Thermistors

Brief information from the theory of thermistors

A thermistor is a semiconductor resistor whose resistance strongly depends on temperature. Specific electrical conductivity of semiconductors:

In impurity (n-type or p-type) semiconductors, one of the terms in the above expression can be neglected.

When heated, the mobility of carriers changes relatively little, but the concentration changes very strongly. Therefore, the temperature dependence of the conductivity of semiconductors is similar to the temperature dependence of the concentration of majority carriers, and the electrical resistance of thermistors can be determined by the formula:

where No is a coefficient depending on the type and geometric dimensions of the semiconductor.

Experimentally, the temperature sensitivity coefficient is determined by the formula:

where T1 and T2 are the initial and final temperatures of the operating temperature range, R1 and R2 are the resistance of the thermistor at temperatures T1 and T2, respectively.

Rice. 1 Graph of the resistance of a semiconductor resistor versus temperature.

Most often, thermistors have a negative temperature coefficient of resistance. Thermistors are also produced that have a positive coefficient in a relatively narrow temperature range and are called posistors. When heated, the resistance value of thermistors decreases, while that of posistors increases hundreds and thousands of times. In reference books, the aR value is given for a temperature of 20 °C.

A thermistor is characterized by a certain thermal inertia, depending on the chemical properties of the semiconductor and the design of the element (radiating surface area). Thermal inertia is estimated by the time constant t - the time during which the difference between the body’s own temperature and the temperature of the environment decreases by e times.

If a thermistor having a certain temperature is placed in an environment with a different temperature, then its temperature will change over time according to the exponential law:

As the thermistor cools, its resistance increases (Fig. 2).

Figure 2. The process of changing the temperature and resistance of the thermistor as it cools

Thermistors or thermistors (TR) are semiconductor resistors with a nonlinear Volt-Ampere Characteristic (VAC). A characteristic feature of temperature-dependent resistors is that the resistance in the body of the element changes due to fluctuations in ambient temperature. Thermistors with negative and positive Temperature Coefficient of Resistance (TCR) are produced.

Their scope of application is quite wide - they are used in circuits and temperature stabilization circuits of resistor amplifier systems, as well as in various types of devices and instruments for measurement, regulation and automation (measurement, level control and automatic adjustment of climate (temperature) and fire alarms).

The main characteristics of thermistors include:

• Nominal resistance Rн

  - electrical resistance, the value of which is indicated on the case or indicated in the regulatory documentation, measured at a certain ambient temperature (usually 20°C). The values ​​are set according to row E6 or E12.

• Temperature coefficient of resistance TKS

  — characterizes a change (reversible) in resistance by one degree Celsius.

• Maximum permissible power dissipation Pmax

  — the greatest power that a TR can dissipate for a long time without causing irreversible changes in characteristics. However, its temperature should not exceed the maximum operating temperature.

• Temperature sensitivity coefficient B

  — determines the nature of the temperature dependence of this type of TR. Known as the “B constant”, it depends on the physical properties of the semiconductor material from which the temperature-sensitive element is made.

• Time constant t

  — characterizes thermal inertia. It is equal to the time during which the resistance of a TR changes by 63% when it is transferred from an air environment with a temperature of 0°C to an air environment with a temperature of 100°C.

  Read also:  The first designs of DC electric motors. Part 1

The dependence of the current that passes through the thermistor on the voltage applied to it (in the case of temperature equilibrium between the resistor and the environment) is determined by the current-voltage characteristic. Inertia shows how quickly the part will respond to changes in ambient temperature, that is, it determines the rate of change in the resistance of the element. Inertia is directly dependent on the design, size of the resistor and thermal conductivity of the environment. Stability determines the period of time during which an element retains its original properties (during use or storage).

Range of nominal resistances at 20°C	Permissible deviation, ±%	Maximum power, at 20° C, mW	Operating temperature range, ° C	TCS at 20°С, %/°С	Constant V, K	Time constant t, s	Application area	Appearance
Rod
KMT-1	22 kOhm…1 MOhm	±20	1000	-60…180	4,2…8,4	3600…7200	85	Temperature measurement and control, temperature compensation
MMT-1	1 kOhm…220 kOhm		600	-60…125	2.4…5	2030…4300
CT3-1	680 kOhm…2.2 MOhm	±10, ±20			3.35…3.95	2870…3395
KMT-4	22 kOhm…1 MOhm	±20	650		4.2…8.4	3600…7200	115
MMT-4	1 kOhm…220 kOhm		560		2.4…5	3060…4300
CT3-6	6.8 kOhm, 8.2 kOhm	±10	150	-90…125	2.8…3.2	1200…2400	35
MMT-6	10 kOhm…100 kOhm	±20	50	-60…125	2.4…5	2060…4300
KMT-10	100 kOhm…3.3 MOhm		250 for 2s	0…125	≥4,2	≥3600	75	Thermal control
KMT-11							10
C9T-1a	150 Ohm…450 Ohm	800	-60…100	1600…2000	110	Temperature regulation
Disk
CT1-2	82 Ohm, 91 Ohm, 100 Ohm, 110 Ohm	±5	700	-60…85	4,4…4,9	3800…4200	60…100	TK, temperature measurement and control
CT4-2	2.1 kOhm…3.0 kOhm	-60…125	4,2…4,8	3170…4120	Temperature measurement
CT4-15	880 Ohm…1.12 kOhm	-60…155	3,4…3,8	2350…3260
KMT-8	100 Ohm…10 kOhm	±10, ±20	600	-60…70	4,2…8,4	3600…7200	900	Temperature compensation
MMT-8	1 Ohm…1 kOhm				2,4…4	2060…3430
MMT-9	10 Ohm…4.7 kOhm		900	-60…125	2,4…5	2060…4300
KMT-12	100 Ohm…10 kOhm	±30	700		4,2…8,4	3600…7200	TK, temperature measurement and control
MMT-12	4.7 Ohm…1 kOhm				2,4…4	2060…3430
MMT-13	10 Ohm…2.2 kOhm	±20	600		2,4…5	2060…4300		100
KMT-17v	330 Ohm…22 kOhm	±10, ±20	300	-60…155	4,2…7	3600…6000		30
CT1-17				-60…100
CT3-17	33 Ohm…330 Ohm				3,0…4,5	2580…3860
CT4-17	1.5 kOhm…2.2 kOhm	±10	500	-80 … 100	3,8…4,2	3260…3600
CT3-23	2.2 Ohm…4.7 kOhm	±10, ±20	0 … 125	3,1…3,8	2600…3200	TK
CT3-28	150 Ohm…3.3 kOhm	±20	-60 … 125	3…4,6	2580…3970
MMT-15	760 Ohm…1.21 kOhm	2,6…4		2230…3430	TK, temperature measurement and control
PT	80 Ohm…400 Ohm	±20		-60 … 150	4,4…4,8	3800…4100		temperature measurement and control
PT-2
PT-1	400 Ohm…900 Ohm	4,1…5,1	3500…4400		Automatic control system sensors
PT-3		±20	4,3…4,8			3700…4100
PT-4	600 Ohm…800 Ohm	4,4…4,9	3500…4200
TR-3	1.2 kOhm, 12 kOhm	±10	1000	-60…125		3,9…4,8	3470…4270
Beaded
KMT-14	510; 680; 910 Ohm; 160; 200; 330 kOhm; 4.3; 7.5 MOhm at 150°C	±20	100	-100…300	2,1…2,5	3690…4510	10…60	Temperature measurement and control
					3,4…4,2	6120…7480
					3,5…4,3	6300…7700
CT3-14	1.5; 2.2 kOhm		30	-60…125	3,2…4,2	2600…3600	4
MKMT-16	2.7; 5.1 kOhm	±30	40	-60…125	3,8…4,2	3260…3600	10
CT1-18	1.5; 2.2; 22; 33 kOhm; 1.5; 2.2 MOhm at 150°C	±20	45	-60…300	2.25…5 at 150°C	4050…9000	1
CT3-18	680 Ohm…3.3 kOhm		15	-90…125	2,6…4,1	2250…3520
CT1-19	3.3…10; 100; 150 kOhm; 1.5; 2.2 MOhm at 150°C		60	-60…300	2.35…4 at 150°C	4230…7200	3
CT3-19	2.2; 10; 15 kOhm;		45	-90…125	3,4…4,5	2900…3850
CT3-22	1 kOhm at 25°C	±30	8	-60…85	3,1…4,2	2700…3700	15	Variable resistance without moving contact
CT3-25	1.5…6.8 kOhm	±20		-100…125	3,05…4,3	2600…3700	0,4	Measurement
CT4-16	10…27 kOhm	±5; ±10	150	-60…155	3,45…4,45	2720…3960	30	Temperature measurement, TC
CT4-16a	6.8; 10; 15 kOhm	±1; ±2; ±5	180	-60…200	4,05…4,45	3260…4100
TR-1	15; 33 kOhm	±10; ±20	20; 50	-60…155	3,8…4,4	3200…3900	5…10
TR-2
TR-4	1 kOhm	±20	70	-60…200	1,8…2,2	1600…1960	3	Temperature measurement and control, TK
Thermistors with negative TCR based on a semiconductor single crystal of synthetic diamond
Type	Range of nominal resistances at 20°C	Permissible deviation, ±%	Maximum power, at 20° C, mW	Operating temperature range, ° C	TCS at 20°С, %/°С	Constant V, K	Time constant t, s	Application area	Appearance
TRA-1	0.01…10000 kOhm	±5; ±10; ±20	500	-200…+350	0,2…2,3	300…2500	1	Measurement of temperature, liquid or gas flow rate, TK
TRA-2	1…100 MOhm				0,55…6,0	600…6000

Thermistors with negative TCR - microwave power meters
Type	Resistance at the main operating point, Ohm	Maximum power, mW	Operating temperature range, ° C	Sensitivity at operating point, Ohm/mW	TCS at 20°С, %/°С	Constant V, K	Time constant t, s	Appearance
T8D	140…160	15	-60…85	20…30	1500	1
T8E		10		30…70				—
T8M	180…220	11		60…110	—			—
Т8Р	115…135	12		10…19	—
Т8C1	110…130	24		10…40	—			—
Т8C2	140…160	19		12…25	—			—
Т8C3	140…160	23		10…50	—			—
T8S1M	110…130	24		10…40	—			—
T8S2M	140…160	19		12…25	—			—
Т8S3М	140…160	23		10…50	—			—
T9	115…135	19		10…40	—			—
TS-1	150	12		—	0,6…3,4	0,8
TS-2		17,5		—	0,3…2,3	1,3
CT3-29	2.2 kOhm at 20°C, 200 when hot	31		10…16	3,15…3,85	2700…3300	0,6…0,7
CT3-32	2.2 kOhm at 20°C, 150 when hot	18,6	-60…70	20…30

Note: The sensitivity of the TP at the operating point at an ambient temperature of 20°C is understood as the change in the resistance of the TP when the dissipated power changes by 1 mW.

Thermistors of direct heating - voltage stabilizer
Type	Rated voltage, V	General voltage stabilization limit, V	Maximum permissible voltage change, V	Average operating current, mA	Current working area, mA	Maximum permissible short-term (2 s) overload, mA	Appearance
TP 2/0.5	2	1,6…3	0,4	0,5	0,2…2	4
TP 2/2				2	0,4…6	12
TP 6/2	6	4,2…7,8	1,2

Thermistors with negative TCS of indirect heating
Type	Nominal resistance range	Rated power, mW	Operating temperature range, ° C	TCS at 20°С, %/°С	Maximum current in the heating circuit, mA	Constant V, K	Time constant t, s	Application area	Appearance
TKP-20	500 Ohm	220	-65…85		40		45	Adjustable non-contact resistors
TCPM-20
TKP-50	2500 Ohm	200			35		79
TCPM-50
TKP-300A	10 kOhm	24		—	20	—	17
TCPM-300A
ST1-21	6.8 kOhm - 150 kOhm	60		3,25…5,75	25	2880…4920	15…40
ST3-21				2,9…4,6		2560…3840
ST1-27	680 Ohm - 1.5 kOhm	70		4,3…5,25	27	3690…4510	4…6
ST3-27				3…4,45	26	2560…3840
ST1-30	33 kOhm	—		4,2…5,1	120	3600…4400	6…12	Measuring velocities of liquids and gases
ST1-31	4.7 kOhm	194		—	44	3690…4510		Adjustable non-contact resistors
ST3-31	680 Ohm	90		3,15…3,85	29,1	2700…3300	4…6
ST3-33		—		3…4		2790…3410	4…10

Thermistors with positive TCS - posistors
Type	Range of nominal resistances at 20°C	Maximum power, W	Operating temperature range, ° C	TCS at 20°С, %/°С	Maximum current in the heating circuit, mA	Multiplicity of measurement in the area of ​​positive TCS	Time constant t, s	Application area	Appearance
ST5-1	20…150 Ohm	0,7	-20…200	100…200	—	—	20	Temperature measurement and control, fire alarm, thermal protection, current limitation and stabilization
ST6-1A	40…400 Ohm	1,1	-60…155	40…155	—	1000 at 25…140°С
ST6-1B	180; 270 Ohm	0,8	-60…125	20…125	—	1000 at 25…100°С
ST6-3B	1 - 10 kOhm	0,2		10…125	15	100 at 25…80°С	10
ST6-4B	100…400 Ohm	0,8		20…125		1000 at 25…100°С	40
ST6-4G	5…25 kOhm			-20…125	2…6	5…15		Temperature measurement and control
ST11-1G	100…300 Ohm				6…9	20…80
ST6-1B1	100…400 Ohm		-60…100	30…100	15	1000 at 25…100°С	20	Heating elements and temperature sensors, thermostatting.
ST6-2B	10…100 Ohm	1,3		10….100
ST6-5B	3…20 Ohm	2,5	-60…125	20…125	15	1000	10	Current limitation and stabilization.
ST6-6B	5…25 Ohm						180
ST10-1	30…100 kOhm	0,3…0,7	-60…70	-20…70	2…4	50	TK
ST14-3	80…200 Ohm	0,5	-60…175	100…175	—	—	—	Self-regulating heating elements of microwave devices
ST15-2-127v	15…35 Ohm	3, Ulim=150V; I=24mA	-60…60	60…160	15	10000 at 25…160°С	—	In circuits for demagnetizing masks of color picture tubes
ST15-2-220v	20…50 Ohm	3, Ulim=150V; I=12mA	-60…85

Description of the experimental setup

The current-voltage characteristics are measured according to the scheme shown in Fig. 3.

Fig.3. Electrical installation diagram

The measuring circuit is powered by a constant regulated voltage source with a built-in voltmeter. The current through the thermistor is measured with a milliammeter.

The MMT-4 thermistor is placed in a demonstration tube with terminals, which does not allow hot water to come into contact with the thermistor body; a liquid thermometer (it is advisable to use a mercury thermometer) can be installed in the tube to control the temperature, directly next to the thermistor.

Variable resistor R2 is only needed if an unregulated power supply is used.

Resistor MMT-4

The directory of precious metal content in radio components was created on the basis of reference data from organizations involved in the processing of scrap radio components, device passports, forms, labels and other open sources. It is worth noting that the actual content may differ by 20-30% downwards.

Content

Content of precious metals in the resistor: MMT-4

Gold: 0 Silver: 0.0150 Platinum: 0 MPG: 0 According to: Handbook of Precious Metals ORDER No. 70

Fixed resistors contain only silver, which is deposited on the terminals. With variable resistors everything is better, they can contain gold, silver, platinum and palladium alloys. Pretentious variable resistors are especially rich in precious metals.

The resistance of a resistor is its main characteristic. The basic unit of electrical resistance is the ohm (Ω). In practice, derivative units are also used - kiloohm (kOhm), megaohm (MOhm), gigaohm (GOhm). Precious metals are mainly found in variable and construction resistors, and palladium is often used in them in the form of runners or flux wires.

Types of resistors

There are three main types of resistors: A variable resistor is a resistor in which the electrical resistance between the moving contact and the terminals of the resistive element can be changed mechanically. Fixed resistors, the resistance of this resistor cannot be changed. As a rule, they have only two outputs. These resistors can only contain silver, in the form of silver-plated leads. Nonlinear. The resistance of components of this type changes under the influence of temperature (thermistors), light radiation (photoresistors), voltage (varistors) and other quantities.

Main characteristics of resistors

Nominal resistance (Ohm, kOhm, mOhm). Maximum power dissipation (0.25 W, 0.5 W, 1 W, etc.) Tolerance or accuracy class (the permissible spread of resistor parameters depends on this value).

Examples of alphanumeric designation of a resistor

Examples of alphanumeric designations for resistance expressed as an integer: 47 Ohm - 47 R; 47 kOhm – 47 K; 47 MOhm – 47 M. If a decimal fraction is used to express the resistance value, then the order of numbers and letters will be different, for example: 0.47 Ohm – R 47; 0.47 kOhm – K 47; 0.47 MOhm - M 47. If the resistance is expressed as a number other than zero and with a decimal fraction, then the letter in the designation plays the role of a comma, for example: 4.7 Ohm - 4R7; 4.7 kOhm – 4K7; 4.7 MOhm – 4M7. The permissible error is indicated in % and is indicated after the nominal value, for example ±7%, ±10%, ±40%. The accuracy class can be determined by a letter, depending on the manufacturer - Russian or Latin.

Share link:

Liked this:

Like

Similar

Work order

3.1. Removing the dependence of R(T) of the thermistor resistance on temperature. The thermistor is placed in a vessel with water, which is heated on an electric stove. Measure the resistance of the thermistor at different temperatures - from room temperature to a maximum of 90°C, with an interval of 10°C. Perform measurements for thermistors MMT-4 and MMT-1. Enter the results of the experiment into the table.

3.2. Determination of the thermal time constant of the thermistor. After measuring the resistance of the thermistor at 90 °C, quickly remove it from the water. Take the moment of extraction as t = 0. Turn off the thermostat.

While recording the time, measure the resistance of the thermistor as it cools until it increases approximately three times. Enter the measurement data into the table.