Philips Components NTC Thermistors Introduction to NTCs GENERAL It can be shown that: Definition and composition /T Negative Temperature Coefficient thermistors (NTCs) are resistive components, of which the resistance decreases as temperature increases. They are made from polycrystalline semiconductors, the composition of which is a mixture of chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co) and nickel (Ni). x e -q2 kT For n, the dependency on temperature can be explained as follows: electrons are bound to atoms by certain energies. As one gives the electron an energy equal to, or greater than, the binding energy (e.g. by raising its temperature), there is a probability that the electron will become free to move. As for many semiconductors, this probability has the form of the well-known Maxwell-Boltzmann distribution. Thus: Manufacture The manufacturing process is comparable to that of ceramics. After intensive mixing and the addition of a plastic binder, the mass is shaped into the required form, e.g. pressing (discs), and fired at a temperature high enough to sinter the constituent oxide. New technologies have led to the sawing of isostatic pressed wafers, the compositions of which are very stable, with as a result, high accuracy and high reproducibility. n/e - q 1 kT The total temperature dependency of the conductivity is: /T -c x e- ( q1 + q2) kT In practice, the exponential factor is the most important. Remembering that resistivity is the inverse of conductivity, the following can be derived: Electrical contacts are then added by burning them in with silver paste or by other methods, such as evaporation. Finally, leads (isolated or not) are fitted. Different encapsulations are possible, depending on the size of the ceramic and the application of the component. R = Axe BT where B = q 1 + q 2 or B log R = A + ---T Miniature NTC thermistors are made by placing a bead of oxide paste between two parallel platinum alloy wires and then drying and sintering. The platinum alloy wires are 60 m in diameter and spaced 0.25 mm apart. During sintering, the bead shrinks onto the wires to make a solid and reliable contact. Miniature NTC thermistors are usually mounted in glass to protect them against aggressive gases and fluids. where A and B are parameters depending on each component (resistivity and shape). Shape of an NTC curve and determination of B-value In Fig.1, the resistance is plotted as a function of the inverse of the temperature. Even in semi-logarithmic scale, it can be seen that this curve is not a straight line. This is due to the fact that A and B are not perfectly constant with temperature. However, over a wide range of temperatures, it may be assumed that these parameters are constant. If this range is defined between T1 and T2, and it is assumed that the curve for this range could be approximated with a straight line, the slope of which will be B, this last value between T1 and T2 can be found as follows: Relationship of resistance with temperature The conductivity () of the material is its capacity to drive a current when a voltage is applied to it. As the current is driven by carriers that are free to move (i.e. which are not bound to atoms), then it follows that the conductivity will be proportional to the number of carriers (n) that are free and also to the mobility () that those carriers can acquire under the influence of electrical fields. Thus: = nxex where e is the unit of electrical charge stored by each carrier. The resistance value is measured at T1 and T2: R1 = A x e B T1 and Both n and are functions of temperature. For , the dependency on temperature is related to the interactions of a carrier with other carriers and with the total net amount of vibrating atoms, the vibration varying with temperature. 1995 Mar 31 -c R2 = A x e 2 B T2 Philips Components NTC Thermistors Introduction to NTCs Voltage (V)/Current (I)-characteristic description Dividing yields: R1 ( B T1 - B T2) ------- = e R2 Figure 2 shows the relationship between the current through and the voltage drop across the NTC thermistor heated by this current to a temperature much higher than the ambient temperature. or 1 1 log R 1 - log R 2 = B ------ - ------ log e T1 T2 With very small values of current, it can be seen that the curve remains straight, following an isoresistive line. Remembering that an isoresistive line is in fact an isothermal line (R = f[T]) it indicates that the power consumption is too small to register a distinct rise in temperature. Hence: ln ( R 1 R 2 ) B = -------------------------------1 T1 - 1 T2 In practice, B varies slightly with increasing temperature. For higher current intensities, the temperature rises by the Joule-effect (P = V x I). The equilibrium temperature is reached when the power dissipated by the NTC is in equilibrium with the power applied to it. It can be seen that as the dissipated power is dependent on the environment, the equilibrium will also depend on it and thus the V/I-characteristic too. The characteristic shown in Fig.2 was measured at a constant ambient temperature after equilibrium had been reached. The temperature coefficient of an NTC may be derived 1 dR B from: = ---- x -------- = - -----R dT T2 For the different materials, the constant B may vary between 2000 and 5500 K; e.g. a value of 3600 K yields = -4%/K at a temperature of 300 K. A and B are assumed to be constant between T1 and T2 B T T . 1 2 Assuming that: In practice, most NTCs are specified with a reference value at 25 C and a constant B-value between 25 C and 85 C. For commodity reasons, the curves printed in this handbook show the resistance as a function of temperature, instead of its inverse. * the heat transfer is proportional to the difference in temperature between thermistor and surrounding medium (which is true for low temperatures); * a constant temperature is present throughout the body of the thermistor; then, in case of equilibrium: W = V x A = ( T - T0) where T0 is the ambient temperature and the dissipation factor (defined in Chapter "Speed of response"). MBD918 5 10halfpage handbook, From this relationship, it is obvious that the temperature of the component will be that of its surroundings if the power P (W) applied to the component is equal to zero (power-off value). If the applied power is not very low (0.01 W), then T is no longer equal to T0 and will be strongly dependent on (power-on conditions). R () 10 4 Because it is not possible to define without any doubt ( is not only dependent on the component itself, but also on special housing if any, convection, turbulence, etc.), all components are specified with their power-off values. 10 3 10 2 2.6 Fig.1 3.0 To choose a component that will be used in a `power-on' application, it is necessary to determine in that application. 3.4 3.8 1/T (10 3 x K 1 ) Resistance as a function of the inverse of temperature for a typical NTC. 1995 Mar 31 3 Philips Components NTC Thermistors Introduction to NTCs V/I-CHARACTERISTIC MBD919 handbook, full pagewidth V (V) 50 k 10 k 0 1k 5k 50 10 2 0 10 o 50 C 50 o o o 200 C 100 C 400 C 10 5W o 300 C 10 0.5 0.5 1 10 1 mW 1m W 10 5m W 1 50 mW 0.1 mW 1W 10 W W W 10 2 10 I (mA) 10 3 Fig.2 Voltage as a function of the current characteristics of a NTC thermistor. SPEED OF RESPONSE This equation yields: Thermal time constant T - T1 = ( T0 - T1) e The thermal time constant is an indication of the time that a component needs to reach thermal equilibrium. This constant depends on two important parameters. where the thermal time constant () is defined as the ratio of the heat capacity (H) of the thermistor with respect to its dissipation factor (). One parameter is the thermal capacity (H) of the component, i.e. the energy that must be applied to the component in order to raise its temperature by 1 Kelvin (or the energy that the component must loose in order to lower its temperature by 1 Kelvin). The units are thus quoted in Joules/Kelvin. The second parameter is called the dissipation factor (). If the temperature of a component rises, it will tend to dissipate energy. This dissipation will depend on the surroundings and also on the component itself. The dissipation factor is defined as the ratio of the change in power dissipation with respect to the resultant body temperature change (units in W/K). The temperature value when the time elapsed (t) is equal to is given by the formula: T -1 T - -----0- - T 0 = 1 - e = 0.632 T 1 This equation gives the following definition: The thermal time constant is the time required for the temperature of a thermistor to change by 63.2% of the difference between its initial and final body temperatures (in accordance with "IEC 539 ": 85 C and 25 C respectively), when subjected to a step function temperature change. If a step change in temperature is applied to a component e.g. from high (T1) to low (T0) temperature, the energy lost by the component (-HdT) is equal to the energy dissipated by it ([T - T0]dt): It is entirely dependent on the component design. The thermal time constant depends on , which varies for different media. - HdT = ( T - T 0 ) dt 1995 Mar 31 -t 4 Philips Components NTC Thermistors Introduction to NTCs The thermal time constants referred to in the data sheets are measured as follows; the method used depends on the application: * By cooling in air under zero power conditions (Tc) handbook, halfpage * By heating or cooling, transferring the thermistor from ambient temperature (25 C) to a bath with fluid with a higher or lower temperature under zero power conditions (Tr, termed `response time' in the data sheets). MBD920 R Tolerances in the nominal NTC specification As already mentioned, an NTC thermistor is normally specified by giving a reference value (generally R25) and the B-value (B25/85). Unfortunately, the manufacturing process dictates that identical components cannot be guaranteed, so there are some tolerances. These tolerances can mean an upward or downward shift in the resistance value, equal at all temperatures due to, for example, variations of mechanical dimensions. The entire curve moves equally up or down (see Fig.3). T This tolerance is usually indicated by giving the shift at the reference temperature; for example, R25 = 10 k 5%. Fig.3 A tolerance also exists on the slope of the curve. Because the B-value is an indication of that slope, it is normally indicated as a tolerance on B25/85. This is covered mainly by variations in the material composition and the effect of sintering on the material (see Fig.4). The effect of the slope or B-value deviation on the resistance at several temperatures can be calculated. handbook, halfpage MBD921 R The fundamental equation of an NTC is: R nT = R ref Effect of variations of mechanical dimensions on the resistance as a function of temperature. B ( 1 T - 1 T ref ) where Rn and B are nominal values (specified values without any tolerance). If B is not a nominal value, it is expressed as: R T = R nT + R T = R ref e ( B + B ) ( 1 T - 1 T ref ) where RT is the absolute deviation at temperature T: R T = R ref e ( B + B ) ( 1 T - 1 T ref ) -e B ( 1 T - 1 T ref ) T If relative deviation is applied: R T B ( 1 T - 1 T ref ) - 1 ----------- = e R nT Fig.4 1995 Mar 31 5 Effect of sintering on the resistance as a function of temperature. Philips Components NTC Thermistors Introduction to NTCs Developing this equation (Taylor's formula), the following simplified expression can be derived: R T 1 1 ----------- ( in % ) = B --- - --------- T T ref R nT 2 T (K) 0 If the `R-deviation due to B-tolerance' is called `Y' and the tolerance at the reference temperature `X', then the total tolerance can be calculated as follows: X Y Z = 1 + ---------- x 1 + ---------- - 1 x 100% 100 100 or, after approximation: ZX+Y 25 0 25 T ( oC) 50 2 Fig.5 If TC is the temperature coefficient and T is the temperature deviation: Z T = -------TC Temperature measurement at more than one point. The 2 or 3-point sensors are particularly suited for applications with the following characteristics: EXAMPLE * Temperature measurement over a certain temperature range At 0 C, assume X = 5%, Y = 0.089% and TC = 5.08%/K, then: * High accuracy 5 0.89 Z = { 1 + ---------- x 1 + ----------- - 1 } x 100% 100 100 * No further calibration for sensor tolerances in the electrical circuitry required. or Z = { 1.05 x 1.0089 - 1 } x 100% = 5.9345% (5.93%) Z 5.93 T = -------- = ----------- = 1.167 C (1.17 K) TC 5.08 Hence, an NTC having a R25-value of 10 k has a value of 32.51 k between -1.17 C and +1.17 C. Resistance specified at more than one temperature (2 or 3-point measurement) Thermistors which are specified at 2 or 3 points of their R/T-characteristic are more accurate. They have a closer tolerance and the spread in B-value has less influence because it is included in the tolerance at the specified points. The tolerances in the reference points can be expressed either as a temperature deviation for the reference resistance or as a resistance tolerance at the reference temperature. This has no influence on the resulting measuring error which is minimum in the temperature region between the reference points, as illustrated in Fig.5. 1995 Mar 31 MBD922 handbook, halfpage This calculation has been performed for all major sensor ranges to be found in this handbook, where `R-deviation due to B-tolerance' values can be found in the electrical data tables. 6 Philips Components NTC Thermistors Introduction to NTCs GLOSSARY OF TERMS HOW TO MEASURE NTC THERMISTORS Resistance The published RT-values are measured at the temperature T. Also called nominal resistance. Formerly specified at only one temperature, or sometimes at two or maximum three. Now new technologies allow the specification of resistance values on all application ranges for several types. The published B-value at 25 C is the result of the measurement at 25 C and that at 85 C. Hence, these values should be used when checking. The following general precautions have to be taken when measuring NTC thermistors: Tolerance on resistance * Never measure thermistors in air; this is quite inaccurate and gives deviations of 1 or 2 K. For measurements at room temperature or below, use petrol or some other non-conductive and non-aggressive fluid. For higher temperatures use oil, preferably silicon oil. The limits of the values that the resistance can take at the reference temperature. B-value The B-value may be calculated using the following formula: ln ( R 1 R 2 ) --------------------------------1 T1 - 1 T2 * Use a thermostat with an accuracy of better than 0.1 C. Even if the fluid is well stirred, there is still a temperature gradient in the fluid. Measure the temperature as close as possible to the NTC. * After placing the NTC in the thermostat, wait until temperature equilibrium between the NTC and the fluid is obtained. For some types this may take more than 1 minute. where R1 and R2 are the nominal values of resistance at T1 and T2 respectively. Tolerance on B-value * Keep the measuring voltage as low as possible, otherwise the NTC will be heated by the measuring current. Miniature NTC thermistors are especially sensitive in this respect. Measuring voltages of less than 0.5 V are recommended. The limits of the value that B can take due to the process variations. R-tolerance due to B-deviation Due to the tolerance on the B-value, the limits of the value that R can take at a certain temperature increase with the difference of that temperature to the reference temperature. * For high temperature measurements it is recommended that stem correction be applied to the thermometer reading. Tolerance on R at a temperature different to Tref The sum of the tolerances on resistance and tolerance due to B-deviation. -value Variation of resistance (in %) for small variations of temperature around a defined temperature. Maximum dissipation Maximum power which could be applied without any risk of failure. 1995 Mar 31 7 Philips Components NTC Thermistors Introduction to NTCs CHOICE OF TYPE Deviating characteristics Selection of an NTC thermistor The following example explains the resistance values resulting from combinations of an NTC thermistor and normal resistors. When selecting an NTC thermistor the following main characteristics should be considered: * Thermal time constant, if applicable Suppose an NTC must have a resistance of 50 at 30 C and 10 at 100 C. A standard type having this characteristic is not included in our programme. The problem may, however, be solved by using a standard NTC and two fixed resistors, e.g. an NTC disc with a cold resistance of 130 mounted in a series and parallel arrangement with two fixed resistors of 6 and 95 respectively. It should be remembered that the temperature coefficient of the combination will always be lower than that of the NTC thermistor alone. * Types best suited to the purpose. Basic forms are chip, disc and bead Remarks on the use of NTC thermistors * Resistance value(s) and temperature coefficient * Accuracy of resistance value(s) * Power to be dissipated: - Without perceptible change in resistance value due to self-heating - With maximum change in resistance value * Permissible temperature range * Protection against undesired external influences, if necessary. Do not use unprotected thermistors in conducting fluids or aggressive and reducing gases which may cause a change in thermistor characteristics. When it is impossible to find an NTC thermistor to fulfil all requirements, it is often more economical to adapt the values of other circuit components to the value of a series-manufactured NTC. Sometimes, a standard NTC can be used with simple parallel and series resistors where otherwise a special type would have been necessary. For temperature measurements do not use too high a voltage on the NTC thermistor, as self-heating may cause incorrect readings. The dissipation constant indicates the maximum permissible measuring power, if an error of 1 C is allowed. If no suitable combination can be found, the development of a special type can be considered. In this event a specification of the requirements is necessary. A description of the circuit in which the NTC is to be used, is most useful. 1995 Mar 31 8 Philips Components NTC Thermistors Introduction to NTCs Tolerance on R25 and the resistance tolerance due to B-value combine to affect the performance of the sensor over its operating temperature range (see Fig.8). HOW NTC TEMPERATURE SENSORS FUNCTION NTC temperature sensors are made from pure metal oxides. They respond quickly to temperature changes, even small temperature increases cause their resistance to decrease significantly, as shown in Fig.6. However, from an operational point of view, it is far better to express sensor tolerance in terms of temperature deviation T over a temperature range. This plot is shown in Fig.9. So, by placing an NTC temperature sensor into one arm of a bridge circuit, accurate temperature measurement is possible. Again we have shown a typical comparison with a similar competitor sensor. Note how Philips outperforms the competitor right across the temperature range. The main characteristics of an NTC temperature sensor are expressed by three parameters: Two other parameters which are important in specifying NTC temperature sensors are the thermal time constant and the response time: * The resistance at 25 C (R25). Tolerances on the R25-value are mainly caused by manufacturing and material tolerances. By using very precise sawing, tolerances on R25 lower than 1% (or 0.25 C) can be achieved. - The thermal time constant is the time required for the temperature of the sensor to change in air by 1-1 --- = 63.2% of the difference between its initial e and final body temperatures, when subjected to a step function temperature change (85 C to 25 C in accordance with "IEC 539" ). * The material constant (B). This constant relates the rate of change of resistance with temperature, and therefore affects the slope of the R/T-characteristic. BT R = Axe where R is the resistance at absolute temperature T (in Kelvin) and A is a first-approximation constant. In practice, B is defined between two selected temperatures. The B-value is very useful for comparing sensors, but in making this comparison, care must be taken to ensure that the same two temperatures are used (normally 25 and 85 C). - The response time is the time the sensor needs to reach 63.2% of the total temperature difference when subjected to a temperature change from 25 C in air to 85 C in silicone oil (MS 200/50). Tolerances on B-value are mainly caused by material tolerances and by the effects of the sintering temperature on the material. Our new materials have tolerances on the B-value as low as 0.75%. * The temperature coefficient of resistance (), expressed in %/K. This coefficient indicates the sensitivity of the sensor to a change in temperature. Values of are given in the "Data sheets" in this "Data Handbook". R For calculation purposes holds = -------- , where R is T the percentage change in resistance at the required temperature (see Fig.7), and T is the temperature deviation (T in Kelvin). So, when R and are known for any temperature, T (the temperature deviation in C) can be calculated. The plot of R as a function of T is known as the butterfly characteristic, which really shows how good a sensor is (see Fig.7). It shows that a typical Philips sensor is far more accurate than a similar competitor sensor. That is why we are renowned as world leaders in high-accuracy sensor technology. 1995 Mar 31 9 Philips Components NTC Thermistors Introduction to NTCs MLC729 10 MSB236 - 1 handbook, halfpage handbook, halfpage R (%) log R () 8 R 25 6 competitor B = 3740 K 4 Philips B = 4570 K 2 0 0 25 0 25 50 75 100 125 T ( o C) 50 Fig.7 Fig.6 25 0 25 50 75 100 125 o T ( C) Typical resistance change as a function of temperature for a 1% Philips NTC temperature sensor compared to a competitor sensor. Typical plot of resistance as a function of temperature for an NTC temperature sensor. MLC730 4 handbook, halfpage MSB296 handbook, halfpage T (K) nominal value total tolerance log R () 3 competitor R 25 2 Philips 1 0 25 Fig.8 0 25 50 75 100 125 T ( o C) 0 The combined effects of tolerance on R25 and resistance tolerance due to B-value. 1995 Mar 31 50 Fig.9 10 25 0 25 50 75 100 125 o T ( C) Temperature deviation as a function of temperature. Philips Components NTC Thermistors Introduction to NTCs APPLICATIONS GENERAL INDUSTRIES General NTC temperature sensors are used in thermal switches, measuring systems and detectors in all segments of industry, notably the following: Temperature is one of the variables that must be measured most frequently. There are as many as nineteen recognized ways of measuring it electrically, most commonly by thermocouples, platinum-bulb thermometers and NTC (Negative Temperature Coefficient) temperature sensors. For general-purpose temperature measurement, NTC temperature sensors are accurate over a wide temperature range (-55 to +300 C). They are stable throughout a long lifetime, have a high impedance and are small and inexpensive. In fact, they are the first choice for most temperature measurements. Typically, they have a negative temperature coefficient of approximately -4.5%/K at room temperature (25 C), more than ten times the sensitivity of a platinum-bulb thermometer of the same nominal resistance at the same temperature. * Aerospace/military * Biomedical/health care * Education/research * Electronics/edp * Energy/environmental * Food processing * Heating and ventilating * Metallurgy * Petrochemical/chemical * Weather forecasting * Fire and smoke detection When you are aiming for accuracy, Philips has the NTC temperature sensors to help you achieving it. We have been making NTC temperature sensors for many years and we have gained an enviable reputation for our value-for-money ranges. Our component manufacturing and marketing activities are represented in more than 60 countries. This worldwide commitment ensures security of supply, guaranteed quality and technical support in every major industrial market. Recent developments in ceramics technology have allowed us to introduce sensors with resistance tolerances lower than 1% and B-value tolerances down to 0.75%. They add precision to your applications and allow you to design-in even more attractive features. And because you are dealing with Philips, you can be sure of excellent quality, design-in support and service. * Battery temperature control * Instrumentation * Air conditioning. DOMESTIC APPLIANCES NTC temperature sensors are used extensively in domestic appliances. You will find at least one NTC temperature sensor in just about anything in the home that gets cold, warm or hot, such as: * Fridges and freezers * Cookers and microwave ovens * Deep-fat fryers * Coffee makers * Food warmers and processors Application areas * Washing machines AUTOMOTIVE SYSTEMS * Electric irons NTC temperature sensors are widely used in cars. For example in: * Dish washers * Electronic fuel injection, in which air-inlet, air/fuel mixture and cooling water temperatures are used to determine fuel concentration for optimum injection * Hair dryers * Electric blankets * Smoke and heat detectors * Central heating * Fan motor control, based on cooling water temperature * Boilers * Oil and water temperature controls * Air conditioning * Climatization systems, such as air-conditioning and seat temperature controls * Aquariums * Water beds. * Frost sensors for outside temperature measurement * Oil level indication * ABS. 1995 Mar 31 11 Philips Components NTC Thermistors Introduction to NTCs APPLICATION GROUPING Examples Applications of NTCs may be classified into three main groups depending on their physical properties: 1. Applications in which advantage is taken of the dependence of the resistance on the temperature, shown in the formula: R = f ( T) This group is split into two sub sections: handbook, halfpage a) The temperature of the NTC thermistor is determined only by the temperature of the ambient medium (or by the current in a separate heater winding). o C - b) The temperature of the NTC thermistor is also determined by the dissipation in the NTC thermistor itself. MBD923 2. Applications in which the time dependence is decisive, when the temperature is considered as a parameter and is written: R = f ( t) This group comprises all applications which make use of the thermal inertia of NTC thermistors. Fig.10 Temperature measurement in industrial and medical thermometers. 3. The third group of applications uses mainly the property of the temperature coefficient being highly negative: <0 Also in this group, applications are listed which take advantage of the fact that the absolute value of the temperature is so high, that a part of the V = f(I) characteristic shows a negative slope. The classifications given above are supported by practical examples in Figs 10 to 23. handbook, halfpage o V C bimetal mA-meter MBD924 Fig.11 Car cooling water temperature measurement with bimetal. 1995 Mar 31 12 Philips Components NTC Thermistors Introduction to NTCs handbook, halfpage handbook, halfpage V o C differential mA-meter - MBD926 MBD925 Fig.13 Temperature measurement with a bridge incorporating an NTC thermistor and a relay or a static switching device. Fig.12 Car cooling water temperature measurement with differential mA-meter. handbook, halfpage handbook, halfpage o C heater flow direction T1 NTC T0 NTC MBD927 MBD928 Fig.15 Flow measurement of liquids and gases. The temperature difference between T1 and T0 is measured for the velocity of the fluid. Fig.14 Liquid level control. 1995 Mar 31 13 Philips Components NTC Thermistors Introduction to NTCs handbook, halfpage handbook, halfpage - Vo Vo - MBD930 MBD929 Fig.17 Basic temperature sensing configuration. The op-amp (e.g. NE532) acts as a Schmitt-trigger. The transfer characteristic is shown in Fig.18. Fig.16 Temperature sensing bridge with op-amp which acts as differential amplifier. The sensitivity can be very high. handbook, halfpage handbook, halfpage MBD931 Vo relay - t MBD933 Fig.18 Transfer characteristic of the circuit shown in Fig.17. 1995 Mar 31 Fig.19 Simple thermostat. 14 Philips Components NTC Thermistors Introduction to NTCs handbook, halfpage RP handbook, halfpage SAWTOOTH TEMPERATURE 0 oC REF. COMP 2 B RS AND GATE Vo COMP 1 Vo COMP 1 A SAWTOOTH GENERATOR Vo COMP 2 CLOCK PULSE GENERATOR MBD935 OUTPUT PULSES OF AND GATE MBD936 Fig.20 Temperature sensing bridge with 0 C offset and ADC. Due to RP and RS the voltage at A varies linearly with the NTC thermistor temperature. The voltage at B is equal to that at A when the NTC thermistor temperature is 0 C. Both voltages are fed to the comparator circuit. See also Fig.21. Fig.21 Pulses occurring at various points in the circuit shown in Fig.20. handbook, halfpage 300 to 500 V handbook, halfpage Rs line deflection driver transistor - MBD942 MBD943 Fig.22 Constant current circuit (sure start-up) for line deflection stage. 1995 Mar 31 Fig.23 Temperature compensation in transistor circuits. Push-pull compensation. 15 Philips Components NTC Thermistors Introduction to NTCs The circuit consists of a 10 V (DC) zener diode stabilized power supply, a Wheatstone Bridge (containing the NTC temperature sensor) and an integrated comparator circuit controlling a triac. The circuit is designed to switch a maximum load current of 2 A off at -5 C and on at +5 C. NTC temperature sensors used as a thermal switch A common use of an NTC temperature sensor is in one of the bridge arms of a thermal switch circuit using an operational amplifier such as the A741. Figure 24 shows a typical thermal switch circuit for a refrigerator thermostat. TEMPERATURE SENSING IN REFRIGERATORS handbook, full pagewidth C1 1.5 F (40 V) R5 1 M R4 100 F1 2A Z1 10 V 400 mW D1 1N4148 R1 10 k R3 120 k A 741 Cb 50 F (16 V) green Rg triac BT136500D 680 RP Rh R2 10 k R6 Rd 390 182 k (1) U LOAD Cd 330 nF (400 V) (1) Catalogue number: 2322 593 32312. All resistors are 0.25 W. Fig.24 Refrigerator thermostat using an NTC temperature sensor. 1995 Mar 31 Vm 220 V 16 MBD944 Philips Components NTC Thermistors Introduction to NTCs HEAT DETECTION IN FIRE ALARMS handbook, full pagewidth R10 R3 TR1 R1 R5 DC supply 17 to 28 V R9 - NTC2 (exposed) alarm D4 TR2 R6 D1 D2 TR4 TR3 Z2 Z1 R7 - NTC1 (insulated) R8 R2 TH1 C1 R4 Z3 Z4 C2 R11 MBD945 Fig.25 Circuit diagram of a typical heat detector using a matched pair of NTC thermistors. 1995 Mar 31 17 Philips Components NTC Thermistors Introduction to NTCs through a linear power regulator to charge the batteries under the control and management of the TEA110X. The BYD13D diode inhibits further charge (and prevents discharge) when the battery pack is full. For further information refer to "Application Note NTC temperature protection of rechargeable batteries, code number 9398 082 91011". NTC temperature protection of rechargeable batteries Figure 26 shows the circuit diagram of an `intelligent' charger designed to charge, within 1 hour, a NiCd or NiMH battery pack containing up to six AA-type cells. The TEA110X allows any type of power regulator to be used. In Fig.26, the unregulated 12 V (DC) supply is passed handbook, full pagewidth unregulated DC BYD13D BD132/ BDT60A BC549/ BC635 6 PH MAINS ADAPTER R3 3 TEA110X(T) - PS MBD946 Fig.26 `Intelligent' charger based on the TEA110X with NTC battery temperature sensing. 1995 Mar 31 18 PS Philips Components NTC Thermistors Introduction to NTCs SELECTING A SENSOR Use the following steps in the specified order to select the required sensor: handbook, 4 columns 1. What temperature range is required? Refer to Fig.27. 633 5.... and 633 7.... high temperature 2. What R25 is required? Refer to the "Data sheets" in this "Data Handbook". This "Data Handbook" gives the resistance/temperature characteristics for each sensor. In practice, the circuit surrounding the sensor will determine the required resistance at room temperature (R25). This value will usually be between 10 and 20 k for the optimum operating temperature range of the sensor. Simply select the sensor having the most suitable R25 value. 3. Are there any other important parameters? Refer to Tables 1 and 2. 4. Can you fulfil your need from our standard ranges (particularly from the Accuracy Line, see Table 1) or do you need a special accuracy (calculation) or encapsulation? 1995 Mar 31 accuracy line 640 5.... and 640 6.... 633 8.... 626 1.... and 626 2.... part of ranges miniatures 633 0.... 633 1.... and 633 2.... chips and discs 640 0.... and 611 9.... moulded 641 6.... 55 640 90059 and 645 90028 standard long leaded 2322 641 2...., 641 3.... and 641 4.... special long leaded 40 0 25 85 125 150 200 T ( oC) 300 MLC268 Fig.27 Temperature selection chart for the standard NTC temperature sensor ranges. 19 1995 Mar 31 17 0.6 3.3 0.5 3740 to 4570 20 - 2675 to 3975 - 22 1 - - - - Lead diameter Min. lead length 21 1 110 5 0.58 110 5 0.3 2.5 to 2.6 1, 2 3993 5 10 645 90028 (non-insulated) 641 2.... (epoxy) 2.2 to 470 400 10 - 6 0.75 to 2.5 3740 to 4570 3 19 0.24 1.6 5 2075 to 4100 5, 10 1 to 1000 5 0.06 0.7 to 1 5 2075 to 4100 5, 10 1 to 1000 20 0.24 3 5 2075 to 4100 5, 10 1 to 1000 400 10 - 6 0.75 to 2.5 3740 to 4570 3 2.2 to 470 641 3.... (water-resistant) 400 10 - 6 0.75 to 2.5 3740 to 4570 3 2.2 to 470 641 4.... (pipe) SPECIAL LONG-LEADED SENSORS 2322 ... ..... 30 0.3 2.5 5 2075 to 4100 5, 10 1 to 1000 633 0.... 633 2.... 633 1.... MINIATURE SENSORS 2322 ... ..... mm mm mm % K % k UNIT mm mm mm % K % k UNIT NTC Thermistors 0.6 2.5 to 2.6 4 0.2 5.2 2.7 Max. body diameter - 0.75 to 2.5 0.75 to 2.5 Tolerance on B-value 3977 - 3740 to 4570 B-value 3.82 3740 to 4570 3 - 1, 2, 3, 5 Tolerance on R25 2.77 640 90059 (insulated) 3500 to 4093 2.2 to 470 2.2 to 470 2.2 to 470 R25 641 6.... 611 9.... 25.4 0.56 1.85 1.3 3977 5, 10 10, 20, 30, 100 STANDARD LONG-LEADED SENSORS 2322 ... ..... 25.4 0.56 - 0.6 MOULDED SENSORS 2322 ... ..... 17 0.6 1.3 3977 5, 10 100, 220 1.85 1.3 3977 5, 10 10, 20, 30, 100 1.7 3.3 0.5 5 0.3 3965 5, 10 0.0033 to 1.5 640 0.... PARAMETER CHIPS AND DISCS 2322 ... ..... Parameter selection chart (continued) 38 Min. lead length Table 2 0.4 Lead diameter 0.75 to 2.5 0.50 to 2.5 0.75 3.4 Tolerance on B-value Max. body diameter 3740 to 4 570 5 B-value 2, 3, 5, 10 1, 2, 3, 5 5 to 10 Tolerance on R25 0.47 to 470 HIGH-TEMPERATURE SENSORS 2322 ... ..... 645 0.... 642 6.... 633 5.... 633 7.... 633 8.... 626 1.... 626 2.... 2.2 to 470 640 6.... ACCURACY LINE 2322 ... ..... R25 640 5.... Parameter selection chart PARAMETER Table 1 Philips Components Introduction to NTCs Philips Components NTC Thermistors Introduction to NTCs Examples The actual resistance tolerance of the sensor is the sum of two components: To illustrate the method of selecting an NTC temperature sensor for your application, consider the following two examples, applying the selection procedure. * The tolerance on the R25-value * The resistance tolerance due to the B-value (being zero at 25 C but increasing at temperatures other than 25 C). EXAMPLE 1 A sensor is required to measure temperatures from 0 to 100 C with an accuracy of 3 C: Considering the lower-cost 5% tolerance sensor first: at 0 C the worst case tolerance is 5% + 0.89% = 5.89%, which is well within the 15.24% imposed by the temperature tolerance. And at 100 C the worst case tolerance is 5% + 2.04% = 7.04%, which again is well within the 8.82% requirement. 1. Step 1 (temperature range) Figure 27 shows that all our sensors will operate over the temperature range from 0 to 100 C. 2. Step 2 (R25 value) and Step 3 (other important parameters) So, assuming that no special encapsulation is required, the 2322 640 63472 sensor fulfils the requirements. The sensor will normally be connected into an arm of a bridge. The resistance of the other arms of the bridge will determine the approximate `cold' resistance value R25. Let us assume that the resistance is 4.7 k and that there are no other critical parameters (response time, etc.). EXAMPLE 2 A sensor is required to measure temperatures from 0 to 250 C. It must be able to measure at 25 C with an accuracy of 2 C. Its R25 value must be 10 k and it must have a radial lead configuration and a minimum body length of 25 mm for encapsulation in a special housing: 3. Step 4 (can you fulfil your need from our standard ranges?) 1. Step 1 (temperature range) Our range of low-cost Accuracy line sensors 2322 640 5.... and 640 6.... should be checked first, referring to Tables 1 and 2. From the data sheets you will find that the 2322 640 6.472 and the 640 5.472 may be suitable and that they are available with a 2%, 3%, 5% and 10% or a 1%, 2% and 3% tolerance on R25. The high-temperature requirement (see Fig.27) restricts the choice of leaded sensors to 2322 626 1.... or 626 2.... or 633 8.... sensors. 2. Step 2 (R25-value) and Step 3 (other important parameters) R25 = 10 k is available for the 2322 626 1...., 626 2.... and 633 8.... ranges. All types have radial lead configurations, but body length dictates the selection of the 2322 626 1.... range, and in particular the 2322 626 1.103 sensor. R From the equation = -------- you can calculate T. But T since depends on temperature, you need to know the temperature coefficients 0 and 100. 3. Step 4 (can you fulfil your need from our standard ranges?) In this "Data Handbook" you will find that 0 = -5.08%/K and 100 = -2.94%/K. In addition, you can find that the resistance tolerance due to the B-value is 0.89% at 0 C and 2.04% at 100 C. The calculation of tolerance is: 2 C accuracy imposes a maximum resistance tolerance at 25 C of 25 x T. So, the 3 C accuracy on temperature imposes a maximum allowable resistance variation at 0 C of R0 = 0 x T = -5.08 x 3 = 15.24% (of nominal resistance at 0 C). For the 2322 626 1.103 sensor it holds that the maximum resistance tolerance is 4.2 x 2 = 8.4% (25 = 4.2). So the 5% sensor 2322 626 13103 will satisfy the requirement. Similarly, the maximum allowable resistance variation at 100 C is 100 x T = - 2.94 x 3 = 8.82% (of nominal resistance at 100 C). 1995 Mar 31 21 Philips Components NTC Thermistors Introduction to NTCs * Chips and discs RANGE SUMMARY - 2322 640 0.... and 611 9.... * Accuracy Line When leaded components cannot be used, there is always the possibility of mechanical fixing. For this purpose we supply metallized square chips with R25-values from 2.2 to 470 k and five types of circular disc sensors - 2322 640 5.... and 640 6.... The flagship of our ranges. The Accuracy Line sensors offer real value for money. They have low tolerances (as low as 1% on the R25-value and 0.75% on the B-value) and an operating temperature range from -40 to +150 C. In addition, they are very stable over a long life. * Moulded sensors - 2322 641 6.... - 2322 645 ..... series Designed for harsh environments, our moulded sensors are ideal where good surface contact is essential. The range has recently been enhanced, and can be extended further on customer request, based on the 2322 640 0.... series. This range is our American standard line with an excellent accuracy over a wide temperature range (0.75% on the B-value). R25-values are available from 5 k to 10 k with an operating temperature range from -40 to +150 C. * Standard long-leaded sensors - 2322 642 6.... series - 2322 640 90059 and 645 90028 This range is mainly used for compensation purposes with R25-values between 3.3 and 1.5 k. These sensors combine the features of the Accuracy Line with long non-insulated or insulated leads for remote sensing applications. On request these sensors can be customized, based on the 2322 640 0.... range. * High-temperature sensors - 2322 633 5...., 633 7.... and 633 8.... This range of high-quality glass-encapsulated NTC temperature sensors are price-competitive for general use. Not only can these sensors be used at up to 300 C, but their glass encapsulation makes them ideal for use in corrosive atmospheres and harsh environments, even down to -40 C. This makes them an attractive alternative to other more expensive sensing methods. In addition, they are very small. Two types of tiny glass envelopes are available: SOD27 for sensors with leads, and SOD80 (the so-called MELF execution) for leadless, surface-mount sensors. * Special long-leaded sensors - 2322 641 2...., 3.... and 4..... For special applications we can supply three types of long-leaded sensors: water-resistant sensors for permanent immersion in water, pipe sensors for use in corrosive atmospheres and epoxy-coated sensors for general use. * Miniature sensors - 2322 626 ..... and 633 ..... These ranges pack extremely high performance in very small size. And they are fast and stable in the temperature range from as low as -55 C to as high as +300 C. 1995 Mar 31 22 Philips Components NTC Thermistors Introduction to NTCs PREFERRED TYPES NTC thermistors for temperature sensing For specific details refer to the relevant section in this data handbook. CATALOGUE NUMBER 2322 ... ..... R25 (k) CATALOGUE NUMBER 2322 ... ..... NOMINAL B-VALUE (K) R25 (k) NOMINAL B-VALUE (K) 2322 642 6.... 10% tolerance 2322 640 6.... 5% tolerance 63471 0.47 3560 0.75% 62338 .0033 2675 63102 1 3528 0.5% 62478 .0047 2750 63152 1.5 3528 0.5% 62229 .022 3025 63202 2 3528 0.5% 62339 .033 3100 63222 2.2 3977 0.75% 62479 .047 3150 63332 3.3 3977 0.75% 62101 .10 3300 63472 4.7 3977 0.75% 62151 .15 3375 63682 6.8 3977 0.75% 62221 .22 3475 63103 10 3977 0.75% 2322 633 5..../7..../8.... 5% tolerance 63153 15 3740 2% 22 3740 2% SMD VERSION 63223 63333 33 4090 1.5% 63473 47 4090 1.5% 63683 68 4190 1.5% 63104 100 4190 1.5% 63154 150 4370 2.5% LEADED VERSION 63224 220 4370 2.5% 73104; nickel-plated 63474 470 4570 1.5% 2322 640 6.... 3% tolerance 66272 2.7 3977 0.75% 66472 4.7 3977 0.75% 53103 10 3977 1.3% 53203 20 3977 1.3% 53303 30 3977 1.3% 53104 100 3977 1.3% 100 3977 1.3% 83103; tinned-coppper 10 3977 1.3% 83203; tinned-coppper 20 3977 1.3% 83303; tinned-coppper 30 3977 1.3% 83104; tinned-coppper 100 3977 1.3% 66103 10 3977 0.75% 2322 641 6.... moulded 66473 47 4090 1.5% 66272 2.7 k 3% 3977 K 0.75% 66104 100 4190 1.5% 66123 12 k 3% 3740 K 2% 4570 1.5% 66153 15 k 3% 3740 K 2% 66223 22 k 3% 3740 K 2% 66104 100 k 3% 4190 K 1.5% 66474 470 k 3% 4190 K 1.5% 66474 470 2322 640 5.... 2% tolerance 54103 10 3977 0.75% 54473 47 4090 1.5% 54104 100 4190 1.5% 55103 10 3977 0.75% 55473 47 4090 1.5% 55104 100 4190 1.5% 2322 640 5.... 1% tolerance 1995 Mar 31 23 Philips Components NTC Thermistors Introduction to NTCs NTC thermistors for temperature sensing (continued) CATALOGUE NUMBER 2322 641 ..... R25 (k) B25/85-VALUE (K) 2.2 k 3% 3977 K 0.75% - 5 k 3% 3977 K 0.75% 46103 10 k 3% 3977 K 0.75% - 47 k 3% 4090 K 2% 46104 100 k 3% EPOXY-COATED TYPE WATER-RESISTANT TYPE BRASS-PIPE TYPE 26222 36222 46222 26502 36502 26103 36103 26473 36473 26104 36104 1995 Mar 31 24 4190 K 1.5%