How thermistors "read" temperature.Led lens company

　　The thermistor doesn't actually "read" anything, rather the thermistor's resistance changes with temperature. How much the resistance changes depends on the type of material used in the thermistor.Unlike other sensors, thermistors are non-linear, which means that the points on a graph representing the relationship between resistance and temperature do not form a straight line. The location of the lines and how much they vary depends on the construction of the thermistor

　　Besides thermistors, several other types of temperature sensors are used. The most common are resistance temperature detectors (RTDs) and integrated circuits (ICs), such as the LM335 and AD590 types. Which sensor is best for a particular application is based on many factors. The table below briefly compares the advantages and disadvantages of each method.

　　Temperature Range: The approximate temperature range over which the sensor type can be used. Some sensors work better than others over a given temperature rangeRelative Cost: Relative cost as these sensors compare to each other. For example, thermistors are cheap relative to RTDs, in part because the material of choice for RTDs is platinum.Time Constant: The approximate time it takes to change from one temperature value to another. This is the time (in seconds) for the thermistor to reach a 63.2% temperature difference from the initial reading to the final reading.Stability: The ability of the controller to maintain a constant temperature based on temperature feedback from a sensor.Sensitivity: The degree of response to temperature changes.

　　What are the shapes of common thermistorsThermistors come in a variety of shapes - discs, chips, beads or rods and can be surface mounted or embedded in a system. They can be encapsulated in epoxy, glass, baked phenolic or painted. The optimal shape usually depends on the material being monitored, such as a solid, liquid or gas.For example, bead thermistors are ideal for embedded devices, while rods, disks, or cylinder heads are best for optical surfaces. Thermistor chips are usually mounted on a printed circuit board (PCB). There are many different shapes of thermistors, some examples are

　　Choose a shape that maximizes contact with temperature-monitored equipment. Regardless of the type of thermistor, the connection to the device being monitored must be made with a high thermal conductivity paste or epoxy. It is usually important that the paste or glue is not conductive.

　　The main purpose of a thermistor is to measure the temperature of a device. In a temperature control system, the thermistor is a small but important part of a larger system. The temperature controller monitors the temperature of the thermistor. Then tell the heater or cooler when to turn on or off to maintain the temperature of the sensor.In the diagram below, an example system is shown with three main components for regulating the temperature of the device: a temperature sensor, a temperature controller, and a Peltier device (labeled here as a TEC or thermoelectric cooler). The sensor head is connected to a cooling plate, which needs to be maintained at a specific temperature to cool the device, and the wires are connected to a temperature controller. The temperature controller is also electrically connected to the Peltier device, which heats and cools the target device. A heat sink is attached to the Peltier device to help dissipate heat.

　　The job of the temperature sensor is to send temperature feedback to the temperature controller. The sensor has a small amount of current flowing through it, called the bias current, sent by the temperature controller. The controller cannot read the resistance, so a control voltage must be generated by applying a bias current across the thermistor using a current source, which converts resistance changes to voltage changes.The temperature controller is the brains of this operation. It takes the sensor information, compares it to the information needed by the unit to be cooled (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 the stability and accuracy of the control system. For best stability, the thermistor needs to be placed as close as possible to the thermoelectric or resistive heater. For best accuracy, the thermistor needs to be close to the device requiring temperature control. Ideally, the thermistor is embedded in the device, but it can also be attached using thermal paste or glue. Even if the device is embedded, use thermal paste or glue to eliminate air gaps.The diagram below shows two thermistors, one directly connected to the device and one remote or remote. Thermal lag time can significantly reduce the accuracy of temperature measurements if the sensor is too far from the device, and placing the thermistor too far from the Peltier device can reduce stability.

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