How do thermistors work

PTC thermistors can function effectively to heat an object to a specific temperature and maintain that temperature value.

The characteristic curve shown in Figure 2 illustrates that in the high resistance state, the device will tend to self-regulate at a constant set temperature. If the temperature decreases, the resistance decreases which will allow more current to flow through the device, dissipating more power and increasing temperature again. Similarly, should the temperature increase from the set value, the device resistance will increase, limiting current flow and causing the temperature to drop.

Manufacturers can change the composition of the ceramic materials used in the construction of the PTC thermistor which can then alter to some degree the transition temperature and the regulated temperature. Applications in which PTC thermistors are used as a heater include diesel fuel heaters to heat the fuel to make cold engine starts easier, as part of wax motors for dishwasher soap dispenser door operation, in bimetallic switches, and in angle-of-attack indicators in aircraft.

Numerous other applications employ PTC thermistors for heating including:. The sudden and dramatic high resistance change above the Curie temperature is the property that enables PTC thermistors to also be used for applications as a resettable fuse, such as to address in-rush current and provide overcurrent protection.

For example, in the case of an electric motor with a set of startup windings, a PTC thermistor can be wired electrically in series with the startup coil. When the motor is initially activated, the PTC thermistor is operating in a low resistance state and allows current to pass into the startup windings.

As the current flows through the device, it dissipates heat and increases in temperature. Once the device transitions to the high resistance state, the current flow to the startup windings is effectively cut off and the startup coil is disengaged from the circuit. Similarly, PTC thermistors can function to limit current in an overcurrent situation. Should a short circuit condition develop, the sudden current flow through the thermistor will cause it to heat up quickly past the transition temperature.

Once in the high resistance state, the device can limit the current flow through the circuit to prevent the short or overcurrent from continuing unabated. Once the overcurrent condition has been corrected, the current flow through the PTC thermistor drops, the device cools, and its resistance decreases as it transitions out of the high resistance state.

PTC thermistors, therefore, behave as self-resettable fuses. Thermistors are defined by a number of key performance specifications and terms; some of the most significant of these are summarized below. This article reviewed the basics of thermistors, including what they are, how they work, the types, and their applications.

The sensitivity of the thermistor is dependent on the temperature. For example, some thermistors are more sensitive at cooler temperatures than at warmer temperatures. The manufacture will specify the voltage limits of the thermistor feedback to a temperature controller. The temperature controller feedback input needs to be in voltage, which comes from the thermistor resistance; this generally needs to be changed to temperature.

The most accurate way to convert thermistor resistance to temperature is by using the Steinhart-Hart equation. The Steinhart-Hart equation is a simple method for modelling thermistor temperatures easily and more accurately. It was a manual calculation that was developed prior to computers but can now be calculated automatically using computer software. The equation calculates the actual resistance of a thermistor as a function of temperature, with extreme accuracy; the narrower the temperature range, the more accurate the resistance calculation will be.

Thermistors change resistance with temperature changes; they are temperature-dependent resistors. They can measure liquid, gas, or solids, depending on the type of thermistor. The closer the thermistor to the device that needs to be monitored, the better the result will be; they can be embedded or surface-mounted to the device.

The table below gives a brief comparison of the benefits and drawbacks of each. Temperature Range: The approximate overall range of temperatures in which a sensor type can be used. Within a given temperature range, some sensors work better than others. Relative Cost: Relative cost as these sensors are compared to one another. For example, thermistors are inexpensive in relation to RTDs, partly because the material of choice for RTDs is platinum.

Time Constant: Approximate time required to change from one temperature value to another. This is the time, in seconds, that a thermistor takes to reach Thermistors come in a variety of shapes-disk, chip, bead, or rod, and can be surface mounted or embedded in a system.

They can be encapsulated in epoxy resin, glass, baked-on phenolic, or painted. The best shape often depends on what material is being monitored, such as a solid, liquid, or gas.

For example, a bead thermistor is ideal for embedding into a device, while a rod, disk, or cylindrical head are best for optical surfaces. A thermistor chip is normally mounted on a printed circuit board PCB. There are many, many different shapes of thermistors and some examples are:. Figure 3: Thermistor Types.

Choose a shape that allows maximum surface contact with the device whose temperature is being monitored. Regardless of the type of thermistor, the connection to the monitored device must be made using a highly thermally conductive paste or epoxy glue.

It is usually important that this paste or glue is not electrically conductive. The main use of a thermistor is to measure the temperature of a device. 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.