Green Color MF11 68 Ohm 5% NTC Thermistor Sensor Probe

Green Color MF11 68 Ohm 5% NTC Thermistor Sensor Probe    

Dimensional Drawing (unit:mm)

Specification

Description

NTC stands for “Negative Temperature Coefficient”. NTC thermistors are resistors with a negative temperature coefficient, which means that the resistance decreases with increasing temperature. They are primarily used as resistive temperature sensors and current-limiting devices. The temperature sensitivity coefficient is about five times greater than that of silicon temperature sensors (silistors) and about ten times greater than those of resistance temperature detectors (RTDs). NTC sensors are typically used in a range from −55°C to 200°C.

The non-linearity of the relationship between resistance and temperature exhibited by NTC resistors posed a great challenge when using analog circuits to accurately measure temperature, but rapid development of digital circuits solved that problem enabling computation of precise values by interpolating lookup tables or by solving equations which approximate a typical NTC curve.

Comparison to other temperature sensors

Compared to RTDs, the NTCs have a smaller size, faster response, greater resistance to shock and vibration at a lower cost. They are slightly less precise than RTDs. When compared to thermocouples, the precision obtained from both is similar; however thermocouples can withstand very high temperatures (in the order of 600°C) and are used in such applications instead of NTC thermistors, where they are sometimes referred to as pyrometers. Even so, NTC thermistors provide greater sensitivity, stability and accuracy than thermocouples at lower temperatures and are used with less additional circuitry and therefore at a lower total cost. The cost is additionally lowered by the lack of need for signal conditioning circuits (amplifiers, level translators, etc.) that are often needed when dealing with RTDs and always needed for thermocouples.

Self – heating effect

The self-heating effect is a phenomenon that takes place whenever there is a current flowing through the NTC thermistor. Since the thermistor is basically a resistor, it dissipates power as heat when there is a current flowing through it. This heat is generated in the thermistor core and affects the precision of the measurements. The extent to which this happens depends on the amount of current flowing, the environment (whether it is a liquid or a gas, whether there is any flow over the NTC sensor and so on), the temperature coefficient of the thermistor, the thermistor’s total area and so on. The fact that the resistance of the NTC sensor and therefore the current through it depends on the environment is often used in liquid presence detectors such as those found in storage tanks.

Heat capacity

The heat capacity represents the amount of heat required to increase the temperature of the thermistor by 1°C and is usually expressed in mJ/°C. Knowing the precise heat capacity is of great importance when using an NTC thermistor sensor as an inrush-current limiting device, as it defines the response speed of the NTC temp sensor.

How to Select and Use an NTC Thermistor ?

NTC Thermistors can be used for a wide variety of tasks and applications. Among other things, NTC thermistors can be used to detect overheating in electronic equipment, to protect circuits from drawing too much electrical current, and to compensate for temperatures in mobile communications equipment. Part of choosing the right thermistor is determining which use best suits your needs.

Thermistor Types
Essentially, there are two types of NTC thermistors to choose from. The difference between these types lies in the way in which the electrodes have been attached to the ceramic body of the thermistor. The first basic type of thermistor is the bead thermistor. These NTC thermistors all have leadwires made of platinum allow that are sintered directly to the body. This type of thermistor includes: bare beads, glass coated beads, ruggedized beads, miniature glass probes, glass probes, glass rods, and bead in glass enclosures.

The second type of thermistor is one that has metallized surface contacts. These NTC thermistors can come with either axial or radial leads or even without leads to make surface mounting or spring contact mounting easy. This type of thermistor includes: disks, chips or wafers, flakes, surface mounts, rods, and washers.

What to Consider
In order to find the best thermistor for your application, you will need to consider all aspects of each thermistor type. You can begin with the basics. Start with the device size and the desired physical characteristics of the thermistor that you need. You may be able to narrow the potential list of NTC thermistors down greatly using just these basic criteria. Next, think about exactly what the NTC thermistor that you are looking for needs to do. This should allow you to further narrow your list of potentials.

Now, consider the resistance temperature curves that you are looking for. Most manufacturers of NTC thermistors will have various tables available of resistance ratios for their products that you can look at. They may even be able to provide you with the necessary coefficients in order to assist you in your decision. Each material system will also have its own limitations regarding the range of nominal resistance values available that will work.

You should also consider the resistance tolerance for the NTC thermistors that are left on your list of potential candidates for your application. The most cost efficient thermistor will have the broadest tolerance. Beta tolerance and resistance limits must also be considered when selecting your thermistor, as this fixes your lowest and highest resistance values. Keep in mind that the structure and composition of the metal oxides as well as the actual process of manufacturing can affect the beta tolerance.

Depending on the desired use for the thermistor, you may also want to consider the issues of curve matching and the interchangeability of the thermistor. Cost and performance may need to be considered when you are looking these factors. You may want to think about metalized surface contact thermistors and those hermetically glass enclosed when considering these elements.

Finally, calibration is something to consider when looking for the right thermistor type. Luckily, there are NTC thermistors available with precise calibrations that offer both broad tolerance and low cost. Testing and calibration of NTC thermistors can be very tricky, so be sure to find out how the calibration and testing of the thermistor is handled before you make your purchase.

Linear Response Elements vs. Thermistor Probes
Some applications require a thermistor with linear response to temperature change, but there is also sometimes the need for a thermistor element that can stand alone. For this purpose, thermistor probes may be considered. Many of these come enclosed in metal tubes making them more rugged.

Ametherm's NTC thermistors are available in an incredible array of designs to fit nearly any desired application. All of Ametherm's NTC Thermistors are consistent and dependable and created using the best materials. for reliable results you can count on. Ametherm's NTC Thermistors are also customizable to fully meet your needs.

Construction and properties of NTC thermistors

Materials typically involved in the fabrication of NTC resistors are platinum, nickel, cobalt, iron and oxides of silicon, used as pure elements or as ceramics and polymers. NTC thermistors can be classified into three groups, depending on the production process used. 

Bead thermistors

These NTC thermistors are made from platinum alloy lead wires directly sintered into the ceramic body. They generally offer fast response times, better stability and allow operation at higher temperatures than Disk and Chip NTC sensors, however they are more fragile. It is common to seal them in glass, to protect them from mechanical damage during assembly, and to improve their measurement stability. The typical sizes range from 0.075 – 5mm in diameter.

Disk and Chip thermistors

These NTC thermistors have metallized surface contacts. They are larger, and as a result have slower reaction times than bead type NTC resistors. However, because of their size, they have a higher dissipation constant (power required to raise their temperature by 1°C) and since power dissipated by the thermistor is proportional to the square of the current, they can handle higher currents much better than bead type thermistors. Disk type thermistors are made by pressing a blend of oxide powders into a round die, which are then sintered at high temperatures. Chips are usually fabricated by a tape-casting process where a slurry of material is spread out as a thick film, dried and cut into shape. The typical sizes range from 0.25-25mm in diameter.

Glass encapsulated NTC thermistors

These are NTC temperature sensors sealed in an airtight glass bubble. They are designed for use with temperatures above 150°C, or for printed circuit board mounting, where ruggedness is a must. Encapsulating a thermistor in glass improves the stability of the sensor, as well as protecting the sensor from the environment. They are made by hermetically sealing bead type NTC resistors into a glass container. The typical sizes range from 0.4-10mm in diameter.

 Typical applications

NTC thermistors are used in a broad spectrum of applications. They are used to measure temperature, control temperature and for temperature compensation. They can also be used to detect the absence or presence of a liquid, as current limiting devices in power supply circuits, temperature monitoring in automotive applications and many more. NTC sensors can be divided into three groups, depending on the electrical characteristic exploited in an application.

Resistance-temperature characteristic

Applications based on the resistance-time characteristic include temperature measurement, control and compensation. These also include situations in which an NTC thermistor is used so that the temperature of the NTC temp sensor is related to some other physical phenomena. This group of applications requires that the thermistor operates in a zero-power condition, meaning that the current through it is kept as low as possible, to avoid heating the probe.

Current-time characteristic

Applications based on current-time characteristic are: time delay, inrush current limiting, surge suppression and many more. These characteristics are related to the heat capacity and dissipation constant of the NTC thermistor used. The circuit usually relies on the NTC thermistor heating up due to the current passing through it. At one point it will trigger some sort of change in the circuit, depending on the application in which it is used.

Voltage-current characteristic

Applications based on the voltage-current characteristic of a thermistor generally involve changes in the environmental conditions or circuit variations which result in changes in the operating point on a given curve in the circuit. Depending on the application, this can be used for current limiting, temperature compensation or temperature measurements.