One of the most commonly used elements in electronics is the resistor. In simple terms, it is called "resistance." With its help, you can limit the current or measure it, divide the voltage, create feedback loops. Not a single circuit can do without resistances. In this article we will talk about what a resistor is, what its principle of operation is, and what this element of the electrical circuit is for.
Content:
- Definition
- Views
- Principle of operation
- Main characteristics
- Where and what it is used for
Definition
The resistor comes from the English "resistor" and from the Latin "resisto", which translated into Russian sounds like "resist". In Russian-language literature, along with the word "resistor", the word "resistance" is used. From the name, the main task of this element is clear - to resist electric current.
It belongs to the group of passive elements, because as a result of its operation, the current can only decrease, that is, unlike active elements, passive ones by themselves cannot amplify the signal. Which of the second
The symbol on the diagram is easy to remember - it is a rectangle, according to GOST 2.728-74 its dimensions are 4x10 mm. There are designations for resistors of different dissipation power.
Views
Resistors are classified according to a number of criteria. If we talk about discrete components, then according to the installation method they are divided into:
- Output. Used for mounting through a printed circuit board. Such elements have terminals located radially or axially. In the people, the conclusions are called legs. This type of resistor was actively used in all old devices (20 and more years ago) - old TVs, receivers, in general everywhere, and now it is used in simple devices, as well as where the use of SMD components is difficult for some reason, either impossible.
- SMD. These are elements that have no legs. The pins for connection are located on the surface of the case, slightly protruding above it. They mount directly to the surface of the PCB. The advantage of such resistors is the simplicity and low cost of assembly on automated lines, saving space on the printed circuit board.
You can see the appearance of two types of elements in the figure below:
We already know what this component looks like, now we should learn about the classification by manufacturing technology. Output resistors are:
- Wire-wound. A wire wound on a core is used as a resistive component; bifilar winding is used to reduce parasitic inductance. The wire is selected from a metal with a low temperature coefficient of resistance and low resistivity.
- Metal-film and composite. As you might guess, metal alloy films are used here as a resistive element.
Since the resistor consists of a resistive material, the latter can be a wire or film with a high resistivity. What it is? Materials such as:
- manganin;
- constantan;
- nichrome;
- nickelin;
- metal dielectrics;
- metal oxides;
- carbon and others.
SMD or chip resistors are thin-film and thick-film, the resistive material is used:
| Material | Features where used |
| Nickel-chromium (nichrome, NiCr) | in thin-film, moisture-resistant |
| Ditantalum nitride (Ta2N). | TCR is 25 ppm / 0С (-55... + 1250С); |
| Ruthenium dioxide (RuO2) | in thick-film |
| Lead ruthenite (Pb2Ru2O6) | in thick-film |
| Bismuth ruthenite (Bi2Ru2O7) | in thick-film |
| Ruthenium dioxides doped with vanadium (Ru0.8V0.2O2, Ru0.9V0.1O2, Ru0.67V0.33O2) | — |
| Lead oxide (PbO) | — |
| Bismuth iridium (Bi2Ir2O7) | — |
| Nickel alloy | In low-resistance (0.03... 10 Ohm) thin-film products |
The figure below shows what the resistor consists of:
By design, they are distinguished:
- Permanent. They have two conclusions, and you cannot change the resistance - it is constant.
- Variables. These are potentiometers and trimming resistors, the principle of which is based on the movement of the sliding contact (slider) along the resistive layer.
- Non-linear. The resistance of components of this type changes under the influence of temperature (thermistors), light radiation (photoresistors), voltage (varistors) and other quantities.
And also by purpose - general and special. The latter are subdivided into:
- High-resistance (resistance range tens of MΩ - TΩ units, at operating voltages up to 400V).
- High-voltage (designed to operate in circuits with voltages up to tens of kV).
- High-frequency (a feature of high-frequency operation is the requirement for low self-inductances and capacitances. Such products can operate in circuits with a signal frequency of hundreds of MHz).
- Precision and super-precision (these are products with a high class of accuracy. They have a tolerance for deviation from the nominal resistance of 0.001 - 1%, while the usual tolerance can be 5% and 10% or more).
Principle of operation
A resistor is installed in an electrical circuit to limit the current flowing through the circuit. The amount of voltage that will drop across it is calculated simply - according to Ohm's law:
U = IR
A voltage drop is the number of volts that appear at the terminals of a resistor when current flows through it. Accordingly, if the voltage across the resistor drops and a current flows through it, it means that a certain power is released into heat on it. In physics, there is a well-known formula for finding the power:
P = UI
Or, to speed up calculations, it is sometimes convenient to use the formula for power through resistance:
P = U2/R=I2R
How does a resistor work? Each conductor has a specific internal structure. When an electric current flows, electrons (charge carriers) collide with various inhomogeneities in the structure of a substance and lose energy, which is then released in the form of heat. If it is difficult for you to understand, then the principle of work of resistance in simple words can be said as follows:
This is a value that shows how difficult it is for an electric current to flow through a substance. It depends on the substance itself - its resistivity.
Where: p - resistivity, l - conductor length, S - cross-sectional area.
Main characteristics
In order to choose the right resistor, it is important to know what characteristics you need to look at when choosing. Its main parameters include:
- Nominal resistance.
- Maximum power dissipation.
- Tolerance or class of accuracy. It depends on how much the percentage of resistance of parts from this class may differ from the declared one.
In most cases, this information is sufficient. Beginners often forget about the permissible power of the resistor, and they burn out. You can calculate how many watts are allocated to the resistor using the formula indicated in the previous section of the article. Buy resistors with a power reserve of 20-30%, more is better, less is not necessary!
Where and what it is used for
We have already considered that the resistor is designed to limit the current in the circuit, now we will look at several practical examples where a resistor is used in electrical engineering.
The first area of application is current limiting, for example for powering LEDs. The principle of operation and calculation of such a circuit is that the power supply voltage is subtracted LED rated operating voltage, divided by rated (or desired) current through Light-emitting diode. As a result, you get the nominal limiting resistance.
Rogre= (Unutrition-Urequired) / Inominal
The second is a voltage divider. Here, the output voltage is calculated by the formula:
Uout= Uin(R2 / R1 + R2)
Also, the resistor has found application for setting the current to transistors. Essentially the same restrictor circuit discussed above.
Finally, we recommend watching a useful video on the topic of the article:
We examined what resistors are, their purpose and principle of operation. This is an important element from which to begin your study of electrical engineering. To calculate circuits with it, Ohm's law and active power are used, and in high-frequency circuits, reactive parameters are also taken into account - parasitic capacitance and inductance. We hope the information provided was useful and interesting for you!
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