Thermistor
What is Thermistor
A thermistor is a semiconductor type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of thermal and resistor. Thermistors are categorized based on their conduction models.
Benefits of Thermistor
High Accuracy and Precision
Accurate temperature measurement is critical in numerous fields, including healthcare, industrial processes, and scientific research. Thermistors are renowned for their ability to provide high levels of accuracy and precision. Their unique construction and material properties enable them to precisely measure temperature variations, ensuring that the data they provide is reliable and consistent. Industries such as pharmaceuticals, electronics, and food processing benefit immensely from the pinpoint accuracy thermistors offer.
Wide Temperature Range
Temperature sensors that can operate across a wide temperature range are indispensable in applications where extreme conditions are a norm. Thermistors excel in this aspect, functioning reliably in both sub-zero environments and high-temperature settings. This versatility makes them ideal for applications in HVAC systems, automotive, and aerospace, where temperature differentials can be substantial.
Long-Term Stability
In the world of temperature sensing, stability over time is of paramount importance. Thermistors exhibit remarkable long-term stability, meaning they maintain their accuracy and calibration over extended periods. This trait is especially valuable in critical applications such as medical devices, where deviations in temperature readings can have significant consequences.
Fast Response Time
The speed at which a temperature sensor responds to changing conditions is a crucial factor in various industries. Thermistors are known for their rapid response times, making them indispensable in applications where real-time temperature monitoring is essential. Industries such as automotive engineering, meteorology, and semiconductor manufacturing benefit from the swift and precise temperature readings provided by thermistors.
Compact and Robust Design
In today's world, where space constraints and durability are often critical considerations, thermistors shine with their compact and robust design. These sensors are crafted to withstand physical stress and environmental factors, making them suitable for applications in tight spaces and harsh conditions. Industries like robotics, renewable energy, and telecommunications rely on thermistors for their resilience and space-saving qualities.
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Thermistors are categorised by how much resistance is measured at ambient room temperature i.e. 25°C; the manufacturer determines certain technical specifications for optimum use.
Temperatures and range:
Thermistors work best when measuring a single temperature within the range between -55°C and +114°C i.e. when measuring within 50°C of ambient; extremely high or low temperatures don't get recorded correctly. Using a thermistor where the setpoint temperature is in the middle of the range is the best way to go.
Depending on the bias current from the controller, each thermistor has an ideal range i.e. the temperature range where small changes in temperature are accurately recorded. The sensitivity of the thermistor is dependent on the temperature. For example, some thermistors are more sensitive at cooler temperatures than at warmer temperatures.
A temperature controller monitors the temperature of the thermistor which then instructs a heater or cooler when to turn on or off, in order to maintain the temperature of the sensor (thermistor), as well as the target device. They are used widely in applications such as air conditioning and display fridges/freezers – upon many more.
The sensor has a small amount of current running through it (bias current), which is sent by the temperature controller. The controller can't read resistance, so it must be converted into voltage changes, by using a current source to apply a bias current across the thermistor to produce a control voltage.
To guarantee the accuracy, the thermistor should be placed close to the device that requires temperature control, either embedded or attached. If the thermistor is located too far away from the device then thermal lag time will drastically reduce the accuracy of the temperature measurement, while placing the thermistor too far from the thermoelectric cooler (heats and cools the target device) reduces the stability. The closer the thermistor is to the device, the more quickly it will react to temperature changes, and the more accurate it will be, which is key when exact temperatures are required.
Once the placement of the thermistor has been determined, then the base thermistor resistance, the bias current, and the setpoint (desired) temperature of the load on the temperature controller need to be determined.
Thermistors vs. RTDs: 4 Key Differences To Consider
Reaction Time
Thermistors and RTDs respond differently to temperature changes. Both are thermally sensitive resistors, but thermistors can typically respond faster due to their smaller size and smaller mass. Thermistors can be coated with a thin protective layer or dipped in a thermally conductive epoxy. RTDs are typically encased in a larger housing to protect the more sensitive RTD element. The smaller mass of the thermistor enables faster response to changes in temperature.
Measurement Range and Sensitivity
The measuring range is an important factor to consider when looking for temperature sensors because some thermal applications can operate across a wide range of temperatures. The RTD is generally the best sensor for measuring a very broad range (100°C or higher) due to its relatively low change in resistance over temperature. However, that broad range also makes the RTD less sensitive than a thermistor in most systems.
Another potential benefit of the RTD is its relative linearity change in resistance. By comparison, the thermistor is highly non-linear. Most digital temperature controllers eliminate this issue by handling the measurement conversion to temperature automatically.
Some temperature controllers offer multiple measurement ranges for thermistor sensors, broadening the operating range significantly and allowing a thermistor to be used in applications that would otherwise require an RTD while maintaining the higher sensitivity of a thermistor.
Cost and Accuracy
Other key differences between thermistors and RTDs are their cost and accuracy. Thermistors are typically the most accurate temperature sensor, with commercially available accuracies as low as ±0.05°C, whereas RTDs are generally ±0.1°C or higher. At comparable accuracies, thermistors are generally available for a significantly lower cost.
Electronics
Due to the low resistance of RTD sensors, to achieve the rated accuracy, a 4-wire (sometimes referred to as Kelvin) measurement must be used, a feature not available on all temperature controllers. Using a 4-wire measurement compensates for the small, but significant, resistance errors introduced by the cables and connectors between the measurement electronics and the RTD.
By comparison, thermistors have a much higher resistance. While the same cable and connector resistances are added to a thermistor measurement, they are typically one to two orders of magnitude less. This makes them much smaller relative impacts, eliminating the need for a 4-wire measurement.
Nominal Resistance at 25°C
It presents the reference needed for the calculation of the resistance at any other condition and permits the selection of the perfect sensor for a particular application.
Temperature Coefficient of Resistance
It determines the sensitivity of the resistance based on temperature response and is introduced as %/°C.
Resistance Tolerance (5%, 3%, 2%, 1%)
It can be found by multiplying the specific temperatures.
Temperature Tolerance (1.0°C, 0.5°C, 0.2°C, 0.1°C)
It presents a deviation in temperature from the common R-T diagram of a thermistor. The tolerance of temperature is consistent across a particular temperature range. Resistance Tolerance is normally determined for these thermistors.
Thermal Time Constant (Secs)
It determines the necessary duration for the Thermistor to modify a particular difference between the initial temperature and the final one.
Temperature Accuracy (ºC)
It can be evaluated as resistance tolerance related to the temperature coefficient.
Maximum Power Rating (mW)
The Thermistor will operate for a particular period while providing the acceptable stability of its properties.
Dissipation Constant (mW/°C)
It determines the ratio of variation in the Thermistor's power to the modification in body temperature at a particular temperature.
Material Constant (°K)
It presents the diagram of the R-T and is a detector of resistance at a specific temperature compared to resistance at another one. It also needs two R-T data sets and is extremely accurate for most industrial applications. It is usually calculated between 25 to 85°C temperature range.
Thermistors have a lot of sizes and shapes, and they are manufactured from a variety of substances based on their considered application and the temperature range they require to work. According to their physical shape, they can be made as flat discs for applications where they require to be in touch with a flat plane. Nevertheless, they can also be constructed in the form of rods for utilization in temperature applications. Actually, the practical form of a thermistor is based on the needs of a specific application.
Metallic oxide thermistors are usually utilized for temperatures between 300 to 700 K. These thermistors are constructed from a tiny powder substance that is sintered and pressed at high temperatures. The most usual materials to be utilized for these thermistors are Nickel oxide, Cobalt oxide, Copper oxide, Manganese oxide, and ferric oxide.
Semiconductor thermistors are employed for extremely lower temperatures. Germanium thermistors are greater utilized than their silicon samples and are employed for temperatures below 100 K. Silicon thermistors can be utilized at temperatures up to 250 K. The Thermistor itself is constructed from an exclusive crystal that has been made in a specific level of chemical materials.
So how can we employ a thermistor to detect the temperature? As discussed before, we understand that a thermistor is a resistive instrument, and therefore according to Ohms law, a voltage reduction will be generated across it by passing a current. A thermistor is a passive kind of a sensor, so it needs an excitation alarm for its function.
The easiest way of applying this is to employ the Thermistor as a section of a potential circuit. A constant voltage is applied through the resistor to supply this. For instance, we use a 5kΩ thermistor with another 5kΩ series resistor. Therefore, the external voltage at the 25oC will be half the supply voltage as 5Ω/(5Ω+5Ω) = 0.5.
The resistance of the Thermistor varies according to the changes in temperature, so the value of the supply voltage within the Thermistor will also be modified, generating an output voltage which is based on the total series resistance in the terminals. Therefore, the practical circuit operates as a simple resistance to voltage converter. The resistance of the Thermistor is detected by temperature. So, the hotter the transducer, the lower the voltage.
If the designers reverse the position of the series resistor, the output voltage will modify in the opposite direction. In this form, the hotter the Thermistor, the higher the voltage.
This is Hefei Jingpu Sensor Technology Co.,Ltd. Jingpu Sensor is a national high-tech enterprise integrating R&D, production and sales of thermistors and temperature sensors. The products include various epoxy-encapsulated and glass-encapsulated thermistors, as well as various temperature sensor assemblies, which are widely used in medical (Eg: Supporting monitors, medical equipment, bacterial incubators, medical refreigerators, etc.), smart wear, Automobile (Eg: Water temperature, oil temperature, air conditioner, filter, intake pressure temperature, steering wheel, rearview mirroe, tire, battery pack, etc.), domestic appliances (Eg: Air conditioner, refrigerator, electric water heater, induction cooker, boiling water boiler, electronic Calendar, etc.), mobile power, fire alarm, meteorology, ocean and other fields.
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Thread Type Temperature Probe, reusable medical temperature sensor, thermal sensor analog