Objectives:
1) To know the operation theory of temperature sensor
2) To find the suitable temperature sensor
Method:
1) By making research in the internet
A temperature sensor basically senses temperature but it does it in a number of ways. A contact temperature sensor will read the temperature of an object that it is attached to physically. A non-contact temperature sensor can detect the temperature of an object that it is not attached to. This means that it can measure temperature of an object that is far away and does not have to touch the object at all. Whether in process industry applications or in laboratory settings, accurate temperature measurements are a critical part of success. Accurate temperature measurements are needed in medical applications, materials research in labs, electrical/electronic component studies, biology research, geological studies and electrical product device thermal characterization. To get the accurate temperature measurements, several consideration when selecting a temperature sensor is needed. These are the consideration that we need to know when selected the temperature sensor:
a) The type of application for the temperature sensor to used. For instance, ask ourselves with this following questions:
- What is the device to be measured?
- Is it ambient air temperature in a room or enclosure?
- Is the electronic component with plastic or metal packaging that may or may not have high voltages present?
Some of these considerations can drive the choice of sensor due to environmental and safety factors, cost budget per sensor, and distance from sensor to instrument.
b) The range of expected measured temperature. For instance, an automobile engine block, when fully warmed up, can generate temperatures of greater than 100⁰C. Most thermocouples can handle this temperature range and the type K is the most general purpose thermocouple for such applications. A type N would be good for the higher temperature because of their stability and resistance to high temperature oxidation.
c) The available area of the sensor to be mounted to adequately sense the temperature to be measured. The device to be measured must have room enough to handle the selected sensor mounting. For example, an integrated circuit, (IC) is a tiny electronic component so the right sensor would depend on what parameter is to be measured, the IC package, or the lead frame or chip device itself. Most sensors come in variety of shapes and sizes and one is sure to fit the application. For the tiny electronic circuit of an IC, an electrically isolated resistance temperature detector (RTD) would probably be the best because of the size, isolation and accuracy of the RTD.
Temperature sensing can be done either through direct contact with the heating source or remotely which means without direct contact with the source using radiated energy instead. There are a wide variety of temperature sensors on the market today including thermocouples, RTD, thermistors, infrared and semiconductor sensors. These are the common five types of temperature sensors:
a) Thermocouple
A type of temperature sensor which is made by joining two dissimilar metals at one end. The joined end is referred to as the HOT JUNCTION. The other end of these dissimilar metals is referred as the COLD END or COLD JUNCTION. The cold junction is formed at the last point of thermocouple material. If there is a difference in temperature between the hot junction and cold junction, a small voltage is created. This voltage is referred to as an EME (electro-motive force) and can be measured and in turn used to indicate temperature.
b) Resistance Temperature Detector (RTD)
A temperature sensing device whose resistance changes with temperature. Typically built from platinum, though devices made from nickel or copper are not uncommon, RTDs can take many different shapes like wire wound, thin film. To measure the resistance across an RTD, apply a constant current, measure the resulting voltage, and determine the RTD resistance. RTDs exhibit fairly linear resistance to temperature curves over their operating regions and any non-linearity are highly predictable and repeatable. The PT100 RTD evaluation board uses surface mount RTD to measure temperature. An external 2, 3, or 4-wire PT100 can also be associated with measure temperature in remote areas. The RTDs are biased using a constant current source. So as to reduce self-heat due to power dissipation, the current magnitude is moderately low.
c) Thermistors
A temperature sensing device whose resistance changes with temperature. Thermistors, however, are made from semiconductor materials. Resistance is determined in the same manner as the RTD but thermistors exhibit a highly non-linear resistance versus temperature curve. Thus, in thermistors operating range we can see large resistance change for a very small temperature change. This makes for a highly sensitive device, ideal for set-point applications.
d) Semiconductor sensors
Classified into different types like Voltage Output, Current Output, Digital Output, Resistance output silicon and Diode temperature sensors. Modern semiconductor temperature sensors offer high accuracy and high linearity over an operating range of about 55⁰C to +150⁰C. Internal amplifiers can scale the output to convenient values like 10mV/⁰C. They are also useful in cold-junction compensation circuits for wide temperature range thermocouples. This type of temperature sensor is a sensor ICs. There are variety of temperature sensor ICs that are available to simplify the broadest possible range of temperature monitoring challenges. These silicon temperature sensors differ significantly from the above mentioned types in a couple of important ways. The first is operating temperature range. A temperature sensor IC can operate over the nominal IC temperature range of -55⁰C to +150⁰C. The second major difference is functionality.
As a silicon temperature sensor is an integrated circuit, it includes extensive signal processing circuitry within the same package as the sensor. There is no need to add compensation circuits for temperature sensor ICs. Some of these are analog circuits with either voltage or current output. The other one is the combining analog-sensing circuits with voltage comparators to provide alert functions. Some of it the analog-sensing circuitry with digital input/output and control register are combined together, making them an ideal solution for microprocessor-based systems.
Digital output sensor usually contains a temperature sensor, analog-to-digital converter (ADC), a two wire digital interface and registers controlling the IC's operation. Temperature is continuously measured and can be read at any time. If desired, the host processor can instruct the sensor to monitor temperature and take an output pin high or low if temperature exceeds a programmed limit. Lower threshold temperature can also be programmed and the host can be notified when temperature has dropped below this threshold. Therefore, digital output sensor can be used for reliable temperature monitoring in micro-processor based systems.
Temperature Sensor
The temperature sensor above has three terminals and the required maximum voltage is 5.5V supply. This type of sensor consists of a material that performs the operation according to temperature to vary the resistance. This change of resistance is sensed by circuit and it calculates temperature. When the voltage increases then the temperature also rises. It can be seen when it operates by using a diode. Temperature sensors directly connected to microprocessor input and thus capable of direct and reliable communication with microprocessors. The sensor unit can communicate effectively with low-cost processors without the need of A/D converters. An example for this temperature sensor is LM35. The LM35 series are precision integrated-circuit temperature sensors, whose output voltage is linearly proportional to the Celcius temperature. The LM35 is operates at -55⁰C to 120⁰C.
e) Digital Temperature Sensors
It eliminate the necessity for extra components such as an A/D converter within the application and there is no need to calibrate components or the system at specific reference temperatures as needed when utilizing thermistors. Digital temperature sensors deal with everything, empowering the basic system temperature monitoring function to be simplified. One of the good thing for this type temeprature sensor is its precision output in degree Celcius. The sensor output is a balanced digital reading. This intends no other components such as an analog to digital converter and much simpler to use than a simple thermistor which provides a non-linear resistance with temperature variation.
Type of sensors with applications
Conclusion:
This week I am going to learned about the temperature sensor as it is one of the component which used in my project. The type of temperature sensor and the application of each type need to take note so that it is suitable to use in the project. For my project, I will used the semiconductor sensors that is LM35 because it is based on voltage output. It is typically 3-pin devices which are power, ground and output. Besides that, this temperature sensor is easily connected to ADC so that the measurement can be read when both of it interface with microcontroller. This can be used to measure the temperature inside the helmet whether it is hot or not so that it can help the paramedics to check on the motorcyclist that had an accident.
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