What is the difference between ntc and ptc probes

The thermistor can also be found functioning in monitoring and maintaining engine temperature. The military uses thermistors within military vehicles, including trucks and tanks.

NTC thermistors also improve the safety of devices they function within. For instance, the devices are used for hot glue, plastic laminating, and fire safety. Industrial soldering iron, which reaches dangerously high temperatures, relies on thermistors to maintain accurate and consistent temperatures.

From winding compensation to gain stabilization, there is almost nothing these devices cannot accomplish. To find out more information about designing and using thermistors, you should give us a call today! Thermistors offer many benefits, which is why they are widely used in many applications and industries. Thermistors are all around us, from life-saving medical equipment to HVAC systems that keep us comfortable.

Although these devices are affordable, powerful, and reliable, they suffer from certain limitations, which means they are not ideal for all applications. To calculate thermistor-resistance measurements, an individual needs a voltage measurement.

The resolution of a voltmeter limits the accuracy of readings. It is also important to keep in mind that input bias currents and input-offset voltage of operation amplifiers also directly affect accuracy.

To properly measure resistance, all currents need to pass through a thermistor that dissipates heat. This results in a small temperature increase, which is labels as self-heating errors. This type of error functions in proportion to the dissipated power plus the thermal resistance of thermistors and the environment they function within. Internal thermal resistance changes depending upon the material and dimensions of the thermistor, whereas external thermal resistance depends on the thermal conductivity level of the medium that surrounds the thermistor.

In many applications, self-heating is considered a serious problem for measurements that are made over an extensive temperature range. Stray thermal influences affect the performance of thermistors. Because of the high thermal resistance that is found between the environment and thermistor, the devices are prone to stray thermal influences.

The two main culprits are the heat that is conducted along lead wires and infrared radiation. The problem is often made worse when there is a poor thermal design. The problem is most often experienced when measuring surface temperature or air.

Using Thermistors in Controlled Systems. Within a controlled system, thermistors have a specific function. A temperature controller is used to monitor the temperature of thermistors that then instruct a heater or cooler to turn on and off. The goal is to maintain a consistent temperature within the thermistor and the target device. Some of the most commonly controlled systems that use thermistors include air conditioning units and refrigerators to name a few.

Sensors have small amounts of currents, called bias currents, running through them. The current is supplied from the temperature controller. Controllers do not read resistance, which means that it must be converted into voltage changes. This is done with the help of a current source that applies a bias current across the entire thermistor, hence producing controlled voltage. A thermistor needs to be placed close to the device, requiring control to guarantee the highest levels of accuracy.

This can be done by attaching or embedding the thermistor. As the thermistor moves further away from the device, users experience greater thermal lag time that negatively affects the accuracy of temperature measurements. Avoid placing thermistors away from thermoelectric coolers because that also reduces stability.

Keeping thermistors close to devices ensures a quick reaction time to temperature changes. This is a key aspect of maintaining consistent temperatures within controlled systems. Placement of a thermistor within a controlled system is the first consideration to make, and once that is finished, individuals can begin to determine base thermistor resistance, setpoint, and bias current. NTC Thermistors and Epoxy.

Epoxy is often used to create a protective barrier for NTC thermistors , howver epoxy is not a waterproof seal. Engineers may use epoxy-protected thermistors to protect thermal conductivity and high dielectric strength. These thermistors are tear-drop-shaped bead that has two radial wire leads.

These thermistors are used in medical devices that measure air temperature and airflow. You may also find epoxy bead thermistors in automotive applications. Protecting NTC thermistors from direct exposure to fluids is critical for the function of different devices. Thermally conductive epoxy can be used within a stainless-steel housing, but is not fully waterproof. Glass parts or laser welded parts that are hermetically sealed would be more ideal for moisture environments.

What is a Thermistor? A thermistor is a common type of semiconductor that is found in a wide range of applications and industries. Engineers turn to thermistors because they are highly accurate while also being cost-effective. NTC thermistors are constructed from various materials that all have known resistance. Materials are the largest factor in determining the shape of resistance and temperature curves.

Most thermistors are subdivided into two categories: Low-Temperature Applications The temperature range for these applications is often considered below 0C. High-Temperature Applications These applications often work in larger devices or systems such as automotive and industrial applications etc. Engineers and designers choose thermistors when their applications demand ruggedness, stability, and reliability. These sensors are ideal for environments that have extreme conditions and the presence of electronic noise.

To meet the demand of engineers, thermistors are available in different shapes, materials, and sizes. The idea shape for an application largely depends on the type of material being measured. NTC thermistor probes are used for measuring temperature and liquid levels for industrial, commercial, and residential purposes.

These sensors are integral tools for many industries, so they are often included in medical technologies, green energy, and automotive electronics. Inrush current is a surge of current when an application is powered-up. It is created by different electrical effects. The powering on of power supplies requires the charging of capacitors. The powering on of a transformer creates an inrush current during its initial magnetizing.

Inrush current can reduce the effective operating life of the equipment and damage the system. Electrical and mechanical stresses can occur from this current surge which can decrease equipment lifespan. The NTC thermistor provides variable resistance based on temperature. As temperature increases, the resistance drops from high to low and allows current to pass through. When used for inrush current mitigation, it provides an additional series resistance at power on.

NTC thermistors are the most common type available for use. The defining characteristic of this thermistor is that its resistance decreases as temperature increases. These sensors are found widespread throughout the HVAC industry, product manufacturing, transportation, appliances, and many other sectors.

By resisting current a thermistor creates the byproduct of residual heat. If an NTC thermistor will be known to operate in temperatures that will cause significant heat, a correction can be applied to sensed values to maintain accuracy. Also, with NTC thermistors, this self-heating effect will happen at low temperatures where it can dissipate much more readily into the surrounding process. Positive Temperature Coefficient means that as temperature increases the resistance of the thermistor also increases.

This category of thermistors is not common but they do perform a particular niche function; that of a safety fuse. In some processes the presence of excessive heat means an undesirable situation is occurring. If a PTC thermistor is present within a circuit it can act like a sort of throttle. It then tells a heater or cooler when to turn on or off to maintain the temperature of the sensor.

In the diagram below, illustrating an example system, there are three main components used to regulate the temperature of a device: the temperature sensor, the temperature controller, and the Peltier device labeled here as a TEC, or thermoelectric cooler. The sensor head is attached to the cooling plate that needs to maintain a specific temperature to cool the device, and the wires are attached to the temperature controller.

The temperature controller is also electronically connected to the Peltier device, which heats and cools the target device. The heatsink is attached to the Peltier device to help with heat dissipation.

Figure 4: Thermistor Controlled System The job of the temperature sensor is to send the temperature feedback to the temperature controller. The sensor has a small amount of current running through it, called bias current, which is sent by the temperature controller. The temperature controller is the brains of this operation.

It takes the sensor information, compares it to what the unit to be cooled needs called the setpoint , and adjusts the current through the Peltier device to change the temperature to match the setpoint. The location of the thermistor in the system affects both the stability and the accuracy of the control system. For best stability, the thermistor needs to be placed as close to the thermoelectric or resistive heater as possible.

For best accuracy, the thermistor needs to be located close to the device requiring temperature control. Ideally, the thermistor is embedded in the device, but it can also be attached using thermally conductive paste or glue.

Even if a device is embedded, air gaps should be eliminated using thermal paste or glue. The figure below shows two thermistors, one attached directly to the device and one remote, or distant from the device. If the sensor is too far away from the device, thermal lag time significantly reduces the accuracy of the temperature measurement, while placing the thermistor too far from the Peltier device reduces the stability. Figure 5: Thermistor Placement.

In the following figure, the graph illustrates the difference in temperature readings taken by both thermistors. The thermistor attached to the device reacted quickly to the change in thermal load and recorded accurate temperatures. The remote thermistor also reacted but not quite as quickly. More importantly, the readings are off by a little more than half a degree. This difference can be very significant when accurate temperatures are required.

Figure 6: Thermistor Location Response Graph. Once the placement of the sensor has been chosen, then the rest of the system needs to be configured. This includes determining the base thermistor resistance, the bias current for the sensor, and the setpoint temperature of the load on the temperature controller.

The device whose temperature needs to be maintained has certain technical specifications for optimum use, as determined by the manufacturer. These must be identified before selecting a sensor. Therefore, it is important to know the following:. What are the maximum and minimum temperatures for the device? If the temperatures are excessively high or low, a thermistor will not work.

Since thermistors are nonlinear, meaning the temperature to resistance values plot on a graph as a curve rather than a straight line, very high or very low temperatures do not get recorded correctly. What is the optimum thermistor range? Depending on the bias current from the controller, each thermistor has an optimum useful range, meaning the temperature range where small changes in temperature are accurately recorded. The table below shows the most effective temperature ranges for Wavelength thermistors at the two most common bias currents.

Figure 7: Thermistor Selection Chart. It is best to choose a thermistor where the setpoint temperature is in the middle of the range. The sensitivity of the thermistor is dependent on the temperature.