NTC temperature sensors Technical Support Note TEMPERATURE CONTROL OF FAT FRYERS For equipment manufacturers needing reliable temperature sensing and control functions, Philips has the answer. Philips' NTC thermistors for temperature sensing are small, accurate and offer excellent long-term stability and reliability. With an operating temperature range between -55 and +300 C, the latest NTC-thermistor series provide attractive and cost-effective alternatives to thermocouples and electro-mechanical temperature controllers, especially in automotive equipment, industrial systems and domestic appliances. Extending its interest to all aspects of temperature control, Philips is now working with specific partners to develop complete solutions to temperature sensing and control, combining its extensive ceramic know-how with their experience of housing and assembly technology. One of the first positive results of this partnership is the development with Energica, France, of a new temperaturesensor for fat fryers. The sensor has already been adopted by the French company SEB in their Universalis fat fryer. Well-defined fat temperature control The figure (overleaf) shows an electronic temperaturecontrol network designed to sense temperatures between 150 and 200 C in a fat fryer with an accuracy better than 5 C, and to maintain the fat temperature during cooking with a variation of less than 0.5 C. For this application, Philips delivers its high-temperature NTC thermistor (100 k at 25 C) in a hermetically-sealed SOD 27 glass encapsulation. This is built into a customized metal housing by Energica. The network comprises a temperature sensing circuit controlling a relay/switch combination (K1a/K1b) that connects the 220 V AC mains voltage across the heater of the fat fryer. A DC voltage VCC derived from the 220 V AC mains via a diode bridge rectifier (D1..D4) powers the sensing circuit. This comprises a comparator (LM358) fed from a Wheatstone bridge containing the NTC thermistor in one of its arms. The arm also contains fixed resistors and a potentiometer to allow external preset of the cooking temperature. The output of the comparator (governed by the degree of bridge imbalance and hence the temperature of the NTC) controls the current into the coil of relay K1a, closing or opening switch K1b to give thermostatic control of fat temperature. Positive feedback (via resistor R7) provides well-defined temperature hysteresis to prevent the relay from switching continuously when the fat temperature has reached its specified value. NTC thermistors offer long-term stability The NTC thermistors supplied by Philips for this application are available with nickel-copper clad iron leads (633 7 in bulk, 633 4 on tape) suitable for clamping or with tinned leads (633 8 in bulk or 633 3 on tape) suitable for soldering. Since the ceramic chips are not soldered to the lead wires (which can give rise to ageing effects due to recrystalization of the solder), the components offer exceptional long-term stability. Moreover, the hermetically-sealed environment in which the ceramic is housed eliminates high-temperature reaction with the surrounding air, ensuring the least possible deviation in resistance characteristics during operation (less than a few tenths of a per cent even after 10 000 hours operation at 200 C). Thermal-cycling tests have also demonstrated that the components are able to withstand more than 105 thermal shocks (between 25 and 200 C) without noticeable change of electrical parameters. This corresponds to more than 20 years' normal use. The preferred electrical values at 25 C are 100 k with a B25/85 of 3977 K or 220 k with a B25/85 of 3795 K. Other values (10 k to 30 k) are also available within the Philips range but for this application (operating temperatures up to 200 C) their resistances would be too low for good temperature discrimination. R2 470 k R1 C1 220 0.5 W 150 nF D1 to D4: 1N4004 X1 220 V D1 D4 D2 D3 VCC D5 X2 neon heating element 24 V C2 K1b D7 VCC X3 K1a VCC 24 V VCC D6 R7 R3 47 k 1% R5 150 k 1% 1N4004 5.1 M 5 R1 + 100 F or 47 F 35 V 6.8 k 1% R8 NTC P1 100 k 100 nF T R6 22 k 1% 6 VCC 8 4 7 LM358 2 3 LM358 1 not used CCA210 Deep fat-fryer temperature control circuit based on a Philips NTC thermistor in metal encapsulation All rights are reserved. Reproduction in whole or in part is prohibited without the prior written consent of the copyright owner. The information presented in this document does not form part of any quotation or contract, is believed to be accurate and reliable and may be changed without notice. No liability will be accepted by the publisher for any consequence of its use. Publication thereof does not convey nor imply any license under patent- or other industrial or intellectual property rights. Printed in The Netherlands (c) Philips Electronics NV 1996 Date of release: July 1996 9398 083 31011