Finkbeiner, J. Franz, S. Hein, A. Junger, J. Muchow, B. Opitz, W. Romes, O.
the semiconductor gas sensing technique
Schatz, and H. Offenberg, H. Schubert, O. Schatz, F. Lutz, W. Golderer, J. Gerstenmeier, J. Marek, B. Mahler, H. Teegarden, G. Lorenz, and R. Thomae, R.
Types of Semiconductor Sensors
Schellin, M. Lang, W. Industrial applications that have high levels of electrical noise use current output IC sensors where output current is proportional to the measured temperature. These sensors produce voltage or current directly proportional to the temperature measured by the sensing-diode-connected transistor. The offset and amplification circuitry allows positive and negative temperature measurements in oC or oF scales. Logic Output Sensors: Many temperature control applications do not need to know what the current temperature is until it reaches a preset level—when some action needs to be taken.
Logic output sensors designed for this type of applications work like a thermostat in our homes. In fact, these sensors are commonly referred to as thermostat sensors. Some devices have user-programmable trip temperature and hysteresis, which is needed to prevent chatter.
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The programming is usually done with resistors. Yet other devices have both the temperature and the hysteresis fixed. Logic temperature sensors are simple, inexpensive and are extremely easy to use. Typical applications include turning on fans and alarms, or setting an interrupt to signal the processor that the temperature limit has been exceeded.
Digital or Serial Output Sensors: A digital temperature sensor is really an analog sensor that contains an ADC and a serial output port. In addition, many sensors have one or several comparators that drive either logic, open collector, or open drain outputs. Digital sensors using the single-wire PWM format vary the ratio of high time to low time duty cycle of a square wave to represent the temperature value.
The processor counts the duration of the high and low times and calculates the temperature based on the ratio. The clock frequency accuracy is not critical since PWM relies on the time ratio, not on the absolute duration. The sensor sends the temperature value to the processor, and the processor uses the same interface to program sensor registers that control temperature limits and other functions.
The SPI interface can be configured as a three-wire or four-wire port used most frequently with microcontrollers and in automotive systems.
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Temperature sensing can be done locally or remotely. Most semiconductor temperature sensors have the diode-connected transistor on board and thus measure the temperature of their own body. Remote-sensing sensors have no sensing element on board and instead connect to a remotely mounted diode-connected p-n-p or n-p-n transistor. The temperature-sensing transistor can be located tens of feet away from the remote sensor IC.
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However, wires leading to the remotely installed sensing transistor require good shielding due to very low signal levels susceptible to noise. PC thermal protection example As computers especially laptops and notepads pack more computing power, thermal management becomes one of the most critical design issues.