How do ntc sensors work

In a temperature controlled system, the thermistor is a small but important piece of a larger system. A temperature controller monitors the temperature of the thermistor.

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. What are the upper and lower voltage limits of the sensor input of the temperature controller?

The voltage limits of the sensor feedback to a temperature controller are specified by the manufacturer. The ideal is to select a thermistor and bias current combination that produces a voltage inside the range allowed by the temperature controller.

This equation is used to determine what bias current is needed. The controller produces a bias current to convert the thermistor resistance to a measurable voltage. The controller will only accept a certain range of voltage. For example, if a controller range is 0 to 5 V, the thermistor voltage needs to be no lower than 0.

To determine if the thermistor can work with the controller, we need to know the usable range of bias currents. When selecting a thermistor and bias current, it is best to choose one where the voltage developed is in the middle of the range. The controller feedback input needs to be in voltage, which is derived from the thermistor resistance. Since people relate to temperature most easily, the resistance often needs to be changed to temperature.

The type of material used in the thermistor will dictate how much the resistance changes, which is changed with temperature. Thermistors are nonlinear i. The change in resistance needs to be converted to temperature, which then produces measurable data.

The other types of temperature sensors that are used include the Resistance Temperature Detectors RTD and integrated circuits. Each type of sensor has its pros and cons, and the application will determine the best instrument to use. From chip to rod-shaped, there are a variety of shapes available for surface mounting or embedding.

For example, thermistor chips are mounted onto circuit boards whereas a bead thermistor can be embedded into a device. 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. The sensor has a small amount of current running through it bias current , which is sent by the temperature controller. 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. Surface-mounted thermistors come with adhesive exteriors that can easily be stuck in place on flat or curved surfaces. They can be removed and re-applied and have several commercial and industrial applications. Once the manufacturing process is complete, thermistors are chemically stable and their accuracy does not change significantly with age.

Common Applications for Thermistors Thermistors are employed in a broad array of commercial and industrial applications to measure the temperature of surfaces, liquids and ambient gasses. Heavy-duty probe mounted thermistors are suitable for immersion in corrosive fluids, and can be used in industrial processes, while vinyl-tipped thermistor mounts are used outdoors or for biological applications.

Thermistors are also available with metal or plastic cage-style element covers for air temperature measurement. Thermistors are very simple to wire. Most come with two-wire connectors. The same two wires that connect the thermistor to its excitation source can be used to measure the voltage across the thermistor. PFA Fluorocarbon Information. Call us at