Kettles, incandescent lamps, machine electrical equipment, and many other electrical appliances contain resistors. They have changed so much that it is sometimes difficult to define them without knowledge of their distinctive features. The reference books give a definition: a resistor is an element with a given constant or variable resistance. In practice, this is a set of elements that are used in the most unexpected designs. To understand what a resistor is made of, you need to know what material it is made of.
Content
- Resistor device from the inside
- Division by type
- Use in electrical circuit
- Areas of use
- Designation in the diagram
- Types of markings
Resistor device from the inside
The simplest resistor is a rheostat. A wire with high resistance is wound on the frame and connected to a power source. Based on this, we can conclude: the first requirement for this element is a high-resistance conductor. For the production of this element, use:
- wire;
- metal film, metal foil;
- composite material;
- semiconductor.
Wire resistances are easy to manufacture, capable of dissipating maximum power, but have a significant drawback: they have the highest inductance. Wire diameters range from a few microns to a few millimeters.
A metal foil made of a high-resistance material is wound on the frame. If it is necessary to increase the resistance, it is cut into a track, thereby increasing the length, and, accordingly, the resistance. A metal film resistor is obtained by sputtering a metal onto a base.
Graphite with organic or inorganic additives is used as a composite material. The resistor can be composed entirely of such material, or of the track on which this material is applied.
With the beginning of the production of microcircuits, new resistors appeared, which are called integral resistors. Manufacturing is done at the molecular level. A thin layer of high-resistance metal is sprayed onto a highly doped semiconductor, which serves as a resistor.
Division by type
Since resistance is one of the most used forms of parts, its application is very diverse. Depending on the purpose of the resistor it can be divided into three categories:
- permanent;
- trimmers;
- regulatory.
The first category - fixed resistors - have a given resistance and are used more than others in electrical circuits. Nevertheless, resistance still depends on external factors. On this basis, they are qualified for the following types:
- linear;
- nonlinear.
Linear ones are named so because their resistance changes smoothly, that is, linearly, depending on external influence. Nonlinear ones do not have such smoothness. For example, if you measure the resistance of an incandescent lamp in a cold state, then it will be one thing, but in a hot one it will be completely different, and 10-15 times more.
If there is such a variety, then a natural question arises - how to understand where the resistor is? In fact, the resistor might look like a circle, tube, or square. They are available in various shapes, sizes, colors. Sometimes, to determine that it is a resistor, you need to look at the electrical circuit diagram.
The second category is trimmers. They have a regulating mechanism that smoothly changes the resistance. Used to fine tune the hardware.
The next category is adjusting. The name speaks for itself here. They are intended for adjustments, which means they must change their resistance. Unlike constants, which have two leads, these have three leads. Two of them are connected to the resistor itself, and the third to the moving contact, which is connected to the rotating element. If you connect power to two terminals, then there will be a different voltage on the moving contact, which will differ from the voltage at the terminals of this element.
If you connect an adjusting (variable) resistor in series with the battery, connect the light bulb with one terminal to the negative terminal batteries, and the other with the output of a movable contact, then when you rotate the handle of the variable resistor, it will be noticeable how the brightness changes light bulbs. Why this happens can be understood if you figure out what the resistor does.
Use in electrical circuit
The brightness of the light bulb depends on the current flowing through the filament - the higher the current, the brighter the light bulb burns. According to Ohm's law, the current can be calculated by dividing the voltage by the resistance, which means that the lower the resistance, the greater the current. In practice, it will work as follows.
Let's say a light bulb is designed for a voltage of 9 V, has a resistance of 70 Ohms (in working, hot state), a 9 V battery and a variable resistance of 100 Ohms. For normal operation, the current passing through the light bulb should be approximately 0.13 A (9 V battery voltage divided by the light bulb resistance of 70 ohms). A 100 ohm variable resistor is connected in series in this circuit, the circuit current will be approximately 0.05 A (battery voltage 9 V is divided by the total resistance of 170 Ohms), this is about a third of the required current and the light bulb, therefore, will not burn.
In this case, the resistor helps to smoothly extinguish the light. A similar principle is used, for example, in cinemas. If the battery is 9 V, and the light bulb is rated for 2.5 V, then a voltage divider or suppressor is needed for its normal operation. What's the point? In the circuit, it is necessary to create a current normal for the light bulb.
If a damper is used, then 2 or more resistors and a light bulb are connected in series to the current source. The total resistance is chosen so that the current flowing through the circuit corresponds to the rated current of the bulb. Let's say there are: a 9 V DC source, a 2.5 V light bulb and a rated current of 0.12 A.
The resistance of the light bulb is calculated, for this the voltage is divided by the current and it turns out about 20.8 ohms. In order for a current of 0.12 A to flow through the circuit, the total resistance is calculated: 9 V divided by 0.12 A gives 75 ohms. Subtract the resistance of the light bulb and get 54.2 ohms - this resistance must be added to the light bulb.
If a divider is used, then two or more resistors are taken and connected in series with the power source. Parallel to some part of the divider, a load is connected, a circuit with a mixed connection is obtained: a source - a part of a divider - a part of a divider connected in parallel and a load - a current source. This is only one option, in fact, there are many connection schemes, but there is always a mixed connection.
Next, the required resistance is calculated. When connected in parallel, the current flows through two circuits, which means that it will be less at the load (the resistor connected in series limits the current). For normal operation of the load, all currents passing through the divider are calculated, and then the limiting one is selected.
With a serial connection, to turn off the light bulb, you need to turn off the power, and when using a divider, it is enough to disconnect the light bulb circuit. If it is necessary to connect several loads with different voltages to the source, then you cannot do without a divider (it is also called a voltage divider).
Areas of use
In addition to their usual purpose - to influence current and voltage, resistors, when using various materials, acquire completely different properties and names. Why they are needed can be seen from the following list:
- depends on voltage, is a varistor;
- from temperature - thermistor, thermistor;
- from illumination - photoresistor;
- from deformation - strain gage;
- from the action of a magnetic field - a magnetoresistor;
- a new one is being developed, called a memristor, the resistance depends on the amount of charge passing through it.
Varistors are most often used as overvoltage protection. Thermistors are used as temperature sensors. If it is necessary to automate the switching on of street lighting, then it will be difficult to do without a photoresistor. The rest of these devices are used in a narrow specialization.
Designation in the diagram
In the electrical circuit diagram, all resistors are indicated by a rectangle. Next is the letter R and a number indicating resistance. If it is constant, then inside the rectangle there can be Roman numerals corresponding to the power of this element in watts. For power less than 1 W, the following conventions apply:
- one longitudinal line inside the rectangle indicates a power of 0.5 W;
- one oblique line indicates a power of 0.25 W;
- two oblique - 0.125 W;
- three oblique - 0.05 watts.
In order to be able to distinguish one device from another, for example, a varistor from a thermistor, the following conventions are also used:
-
a fixed resistor is indicated only by a rectangle; - adjusting - the arrow crosses out the rectangle, the central terminal is connected to one of the terminals of the resistor;
- variable - an arrow approaches the rectangle from above at a right angle, other devices are connected to it;
- trimmer - the letter "t" lies on top of the rectangle, other devices are connected to this output;
- a trimmer, like a rheostat, the central terminal is connected to one of the device terminals - the rectangle crosses out the oblique letter "t";
- thermistor (thermistor) - a hockey stick lies on the rectangle at an angle;
- varistor - referred to as a thermistor, but the letter U is placed above the working surface of the club;
- photoresistor - two oblique arrows fit to the rectangle from above.
Types of markings
On large fixed resistors, power, resistance and tolerance are written in abbreviated form (by what percentage the specified value can deviate). Small parts are color-coded, alphabetic or numerically coded, and letters and numbers can complement each other. Each manufacturer chooses the marking method himself.
