November 2011

FormFit™ PTC thermistors for vaporizers

Reliable energy-saving insect repellers

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In tropical regions, mosquitoes are carriers of sometimes lethal pathogens. Effective protection is offered by small electrical devices that vaporize repellents. The heart of these vaporizers is a heating module based on an EPCOS FormFit™ PTC thermistor as the heating element, which reduces their energy consumption by almost 50 percent compared with conventional heating modules.

Insect repellers vaporize liquid repellents by an electrical heating module. The vaporizer consists of a heating module, which surrounds the wick that transports the liquid by capillary effect from its container into the heating zone. This zone is maintained at a specific temperature which is precisely matched to the specific repellent so that just the required quantity is vaporized.

Figure 1: Insect repellent vaporizers
Electrical vaporizers heat up liquids which repel pests by their smell. Identically designed devices can also be used as air fresheners.

In order to ensure that only small quantities of the highly effective substances are released, the temperature in the heating zone must be kept within a narrow range. Neither too little nor too much liquid may be vaporized. The parameters of the heating modules are typically designed so that the liquid reserve suffices for 25 to 30 days as a rule. Because the manufacturers of these devices advertise them as healthcare products, they must carry out extensive testing in order to assure the required temperature in the heating zone.

To achieve the optimal vaporization rate, which varies from one substance to another, the quantity of heat produced in the heating zone must exactly equal the vaporization energy of the insect repellent. Although this may sound simple, it is in reality the most demanding technical challenge for these devices.

In conventional heating modules, extensive final testing is needed in order to determine whether the required temperature is reached. This is due to their design, because the heat-generating component in these heating modules, either a broadly toleranced fixed resistor or a PTC disk (Figure 2), is not in direct contact only with the wick. Thus, the thermal resistance of the heat-generating component to the wick is, on the one hand, very high and, on the other, not exactly determined due to the total mechanical tolerances. Under these circumstances testing of the produced modules is the only way to ensure that the specified temperature is reached in the heating zone.

Figure 2: Design of conventional insect repellent vaporizers
Insect repellent vaporizers with fixed resistors and a PTC resistor. With the fixed-resistor design (left) it is impossible to achieve a precise temperature at the repellent wick and the resistors run the risk of uncontrolled overheating. Despite the safer design of the PTC-based vaporizer (right), significant heat losses occur due to air gaps and the various materials between the heating element and the wick.

Reliability thanks to self-regulation

The increased testing required for the conventional heating modules is both time and energy intensive and consequently a significant cost factor. Moreover, modules that use a fixed resistor as the heat-generating component involve a considerable risk factor. Fixed resistors can easily overheat and, in the worst case, cause a fire. Such uncontrolled overheating can be prevented by using ceramic PTC thermistors, which are self-regulating heating elements. Heating modules with PTC thermistors thus have clear advantages over those with fixed resistors.

Design with conventional PTC heaters

The conventional production methods of PTC elements limit their design to the basic geometries of disk and rectangle. Thus, the design of the systems with these PTC components as a heating element is identical to the design of systems with resistors. In contrast, the ceramic injection molding process developed by EPCOS permits a free choice of shape. Figure 3 shows the conventional shapes compared with the horseshoe-shaped EPCOS FormFit PTC thermistor, which is the heart of the heating module.

Figure 3: Various designs of EPCOS PTC thermistors
PTC elements were previously manufactured only in rectangular or disk form. As a comparison, on the right, the horseshoe-shaped EPCOS FormFit PTC for repellent vaporizers.

FormFit optimizes heat transfer

In order to compensate the poor thermal junction of the heating element, as shown in Figure 2, and to have enough vaporization energy available where it is required, therefore, the heating element needs more power. The typical power input of conventional heating modules is between about 5.3 and 6 W. TDK-EPC has designed a heating module which requires almost 50 percent less input power. One of the main considerations here was to generate the thermal output where it is actually needed, i.e. close to the central bore of the heating module – the resulting design is shaped like a horseshoe.

In contrast to conventional heating modules, the PTC thermistor now surrounds almost 70 percent of the module’s central bore (Figure 3). This solution significantly reduces the thermal resistance from the PTC to the inside of the bore, thus optimizing the heat transfer from the PTC to the wick. Thanks to the dimensional accuracy of the injection-molding process, the mechanical tolerances and thus also the thermal contact resistances were practically reduced to zero. This solution allows the power input to be reduced by almost half compared with conventional heating modules. Another technical advantage of this design is that the final temperature of the heating module is fixed within a very narrow temperature range. This means that the time-consuming check of the temperature in the heating zone of the module at the final inspection is unnecessary and can be replaced by a simple resistance measurement of the module at room temperature.

Table: Key data of the heating module with EPCOS FormFit-PTC thermistors

The injection molding process allows FormFit PTC thermistors to be adapted exactly to the respective application. The mechanical attachments are standardized and can be used as a mechanical form-fit equivalent in most available vaporizers. Parameters such as temperature and resistance can be designed to customer requirements.

Version Low-temperature  High-temperature 
Dimensions [mm³]41.4 x 16.75 x 5.6
Maximum operating voltage VOP [V]265
Rated voltage VR [V]230
Rated resistance RR at Ta=25 °C [Ω]2000
Core temperature TS at V = VR [°C]162178
Heat output P [W] at 230 V2.93.7

In view of the low thermal mass of the newly developed heating module (Table), thermal equilibrium is reached significantly more quickly than in conventional modules. Figure 4 shows the temperature and inrush current curves over time. It can be clearly seen that the final temperature of the EPCOS FormFit heating module has already been reached, whereas the comparison module still lags considerably behind in reaching the desired operating temperature. This was made possible by the patented injection molding process for function ceramics, which permits any desired shape. The FormFit technology allows the design to be adapted directly to the application. The case for the heating module is available in two suitable plastics: PBT (polybutylene terephthalate) and LCP (liquid crystalline polymers). 

Figure 4: Temperature and inrush-current curves of the heating modules

The final temperature of the EPCOS FormFit PTC heating module is reached after a few seconds. The temperature rise of the comparison module shows a significant delay. The steady state current and thus also the power consumption are significantly lower for the FormFit modules.

Requirements optimally met

EPCOS FormFit-PTC thermistors, which are used as heating modules for insect repellent vaporizers, offer a series of clear advantages in comparison with conventional heating elements. Their individual shaping allows a larger area around the wick to be heated, thus saving energy. Moreover, the distance from the heating source to the wick is shorter, which also leads to a lower power input. The self-regulating property of the PTC thermistor prevents uncontrolled overheating and additionally assures that the required temperature is maintained with constant accuracy and high reliability over the entire operating life of the heating element.

FormFit technology allows an optimal, material-saving design of the PTC and consequently a reduced thermal mass. The result: better thermal conductivity to the wick, faster heating up, and significant energy saving.

Several hundred millions of these vaporizers are in use in the affected tropical regions. If, say, 100 million of these devices were equipped with EPCOS heating modules instead of with conventional designs, this would result in energy savings of about 250 MW. The development of EPCOS FormFit PTC thermistors consequently not only makes a valuable contribution to health but also to saving energy.

Function of PTC thermistors

Known as PTC (Positive Temperature Coefficient) resistors or PTC thermistors, these conductive materials have the property that their conductivity declines as the temperature increases (see Figure). The figure shows the dependence of the resistor on temperature (R/T curve) of a ceramic PTC thermistor.

It can be clearly seen that the resistance gradient jumps suddenly at a specific temperature threshold, known as the reference -temperature Tref or the switching temperature.The PTC thermistors used in insect repellent vaporizers are ceramic components made of barium titanate BaTiO3. As pure BaTiO3 is an insulator with a gap of about 3 eV between the valence and conductance bands, the ceramic must be doped to close this gap. This produces a higher charge carrier density, i.e. free electrons at the grain boundaries, and the ceramic already becomes electrically conductive at low temperatures.

At high temperatures, beyond the reference temperature, the conducting electrons are attracted by the acceptor points in the regions close to the grain boundaries to produce a depletion layer that leads to the sudden jump of resistance. With various dopants and suitable temperature control during the sintering process, the required resistance characteristic can be designed as required by the specific application.

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