PTC Thermistors As Self-Regulating Heating Elements 1

170 PHILIPS TECHNICAL REVIEW VOLUME 30

PTC thermistors as self-regulating heating elements

E. Andrich

The resistance of PTe thermistors varies with temperature in such a way that they can be used as heating elements with particularly interesting characteristics. When a sufficiently large voltage is used, they reach a temperature near their ferroelectric Curie point with a high initial current in a few seconds, assume a high value of resistance at this temperature and act from then on like thermostats. These heating elements thus automatically regulate their power consumption to suit the heat requirements. Investigations with laboratory models of soldering irons and domestic hotplates have given impressive demonstrations of the special advantages of these elements.

Introduetion PTC thermistors are resistors made of semiconducting materials with large positive temperature coef- Rmax 5 7 ficients. Ceramic semiconductors like barium titanate or / a mixture of barium titanate with strontium and lead 1£ titanate are usually used. These materials have a transition temperature (Curie temperature) at which the crystallites change from the tetragonal to the cubic I phase. This transition is accompanied by a marked change in electrical properties. In particular, the resist- 5 ance increases by several powers of ten when the 1 temperature is raised to the vicinity of the Curie point. 2 The abrupt increase in resistance is steepest in semiconducting barium titanate, whose Curie point lies at 120°C. The temperature coefficient in this case 5 reaches a maximum of 60 % per degree (fig. 1). In mixed titanates, whose PTC characteristics lie in other temperature ranges, depending upon the composition of 2 the material, the maximum change of resistance per degree is generally between 10and 20 % (fig. 2). An obvious application of these substances with 5 this sharp increase in resistance is in temperature- dependent switching processes: if the temperature, of the thermistor increases to the neighbourhood of the 2 Curie temperature, perhaps because of an increase 1 in the ambient temperature or an increase in the electrical power converted in the device,it then restricts the 5 current flowing through it. This effect can be used, for Rmin example, to reduce the power consumption of a load connected in series with it, or to switch over a relay. 2

A special case is encountered when the thermistor 1 itself is the load and the power supplied to it is converted 100 20 40 60 80 100 120 140 160 180oe -T into a useful heat output. In this case it combines various functions and operates as a self-regulating Fig. 1. Re~istance-temperature characteristic of a PTC thermistor of semiconductor BaTiOa. Its Curie temperature is 120°C. Dipl-Phys. E. Andrich is with the Aachen laboratory of Philips The temperature coefficient also has its maximum here, with Zentrallaboratorium GmbH. about 60% change in resistance per degree. 1969, No. 6/7 PTC HEATING ELEMENT 171 heat source, in other words as a thermost~t. At a given The use of PTC thermistors as heating elements pre- voltage the PTC thermistor initially draws a high sents a number of technical problems. The elements current, since at first it is still cold and its resistance is should not crack when heated up quickly, they should low. Its temperature thenrises because of Joule heat- be capable of withstanding high electrical fields and ing and it would become white hot in a few seconds if should have an adequate life at operating temperatures the sharp increase in resistance at the Curie tempera- between 300 and 400°C. These requirements are partly ture did not immediately restrict the' power absorbed. met by using special methods for fabricating these A state of equilibrium then arises in which the ceramic semiconductors and applying the contacts. power absorbed adjusts itself to be equal to the heat The work at the Aachen laboratories has eliminated dissipated; the thermistor tries to keep its temperature in the problems encountered in the reproducible

I---- 1 .1 I / (Ba,_ySry) Tt03 I I r:;-- Y=O.~ 1/ / Ba!f_03 I (Ba,_xPbx)Ti 03 D,, sf / J 0.5 I A v- x=as/ I I 1(·4 /J ~)I/ Q3 j '/Ix=aos!n!( / ?.: IL,...-- / 1ïl0.1 /( jQ?f Q3/ 1 I / 10 -IjJ / /" '\ r-, V Vj / / / 1/ / / I ;::::" ./ ./ _ ....,/ - .LLV j-i /~~ as 1 1-::- 'So -100 -50 0 50 100 150 200 250 30o -:350 400 45 _T

Fig. 2. Resistance-temperature characteristics of PTC thermistors sintered in the laboratory. The distribution of the curves depends on the composition of the material, indicated by the value of the subscripts x and y. The two curves for x = 0.5 differ only because different sintering methods were used for the two thermistors.

the vicinity ofthe Curie point. This can often be a useful manufacture of these soldering irons with their heating feature of a heating element that is subjected to a vary- thermistors. ing heat drain. For example, a soldering iron fitted In this article we shall show how many of the with such an element adjusts its power intake to suit characteristics of PTC thermistors make them particu- the conditions of the soldering operations, It heats up larly suitable for application as heating elements. We quickly but does not become overheated when not in shall also indicate that there are certain limitations in use. Because ofthis there is less corrosion ofthe bit and such applications because of particular properties of also, where there is a large loss of heat, as when solder- the materials. ing a chassis, the heat loss is quickly compensated [11. Experimental soldering irons of this type have been Properties of semiconducting titanate made with some success at the Philips laboratories in By mixing BaTiOa with SrTiOa or with PbTiOa in Aachen (there is no production of these irons as yet). the manufacture of the ceramic elements, materials are Other applications now being studied include a fast- obtained with a wide range of Curie temperatures and heating hotplate for domestic use, and a simple ther- thus a corresponding range of different-P'ï'C character- mal conductivity meter incorporating a PTC thermistor istics. Depending on the proportion of SrTiOa or as a heat source of virtually constant temperature for PbTiOa in these mixed titanates the Curie point is laboratory use. This instrument can be so designed shifted either downwards or upwards (fig. 2). The in- that the power or current consumption are exactly clusion of 0.3 mol % of LaTiOa makes these substances proportional to the thermal conductivity of the test [1] E. Andrich, Properties and applications of PTC thermistors, object. Electronic Appl. 26, 123-144, 1965/66. 172 PHILlPS TECHNICAL REVIEW VOLUME 30 semiconducting. The sharp increase of resistance This situation corresponds to the operating point at the Curie temperature does not take place uni- P2' in fig. 4. It should be borne in mind that this is formly throughout the whole ceramic body but only usually a single switching process; when the ambient at the grain boundaries between the densely sintered temperature goes back to Tl the operating point does crystallites [21. Barrier layers are formed at these not go back beyond P2. Generally speaking, it is only boundaries, whose resistance depends not only on the possible to switch the thermistor back to the cold temperature but also on the applied voltage. The resistance with the operating point PI if the supply of resistance increase with positive temperature coefficient current is temporarily disconnected or the ambient extends over a range of about 100 degrees. Outside this temperature falls far below freezing point, so that the range the temperature coefficient is negative (fig. 3). complete current-voltage characteristicofthe thermistor rises above the load-line of the load. This means that there is little point in using a series-connected thermistor to give temperature-dependent control of the (- ----... r-, power in a resistive load. In such a circuit the thermis- <, tor works merely as a delayed-action fuse [11. This brings us to another important characteristic / of this device, the current-time characteristic. In fig. 5 it can be seen that the relatively high initial current 1\ I drops steeply after a certain time to a low residual value; this happens when the Curie temperature is ) reached, and the higher the initial current the earlier it -.l- occurs. Measurements of this time show that it may be as short as a fraction of a second [11. If such short t self-arresting heating-up processes are possible, and o 100 200 300 -- ... r PTC thermistors with Curie points higher than 400 °C can be made (see fig. 2), then it should be possible to Fig. 3. Variation of the resistance R of a PTC thermistor in the build heating units which reach their operating temper- temperature range from -100 to +390°C. Outside the PTC range (50°C to 180 "C) the temperature coefficient is negative. ature immediately after being switched on and then hold this temperature constant. We shall see presently (page 174) that in fact it is not possible to heat up It should be noted that the curves in figs. 1, 2 and 3 were obtained with a test voltage that was smaller than 2 V. At the higher voltages that are applied when the device is used as a heating element there is a significant I decrease in the resistance with increasing voltage, as in the well-known voltage-dependent resistor (VDR). I The behaviour of the PTC thermistor at higher voltages can be readily understood if it is assumed to have a VDR shunted across it. The resistance ratio Rmax/Rmln (fig. 1) of the thermistor whenitis being used as a heating element may therefore be considerably smaller than that observed at the test voltage; nevertheless it can still be greater than 100:1, and this is still perfectly adequate for temperature-dependent control of the power in the heating element. The current-voltage characteristic of the PTC thermistor, which shows the behaviour of the device when it is carrying a current, is therefore of importance; this Fig. 4. Current-voltage characteristic of a PTC thermistor at fig. is; the non-linear curve in 4. The dashed curve in different ambient temperatures Tl and T2. At low ambient tem- fig. 4 indicates the dependence of this characteristic on perature Tl the characteristic has two stable points of intersection P; and P2 with the load-line of a load R connected in series ambient temperature or heat loss. The PTC thermistor with the thermistor. At the operating point Pi a relatively high can thus be used às a switching element that severely current flows through the circuit. At higher ambient temperature T2 there is only one point of intersection P2' at a low current, limits the current through the resistive load R in series indicating that the thermistor has severely restricted the current with it as the ambient temperature rises from Tl to T2. through the load. 1969, No. 6/7 PTC HEATING ELEMENT 173

600mA grey ceramic. The volume of the pellets shrinks by about 20 % during this process. I I Difficulties arise during sintering since PbO evapor- 400 , ates readily at these high temperatures. The PbO also reacts with the ceramic P. elements usually used for -, substrate, boat and furnace <, 200 -_ tube in the sintering pro- .- cess. To avoid such reac- tions it is necessary to use platinum substrates in the preparation of these titanates and to use closed plat- 200 400 600 800s _ï inum vessels similar to Fig. 5. Current-time characteristics of a PTC thermistor. The higher the initial current the thoseusedin the production sooner the thermistor reaches its Curie temperature, at which the current drops to a small resid- of piezoelectric ceramics ual value. The dashed curve represents the switching time T as a function of theinitial currentI. containing PbO. The sintering conditions (i.e. the temperature and the entire system within fractions of a second, for we atmosphere) and the preliminary treatment of the have not yet taken into account the thermal resistance powder all have a considerable influence on the end- of the thermistor and the material that only plays a product of the mixture (Ba,Pb )Ti03 (more so than with passive part in the heating-up process, or their heat pure BaTi03 semiconductors) and very many experi- capacities. Moreover, the initial electrical power avail- mental runs were therefore needed before a reprodu- able is usually limited. cible method of preparation was found.

The technology of (Ba,Pb)Ti03 The PTC heating element The (Ba,Pb) titanates have high Curie temperatures and are therefore particularly interesting for use as Accuracy of temperature control heating elements. The method of producing these The operating temperature of a PTC heating element mixed titanates will now be considered in some detail. is not very much affected by a variation in operating For technical reasons the two components BaTi03 and voltage. For example, a thermistor rated at '10 V can PbTi03 are prepared separately. The BaTi03 ispre-fired still be operated at 20 V with no significant increase in at about 1100 °C to give the reaction its temperature. An example considered in conjunction

BaC03 + Ti02 -+ BaTi03 + CO2, and the PbTi03 is pre-fired at about 950°C to give the weighing reaction PbO + Ti02 -+ PbTi03 wet mixing 16 h wet mixing 16 h

(a diagram of the process is given infig. 6). The pre- pre-f;ring firing processes produce a powder consisting of fine grains of doped titanates; these are ground in a ball mill for about 16hours and at the same time thoroughly wet mixing+grinding 16h mixed in the specified barium-lead ratio. The powder is I then compacted by cold pressing into pellets for the pressing 3000 kgjcm2 heating elements. The pellets are finally sintered at a temperature ,from 1300 to 1400 °C to form a bluish- sirjtering 13400C

[2] E. Andrich and K. H. Härdtl, Investigations on BaTiOa Fig. 6. Diagram of the method of preparing lanthanum-doped semiconductors, Philips tech. Rev. 26, i19-127, 1965. (Ba,P,b)TiOa. 174 PHILlPS TECHNICAL REVIEW VOLUME 30

response to the temperature 104 stabilization of the ther- Dm mistor. The measures taken 103 p 1 to minimize the adverse 702 effects of these factors on 1 Î the temperature control 10' include the use of a thin PTC element and the care- -- ful choice of insulator and Ts=3500C ~ax=500OC Umax=72V metal jacket, but obvious- -T _U ly some compromises have to be made.

Fig. 7. a) Resistance-temperature characteristic and b) current-voltage characteristic of a The various layers - semi- (BaD.sPbo.s)TiOa thermistor. The temperature interval Tmu-Ts in (a) corresponds to the voltage conductor, insulation and metal interval Vmnx- Vs in (b). - should not be rigidly bonded, although this would give better with the two characteristics in fig. 7 will make this heat transfer. However well the thermal coefficients of expansion clear. From the resistance-temperature characteristic are matched to one another, there would still be relative move- ment of the materials against one another because of the con- (R-Tcharacteristic) of a thermistor of (Bao.5Pbo.5)Ti03, siderable temperature differences that can arise between them as shown in fig.7a, we find that the temperature inter- during heating-up or when heat drain is large. The heat-trans- val over which the temperature coefficient is positive is ferring surfaces between the different materials must therefore LIT = 150°C, and from the current-voltage character- fit flush with each other, with no air gaps between them. istic (1- V characteristic) shown in fig. 7b, we find that the corresponding voltage interval is LIV = 66V. The Even with an ideal design for the soldering iron, the ratio LlTjLl V = 2.3 °c/v then givesthe average change two unavoidable, passive quantities - thermal resist- in temperature per volt. If the maximum current is ance and heat capacity-are still present in the system. reached at 350°C with 6 volts, then for 10V we can These quantities cause a time delay in the heat control, calculate an operating temperature of about 359°C in and a lower temperature at the bit; the temperature of the thermistor and an operating temperature of 382 °C the bitis also highly dependent upon the heat loss to the for 20 V. work. A controlled heating element does not therefore On the other hand, it can be seen that a fine adjust- of itself givea stable temperature at the bit, independent ment of the temperature in the thermistor can be at- of the heat loss to the work. This is a fact that is easily tained by varying the applied voltage. Of course, a sim- overlooked and which applies equally to soldering plified calculation like this can only give an approxi- irons with other kinds of temperature control, such as mate picture of the behaviour of the thermistor. A a bimetallic strip. complete picture would have to take into account the Operating voltage and thermal strength heat lost to the environment, the non-linearity of the characteristic and also the geometry of the thermistor. The limits of the operating voltage are not simply However, although the calculations can be made more set by the maximum voltage that can be applied Vmax complete in this way they still have to be based on (see fig. 7b). If the minimum resistance of a heating certain simplifying assumptions, and this means that thermistor below its Curie point is 1 n, it will absorb there is not much improvement in the agreement with 1 kW ofpower when the operating voltage is still only the measured results. 32 V, which means that the element heats up in less The temperatures we are referring to are only to be than 1 second. Owing to the thermal contact with the found inside the thermistor, since the thermal conduc- heat-dissipating metal jacket, this gives rise to a high tivity of the thermistor material is low (ÄR:ll Wjm°C). temperature gradient in the semiconductor which can The temperature stabilization at the surface is therefore cause the ceramic material to crack. To avoid this the not as perfect as one might expect from the electrical initial power when the element is switched on must characteristics of the thermistor. Moreover, when the not be greater than 300 to 400 W per element; this device is incorporated in an actual instrument, such as a value corresponds'" to a heating element with a soldering iron, it has to be surrounded with electrical surface area of 1 cm-, insulation, which is usually also a poor conductor of If, on the other hand, we apply a lower voltage and heat. Finally, the heat-dissipating metal casing has a wait until the thermistor and the system to be heated fairly high heat capacity and consequently a delayed have reached an almost steady temperature near the 1969, No. 6/7 PTC HEATING ELEMENT 175

Curie point, we can then raise the voltage nearly to the mistor can be thought of as a series arrangement of limit of the breakdown voltage, i.e. to about 70 volts in very thin layers of PTC resistance. Since the greater our case. part of the voltage concentrates at a few grain bound- It has also been found possible to make (Ba,Pb)Ti03 aries, very high fields will arise, particularly with mains semiconductors that can be operated at the mains operation: there will be local overheating and finally voltage. Laboratory models made from semiconduct- the ceramic will fracture because of mechanical stresses ing (Ban 5Pbo.5)Ti03 withstand fields up to 600 V/mm at the places where the temperature gradients are without breakdown. The R-T characteristic of these steepest. Even with a thickness of one millimetre these thermistors shows a resistance increase above the elements can still split into two layers. The investiga- Curie temperature of greater than 105. Unfortunately tions of these effects have not yet been completed. certain other features of these thermistors have made Because of these effects the application of mains- them unsuitable so far for certain applications such as operated PTC devices appears at present to be limited soldering irons. In particular, these thermistors usually have too Iowa resistance below their Curie temperature: 5 Q at 1 mm electrode spacing and 1 cm- cross-section. At an operating voltage of 220 V this would require an initial power consumption of almost 10 kW, which is definitely too high for these semiconductors. On the other hand, if a high-resistance material with a e of several tens of Dm is used, the negative temperature coefficient will have a marked effect between room température and the high Curie temperature. For example, if a (Ba,Pb)Ti03 thermistor has a resistance of 300 Q just below a Curie ternperature of 340°C, its resistance at room température will be as much as

ten times higher, i.e. about 3000 D [3l. This behaviour Fig. 8. Temperature curve (low- increases the heating-up time, so that this is no longer er curve) and resistance curve (upper curve) in a PTC heating just a few seconds. thermistor along the current Another feature that causes difficulties, particu- path for the sarne heat loss on left-hand and right-hand sides. d larly in PTC thermistors supplied from the mains, is the cl thickness of the thermistor. non-uniform distribution of the voltage along the current path. Because of the low thermal conductivity to cases in which the initial power is limited and the of this ceramic material there is a fairly high tempera- required heat dissipation per unit surface area is not ture gradient inside the thermistor (fig. 8, lower curve), too great. It will be shown below that such PTC devices since the heat is only conducted away via the contact are however suitable for use in domestic hotplates and surfaces. As can be seen from fig. I, the resistance in that they are more suitable for this application than the the PTC region increases more or less exponentially conventional heating elements in spite of the various with the temperature. When it is drawn on a linear difficulties we have noted. scale theresistancecurvelooks like the oneshown infig. 8. Consequently most of the voltage drop appears across Sensitivity to ambient atmosphere; contacts a fraction of the semiconductor thickness inside the At operating ternperatures above 250°C the PTC material. Most of the heat is generated here, and this thermistor is highly sensitive to the surrounding atmo- has the result that the ternperature gradient increases sphere presumably because at higher temperatures there still further in the heating process, which in turn leads to is a significant diffusion of oxygen in the semiconductor. a further narrowing of the zone where the heat is gener- If it reaches such temperatures when it is carrying a ated: the whole device is unstable until almost all of current and it is kept in a reducing atmosphere or in a the voltage drop is confined to a thin layer inside vacuum, it will soon lose its positive temperature the thermistor [4l. The same instability is found in the series arrange- [3] The resistance of low-resistivity thermistors (f! "'" 0.1 Om) at room temperature, on the other hand, is only about 2.5 times ment of two similar PTC thermistors; when a voltage higher than the resistance minimum below the Curie point. is applied, the one that was initially a little warmer or [4] The NTC resistor behaves in the opposite way; when a voltage is applied, the current becomes locally concentrated slightly better insulated heats up while the other re- inside the device and can even form a breakdown path. This mains cold. Bearing in mind that the abrupt increase of is discussed in E. Andrich and P. L. Gillessen, Eigenschappen en toepassingsmogelijkheden van PTC-thermistors, Polytechn, resistance takes place at the grain boundaries, the ther- T. E 19, 592-597, 613-619, 1964 (in Dutch). 176 PHILlPS TECHNlCAL REVIEW VOLUME 30

Fig. 9. Three laboratory models of soldering irons with thermistor heating for 5-20 V operating voltage. The maximum powers are a) 23 W, b) 50 W, and c) 120 W.

coefficient because of reduction, greatly exceed its Curie per tube. The end of the tube tapers to a point. temperature and be destroyed by the avalanche-like The soldering irons are rated for an operating volt- increase in current. On the other hand, at the same age that may vary from 5 to 20 V; at a voltage of 10 V current in an oxidizing atmosphere such as air the the medium sized soldering iron (b in fig. 9) requires a semiconductor does not lose its electrical characteris- warming-up power of 100 W, which brings it to the re- tics. After slow cooling following sintering, or after quired operating temperature within 10 seconds. The tempering in air at about 550°C, it reaches the peak of maximum power consumption, e.g. for soldering to a its PTC characteristic and then its characteristics re- main stable, even after months of use, provided air is still used as the surrounding atmosphere. This has been verified in life tests of several months on semiconductors of the composition (Bal-xPbx)Ti03, x having values up to 0.6. The only sign of ageing observed was

at the metallic contacts, which oxidize slowly. Noble Fig. 10. Design of the soldering iron. E PTC heating element. metals cannot be used for the contacts of PTC thermis- M metal strips serving as current leads. G mica. L bit. tors because barrier layers at the surface of the thermistor give rise to contact resistances which in some cases can be quite large. Experience has shown that the chassis, is 50 W, and when the iron is idle (i.e. not being contact metal that ensures a good ohmic connection used for soldering but switched on) it draws 10 W. The must have a certain affinity to oxygen. Although this temperature at the bit is then about 300°C. The other implies a gradual oxidation of the contact at high two versions shown, a and c, draw maximum powers of operating temperatures and hence a deterioration of the 23 Wand 120 W respectively and the idling powers connection, experiments in the laboratory have shown are 7 Wand 20 W. A heating power of up to 30 W /cm2 that a life of more than 3000 hours can nevertheless can be obtained from these thermistors with careful be achieved with vacuum-evaporated nickel-chrome design. The life ofthe irons is determined by the gradual contacts. oxidation of the contacts mentioned above, but as we noted earlier is greater than 3000 hours. Soldering irons and hotplates using PTC heating elements There are also temperature-controlled soldering irons of a Soldering irons using PTC heating elements have different design, in which the control is effected by means of a been made in the laboratory in various versions for bimetallic strip, a thermocouple or the ferromagnetic Curie different power ratings; three of them can be seen in point. These have the same good features as the soldering iron fig. 9. The design is the sarne in all cases; a rectangular with thermistor heating: fast heating-up, automatic cernpensa- (Bao.5Pbo.5)Ti03 pellet 1 mm thick with vacuum- tion of high heat losses and a low idl ing temperature. A n advantage of the PTC element which these others do not possess is that all evaporated metal electrodes is placed between two these useful functions are characteristic of the heating element metal strips which act as the current leads (seefig. la). itself; moreover they give continuous regulation, they have no The electrical insulation is now provided by mica on switching contacts and thus do not cause interference, and only both sides, and the whole assembly is placed in a cop- two leads from the current source are required. 1969, No. 6/7 PTC HEATING ELEMENT 177

A second application of PTe heating elements, now the subject of experiments in the laboratory, is a domestic hotplate fitted with mains-operated therrnis- tors. These, however, are still in the laboratory stage. Fig. // shows a laboratory model and the two elements that heat the plate. Since these heati ngelements automatically limit their own temperature, the hotplate can be mass-prod uced in an inexpensive thermoplastic housing whose resistance to deformation can be guaran- Fig. 11. Laboratory model of a domestic hotplate for mains-voltage operation. Continuous load 33 W, plate temperature 120°C. The plate is heated by two PTC thermistors (shown in teed only up to a specified the foreground). temperature. If a resistive I I heating element without ex- ___ ------~90WI ternal temperature control I I is used, this is only pos- I I P sible if the element IS 60 given a sufficiently high 1 r resistance, but it then ,~c i takes a relatively long __ R:.:.::======-= -II -130 time for the plate to reach I the required temperature. I 20 1 I Fig. /2 shows that the use I of a thermistor permits fast- °0~------~5~------~ILO------~~~------2LO------2~5-m-i-n~10 er heati ng up. Thermistor heating is here compared Fig. 12 Ternper ature variation (solid curves) and power consumption (dashed curves) of the hotplate after being switched on. PTe: Temperature variation and power consumption with the maxi mum permissi- when heating with PTC thermistors. Q: Temperature variation for constant power consurnp- ble ohmic heating of the tion, corresponding to ohmic heating. same system, i.e. with a constant power, cortesponding to the steady-state power of principle again has some definite advantages in this the thermistor. It can be seen that the tem perature of application. the thermistor-heated plate reaches 90 % of its final tem- In conclusion we can say that the experience gained perature of 120 oe in slightly more than 4 minutes, in our laboratory indicates that the PTe thermistor whereas the plate with ohmic heating takes 25 minutes is clearly a useful new type of self-regulating heating to reach this temperature. Since the insulation has to be element. It will probably be most suitable where Iow better on account of the higher voltage and the plate heating powers, i.e. up to about 100 W, are required. has a much greater heat capacity, it takes longer to Thermistors for low-voltage operation with working heat up than the thermistor-heated soldering iron and ternperatures up to 400 oe can already be made with its control action is not nearly as good; nevertheless the readily reproduci bie characteristics.

Summary. There are several interesting applications as self- rated working voltage with no significant change in its tem- regulating heating elements for PTC thermistors, which are perature. These devices are particularly useful and economical for ceramic semiconductors that give a sharp increase in resistance low-voltage operation (between 6V and 40 V) and for powers above their crystallographic transition temperature. When a lower than 100 W. lmpressive results have been obtained with voltage is applied they heat up quickly, drawing a high current thermistor-heated soldering irons. They are ready for use within which is then severely restricted when the Curie temperature is 10 seconds,and ifrequired can increase their powerconsumption by reached. The thermistor then automatically regulates its power five times to cornpensate for the heat drain through the soldering consumption to match the heat loss. The device gives continuous iron la the work. Other possible applications are under investi- control, and acts as heating element, temperature sensor and gation, including a domestic hotplate for mains operation and an power controller at the same time. Lt can be operated at twice the instrument for measuring thermal conductivity.