Resistor, types and markings

The passive element of an electrical circuit, necessary to regulate the current strength in it, can be either with a constant value (nominal resistance) or with a variable value. From previous lessons on the basics of electronics, we solved problems with an incandescent lamp, where we calculated its resistance as a constant value, but in fact its resistance during operation (when illuminated) is several times lower than when the voltage is turned off, i.e. the lamp (essentially a resistor) has a nonlinear relationship
Рис 4. Обозначение резисторов .jpeg
Рис 4. Обозначение резисторов .jpeg

Main characteristics of resistors

1. Nominal resistance, Ohm
2. Limit power dissipation, Watt
3. Accuracy or tolerance, %

There are many types of resistors, but let's look at several of their varieties for indirect use, i.e. when the main purpose of the resistor is not to regulate the current in the circuit
Таблица 1. Мощность резисторов.jpg

Strain gauge

This is a thin film resistor that is glued to an aluminum square that has several mechanical breaks. It is used in household scales, where it consists of four strain gauges combined into a resistor bridge (On strain gauges, the HOME button starting from iPhone 7 and all kinds of pressure sensors)
Рис 5. Тензорезисторный мост в весах.jpg

Varistors

These are resistors whose resistance depends on the applied voltage. Most control models of washing machines have varistors; as a rule, they are located at the power input, since when the voltage surges the resistance of the resistor drops, we get a smoother input voltage, which has a positive effect on service life.

Thermistor and photoresistor

Thermistors (thermistor) are a resistor whose resistance depends on temperature.

A photoresistor is a resistor whose resistance depends on the illumination.

The resistance ratings of fixed resistors are reduced to the so-called resistor series, which are the result of standardization of the nominal resistances of resistors.

Fixed resistors have six rows: E6, E12, E24, E48, E96 and E192; where the number following the letter E indicates the number of values ​​in the series for each decimal interval.

Below is a table of all nominal values ​​of resistor resistance
which correspond to the E24 series.
E24 ряд резисторов.jpg

Color coding of resistors

For convenience, a color scheme with rings was used in the marking of resistors, which are visible regardless of the installation of the resistor on the board, while for example, diodes have alphanumeric markings, which complicates repairs, since their markings are not always on top.

In resistors, 4, 5 and 6 ring markings are used; as a rule, the rings are offset, which allows you to understand where the first ring is. There is no third element in a four-ring resistor, while the first two mean the nominal value, the third multiplier and the fourth tolerance
Рис 6. Цветовая маркировка резисторов.jpeg

Marking of SMD resistors

Almost all modern technology is made on SMD elements; these are surface-mounted radio components, i.e. There is a special pad on the printed circuit board where the element is soldered without going through the board.

SMD resistors have a numerical designation where the first two numbers are a pointer to the number, in Fig. 7 it is 22, 22, 56, the presence of four numbers indicates a tolerance of 1% (deviation from the specified value) or 5% for a three-hour marking. Having an R at the beginning reduces the factor, which is always the last digit and is a power of ten.
Рис 7. SMD резисторы.jpeg

SMD resistor housings

Don’t forget about the zero jumpers, this is a resistor on the body of which is marked 0 or 000, such resistors are necessary in circuits with SMD markings in order to bypass (jump over) the track
Таблица 2. Корпуса SMD резисторов.jpg

Voltage divider

One of the simplest resistor circuits is the voltage divider circuit. From the name it is already clear what we will do. Suppose we have a voltage source of 15 volts, but we need 12.7 volts, for this we can use a voltage divider circuit, which consists of two resistors. We know that the current will be the same, and the voltage will depend on the selected resistors on the shoulders. We need 12.7 on the lower arm (it is customary to take the lower arm), therefore, on the first resistor there should be a drop of 2.3 V so that the current is small, say 0.01 Ampere, we take resistor R1 = 220 ohms, then it remains to calculate resistor R2 = 12.7/0.01 = 1270 Ohm

From 24 rows 1.3 Com

If our load creates resistance, and as a rule, any load does this, then resistor R2 needs to be recalculated using the rule for connecting resistors in parallel
Делитель напряжения на резисторе.jpg

Inductive reactance

Inductive reactance is a physical phenomenon that occurs in inductor coils when high-frequency currents (often changing their direction) do not have time (or almost do not have time) to pass through the coil until the current changes its direction; these properties are actively used in network filters that protect the circuit from high frequency currents.

Any inductor also has active resistance, but it is so insignificant that it is simply neglected and for our purposes it is not important.

The coils are checked with a multimeter for openness and an L meter for inductance; in practice, in most cases, only a multimeter is used and it is extremely rare to find a malfunction caused by a loss of quality factor of the coil

Capacitance

Capacitive reactance is exactly the same “special” resistance that flows in alternating current circuits. If we remember how a capacitor behaves in a direct current circuit, then it will be an open circuit, while in alternating current circuits the capacitor is an ideal conductor, although physically the charge carriers do not pass through the dielectric of the conductor, but due to a change in the direction of movement, part of the accumulated charge “helps” changes, therefore current flows faster, hence the concept of an ideal conductor

Impedance

Impedance or complex electrical resistance in alternating current is essentially R eq or total resistance, but only in a circuit with alternating current, where the resistance can be not only resistors, but also capacitors and inductances

Complex electrical resistance cannot be calculated based on the principle of series or parallel connection in DC circuits. Since impedance is characteristic of alternating current and the presence of capacitive or inductive reactance, a phase shift will certainly occur, so you cannot add the resistances as in DC circuits
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