Thermally Coupled Thick-Film Thermistors: Main Properties and Applications
Milan Bodić, Stanko O. Aleksić, Vladimir Rajs, Mirjana Damnjanović, Milica Kisić
Abstract
A pair of thick-film segmented thermistors with negative temperature coefficient (NTC) were joined together (back-to-back) to form thermally coupled device. The alumina substrate enables very high galvanic insulation of thermistors and thermal coupling at the same time. The input thermistor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}_{{1}}$ </tex-math></inline-formula> in the coupled pair was self-heated at constant voltages and the output thermistor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}_{{2}}$ </tex-math></inline-formula> was a heat receiver through the alumina substrate. Custom-designed NTC thick-film thermistor pastes were made out of nickel manganese and modified nickel manganese. Thermistor electrical resistances were measured in a climatic test chamber and the thermistor exponential temperature factor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${B}$ </tex-math></inline-formula> was determined for both pastes. The TCT device was electrically characterized at different ambient temperatures. The input power, heat transfer, resistance, and temperature of both thermistors were measured as a function of input voltage and time. The input and output thermistor temperatures <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}_{{1}}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}_{{2}}$ </tex-math></inline-formula> were obtained using the thermistor resistance and the Steinhart–Hart equation. The device sensitivity and measuring inaccuracy were analyzed. The main advantage of TCT is thermal coupling/electrical decoupling. The applications of TCT device are seen in automotive electronics, home appliances, and power converters to measure output temperature versus input power.