To evaluate the properties of resistors, the following main parameters are used: nominal resistance, permissible deviation of the resistance value from the nominal value (tolerance), rated power dissipation, limit voltage, temperature coefficient of resistance, voltage coefficient, self-noise level, self-capacitance and inductance.
The nominal resistance Rn is the electrical resistance, the value of which is indicated on the resistor or indicated in the accompanying documentation.
In EVA, resistors with resistance from a few ohms to several megaohms are used. The nominal resistances of the resistors are standardized. The numerical values of the nominal resistances are determined by the series of preferred numbers: E6, E12, E24, E48, E96, E192 (the number indicates the number of nominal resistances in a row).
Rows E6, E12, E24 are used for general purpose fixed resistors. The scale of the nominal values of variable resistance resistors is determined by the E6 series.
Multiple and submultiple resistance values are obtained by multiplying or dividing this series by 10.
The nominal resistance scale for fixed resistors of general use in the series E6, E12, E24 is given in Table 2.
Table 2. Rated resistances for the series E6, E12, E24
|Row index||Numeric odds multiplied by any multiple of 10|
|E6||1.0; 1.5; 2.2; 3.3; 4.7; 6.8|
|E12||1.0; 1.2; 1.5; 1.8; 2.2; 2.7; 3.3; 3.9; 4.7; 5.6; 6.8; 9.1|
|E24||1.0; 1.1; 1.2; 1.3; 1.5; 1.6; 1.8; 2.0; 2.2; 2.4; 2.7; 3.0; 3.3; 3.6; 3.9; 4.3; 4.7; 5.1; 5.6; 6.2; 6.8; 7.5; 8.2; 9.1|
Permissible deviation is the maximum permissible deviation of the real resistance value of the resistor from its nominal value, expressed as a percentage.
According to GOST, a number of tolerances are established: ± 0.001; ±0.002; ±0.005; ±0.01 ±0.02; ±0.05; ±0.1 ; ±0.25; +0.5; ±1; ±2; ±5; ±10; ±20; ±30.
The most common resistors with a tolerance of ± 5; ±10; ±20%.
Variable resistors have tolerances of ±5, ±10, ±20, ±30%.
The rated power dissipation РН is the maximum power created by the current flowing through the resistor, at which it can work reliably for a long time.
The value of РН depends on the design of the resistor, the physical properties of the materials and the ambient temperature.
Resistors are operated, as a rule, at dissipation powers 3–10 times less than the nominal ones, which ensures higher reliability of the devices.
Specific values of nominal power dissipation in watts are set according to GOST and are selected from the range: 0.01; 0.025; 0.05; 0.062; 0.125; 0.25; 0.5; 1.0; 2; 3; 4; eight; ten; sixteen; 25; 40; 63; 80; 100; 160; 250; 500.
The value of the rated power dissipation is indicated on the cases of large resistors, and for small ones it is determined by the dimensions of the case.
The dissipated power P can be calculated by the formulas:
P=UI=I 2 R=U 2 /R.
If the resistor is dissipated more power than intended, its temperature will rise, which can lead to burnout of the conductive element and thus to the sudden failure of the resistor.
Limit voltage Upred . is the maximum voltage at which the resistor can operate. It is limited by thermal processes, and for high-resistance resistors – by the electrical strength of the resistor.
The temperature coefficient of resistance (TCR) is the relative change in the resistance value of the resistor when the temperature changes by 1ºС: TCR = ΔR / (Ro ΔT),
where Ro is the initial value of the resistance value of the resistor,
ΔR is the change in resistance in the temperature range ΔT.
The TCR value of precision resistors ranges from units to 100 × 10 -6 1 / ºС, and for general purpose resistors – from tens to 2000 × 10 -6 / ° С.
The inherent noise of resistors is the sum of thermal and current noise. Current noise is most characteristic of non-wire resistors. The most noisy are composition resistors, so they are used in receivers to a limited extent. According to the noise level, resistors are divided into two groups A (1 μV / V) and B (5 μV / V).
Frequency properties of resistors. When resistors operate in the AC frequency range, the resistance can change relative to its nominal value at direct current, which leads to a change in the output parameters and the stability of the devices.
A simplified equivalent circuit of a resistor for high frequencies (Fig. 5), in addition to the actual active resistance R, includes reactive components – inductances L pairs and capacitance C pairs , which worsen the frequency properties of resistors and therefore they are often called parasitic. For different types of resistors, parasitic inductances and capacitance are formed in different ways.
Fig.5. The equivalent circuit of a resistor.
In wirewound resistors, parasitic inductance is formed due to the winding of the wire and the inductance of the leads, and parasitic capacitance is due to the turn-to-turn capacitance. Wirewound resistors are much less high-frequency than non-wirewound ones, and their use without special measures is limited to the DC region and the audio frequency range.
Fig. 6. Functional characteristic of the resistance of variable resistors.
Unlike fixed resistors, variables have, in addition to the above. additional options. This is a functional characteristic (Fig. 6.). It determines the dependence of the resistance of the variable resistor on the position (angle of rotation) of the movable contact. The most common relationships are: linear – A, logarithmic – B, inverse logarithmic – C.
General Purpose Resistors
The general purpose group includes resistors used as anode and collector loads, resistances in emitter and base circuits, etc.
Carbon resistors are designed to operate in DC, AC and pulse current circuits in electronic equipment.
Resistors are cylindrical and have radial or axial leads. Outside – green glurophobic enamel.
Carbon resistors are characterized by high resistance stability, low self-noise, low negative TCR, and weak dependence of resistance on the applied voltage frequency.
The main types of carbon resistors are: general-purpose resistors of the C1-4 VS type, special-purpose resistors of the BLP type, semi-precision of the ULI type, which are designed to operate in RF circuits as active loads. Due to the widespread use of metal film and the rapid development of microwire high stability resistors, the use of carbon resistors in our time has become more limited.
Metal film resistors are designed to operate in DC, AC and pulse current circuits. They are heat resistant, moisture resistant, have high mechanical strength.
They are widely used in small-sized equipment, because. they are dimensionally compatible with ICs. These resistors have better electrical parameters compared to carbon and composite ones at a relatively low cost, which explains their wide application.
Disadvantages: relatively low resistance to impulse loading and a smaller frequency range of application than carbon ones.
Metal film resistors contain a resistive element in the form of a very thin (tenths of a micrometer) metal film deposited on a base made of ceramic, glass, laminated plastic, ceramic glass or other insulating material. Hydrophobic enamel – red.
The main types of metal-film resistors: C2 MLT – heat-resistant; OMLT – special with increased reliability; MT – with increased heat resistance; MGP – hermetic, precision; C2 -10 – ultrahigh-frequency precision; SP2-3 – variables of a closed design.
Composition resistors are used for the same purposes as metal film resistors. Distinctive features of the resistors of this group are high vibration resistance due to pressing the leads into the base of the resistors, a high level of intrinsic noise and the dependence of the resistance on the applied voltage.
The resistive element of these resistors is made on the basis of compositions consisting of a mixture of a powdered conductor (soot, graphite, etc.) and an organic or inorganic dielectric.
The main types of composite resistors: S3-3, S3-3P, S3-4, SKIM – lacquered; S3-13, S3-14, KVM, KIM, KLM – high-megaohm lacquer-film; SP, SP3-1, SP3-22, SP3-27, SP3-26, SP3-39 – trimming varnish-film; SP3-24, SP3-36, SP3-40, SP3-37, RP1-53, RP1-48 – trimmers with a rectilinearly moving system; RP1-52 – subminiature trimmers; SP4-1a, SP4-2Ma – volumetric adjustment.
Wirewound resistors have increased temperature stability and heat resistance. These resistors have a high power dissipation capacity (tens of watts) with relatively small dimensions. The main disadvantages of wirewound resistors are limited resistance range and high cost, as well as large inductance and self-capacitance.
Structurally, they are made by winding wire from nichrome, manganin, constantan on an insulated cylindrical frame.
Resistors PE, PEV, PEVR, PEVT (PE – wire enameled, V – moisture resistant, R – adjustable with a clamp, T – heat-resistant) – previously released modifications. Modern – C5-35, C5-36, C5-378. C5-31 – microwire miniature.
Resistors of variable resistance are divided into adjusting and tuning.
If a fixed resistor has two terminals, then a variable (adjusting and trimmer) has three. The middle output is a slider that is moved by a handle (axle) protruding out of the case.
The adjusting resistor is used relatively often, for example, to control the sound volume. With a tuning resistor, some kind of construction mode is selected or during adjustment. The handle (axis) of its engine is short, designed for adjustment with a screwdriver.
The diagrams indicate the resistance between the extreme terminals of the variable resistor, while the resistance between the middle and the extreme changes with the rotation of the protruding axis of the resistor.
Most often in designs use adjusting resistors SP (variable resistance), SPO (variable volume resistance). The power of variable resistors is not set on the circuit. Most general purpose variable resistors are composite non-wire resistors. Can be single or twin design, with or without switch, with or without screen, etc.