There are a lot of options out there for sensing and monitoring temperature. As described by Johnson, “industrial temperature regulation has always been of paramount importance and becomes even more so with the advance of technology” (Johnson, 2006). This advance of technology has also improved the amount and quality of temperature sensors that can be used, but with all the options out there, making a decision on which one to use can be difficult. Understanding the system that you are trying to monitor will make that decision easier, but it is also important to know how the sensors function. In this paper, I will give a basic operation of one popular type of temperature sensors, integrated circuit sensors, or sometimes referred to as solid state-state temperature sensors. Once the basic operation is covered, three specific examples of temperature sensors are presented in greater detail to try and assist you in making the correct decision for your application.
Integrated circuit (IC) temperature sensors work based on the properties of the PN junction of silicon as a function of temperature. Johnson, in his text book on instrumentation technology, states that “one common version is essentially a Zener diode in which the Zener voltage increases linearly with temperature” (Johnson, 2006). A Zener diode is a heavily doped silicon PN junction that will increase the voltage output as temperature increases. The diodes are easily incorporated into circuits and can be used to monitor temperature, control a circuit function, or both.
The advantages of using this type of sensor is that it is generally the cheapest option, produces a very linear output that is proportional to temperature and are generally easy to incorporate into the circuit design (Lacanette, 2009). The design also leads to some disadvantages though. The sensors have a limited range of temperature, slow to react, limited configurations, required a power source, and because it uses power, is susceptible to self-heating. Because of the self-heating care must be taken to correct for this or provide some sort of heat dissipation (Comparison of Temperature sensors, n.d.). The unique advantage and disadvantages make this type of sensor perfect for use in control systems and computer boards.
Understanding the basic operation of some common IC temperature sensors, we can now look into the specifics of three types that could be considered for application. The first sensor that will be discussed is the LM135 produced by Texas Instruments. The LM135 comes inside two available small, hermetic, plastic bodies, both under 5mm, and operates from a range of -55˚C to 150˚C. It operates as a two terminal Zener that has a third pin that outputs an analog voltage directly proportional to temperature. It comes calibrated to the Kalvin Temperature scale and has a 1˚C available accuracy. The supply voltage required for the sensor is 5V and operates from 400 mA to 5 mA, with a dynamic impedance less than 1-W (LM135, n.d.) This type of sensor is ideal for applications like power supplies, HVAC, appliances and battery management. Placing the sensor close to the temperature critical component would allow the IC to control the fan and supply cooling.
The second sensor that will be looked at, which is very similar to the LM135, is the MCP9700 produced by Microchip Technology Inc. The MCP9700 has a similar temperature range, from -40˚C to 125˚C, but the accuracy is slightly less than the LM135. The MCP9700 has an accuracy band of plus or minus 4˚C, which makes it slightly less accurate but does have a much smaller operating current, typically 6mA and 12mA max. The MCP7000 also uses 5V to operate and puts out an analog signal that is proportional to the temperature and like the LM135, it comes in a space saving package. The MCP9700 also features a 500mV DC offset that allows it to read negative temperatures without the need for a negative power supply (MCP9700, 2017). It has very similar applications to the LM135 and its typical applications include entertainment systems, home appliances, hard disk drives and other peripherals. The major advantage of the MCP970 comes from its cost, less than $0.25 per unit compared to $13.99 for the LM135. The much cheaper unit sacrifices some accuracy but does consume a lot less power. The decision between these two sensors will come down to a cost benefit analysis.
The last type of sensor we will discuss in the OM-2628 from Omega Engineering. This particular sensor is useful because it is actually a temperature probe that can be connected to the IC. The probe is maneuverable and that allows it to be placed in a separate location from the circuit board. This is a useful feature where there are a lot of temperatures being monitored all to a central location. An example of this would be a refrigerator freezer combination. One probe could be located in each compartment and report back to one centrally located board. The OM-2628 operates in a temperature band of -25˚C to 105˚C with a plus or minus 5˚C accuracy. Like the other sensors it puts out a linear voltage signal that is proportional to the temperature that produces a temperature monitoring with excellent stability and repeatability. The probe comes in two sizes 38mm and 153mm, that makes this particular probe very versatile. The heretically sealed probe also allows the probe to be used as an immersion probe and the high impedance current output makes the probe immune to voltage noise and drops over the long lines (Integrated Circuit Temperature Probe, 2016). This style of sensor would be particularly useful in remote sensing applications, but is pricier than the other to at $47.50. The versatility in applications may make this a better solution for your needs.
These are just three of a seemingly endless amount of temperature sensors available out there. Integrated circuit temper sensors have a reduced range but provide a very cost effective and reliable solution to temperature indication and control. It also illustrates the need to understand the application in which the sensor will be used and what features in a sensor you will need. Knowing the application, budget, and desired feature will help narrow down the selection and make the selection process easier.
Comparison of Temperature Sensors. (n.d.). Retrieved January 07, 2017, from http://www.industrial-electronics.com/DAQ/industrial_electronics/input_devices_sensors_transducers_transmitters_measurement/Comparison_Temperature_Sensors.html
Integrated Circuit Temperature Probe. (2016, June 11). Retrieved January 07, 2017, from http://www.omega.com/pptst/OM-2628.html
Johnson, C. D. (2006). Process Control Instrumentation Technology (8th ed.). Upper Saddle River, NJ: Pearson/Prentice Hall.
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