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Semiconductor resistors Semiconductor resistors include:

• thermistors
• Bolometers
• posistors
• varistors
• photoresistors.

Thermistors. They are semiconductor thermal devices with a negative temperature coefficient of resistance TKS. As the temperature increases, thermal generation of charge carriers occurs in the semiconductor material, as a result of which the electrical resistance of the thermistor TP decreases. There are TRs that respond to changes in ambient temperatures and to heating caused by current passing through them. The properties of the first group TR are determined by the temperature characteristic Rr=Ф(t°), which expresses the dependence of the device resistance on the ambient temperature (curve 2). The properties of the second group TR are assessed by the current-voltage characteristic U=ph(I), which reflects its heating by passing current and determines the nonlinear properties of the device

Parameters thermistors are:

• resistance (ohms) Rt at a temperature of 20 °C;
• temperature coefficient of resistance, expressing as a percentage the change in resistance of the device when the temperature changes by 1 ° C
• dissipation power Pti at which the temperature does not exceed the permissible limit;
• time constant t, characterizing the thermal inertia of the thermistor TR (t=St/Рр, where St is the heat capacity, representing the energy required to heat the TR by 1 °C, W*s/°C; PP is the dissipation coefficient, i.e. . power dissipated by the TR at a temperature difference between it and the environment of 1 °C, W/°C).

The designation of thermistors consists of three or four elements, for example, SP-21, ST2-26, STZ-27, ST4-15, etc. The letters of the first element ST mean temperature-sensitive resistance, the numbers of the second element characterize the type of semiconductor material used ( 1 - cobalt-manganese, 2 - copper-manganese, 3 - copper-cobalt-manganese, 4 - cobalt-nickel-manganese), the third element is the design code, the letters of the fourth element indicate the code for the operating temperature range (these letters -you don’t have to bet).

Thermistors of groups ST1-21, STZ-21, STZ-27 and others are used as adjustable non-contact resistors in automation circuits; groups MMT, KMT and others - for measuring and regulating temperature, as well as for thermal compensation of elements of electrical circuits; groups T8D, T8E, T8S2M and others - as a temperature-sensitive element when measuring the power of microwave oscillations.

Bolometers. They are a special type of thermistors used as receivers of radiant energy. The action of bolometers is based on a change in the resistance of the sensitive element when it is heated as a result of the absorption of energy from radiation.

Semiconductor bolometers contain two (active and compensation) thermistor elements. The active one is directly exposed to the measured radiation, and the compensating one is shielded from radiation and serves to compensate for the influence of changes in ambient temperature. The designations of semiconductor bolometers consist of letters and numbers (for example, BKM-1, BKM-2), indicating the serial number of the device type.

Table 46

Note. The parameters of thermistors and posistors are indicated for an ambient temperature of 20 °C, and ST1-19 - for 150 °C.

Bolometers are used for non-contact remote temperature measurement as radiant energy receivers in spectral instruments and in various orientation systems. Immersion semiconductor bolometers (for example, BP1-2) are used as infrared radiation receivers in automatic monitoring equipment of critical components of railway rolling stock (wheel pairs, bearings, etc.)

PTC resistors. They are thermistors with a positive temperature coefficient of resistance. TCR of posistors made on the basis of barium titanate reaches tens of percent per 1 °C

PTC posistors are used to limit and stabilize current in electrical circuits, auto-gain control in thermal compensation circuits, to protect circuit elements and devices from overheating, temperature control, etc. CT5-1, ST6-1A, ST6- devices are used as posistors. 1B, ST6-5B, ST6-4V, ST64G, etc.

The main parameters of some thermistors and posistors are given in Table 46.

CVTs. These devices are volume-type semiconductor resistors with nonlinear current-voltage characteristics. For voltages of different polarities, the current-voltage characteristics are symmetrical. Varistors can be used in DC, AC (with frequencies up to several kilohertz) and pulse current circuits. Rod and disk varistors are made from powdered silicon carbide.

The main parameters of varistors are the following.

• Rated classification voltage Ucl is a constant voltage at which a given current Icl passes through the varistor.
• Maximum permissible pulse voltage Ui max [for rod varistors
• Ui,max = (1.2-2) Ucl, and for disk ones Ul max = (3H-4) UKa].
• Nonlinearity coefficient P is the ratio of the varistor's resistance to direct current to its resistance to alternating current.
• Rated dissipation power Raon - 1 kyaUkl at a given ambient temperature.
• The symbol for va-ristors consists of letters and numbers (for example, CH1-1-1-1500).

The letters CH indicate non-linear resistance, the first number indicates the material used, the second - the design (1 - rod, 2 - disk), the third - the serial number of the development; the number at the end of the designation characterizes the magnitude of the voltage drop.

The parameters of some types of varistors are given in table. 47.

Varistors used in devices for stabilizing high-voltage voltage sources of television receivers, for stabilizing currents in deflecting coils of picture tubes, in demagnetization systems for color picture tubes, and automatic control systems.

Photoresistors. They are semiconductor devices whose electrical resistance changes under the influence of electromagnetic (light) radiation. The nature of the change in resistance is determined by the intensity and composition of the irradiating light.

Parameters photoresistors The FRs are as follows.

• Operating voltage at which the DF can be used during the specified service life while maintaining its parameters.
• Permissible power dissipation of the RF - the maximum power dissipated by the RF without its thermal damage
• Dark electrical resistance RT - at 20 °C 30 s after removing the illumination of 200 lux.
• Rate current It passing in the FR circuit with an applied operating voltage 30 s after removing the illumination of 200 lux.
• Luminous current Ic passing through the DF at a voltage and illumination of 200 lux from a light source with a color temperature of 2850K. Table 47

To measure temperatures, thermistors are used from materials that have a highly stable TCR, a linear dependence of resistance on temperature, good reproducibility of properties and inertness to environmental influences. Such materials primarily include platinum. Due to their low cost, copper thermistors are widely used; tungsten and nickel are also used.

The resistance of platinum thermistors in the temperature range from 0 to +650 °C is expressed by the relation R = R 0 (1 + A + B 2), where R 0 -- resistance at 0 °C; -- temperature, °C. For platinum wire with ratio R 100 /R o = 1.385 values ​​of A = 3.90784·10 -3 Kg -1 ; IN= 5.7841-10 -7 K -2. In the temperature range from 0 to -200 °C, the dependence of platinum resistance on temperature has the form R = R 0 , Where WITH= = --4.482-10 -12 K -4 . Industrial platinum thermometers in accordance with GOST 6651--78 are used in the temperature range from -260 to + 1100 °C.

Miniature high-resistance platinum thermistors are made by burning or otherwise applying a platinum film onto a ceramic base 1-2 mm thick. With a film width of 0.1--0.2 mm and a length of 5--10 mm, the resistance of the thermistor lies in the range of 200--500 Ohms. This kind of heat-sensitive elements, when applying a film on both sides, are used to measure the temperature gradient and have a sensitivity threshold of (1 5)10 -5 K/m.

When calculating the resistance of copper conductors in the temperature range from -50 to +180 °C, you can use the formula R = R 0 (1 +), where = 4.26-10 -3 K -1 ; R 0 -- resistance at 0 °C. If you need to determine the resistance for a copper thermistor R,(at temperature 2) by known resistance R 1

(at temperature 1), then you should use the formula

R 2 = R 1 (1 + 2)/(1 + 1 ).

The copper thermistor can only be used up to a temperature of 200°C in an atmosphere free of humidity and corrosive gases. At higher temperatures, copper oxidizes. The lower temperature limit for copper resistance thermometers is -200°C, although with the introduction of individual calibration they can be used down to -260°C.

Errors that arise when measuring temperature with resistance thermometers are caused by instability over time of the initial resistance of the thermometer and its TCR, changes in the resistance of the line connecting the thermometer to the measuring device, overheating of the measuring thermometer

electric shock In particular, V.I. Lakh gives the following relation to determine the permissible measuring current through a thermometer in the range of measured temperatures up to 750 °C

I = 2d 1.50.5, where I is current, A; d -- thermometer wire diameter, mm; -- permissible increase in thermometer readings due to its heating by current. In the temperature range from -50 to +100 °C, overheating of a wire with diameter d in calm air = 0.05 0.1 mm is determined from the formula = 5I 2 /d 2 .

Semiconductor thermistors differ from metal ones in smaller dimensions and higher TCR values.

The TCR of semiconductor thermistors (STR) is negative and decreases in inverse proportion to the square of the absolute temperature: = B/ 2. At 20 °C TCS is 0.02--0.08 K -1.

Temperature dependence of PTR resistance (Fig. 11, curve 2) is described quite well by the formula R = Ae H/T , Where T-- absolute temperature; A-- coefficient having the dimension of Resistance; IN-- coefficient having the dimension of temperature. In Fig. For comparison, Fig. 11 shows the temperature dependence for a copper thermistor (straight line 1).

If the coefficients for the applied PTR are not known A And IN, But the resistances R 1 and R 2 at T 1 and T 2 are known, then the resistance and coefficient IN for any other temperature can be determined from the relations:

The disadvantages of semiconductor thermistors, which significantly reduce their performance, are the nonlinearity of the dependence of resistance on temperature (Fig. 11) and a significant scatter from sample to sample of both the nominal resistance and constant IN

Structurally, thermistors can be manufactured in a wide variety of shapes. In Fig. Figure 12 shows the design of several types of thermistors. Thermistors of type MMT-1 and KMT-1 are a semiconductor rod coated with enamel paint, with contact caps and leads. This type of thermistor can only be used in dry rooms.

Thermistors of types MMT-4a and KMT-4a are enclosed in metal capsules and sealed, so they can be used at any humidity and even in liquids that are not aggressive to the thermistor body.

Of particular interest are miniature semiconductor thermistors, which make it possible to measure the temperature of small objects with minimal distortion of the operating mode, as well as temperature that changes over time. Thermistors ST1-19 and STZ-19 have a teardrop shape. For sealing, the sensitive element in them is fused with glass and equipped with leads made of wire having low thermal conductivity. The thermistor STZ-25 is sensitive! the element is also placed in a glass shell, the diameter of which is increased to 0.5-0.3 mm. The thermistor is attached to the traverses using leads.

The thermistor ST4-16, in which the thermosensitive element in the form of a bead is fused with glass for sealing, has increased stability and a relatively small spread of the nominal value; resistance (less than ±5%). The ST17-1 thermistor is designed to operate in the low temperature range (from -258 to +60 °C). "At the boiling point of liquid nitrogen (--196 °C) its TKS ranges from -0.06 to

0.12 K -1 at a temperature of -252.6 °C TKS increases and reaches values ​​from -0.15 to -0.30 K -1 , the time constant when immersed in liquid nitrogen does not exceed 3 s. The ST18-1 thermistor is designed to operate in the temperature range from +200 to +600 "C, its TKS at +250 °C is -0.034 K -1, at 600 °C it is equal to -0.011 K -1" 1.

In table 11-5 shows the characteristics for some types of PTR, taken from the relevant standards. The column “nominal resistance” shows the extreme values ​​of the series of nominal resistances.

Table 5

Minimum power dissipation R min is the power at which a thermistor located in calm air at a temperature of (20 ± 1) °C, resistance decreases from heating by current by no more than 1%. The maximum power is called Pmax, at which the thermistor, located in the same conditions, is heated by current to the upper permissible temperature. In addition, the permissible power P permissible at the maximum permissible temperature is indicated. According to the standards for most thermistors, deviations from the nominal values ​​of the initial resistance are allowed within ± 20%; with long-term exposure of the PTR at the maximum permissible temperature, a change in resistance is allowed within ± 3%; when stored for 18 months, the change in resistance should not exceed ± (1 3)%, when stored for up to 10 years, the change in resistance can reach ±30%. However, experience with PTR shows that the stability of PTR characteristics is in most cases significantly higher than that specified in the standards.

Currently, there are not standards for all types of manufactured anti-tank guns. The main characteristics of some of these types of anti-tank guns, not included in the table. 5, are given in table. 6. In the column “constant IN" two ranges of possible values ​​are given IN: the first line refers to low temperatures, and the second to high temperatures. The nominal resistances of PTR types KMT-14, ST1-18, ST1-19 are standardized for 150 °C, the rest - for 20 °C.

Table 6

Page 5

Errors that arise when measuring temperature with resistance thermometers are caused by instability over time of the initial resistance of the thermometer and its TCR, changes in the resistance of the line connecting the thermometer to the measuring device, and overheating of the thermometer by the measuring current.

Resistance thermometers are among the most accurate temperature transmitters. For example, platinum theomoresistors make it possible to measure temperature with an error of the order of 0.001 ° C.

Semiconductor thermistors differ from metal smaller dimensions and higher TCS values.

The TCR of semiconductor thermistors (STR) is negative and decreases in inverse proportion to the square of the absolute temperature: a = B/Θ2. At 20°C the TCR value is 2-8 percent/K.

Temperature dependence of PTR resistance ( rice. 7, curve 2) is described quite well by the formula RT = AeB/Θ, where Θ is the absolute temperature; A is a coefficient having the dimension of resistance; B is a coefficient having the dimension of temperature. In Fig. rice. 7 For comparison, the temperature dependence for a copper thermistor is shown (curve 1). For each specific PTR, coefficients A and B are usually constant, with the exception of some types of 1 PTR (for example, ST 3-14), for the latter B can take two different values ​​depending on the range of measured temperatures.

If coefficients A and B are not known for the applied PTR, but resistances R1 and R2 are known at Θ1 and Θ2, then the resistance value and coefficient B for any other temperature can be determined from the relations

"

Structurally, thermistors can be manufactured in a wide variety of shapes. On rice. 8 the device of several types of thermistors is shown. Thermistors of type MMT-1 and KMT-1 are a semiconductor rod coated with enamel paint with contact caps and leads. This type of thermistor can only be used in dry rooms.,

Thermistor types MMT-4 and KMT-4 are enclosed in metal capsules and sealed, so they can be used in conditions of any humidity and even in liquids that are not aggressive to the thermistor body.

Of particular interest are miniature semiconductor thermistors, which make it possible to measure the temperature of small objects with minimal distortion of the operating mode, as well as temperature that changes over time. Thermistors ST1-19 and STZ-19 have a teardrop shape. The sensitive element in them is sealed with glass and equipped with leads made of wire having low thermal conductivity. In the STZ-25 thermistor, the sensitive element is also placed in a glass shell, the diameter of which is increased to 0.5-0.3 mm. The thermistor is attached to the traverses using leads.

Rice. 8

In table 4 presents the main characteristics of some PTRs. The column “nominal resistances” shows the extreme values ​​of the series of nominal resistances standardized for most PTRs at 20° C. The exception is PTR types

Table 4

The device is universal and is designed to maintain a fixed value of a given positive temperature in the range of +1...80 °C with an accuracy of 0.2 °C.

The heat stabilizer can be used in an artificial incubator for hatching chickens from eggs (+37.5 °C), a drying cabinet (+60 °C), a home bath, or maintain a positive temperature (+2 °C) in an insulated storage for vegetables on the balcony at negative ambient temperature. In this case, the operation of the device is not affected by possible instability of the mains voltage.

The device is powered using a transformerless circuit directly from a 220 V network, which makes it possible to significantly reduce its dimensions.

The principle of operation of the circuit on comparator D1 does not need any special explanation - it is often used in various devices and is described in the literature. A special feature of this comparator connection is the control of the output load using the emitter output of the microcircuit. The use of transistor VT1 improves the operation of the comparator and simplifies the thyristor control circuit.

Any load with a power of no more than 1000 W is suitable as a heater (I used a 500 W “air” heating element - it is more durable than a heater in the form of a light bulb). If you need to control a more powerful load, then the VD3...VD7 diodes must be used for a higher permissible operating current (for example, D246A, B, D247A, B) and connect an additional thyristor together with another KT940A transistor in the same way as the circuit shown. The control signal for the second load (it is connected to separate sockets) is removed from pin D1/1.

To control a load with a power of more than 1000 W, you can use one thyristor type T122-20-4 or T122-25-4 (the last digit in the designation may be greater).

Indicators of the operating modes of the circuit are LEDs HL1, HL2. So, when you turn on the device with toggle switch S2, if heating element A1 is not connected (or it has burned out), then both LEDs will light up at the same time, and during normal operation of the device, the glow between the indicators will alternate: when A1 heats up, the red LED HL1 lights up (the thyristor is open), when cooling, HL2 is green.

The circuit uses an STZ-19 type thermistor as a temperature sensor (it has small dimensions and weight), but other types are also suitable (this may increase the inertia of thermal stabilization).

For ease of operation of the thermostabilizer, a switch (S1) is used, which allows you to have 5 fixed temperature values ​​and one variable. In the sixth position of the switch, variable resistor R2 allows you to set any temperature in the specified range.

It is convenient to set the most frequently used temperature values ​​with resistors R3, R6...R8, R10 (multi-turn, type SP5-2) in the corresponding switch positions.

The circuit uses constant resistors of type C2-23; variable resistor R2 type SP2-2; capacitor C1-K50-15, C2 - K10-7V; switch S1 type PG2-5-6P2N; toggle switch S2 type TZ; connector X1 - RS-4; sockets X2, XZ type G4.0.

When manufacturing the structure, it is necessary to provide a heat sink for the VSI thyristor and VD3...VD7 diodes.

The appearance of the housing structure is shown in the figure above. It is made from dielectric materials.

The connecting cable from socket X1 to the temperature sensor can be up to two meters long and is made with wires intertwined with each other - this will reduce the influence of noise and interference on the input of the circuit.