volume, fan speed. The most common model is with manual adjustment, but there is also an automatic integrated potentiometer.

PT is also used when it is necessary to establish the desired mode of equipment in difficult conditions, when a certain level of electrical parameters can disable the application, or for research, for the purposes of maintenance, repair, experiments, and adjustment.

An increase/decrease in U supplied to the load, which also entails changes in current, is carried out by potentiometers or rheostats. We will look at the difference between them below. In fact, these terms do not denote the part itself (in all cases this is a variable resistor), but the modes of its inclusion in the circuit.

The most typical examples of what is regulated:

• power and other parameters (adjustment by equalizers) of sound, video brightness/shades, light (dimmers);
• speed of low-power electric motors of household appliances and toys;
• Fans with variable speed settings have voltage dividers. Even those whose rotation intensity is meant to be set to constant operation with a certain value? often have a trimmer on the chip;
• generator frequency;
• calibration of electrical circuits, on microcircuits for adjusting electrical parameters by voltage (its output power).
• Precision, including automatic high-precision potentiometer is used in angular and linear displacement sensors.

Variables/trimmers are used wherever adjustment of the output voltage is required. But you need to understand that such a device is needed only for high-resistance loads and low currents. Where these parameters are large, rheostats are used. For example, the dimmer may contain a DC, but if the incandescent lamp is powerful, then it will be useless and a PC must be used. The same applies to electric motors: low-power ones can be regulated by PT, but powerful power plants in vehicles have RS. In order to study where to apply what, you need to do calculations using Ohm's formulas.

Potentiometer and rheostat: what is the difference

Let's consider RT in detail, since the properties of potentiometers will also be revealed in the process. So, the same variable resistor can be mounted on the circuit in two ways, creating two modes:

• parallel connection - PT. The potentiometer connection usually uses all 3 pins.
• sequential - rheostat. Only 2 contacts are used.

“Potentiometer” and “rheostat” are simply different options for including the same variable resistor in a circuit, respectively, in series or parallel. Both parts work specifically with R, U, I. But the proportionality of the changes is different, in the first case the voltage is regulated to a greater extent, in the second - the current.

The rheostat has two outputs, the potentiometer has three (if used as the first, then only two contacts are connected). That is, the PC is included in the circuit like an ordinary resistor. Both are non-polarized and can work in reverse.

PT and PC are connected differently. The second, unlike the first, is usually an industrial device or on powerful equipment. Some schools taught lessons with a rheostat, so some may remember its shape: a large ceramic tube with a nichrome winding and a slider on the middle terminal that is not connected anywhere. RS has high power (passes powerful current) and low resistance (up to tens of ohms). It has significant inductance, which is taken into account in HF devices.

Voltage dividers are usually low-power, so they are rarely suitable for the role of PCs; in production, variables up to 10 W are positioned as the first, and from 10 W - as the second.

Variable as a rheostat

RS changes the overall resistance. circuits - it is this property that is important here; it is used in its full, most effective form to control (limit) the current.

It is connected to the circuit only in series: the switched-on variable is called a rheostat (this is the operating mode).

You can imagine the circuit as such, consisting of two ordinary resistors connected in series, that is, the slider divides the PC coil into the indicated elements. By adjusting R, the parameters of these resistors and, accordingly, the current in the circuit are reduced/increased.

What kind of resistor is this

A trimmer resistor is a miniature version of a standard variable resistor. They are designed to be mounted directly on the PCB and are only adjustable when setting up the circuit. For example, to adjust the sensitivity of a sensor or set the gain of a power amplifier.

In the literature, trimming resistors and variables are often understood as different circuit elements, although, strictly speaking, any trimming resistor is also variable due to the fact that its resistance can be changed.

To operate the trimmer, you need a small screwdriver or something similar. Just like trimming capacitors, trimming resistors can be single-turn or multi-turn, made on the principle of a worm gear.

But unlike them, you don’t need a special adjusting screwdriver to work with a trimming resistor. Placing your hand or a steel screwdriver close to the resistor does not affect its resistance in any way. The trimming resistor is adjusted with a regular screwdriver, which is inserted into a special groove in the adjustment mechanism associated with a circular slider.

Multi-turn trimmers are used in those parts of the circuit where precision is needed in setting the required resistance. It is impossible to achieve high tuning accuracy with single-turn trimmer resistors.

Trimmer resistors are used for one-time adjustment of resistance; for example, trimmer resistors can always be found as potentiometers in feedback circuits of switching power supplies. There are also multi-turn trimmer resistors.

Trimmer resistors have small overall dimensions and are designed for only a few adjustment cycles for the purpose of preliminary or preventative adjustment of equipment, and, as a rule, they are not touched anymore.

Therefore, trimming resistors are not very stable and durable compared to variable resistors, and are designed for a maximum of several tens of adjustment cycles. It is obvious that a tuning resistor will never replace a variable one, and if this principle is violated, then you can pay for the low reliability of the device being designed.

Areas of application of trimming resistors.

Variable resistor as potentiometer

An appropriate and more correct other name for PT is a voltage divider. If we take the above diagram, then these are also 2 or more resistors with a serial connection, but such a node of them (chain) is connected in parallel to the source, which allows adjusting their resistance to obtain exactly the voltage required for the load.

Difference in scope

The potentiometer has low power and is used for relatively low-power devices: TVs, audio equipment, low-power dimmers, underfloor heating controllers, boilers, as converters, for adjusting the speed of weak motors, for fans, for example, computer coolers, ventilation systems.

We will characterize the use of RS with a selection from a thematic site:

The areas of use at first glance are similar to PTs, but this is not the case: RSs are used where there are high currents and the operation of devices depends on them: powerful power tools, electric motors of vehicles and production ones, in industry.

We can say that the alternator for lamps operating with high currents and the same loads in the form of electric motors, for electric furnaces, and machine tools is used only in rheostat mode.

The clearest explanation for the difference in application

With a potentiometer, the current from the source is spent several times higher than the load needs. At PC, the value of this value is equal to that at the load. Therefore, the latter is used to adjust I and U on low-resistance loads, they have a pattern - they consume relatively more powerful currents, and potentiometers - for high-resistance loads, since they are usually powered by this value with a small value.

Features in appearance

The variable can be both, but if it is manufactured for rheostat mode, then it has a typical size for it: with two terminals, with a large resistive part (winding), usually a large, thick, heavy wire resistor and its shape is much larger, what about the parts for PT.

It is necessary to distinguish between terms, since sometimes confusion arises in different sources: for example, the phrase “potentiometer in rheostat mode” is not entirely correct, since this designates two different connections, but the phrase “variable resistor in rheostat (or potentiometer) mode” is correct. Although erroneous lexical formations are often found even on technical websites, the main thing here is that the user distinguishes what is being said.

If a part has two outputs, then its state is only PC, but if there are three, then such a part can theoretically be used as it (we described this above), but in reality it is intended specifically for the DC mode.

Inclusion of variable resistors in an electrical circuit.

In electrical circuits, variable resistors can be used as a rheostat (adjustable resistor) or as a 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 by the 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 turning on the resistor with a 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 - nonlinear resistors. 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

Algorithm for the operation of a potentiometer, comparing it with that of a rheostat

It is appropriate to reveal the principle of operation simultaneously for PT and RS, since we are essentially talking about one part in different modes:

The principles of operation are revealed by the laws of the process of changing current and voltage. For the rheostat, let's take a light bulb as the load (in the diagrams above). With increasing resistance on PC the same thing happens with the general resistance. (Rtotal), and the same current decreases. Consequently, both I at the load and the voltage across it drop.

The decrease/increase in current in the circuit is not inversely proportional to that of the resistor. RS, because in addition to the adjustable R rheostat, there is also R, but unchanged - at the load. Only when Rreost >>Rн will these quantities change with close to inverse proportionality. On the contrary, if Rreost

Now we will explain the action in the process of describing how U of the load changes: the total value at the current source (Uist = Un + Upeost) between RT and U is proportional to their R:

When this decreases on the PC, a redistribution of the total U occurs and at the same time the U of the load, and, consequently, I through it increases.

Now let's move on to the potentiometer. This is the same rheostat, but connected differently and the name “voltage divider” most accurately reflects its essence.

The PT regulates current and voltage on high-resistance (we focus on this) equipment, that is, with this parameter, the use of RS is inappropriate, if not completely impossible. A variable resistor as a PT is connected to the source by the lower terminals A and B, which are the ends of the winding, enclosed (crimped) in a way convenient for inclusion in the circuit. Unlike a PC, terminal C is connected to the consumer and A and B are also connected to it.

Operating principle of PT:

• voltage is applied to the entire resistor;
• but for the consumer only part of it is removed, which can be adjusted by moving the slider D between points A and B;
• in this case, the indicated load value Un will vary from 0 to the maximum U of the source.

U at the consumer can fluctuate in direct proportion to the length of the segment ^AC, but it can also have a more complex relationship U = f(l), determined by the ratio of R load and R potentiometer. There are such patterns:

Basic properties of variable resistors

When a person has a clear idea of ​​the conventional elements of graphic display on diagrams, then he has the problem of transferring the drawing into reality. You need to find or purchase individual components of a ready-made circuit. Today there are a large number of stores that sell the necessary parts. You can also find elements in old broken radio equipment.

A variable resistor must be present in any circuit. It is found in any electronic devices. This design is a cylinder that includes diametrically opposed terminals. The resistor creates a limit on the flow of current in the circuit. If necessary, it will perform resistance, which can be measured in ohms. A variable resistor is indicated on the diagram in the form of a rectangle along with two dashes. They are located on opposite sides inside the rectangle. Thus, a person denotes power on his diagram.

The equipment, which is found in almost every home, includes resistors with a certain value. They are located along the E24 row and conventionally indicate the range from one to ten.

Calculation, selection of potentiometer parameters

So, the potentiometer is designed to regulate the voltage specifically on a high-resistance load - it must have resistance. higher than PT, otherwise the number of Volts will be determined by it, and the adjustment function will disappear.

The main features for calculating PT are:

• resistance The PT should be much smaller (Rpot<<Rн) than that of the load. This is not necessary, but if not observed, further calculations will become more complicated - you will have to take into account the current on it. The recommended values ​​are at least 10 times lower, but better - 20, 30, 100. The lower, the better, but not excessively, otherwise the requirements of the following points will not be met;
• U of the current source must be suitable, the PT must withstand it (Inom.pot×Rpot) > Usource. In this case, the number of Amperes flowing through the alternator (Ipot = Uust/Rpot) should be less than the current rating of the part;
• the current passing through the PT (Ipot = Uust / Rpot) should not be higher than the nominal value according to that of the source (Ipot < Inom. source);
• if there are several PTs and they all fit the above conditions, then take a product with higher resistance - it will consume less current, which is especially significant when used with galvanic batteries.

Another nuance of adjusting current and voltage with a rheostat and potentiometer:

• both make it possible to obtain U at the load equal to or lower than U of the source;
• but with PT you can lower the above value to 0, which is extremely difficult, almost impossible, to achieve from RS.

The Importance of Power Dissipation

When selecting a variable resistor, the resistance rating is taken into account first, but current rating, in other words, power dissipation, is no less important to pay attention to. The two parameters are interrelated. Let's explain with an example. The circuit contains a resistor with a certain R, but it turns out that this value should be significantly lower, that is, the part must be replaced.

They put in an element with a significantly lower R, and it would seem that the problem is solved, but here there is a danger associated with ignoring Ohm’s law. R on the resistor was significant, U of the circuit was fixed. When the value of the variable was lowered, the total R of the line fell, as a result, the current increased. If you install a PT with the same dissipation power, then with increased I it may not withstand the load, the consequences are traditional - overheating, even to the point of fire.

Approximate norm: with a nominal value of 10 Ohms, a current of about 1 A should flow through the circuit - this is the power dissipated by the resistor. When choosing, be sure to look at this permissible value for the part.

Design and types of variable resistors

A potentiometer is primarily an analog electromechanical part; there are also digital types, but they are still not very common. The wiper can also be moved using electrical means, not only manually. Movement can be angular (rotation) or linear (straight); Usually it is manual, but there is also an automatic, application-configurable potentiometer.

Potentiometer structure (the first two parts are main):

• resistive element;
• a plate (wiper, scraper) with a sliding contact moved by a handle (selector, lever) along the above part;
• terminals at each end of the part;
• mechanism that moves the wiper (shaft, slider block), bushing, bearing
• a housing in which the resistive part is enclosed.

The described device can be taken as a general principle, which is implemented in one form or another in all types of radio components; string and digital PTs are also based on it, but with their own specifics.

The resistive part of inexpensive variables is often made of graphite. Sensitive wire, plastic with carbon particles, and a mixture of ceramics and metal (cermet) are also used. For options with conductive tracks, polymer conductive pastes with carbon, wear-resistant resins, solvents, and lubricants are used. To summarize, according to the design of the case, the potentiometer has its own types and types in the form of pots (barrels, boilers), strips and chips (trimmers).

According to the material of the sensitive part

Wire - Constantine or Manganin wires are laid evenly in a horseshoe in the body. The slider slides along the turns, touches the next one before leaving the previous one - this is how smooth adjustment is achieved.

Thin film. The sensitive part is a horseshoe-shaped frame of a dielectric plate with a thin film: carbon, boron, metallized, composite materials. Trimmers and trimmers are often like this

By number of contacts

There are single-element models - these are standard variables. There are multi-element ones - double, triple, and so on - then there are more contacts for each such part. There are also products with contacts for the switch (below in the figure).

Rotary (circular, disk) variables

The most common variables are rotary ones. They can be equipped with a switch, usually triggered in the extreme counterclockwise position. Thus, you can immediately, without a separate element, turn off/on radio receivers such as equipment: for example, the device starts after a click when turning the selector at minimum volume, then it can be increased.

Pins 1 and 2 are also on regular resistors - of a constant value. Resistance is created by a special coating, an array of conductive alloy (including in the form of film, spraying), and a wire winding (nichrome and the like) on the “body” between them. For variable type, a third terminal is added (for the scraper), connected to the “engine”, a movable plate (wiper), moved with a sliding contact along this segment; for rotary types it is in the form of a horseshoe (arc).

If you turn the knob, R changes between 1 and 3, from 0 to the value stamped on the device body. The same thing happens between 2 and 3 “upside down”: when R between 1 and 3 increases, then between 2 and 3 it decreases and vice versa.

Linear, with sliders

Linear (sliders) have the shape of a plate, bar, that is, a longitudinal resistive element. The adjustment tool is a slider that slides along rather than rotating. The disadvantage of such models is that they are less protected from dirt: dirt can get inside through any part of the slot, even though there is a slider seal. The advantage is better visualization of the settings indication.

For rotary models, the position of the handle is often impossible to determine, especially if a “bare” resistor is mounted - for this you need to do calibration. If there is a scale, then it is perceived less conveniently. On the linear type, the setting position is visible even without the indicated one - by the position of the slider. For example, for equalizers and faders, you can determine the adjustment positions, evaluate the overall picture of the settings of a dozen or more sliders on the control panel, without looking too closely. In addition, linear FETs are usually more accurate because the resistive part is longer.

Multi-turn

A conventional circular variable makes complete movement of the adjusted point in 1 turn of the adjustment screw and is incomplete; the horseshoe should not close. For some tasks they are not accurate enough. For special sensitivity there are multi-turn options. They complete the described cycle in a certain number of revolutions, which greatly reduces errors. For example, turning the selector of a single-turn model with a nominal value of 10 kOhm half a turn will change the resistance. at 5000 Ohms, if the tolerance is 10%, then it will give an error of 500 Ohms. Multi-turn potentiometer 10 rpm with the same parameters, with a similar rotation of the lever, it gives a deviation of only 50 Ohms - 0.5% of the nominal value.

On average, turning the selector by the same angle gives an accurate setting one-tenth better than single-turn models.

Strings

A special subtype are string voltage dividers; they are controlled by a flexible cable and a spring-loaded coil. They are used for measurements on moving objects, for measuring linear position in industry, manufacturing, medicine, robotics, and on automated production lines.

Particularly accurate

Rotary or linear PTs with fixed positions marked with clicks are special discrete precision models made up of several resistors. They are used in testing equipment and on particularly precise equipment.

Automatic

The adjustment is not done manually, but by automatic application.

Twin

Several resistors can be located on the same shaft with their sliding scrapers, which makes it possible to regulate 2 or more channels in parallel. Used in audio amplifiers.

Trimmers (trimmers, presets)

Trimmers, also known as trimmers or pre-installed ones, are standard sizes for soldering onto a board. When a manufacturer creates a microcircuit, the part is soldered in and the required position is immediately set. It is assumed that it is the best for the device and it is not recommended to change it, except for the purposes of repair or special configuration cases.

Although there are also parts that require constant adjustment, for example, when such a standard size is appropriate for a particular device.

Digital

Digital PTs are becoming popular, which are integrated circuits without moving parts that allow you to adjust your own R programmatically with a given step.

Types and types of device

There are many types of trimming resistors on the market today. These are non-separable trimming resistors of type SP4-1, filled with epoxy compound, and intended for defense equipment, and trimmers of type SP3-16b for vertical mounting on a board.

It will be interesting➡ Diode bridge - what is it?

In the manufacture of household equipment, small trimming resistors are soldered onto the boards, which, by the way, can reach 0.5 watts in power. In some of them, for example in SP3-19a, metal ceramics are used as a resistive layer.

There are also very simple tuning resistors based on varnish film, such as SP3-38 with an open case, vulnerable to moisture and dust, and with a power of no more than 0.25 watts. Such resistors are adjusted with a dielectric screwdriver to avoid accidental short circuits. These simple resistors are often found in consumer electronics, such as monitor power supplies.

Some trimmer resistors have a sealed housing, for example R-16N2, they are adjusted with a special screwdriver, and are more reliable because dust does not get on the resistive track and moisture does not condense.

Powerful three-watt resistors of the SP5-50MA type in the case have holes for ventilation, in them the conductor is wound in the shape of a toroid, and the contact slider slides along it when the handle is turned with a screwdriver.

In some CRT TVs you can still find high-voltage trimming resistors, such as HP1-9A, with a resistance of 68 MOhm and a nominal power of 4 watts. Essentially, this is a set of cermet resistors in one package, and the typical operating voltage for this resistor is 8.5 kV, with a maximum of 15 kV. Today, similar resistors are built into TDKS.

In analog audio equipment you can find slider or slide variable resistors, such as SP3-23a, which are responsible for adjusting volume, timbre, balance, etc. These are linear resistors, which can also be double, such as SP3-23b.

What do linear resistors look like in a diagram?

Trimmer multi-turn resistors are often found in electronic equipment, measuring instruments, etc. Their mechanism allows you to precisely regulate the resistance, and the number of turns is measured in several tens.

The worm gear allows the sliding contact to turn slowly and move smoothly along the resistive track, allowing the circuits to be tuned very, very accurately.

It will be interesting➡ Varistors - what they are, operating principle, characteristics and parameters.

For example, the SP5-2VB multi-turn tuning resistor is adjusted precisely by means of a worm gear inside the housing, and to completely pass the entire resistive track you need to make 40 turns with a screwdriver. Resistors of this type in various modifications have a power from 0.125 to 1 watt, and are designed for 100 - 200 adjustment cycles.

This is not a complete overview of the types and types of parts. As we can see from the previous description, tuning resistors are inherently close to variables, but strictly speaking, they are not. This video briefly but clearly describes how to convert a trimmer resistor into a variable resistor.

Types according to the “cone” - the nature of the change in resistance

“Cone” or “law” is the relationship between resistance. and the position of the scraper. Manufacturer controlled. Any ratio is possible, but for most tasks linear and logarithmic PTs (“sound cone”) will suffice.

A letter code may be used, but this is not standardized; it may be different for different manufacturers, but usually the marking is as follows:

• Asia and the USA. A - for logarithmic, C - for inverse logarithmic (rare, exponential) and B - for linear taper;
• Europe - A for linear, C and B for logarithmic, F for its inverse version.

Explanation

The percentage relating to the non-linear cone refers to the resistance index. at the midpoint of shaft rotation. The 10% cone of a log measures 10% of the total R on it. That is 10% logarithm. a cone on a 10 kOhm PT gives 1 kOhm at the indicated mark. The higher the percentage, the steeper the logarithm. curve.

How voltage dividers are shown in diagrams

In order for the reader to distinguish the details more deeply, we indicate the graphics for PT and RS.

Designation of rheostats:

Designation of voltage dividers:

Designation of a simple (fixed) resistor:

Potentiometer on the diagram

When using a potentiometer, connections are made at both ends and also to the contact brush as shown in the figure. The position of the contact brush provides a corresponding output signal (pin 2) that will vary between the voltage level applied to one end of the resistive trace (pin 1) and the voltage level at the other (pin 3).

The potentiometer is a three-wire resistive device that acts as a voltage divider, producing a continuously varying voltage output that is proportional to the physical position of the contact brush along the trace.

Potentiometer connection

To begin with, we present a block of the most typical schemes. It must be said that the PT can be connected not only as a PC, but also as a simple fixed resistor (options in Fig. 3):

Below are the most common schemes (notations according to Western standards):

It must be said that the traditional wiring diagram for a potentiometer frequency switch always recommends connecting an “extra” pin; a break in the “moving contact - horseshoe” line is not excluded, which can lead to unpleasant consequences.

The diagrams for how to connect a PT are extremely simple; in fact, there is only one option - in parallel to one of the power wires.

For example, this is what the regulator on a computer cooler looks like. In this case, polarity does not matter. Take any cooler power supply wire, cut it, one end is soldered immediately to the first and second (middle) contacts, the second to the remaining one. That is, on the first 2 contacts there is one end of the wire (they are soldered to the same conductor), the third contact is the other end, as if standing alone.

The complexity of some circuits: you need to know which wire to connect to, that is, which power line to regulate, for example, if you are connecting an external potentiometer for variable-frequency electric drives to adjust the intensity of rotation of electric motors, when adjusting PID controllers.

In such cases, they are guided by the schemes of the manufacturers or authors of such improvements, the recommendations of the masters; all the information is available online on special forums and thematic sites. Below is an example of connecting to a frequency converter:

How to check serviceability with a multimeter

To measure resistance you will need a digital multimeter. In order to measure resistance, we need to turn the knob to “measure resistance”. Using a stick, we can twist the resistor clockwise or counterclockwise, thereby changing the resistance between the middle contact and the two outer contacts. Rules for measuring resistance:

1. Press the probes with some force onto the resistor terminals. In this way, you will eliminate the appearance of contact resistance, which, when pressed lightly, will add up to the measured resistance.
2. When measuring the resistance of a resistor on a printed circuit board, double-check that the board is de-energized. Then unsolder one end of the resistor and then measure its resistance.
3. Do not touch the resistor leads when measuring its resistance! The average human body has a resistance of about 1 KiloOhm and depends on many factors. Therefore, by touching the resistor terminals when measuring resistance, you introduce an error into the measurements.
4. If you want to measure the resistor's resistance as accurately as possible, clean its terminals either with a knife or with the mildest sandpaper. In this case, you will remove the oxide layer, which in some cases introduces a noticeable error in the resistance measurement.

Do not measure resistance under voltage! This could damage the multimeter or cause you an electric shock!

We place the probes at the extreme contacts. We measure the total resistance of the variable resistor. In order to check whether it is working, turn the variable resistor knob all the way counterclockwise and measure the resistance between the left and middle contacts. It should be close to zero.

Checking the tuning resistor with a multiprobe.

Next, turn the handle clockwise, but not all the way. We measure the resistance again between the middle and left contacts, then between the middle and right. The total should be the result of the resistance of the two extreme contacts.

We also suggest reading interesting material about little-known facts about DC motors in our other article.

Examination

Checking the potentimeter is done with a multimeter:

• the resistance measurement mode is set, the probes touch the two outer contacts - the tester should show a value equal to the nominal one with a permissible deviation. Move the slider and observe how R changes on the multimeter display;
• the wire winding may be torn, the contacts may come loose - then the break is checked as standard - the tester is set to “continuity”, the two outer contacts are touched with probes: 1 - break; 0, digital values ​​tending to it or beeping (if there is a buzzer) - the circuit is intact.

Variable resistor in the diagram

When using a variable resistor, connections are made to only one end of the resistive track (pin 1 or 3) and the contact brush (pin 2), as shown in the figure. The position of the contact brush is used to change the amount of effective resistance connected to each other, the moving contact and the fixed end.

Sometimes it is advisable to make an electrical connection between the unused end of the resistive trace and the contact brush to prevent open circuit conditions.

A variable resistor is then a two-wire resistive device that provides an infinite number of resistance values ​​that control the current offered to the connected circuit, proportional to the physical position of the contact brush along the trace. Note that variable resistors used to control very high circuit currents found in lamps or motor loads are called rheostats.

How to choose

The process of how to select a voltage divider involves studying the following parameters (some of the data is in GOST 10318):

• priority, ratings for: resistance;
• to the utmost slave tension;
• by power (dissipation);

How are they labeled?

Traditional markings, known since Soviet times: PTP, PLP, PPML, PLP, RPP, PPBL, PPMF. What should be indicated on the product is regulated by GOSTs 9245, 8.478-82. The alphanumeric characters themselves are found in OST 11.074.009 (current), GOST 13453 and 3453 (outdated).

New marking:

But it must be said that in modern conditions there is no single standard, so you need to look at the product specification from the manufacturer.

Repair

If a contact falls off, it can be soldered, but this is usually difficult to do, much less it is impossible to repair a mechanically damaged track or wire. In case of such breakdowns of functional parts, especially the resistive segment, the variables are not repaired.

If the functional parts are without mechanical damage, then you can try the following methods:

• restore the sensitive track: o lightly bend the spring of the movable contact with the lead of a simple pencil (consists of carbon) and draw along the sensitive layer. Method for thin film models;
• o grind the same stylus, mix with lithol or a similar lubricant, lubricate the path along which the slider walks;

Cleaning the trimmer with regular alcohol

The resistor in circuits can become dirty; its slider track becomes covered with a layer of dust over time. And in order to return the electrical resistance to its previous performance, you just need to clean it.

Cleaning trimmer resistors is quite simple and quick. It is best to use pure alcohol for these purposes. It is better not to use various products such as nail polish remover, moonshine, or cleaners, as they may contain impurities that negatively affect the cleanliness of the resistor.

To better master the material, we also recommend reading the following material: everything you need to know about stepper motors.

So, we disassemble the resistor (if it has a protective casing), for this it is usually enough to unbend the small metal clips on the resistor body itself, after which you need to remove this cover. Inside the resistor we will see a track along which the slider of the middle terminal of the resistor moves. It is this path that needs to be cleaned from dirt with alcohol.

It’s convenient to do this: take a syringe (let’s say 2 cc), fill it with alcohol, and carefully apply a few drops through the needle of the syringe directly onto the resistor track. After this, we begin to rotate this resistance in different directions so that the alcohol spreads throughout the entire track and thereby clears the way for the slider.

How to clean a resistor at home.

In principle, this is enough so that after assembling and installing the tuning resistor in our circuit workplace, we can enjoy its normal operation without previous problems. Although, if space on the resistor itself allows, you can also carefully go over it with a cotton swab, which will completely remove all dirt from the slider track.

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Well, then we need to put our updated resistor back together and put it in our workplace. In most cases, after such cleaning, the electrical resistance is completely restored, and the intermittency of its operation disappears.

Difficult cleaning cases

In very rare cases, it is not a matter of dirt, but, for example, the destruction of this path as a result of excessive overheating. This can happen when too much voltage is accidentally applied to this resistor and the power of this resistor is not large enough to quickly dissipate the generated heat from the high current. This is where the variable resistor track heats up greatly, followed by its destruction. Cleaning with alcohol won't help here.

This resistor needs to be completely replaced with a new one that is known to work. And, of course, before installing a new resistor on the old circuit, check it so that the process of destroying the track does not repeat with a new resistance.

Unfortunately, not all types of variable and trimming resistors can be cleaned using the above method. Sometimes there is resistance in the solid body, which makes it impossible to reach the slider track.

Here you can go to extreme measures. Make a small hole in the body (with a 0.8-1 mm drill). Well, pour alcohol through it with a syringe through a needle. Next, again turn the resistor knob in different directions and then you need to wait until the alcohol has completely evaporated.

You can warm up this variable resistor a little (up to 50 degrees), this will speed up the evaporation of alcohol. Although pure alcohol is a dielectric, it does not conduct current through itself. Consequently, it will not negatively affect the operation of the variable resistor, even if there is some alcohol left on it, which will still evaporate.