The principle of operation of a variable resistor. variable resistors. Functional characteristic form

You will need

The performance of these works will require basic knowledge of radio engineering, methods of working with measuring instruments (tester, ohmmeter), as well as skills in handling a screwdriver, soldering iron, and pliers.

Instruction

Determine with the help of technical documentation or circuit diagrams what function the variable resistor performs in the device (it is an adjustable resistance or a potentiometer). Set, using the specification or by calculation, the nominal / value of the variable resistance and its type. Then select the required type and value of the variable resistor or its equivalent.

Check its performance with a resistance measuring device (ohmmeter) and find the terminal where the resistance changes. It's called a "slider".

Switch the pins of the variable resistor according to its function: connect the "slider" pin of the resistor to one of the two remaining pins to get a variable resistor, or use all the pins of the resistor to use it as a potentiometer.

Install the device in the device or on the mounting panel and connect its outputs, in accordance with the circuit diagram. Check the fuse ratings and turn on the device in compliance with safety standards to check its operation.

Useful advice

Variable resistors are used in devices where it is necessary to vary the resistance value. With a change in resistance in the circuit, the current will change, in accordance with Ohm's law. And at the output of the potentiometer, you can get any voltage value, but it will always be no more than the input voltage. Potentiometers are used to adjust parameters such as output voltage, power, volume, etc. in devices.

Today, LEDs are used everywhere: as indicators, lighting elements, in flashlights and even traffic lights. There are thousands of models of these devices. On their basis at home, you can easily assemble entertaining devices. LEDs are freely sold in radio parts stores. Unlike incandescent lamps, they cannot be connected directly to a current source - LEDs fail. You need a limiting resistor. Therefore, the question of how to calculate the resistance to the LED arises immediately before using it.

You will need

A reference book on light-emitting semiconductor devices, knowledge of standard resistor values (series E6, E12, E24, E48), or Internet access to obtain the necessary data. A piece of paper with a pen or a calculator.

Instruction

Find out the electrical parameters of the LED used. For resistance, you need to know the forward voltage and rated current of the device. Knowing the model, in the directory or on the Internet, find the required parameters. Memorize or write down their meanings.

Determine the voltage from which the LED will be powered. If you intend to use galvanic cells or batteries as a power source, find out their nominal voltage. If the LED is to be powered from circuits with a large voltage spread (such as the mains), determine the maximum possible voltage for the circuit.

Calculate the resistance to the LED. Calculate using the formula R = (Vs - Vd) / I, where Vs is the power supply voltage, Vd is the forward voltage of the LED, and I is its rated current. Select the nearest higher resistance value in one of the nominal resistance series. It makes sense to use the E12 series. The tolerance in the resistance ratings of this series is 10%. So, if the calculated value of the resistance R = 1011 Ohm, the value of 1200 Ohm must be chosen as the real resistance.

Calculate the minimum required power of the damping resistor. Calculate the value using the formula P = (Vs - Vd)² / R. The values of the variables Vs and Vd are the same as the values of the previous step. The R value is the resistance calculated earlier.

note

Do not connect LEDs in parallel using a single quenching resistor. Due to the natural scatter of device parameters, some of them will have an increased load, which can cause them to fail.

Useful advice

If the LED model is not known, a variable resistor can be used to experimentally determine the required value.

Sources:

how to calculate resistor for led

The LED is a semiconductor device that has firmly entered our lives and slowly began to replace traditional light bulbs. It has low power consumption and small size, which has a positive effect on its areas of application.

Instruction

Remember that any LED connected to the network must have a resistor connected in series, which is necessary to limit the amount of current flowing through the semiconductor device. Otherwise, there is a high probability that the LED can quickly fail.

Therefore, before assembling a circuit containing LEDs, carefully calculate the resistance value, which is defined as the difference between the supply voltage and the forward voltage, which is calculated for a particular type of diode. It ranges from 2 to 4 volts. Divide the resulting difference by the current of the device and as a result get the desired value.

Remember that if you cannot accurately select the resistance value of the resistor, then it is better to take a resistor with a slightly larger value than the desired value. You will hardly notice the difference, because the brightness of the emitted light will decrease by an insignificant part. Also, the resistance value can be calculated using Ohm's law, in which the voltage flowing through the diode must be divided by the current.

When connecting several LEDs in series at once, it is also necessary to set the resistance, which is calculated in a similar way. Remember that the total voltage from all diodes is taken here, which is taken into account in the formula for determining the parameters of the resistor.

Also, do not forget that it is forbidden to connect LEDs in parallel through one resistor. This is due to the fact that all devices have a different spread of parameters, and one of the diodes will glow brighter, therefore, a large amount of current will pass through it. Eventually, this will cause it to fail. Therefore, when connected in parallel, set the resistance for each separately.

There are various connection schemes, depending on which the variable resistor can be both a source of variable resistance and a potentiometer. It all depends on the type of connection of its third output.

Resistors are passive elements of electrical circuits. These elements are used to linearly convert current to voltage or vice versa. When the voltage is converted, the current strength may be limited, or the absorption of electrical energy may occur. Initially, these elements were called resistances, since it is this value that is decisive in their use. Later, in order not to confuse the basic physical concept and the designation of radio components, the name resistor began to be used.

Variable resistors differ from others in that they can change resistance. There are 2 main types of variable resistors:

potentiometers that convert voltage;
rheostats that regulate the current.

Resistors allow you to change the volume of the sound, adjust the parameters of the circuits. These elements are used to create sensors for various purposes, alarm systems and automatic switching on of equipment. Variable resistors are needed to adjust the speed of engines, photo relays, converters for video and audio equipment. If the task is to debug the equipment, then tuning resistors will be required.

Potentiometers

The potentiometer differs from other types of resistance in that it has three outputs:

2 permanent, or extreme;
1 movable, or medium.

The first two leads are located at the edges of the resistive element and are connected to its ends. The middle output is combined with a movable slider, through which there is movement along the resistive part. Due to this movement, the resistance value at the ends of the resistive element changes.

All variants of variable resistors are divided into wire and non-wire, it depends on the design of the element.

To create a non-wire variable resistor, rectangular or horseshoe-shaped plates of isolate are used, on the surface of which a special layer is applied that has a given resistance. Usually the layer is a carbonaceous film. Less commonly used in the design:

microcomposite layers of metals, their oxides and dielectrics;
heterogeneous systems of several elements, including 1 conductive;
semiconductor materials.

Attention! When using carbon film resistors in the power circuit, it is important to prevent the element from overheating, otherwise sudden voltage drops may occur during the adjustment process.

When using a horseshoe-shaped element, the slider moves in a circle with a rotation angle of up to 270°C. These potentiometers are round in shape. The rectangular resistive element has a translational slider movement, and the potentiometer is made in the form of a prism.

Wire versions are built on the basis of high-resistance wire. This wire is wound around an annular contact. During operation, the contact moves along this ring. In order to ensure a strong connection with the contact, the track is additionally polished.

The material of manufacture depends on the accuracy of the potentiometer. Of particular importance is the diameter of the wire, which is selected based on the current density. The wire must have high resistivity. In production, nichrome, manganin, constantin and special alloys of noble metals are used for winding, which have low oxidizability and increased wear resistance.

In high-precision devices, ready-made rings are used, where the winding is placed. Such winding requires special high-precision equipment. The frame is made of ceramic, metal or plastic.

If the accuracy of the device is 10-15 percent, then a plate is used, it is folded into a ring after winding. Aluminum, brass or insulating materials are used as a frame, for example, fiberglass, textolin, getinaks.

Note! The first sign of a failed resistor may be crackling or noise when the knob is turned to adjust the volume. This defect occurs as a result of wear of the resistive layer, and, therefore, loose contact.

Main characteristics

Among the parameters on which the operation of a variable resistor depends, not only the total and minimum resistances, but also other data are of great importance:

functional characteristic;
dissipation power;
wear resistance;
existing degree of rotational noise;
dependence on environmental conditions;
sizes.

The resistance that occurs between the fixed conclusions is called total.

In most cases, the nominal resistance is indicated on the case and is measured in kilo- and megaohms. This value can fluctuate within 30 percent.

The dependence according to which the resistance changes when the movable contact moves from one extreme output to another is called the functional characteristic. According to this characteristic, variable resistors are divided into 2 types:

Linear, where the value of the resistance level is transformed in proportion to the movement of the contact;
Non-linear, in which the resistance level changes according to certain laws.

The figure shows different types of dependencies. For linear variable resistors, the dependence is shown in graph A, for non-linear ones that work:

according to the logarithmic law - on curve B;
according to the exponential (inversely logarithmic) law - on graph B.

Also, non-linear potentiometers can change resistances, as shown in graphs I and E.

All curves are built according to the readings of the total and current angle of rotation of the moving part - αn and α from the total Rn and current R resistances. For computers and automatic devices, the resistance level can vary in cosine or sine amplitudes.

In order to create wirewound resistors with the required functional characteristics, use a frame of different heights or change the distance in steps between the turns of the winding. For the same purposes, in non-wire potentiometers, the composition or thickness of the resistive film is changed.

Basic designations

In current-carrying circuit diagrams, a variable resistor is denoted as a rectangle and an arrow, which is directed to the center of the case. This arrow shows the middle or moving control output.

Sometimes the circuit requires not smooth, but stepped switching. To do this, use a circuit consisting of several fixed resistors. These resistances turn on, depending on the position of the regulator knob. Then a step switch sign is added to the designation, the number on top indicates the number of switch steps.

Dual potentiometers are integrated into the high-precision hardware for gradual volume control. Here, the resistance value of each resistor changes as one knob moves. This mechanism is indicated by a dotted line or a double line. If the variable resistors are far from each other in the diagram, then the connection is simply highlighted with a dotted line on the arrow.

Some dual options can be controlled independently of each other. In such circuits, the axis of one potentiometer is placed inside another. In this case, the double bond designation is not used, and the resistor itself is labeled according to its reference designation.

A variable resistor can be equipped with a switch that supplies power to the entire circuit. In this case, the switch handle is combined with the switching mechanism. The switch is activated when the movable contact is moved to the end position.

Features of trimmer resistors

Such radio components are necessary for setting up equipment elements during repair, adjustment or assembly. The main difference between trimmers and other models is the existence of an additional locking element. The operation of these resistors uses a linear relationship.

To create components, flat and ring resistive elements are used. If we are talking about the use of devices under heavy load, then cylindrical structures are used. In the diagram, instead of an arrow, a trim adjustment sign is placed.

How to determine the type of variable resistor

The general marking of potentiometers and trimmers contains a digital and letter designation of the model, which indicates the type, design feature and rating.

The first resistors at the beginning of the abbreviation had the letter "C", that is, resistance. The second letter "P" denoted a variable or trimmer. Next came the group number of the current-carrying part. If we were talking about non-linear models, then the marking began with the letters CH, ST, SF, depending on the material of manufacture. Then came the registration number.

Today, the designation RP is used - a variable resistor. Then follows a group: wire - 1 and non-wire - 2. At the end, there is also a development registration number through a dash.

For ease of notation, miniature resistors use their own color palette. If the radio component is too small, 5, 4 or 3 colored rings are marked. The resistance value comes first, then the multiplier, and finally the tolerance.

Important! Radio components are produced by many trading companies around the world. The same designations may refer to different parameters. Therefore, models are selected according to the characteristics attached to the description.

The general rule for choosing a resistor is to study the official designations on the manufacturer's website. This is the only way to be sure of the necessary markings.

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Variable and tuning resistors. Rheostat. Variable resistor in the diagram

operating principle. How to connect a variable resistor? :: SYL.ru

A large number of people turn to radio shops to do something with their own hands. The main task of those who like to collect radios and circuits is to create useful items that will benefit not only themselves, but also those around them. A variable resistor helps to repair or create a device that runs on the mains.

Basic properties of variable resistors

When a person has a clear idea of the conditional elements of the graphic display on the diagrams, then he has the problem of transferring the drawing to reality. It is required to find or purchase individual components of an already finished circuit. Today there are a large number of stores that sell the necessary parts. You can also find elements in the 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 diametrical opposite leads. The resistor creates a current limit in the circuit. If necessary, it will perform resistance, which can be measured in ohms. A variable resistor is indicated on the diagram as a rectangle along with two dashes. They are located on opposite sides inside the rectangle. Thus, a person designates power on his diagram.

The equipment that is available in almost every home includes resistors with a certain rating. They are located along the E24 row and conditionally designate a range from one to ten.

Varieties of resistors

Today, there are a large number of resistors that are found in modern household electrical appliances. The following types can be distinguished:

Resistor metal lacquered heat resistant. It can be found in lamp devices that have a power of at least 0.5 watts. In Soviet equipment, you can find such resistors that were produced in the early 80s. They have different power, which directly depends on the size and dimensions of the radio equipment. When there is no power symbol on the diagrams, then it is allowed to use a variable resistor of 0.125 watts.
waterproof resistors. In most cases, they are found in lamp electrical appliances that were produced in the 1960s. In black and white TV and radio, these elements are sure to occur. Their marking is very similar to the designation of metal resistors. Depending on the rated power, they can have different sizes and dimensions.

Today, the generally accepted marking of resistors is widely used, which are divided into different colors. This way, you can quickly and easily determine the value without having to solder the circuit. Thanks to the color marking, you can significantly speed up the search for the required resistor. Now a large number of foreign and domestic firms are engaged in the production of such elements for microcircuits.

The main characteristics and parameters of the variable resistor

Several main parameters can be distinguished:

Rated resistance.
Power dissipation limits.
Temperature coefficients of resistance.
Permissible resistance deviation values. It is calculated from nominal values. When such resistors are made, manufacturers use technological variation.
Limit indicators of operating voltage.
Excessive noise.

During the design of the presented devices, specific characteristics are used. These parameters apply to devices that operate at high frequencies:

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

Classification by common 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:

Permanent.
Variables, adjustable.
rigged variables.

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

With insulation.
Without isolation.
Vacuum.
Sealed.

Connecting a Variable Resistor

A large number of people do not know how to connect a variable resistor. These elements often have two connection schemes. This work can be done by a person who is at least a little versed in electronics and has dealt with soldering microcircuits.

The first connection option is that the top output must be connected to the main power source. The lower one is soldered to a common wire. Experts call it "land". It is worth noting that the middle pins are connected exclusively to the control elements of the circuit. This may be the base or main gate of the transistor. In this case, these structures will play the role of a potentiometer.
There is a second way that will help you learn how to connect a variable resistor. The top terminals must be connected to the main power supply. The lower ends of the structure are soldered to a general purpose wire, and the middle ends are connected to the lower or upper terminals. It is they who are able to supply the necessary power to the control elements of the circuit. This connection method lies in the fact that variable resistors will play an important role and regulate the incoming current.

Manufacturing technology of variable resistors

There is a classification that depends on the manufacturing technology of resistors. During the production process, different stages and schemes are used. Today we can distinguish the following designs:

Today in the radio markets you can find a large number of elements for drawing up a diagram. The most popular is a variable resistor of 10 kOhm. It can be variable, wire or adjusting. Its main distinguishing feature is a single single turn. This type of resistor is designed to work in an electrical circuit where there is direct or alternating current.

The nominal power indicators are 50 volts, and the resistance is 15 kOhm. These elements were produced in the mid-eighties, so today they can be found not only in specialized stores, but also in old radio circuits. The 10 kΩ variable resistor has several functional and possible analogues.

Variable resistor noise

Even new and reliable resistors at high temperatures, which are well above absolute zero, can become a major source of noise. The variable dual resistor is used in an electrical circuit in a microcircuit. The appearance of noise became known from the fundamental fluctuation-dissipation theorem. It is commonly known as the Nyquist theorem.

If the circuit has a variable joint venture resistor with high resistance values, then a person will observe the effective noise voltage. It will be directly proportional to the roots of the temperature regime.

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Interlinear marking of variable resistors

Variable resistors differ from others in that they can change resistance. There are 2 main types of variable resistors:

potentiometers that convert voltage;
rheostats that regulate the current.

The potentiometer differs from other types of resistance in that it has three outputs:

2 permanent, or extreme;
1 movable, or medium.

All variants of variable resistors are divided into wire and non-wire, it depends on the design of the element.

How a resistor works

microcomposite layers of metals, their oxides and dielectrics;
heterogeneous systems of several elements, including 1 conductive;
semiconductor materials.

Attention! When using carbon film resistors in the power circuit, it is important to prevent the element from overheating, otherwise sudden voltage drops may occur during the adjustment process.

What does a non-wire variable resistor look like?

In high-precision devices, ready-made rings are used, where the winding is placed. Such winding requires special high-precision equipment. The frame is made of ceramic, metal or plastic.

Note! The first sign of a failed resistor may be crackling or noise when the knob is turned to adjust the volume. This defect occurs as a result of wear of the resistive layer, and, therefore, loose contact.

Main characteristics

Among the parameters on which the operation of a variable resistor depends, not only the total and minimum resistances, but also other data are of great importance:

functional characteristic;
dissipation power;
wear resistance;
existing degree of rotational noise;
dependence on environmental conditions;
sizes.

The resistance that occurs between the fixed conclusions is called total.

In most cases, the nominal resistance is indicated on the case and is measured in kilo- and megaohms. This value can fluctuate within 30 percent.

The dependence according to which the resistance changes when the movable contact moves from one extreme output to another is called the functional characteristic. According to this characteristic, variable resistors are divided into 2 types:

Linear, where the value of the resistance level is transformed in proportion to the movement of the contact;
Non-linear, in which the resistance level changes according to certain laws.

The meaning of the functional characteristics of potentiometers

The figure shows different types of dependencies. For linear variable resistors, the dependence is shown in graph A, for non-linear ones that work:

according to the logarithmic law - on curve B;
according to the exponential (inversely logarithmic) law - on graph B.

Also, non-linear potentiometers can change resistances, as shown in graphs I and E.

Basic designations

In current-carrying circuit diagrams, a variable resistor is denoted as a rectangle and an arrow, which is directed to the center of the case. This arrow shows the middle or moving control output.

Designations of variable resistors

Features of trimmer resistors

How to determine the type of variable resistor

The general marking of potentiometers and trimmers contains a digital and letter designation of the model, which indicates the type, design feature and rating.

Today, the designation RP is used - a variable resistor. Then follows a group: wire - 1 and non-wire - 2. At the end, there is also a development registration number through a dash.

Resistor color coding

Important! Radio components are produced by many trading companies around the world. The same designations may refer to different parameters. Therefore, models are selected according to the characteristics attached to the description.

The general rule for choosing a resistor is to study the official designations on the manufacturer's website. This is the only way to be sure of the necessary markings.

Video

elquanta.ru

Variable resistor | Electronics for everyone

It seems to be a simple detail, what could be complicated here? But no! There are a couple of tricks to using this thing. Structurally, the variable resistor is arranged in the same way as it is shown in the diagram - a strip of material with resistance, contacts are soldered to the edges, but there is also a movable third output that can take any position on this strip, dividing the resistance into parts. It can serve as both a resettable voltage divider (potentiometer) and a variable resistor - if you just need to change the resistance.

Constructive trick: Let's say we need to make a variable resistance. We need two conclusions, and the device has three of them. It seems that the obvious thing suggests itself - do not use one extreme conclusion, but use only the middle and the second extreme. Bad idea! Why? Yes, just at the moment of movement along the strip, the movable contact can bounce, tremble and lose contact with the surface in every possible way. At the same time, the resistance of our variable resistor becomes infinity, causing interference during tuning, sparking and burnout of the graphite track of the resistor, the withdrawal of a custom device from an acceptable tuning mode, which can be fatal. Solution? Connect the end lead to the middle one. In this case, the worst thing that awaits the device is a short-term appearance of maximum resistance, but not a break.

Fighting limit values. If the current is regulated by a variable resistor, for example, the power supply of the LED, then when brought to the extreme position, we can bring the resistance to zero, and this is essentially the absence of a resistor - the LED will char and burn out. So you need to introduce an additional resistor that sets the minimum allowable resistance. And there are two solutions here - the obvious and the beautiful :) The obvious is clear in its simplicity, and the beautiful is remarkable in that we do not change the maximum possible resistance, if it is impossible to bring the engine to zero. When the engine is in the highest position, the resistance will be equal to (R1 * R2) / (R1 + R2) - the minimum resistance. And in the extremely lower one it will be equal to R1 - the one that we calculated, and there is no need to make an allowance for an additional resistor. It's beautiful! :)

If you need to stick a restriction on both sides, then just insert a constant resistor from above and below. Simple and effective. At the same time, you can also get an increase in accuracy, according to the principle below.

Increasing accuracy. Sometimes it is necessary to adjust the resistance by many kOhm, but adjust just a little bit - by a fraction of a percent. In order not to catch these microdegrees of rotation of the engine with a screwdriver on a large resistor, they put two variables. One for a large resistance, and the second for a small one, equal to the value of the intended adjustment. As a result, we have two twists - one “Roughly”, the second “Exactly”. We set the Large one to an approximate value, and then we finish it with a small one to the standard.

easyelectronics.ru

How to connect a variable resistor 🚩 variable resistor connection 🚩 Apartment renovation

The term "resistor" comes from the English verb resist, which means "to resist", "to prevent", "to resist". In a literal translation into Russian, the name of this device means "resistance". The fact is that a current flows in electrical circuits, which experiences internal resistance. Its value is determined by the properties of the conductor and many other external factors.

This current characteristic is measured in ohms and is related to current and voltage. The resistance of a conductor is 1 ohm if a current of 1 ampere flows through it, and a voltage of 1 volt is applied to the ends of the conductor. Thus, with the help of resistance artificially created and introduced into the electrical circuit, it is possible to regulate other important parameters of the system, which can be calculated in advance.

The scope of application of resistors is unusually wide, they are considered one of the most common mounting elements. The main function of the resistor is to limit and control the current. It is also often used in voltage division circuits when it is required to reduce this characteristic of the circuit. Being passive elements of electrical circuits, resistors are characterized not only by the value of the nominal resistance, but also by the power, which shows how much energy the resistor is able to dissipate without overheating.

In electronic devices and household electrical circuits, many resistors of various shapes and sizes are used. These miniature devices differ from each other not only in appearance, but also in face value and performance. All resistors are conditionally divided into three large groups: constant, variable and tuning.

Most often, in devices you can find constant-type resistors, resembling oblong "barrels" in appearance with leads at the ends. The resistance parameters in devices of this type do not change significantly from external influences. Slight deviations from ratings may be caused by internal noise, temperature changes, or the effects of power surges.

With variable resistors, the user can arbitrarily change the resistance value. For this, the device is equipped with a special handle that looks like a slider or can rotate. The most common representative of this family of resistors can be seen in the volume controls that are equipped with audio equipment. Turning the knob can smoothly change the parameters of the circuit and, accordingly, increase or decrease the volume. But tuning resistors are intended only for relatively rare adjustments, therefore they do not have a handle, but a slotted screw.

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Variable and tuning resistors. Rheostat.

In one of the previous articles, we discussed the main aspects related to working with resistors, so today we will continue this topic. Everything that we discussed earlier concerned, first of all, fixed resistors, the resistance of which is a constant value. But this is not the only existing type of resistors, so in this article we will pay attention to elements that have variable resistance.

So, what is the difference between a variable resistor and a constant one? Actually, here the answer directly follows from the name of these elements 🙂 The resistance value of a variable resistor, unlike a constant one, can be changed. How? And this is exactly what we will find out! First, let's look at the conditional circuit of a variable resistor:

It can be immediately noted that here, unlike resistors with constant resistance, there are three outputs, not two. Now let's figure out why they are needed and how it all works 🙂

So, the main part of the variable resistor is a resistive layer with a certain resistance. Points 1 and 3 in the figure are the ends of the resistive layer. Also an important part of the resistor is the slider, which can change its position (it can take any intermediate position between points 1 and 3, for example, it can end up at point 2 as in the diagram). Thus, in the end we get the following. The resistance between the left and center terminals of the resistor will be equal to the resistance of section 1-2 of the resistive layer. Similarly, the resistance between the central and right terminals will be numerically equal to the resistance of section 2-3 of the resistive layer. It turns out that by moving the slider we can get any resistance value from zero to . A is nothing more than the impedance of the resistive layer.

Structurally, variable resistors are rotary, that is, to change the position of the slider, you need to turn a special knob (this design is suitable for the resistor shown in our diagram). Also, the resistive layer can be made in the form of a straight line, respectively, the slider will move straight. Such devices are called slide or slide variable resistors. Rotary resistors are very common in audio equipment, where they are used to control volume/bass, etc. Here's what they look like:

A sliding-type variable resistor looks a little different:

Often, when using rotary resistors, resistors with a switch are used as volume controls. Surely you have come across such a regulator more than once - for example, on radios. If the resistor is in its extreme position (minimum volume / device off), then if you start to rotate it, you will hear a noticeable click, after which the receiver will turn on. And with further rotation, the volume will increase. Similarly, when the volume is reduced - when approaching the extreme position, there will be a click again, after which the device will turn off. A click in this case indicates that the receiver's power has been turned on/off. Such a resistor looks like this:

As you can see, there are two additional conclusions here. They are just connected to the power circuit in such a way that when the slider is rotated, the power circuit opens and closes.

There is another large class of resistors that have a variable resistance that can be changed mechanically - these are tuning resistors. Let's take some time for them too 🙂

Trimmer resistors.

Just to begin with, let's clarify the terminology ... In fact, the trimmer resistor is variable, because its resistance can be changed, but let's agree that when discussing trimmer resistors under variable resistors, we will mean those that we have already discussed in this article (rotary, slider, etc. e). This will simplify the presentation, as we will be contrasting these types of resistors with each other. And, by the way, in the literature, trimmers and variables are often understood as different elements of the circuit, although, strictly speaking, any trimmer is also variable due to the fact that its resistance can be changed.

So, the difference between trimmers and variables that we have already discussed, first of all, is the number of cycles of moving the slider. If for variables this number can be 50,000 or even 100,000 (that is, the volume knob can be turned almost as much as you like 😉), then for tuning resistors this value is much less. Therefore, tuning resistors are most often used directly on the board, where their resistance changes only once, when setting up the device, and during operation, the resistance value does not change. Externally, the tuning resistor looks completely different from the variables mentioned:

The designation of variable resistors is slightly different from the designation of constants:

Actually, we discussed all the main points regarding variables and trimmers, but there is one more very important point that cannot be ignored.

Often in the literature or in various articles you can find the terms potentiometer and rheostat. In some sources, variable resistors are called so, in others some other meaning may be attached to these terms. In fact, there is only one correct interpretation of the terms potentiometer and rheostat. If all the terms that we have already mentioned in this article related, first of all, to the design of variable resistors, then the potentiometer and rheostat are different switching circuits (!!!) of variable resistors. That is, for example, a rotary variable resistor can act both as a potentiometer and as a rheostat - it all depends on the switching circuit. Let's start with the rheostat.

A rheostat (a variable resistor connected in a rheostat circuit) is mainly used to regulate the current. If we turn on the ammeter in series with the rheostat, then when we move the slider, we will see a changing value of the current strength. The resistor in this circuit plays the role of a load, the current through which we are going to regulate the variable resistor. Let the maximum resistance of the rheostat be equal, then, according to Ohm's law, the maximum current through the load will be equal to:

Here we took into account the fact that the current will be maximum at the minimum resistance value in the circuit, that is, when the slider is in the leftmost position. The minimum current will be:

So it turns out that the rheostat acts as a regulator of the current flowing through the load.

There is one problem in this circuit - if contact is lost between the slider and the resistive layer, the circuit will be open and current will stop flowing through it. You can solve this problem in the following way:

The difference from the previous scheme is that points 1 and 2 are additionally connected. What does this give in normal operation? Nothing, no changes 🙂 Since there is a non-zero resistance between the resistor slider and point 1, all the current will flow directly to the slider, as in the absence of contact between points 1 and 2. And what happens if contact is lost between the slider and the resistive layer? And this situation is absolutely identical to the absence of a direct connection of the slider with point 2. Then the current will flow through the rheostat (from point 1 to point 3), and its value will be equal to:

That is, if the contact is lost in this circuit, there will only be a decrease in the current strength, and not a complete break in the circuit, as in the previous case.

We figured out the rheostat, let's look at a variable resistor connected according to the potentiometer circuit.

Do not miss the article about measuring instruments in electrical circuits - link.

A potentiometer, unlike a rheostat, is used to adjust the voltage. It is for this reason that in our diagram you see as many as two voltmeters 🙂 The current flowing through the potentiometer, from point 3 to point 1, remains unchanged when the slider is moved, but the resistance value between points 2-3 and 2-1 changes. And since the voltage is directly proportional to the current strength and resistance, it will change. When the slider is moved down, the resistance 2-1 will decrease, respectively, the readings of the voltmeter 2 will also decrease. With this movement of the slider (down), the resistance of the section 2-3 will increase, and with it the voltage on the voltmeter 1. In this case, the total readings of the voltmeters will be equal to the voltage of the power source, that is, 12 V. In the uppermost position on voltmeter 1 it will be 0 V, and on voltmeter 2 - 12 V. In the figure, the slider is in the middle position, and the voltmeter readings, which is absolutely logical, are equal 🙂

This concludes our consideration of variable resistors, in the next article we will talk about the possible connections of resistors to each other, thanks for your attention, I will be glad to see you on our website! 🙂

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Electronic variable resistor - Diodnik

In their homemade crafts, radio amateurs almost always use variable resistors to adjust the volume or voltage and, of course, any other parameters. But a device with buttons on the front panel looks much more interesting and modern than with ordinary knobs. The use of microcontroller control is not always advisable in simple crafts, and it is also difficult for a beginner, but probably everyone can repeat the electronic variable resistor described below.

The circuit has such small dimensions that it can be crammed into almost any home-made device. It fully performs the function of an ordinary variable resistor, does not contain scarce and specific components.

It is based on the field effect transistor KP 501 (or any other analogue thereof).

By pressing the SB1 button, we accumulate a charge on the electrolytic capacitor C 1, which allows us to slightly open the transistor and affect the resistance at the output terminals of the circuit. By pressing the SB2 button, we discharge the capacitor C 1, which leads to the gradual closing of the transistor. With constant clamping of any of the buttons, changes in resistance are made smoothly.

The smoothness of adjustment of such an electronic variable resistor depends on the capacitance of the capacitor C 1 and the value of the resistor R 1. The maximum resistance that the circuit can simulate depends on the tuning resistor R 2. The circuit starts working immediately and does not require additional settings, except for adjusting the maximum resistance with the resistor R 2 .

After turning off the power to the circuit, such an electronic variable resistor does not reset the settings immediately, but the resistance of the circuit increases gradually, which is associated with the self-discharge of the capacitor C 1. When using a new and high-quality capacitor C 1, the circuit settings can last about a day.

Probably the most popular application of this circuit will be an electronic volume control. This electronic volume control is not without its drawbacks, but ease of repetition is sure to be the most important factor for radio amateurs.

We see a demonstration of the operation of this scheme below, put a like, and also subscribe to our pages in the social. networks!

Note. In the video, the electronic analogue of the variable resistor is set to 10 kOhm. The Bside ADM01 multimeter used has automatic range switching and, when switching them, does not always immediately determine the current resistance of the circuit.

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Last time, to connect the LED to a 6.4 V DC source (4 AA batteries), we used a resistor with a resistance of about 200 ohms. This, in principle, ensured the normal operation of the LED and did not allow it to burn out. But what if we want to adjust the brightness of an LED?

To do this, the easiest option would be to use a potentiometer (or trimmer). It is in most cases a cylinder with a resistance adjustment knob and three contacts. Let's see how it is arranged.

It should be remembered that it is correct to regulate the brightness of the LED by PWM modulation, and not by changing the voltage, since there is an optimal operating voltage for each diode. But for the sake of demonstrating the use of a potentiometer, such use (of a potentiometer) for educational purposes is permissible.

Having pressed the four clamps and removed the bottom cover, we will see that the two extreme contacts are connected to the graphite track. The middle contact is connected to the ring contact inside. And the adjustment knob simply moves the jumper connecting the graphite track and the ring contact. When the knob is rotated, the arc length of the graphite track changes, which ultimately determines the resistance of the resistor.

It should be noted that when measuring the resistance between the two extreme contacts, the multimeter readings will correspond to the nominal resistance of the potentiometer, since in this case the measured resistance corresponds to the resistance of the entire graphite track (in our case, 2 kOhm). And the sum of the resistances R1 and R2 will always be approximately equal to the nominal, regardless of the angle of rotation of the adjustment knob.

So, by connecting a potentiometer in series with the LED, as shown in the diagram, changing its resistance, you can change the brightness of the LED. In fact, when changing the resistance of the potentiometer, we change the current passing through the LED, which leads to a change in its brightness.

True, it should be remembered that for each LED there is a maximum allowable current, above which it simply burns out. Therefore, in order to prevent the diode from burning out if the potentiometer knob is unscrewed too much, one more resistor with a resistance of about 200 ohms can be connected in series (this resistance depends on the type of LED used) as shown in the diagram below.

For reference: LEDs must be connected with a long "leg" to +, and a short one to -. Otherwise, the LED at low voltages simply will not burn (it will not pass current), and at a certain voltage, called the breakdown voltage (in our case, it is 5 V), the diode will fail.

Let's take a look at the variable resistor... What do we know about it? So far, nothing, because we still do not even know the main parameters of this very common radio component in electronics. So let's learn more about the parameters of variables and trimmers.

To begin with, it is worth noting that variables and trimmers are passive components of electronic circuits. This means that they consume the energy of the electrical circuit in the course of their work. Passive circuit elements also include capacitors, inductors and transformers.

Parameters, with the exception of precision products that are used in military or space technology, they do not have too many:

Rated resistance. Without a doubt, this is the main parameter. The impedance can be in the range from tens of ohms to tens of megaohms. Why total resistance? This is the resistance between the extreme fixed terminals of the resistor - it does not change.

With the help of the adjusting slider, we can change the resistance between any of the extreme pins and the pin of the moving contact. The resistance will vary from zero to the full resistance of the resistor (or vice versa - depending on the connection). The nominal resistance of the resistor is indicated on its case using an alphanumeric code (M15M, 15k, etc.)

Dissipated or rated power(resistor power). In conventional electronic equipment, variable resistors are used with a power of: 0.04; 0.25; 0.5; 1.0; 2.0 watts or more.

It should be understood that wire-wound variable resistors, as a rule, are more powerful than thin-film ones. Yes, this is not surprising, because a thin conductive film can withstand much less current than a wire. Therefore, the power characteristics can be tentatively judged even by the appearance of the "variable" and its design.

Maximum or limit operating voltage. Everything is so clear here. This is the maximum operating voltage of the resistor, which should not be exceeded. For variable resistors, the maximum voltage corresponds to the series: 5, 10, 25, 50, 100, 150, 200, 250, 350, 500, 750, 1000, 1500, 3000, 8000 Volts. Limit stresses of some instances:

SP3-38 (a - e) for a power of 0.125 W - 150 V (for operation in AC and DC circuits);

SP3-29a- 1000 V (for operation in AC and DC circuits);

SP5-2- from 100 to 300 V (depending on modification and nominal resistance).