PTC thermistors have the property that their resistance rises significantly when a certain temperature is exceeded. This property makes them suitable for use as temperature protection devices that detect defined high temperatures to protect circuits from overheating.
TDK offers various types of PTC limit temperature sensors and motor protection sensors that incorporate PTC thermistors and serve as temperature protection devices. This article describes typical applications of such sensors.
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PTC thermistors are temperature-dependent resistors based on special semiconductor ceramics with a high positive temperature coefficient (PTC). They exhibit relatively low resistance values at room temperature, but their resistance rises significantly when they are heated by an external heat source to above a certain temperature (Curie temperature).
With this unique property PTC thermistors are used to protect components and devices from overheating.TDK's broad portfolio of high-quality and highly reliable PTC limit temperature sensors includes SMD, leaded disk and screw-on types, so that a suitable component is available for nearly every overtemperature detection requirement. Motor protection sensors are PTC limit temperature sensors with a special designed shape, allowing them to be directly assembled within the windings (stator side) of electric motors.
The following are applications for PTC limit temperature sensors and motor protection sensors.
* In the following articles and diagrams PTC limit temperature sensors and motor protection sensors are referred to as PTC thermistors for short. Also, circuit diagrams are simplified.
PTC limit temperature sensors can be used as a simple and convenient way to protect transistor circuits from a rise in the ambient temperature or from heat generated by the transistor itself.
With transistor amplifier circuits like the one shown in the following diagram, temperatures higher than the rated temperature may degrade the characteristics of transistors.
A PTC thermistor used in such circuits detects a rise in the ambient temperature and enters a high ohmic state. As a result, the base-emitter voltage of the transistor drops and the transistor cuts-off the load current. When the ambient temperature returns to the normal level, the PTC thermistor returns to its original low resistance state.
Figure 1&#;Overtemperature protection for transistor circuits
The diagram below is an example of an overtemperature protection circuit, in which a PTC thermistor is thermally bonded to a heatsink (radiator). When a certain temperature is exceeded, the resistance of the PTC thermistor rises significantly and the base voltage of the power transistor drops. As a result, the collector current decreases and heat generation is suppressed, thereby protecting the power transistor from overheating.
Figure 2&#;Overtemperature protection for power semiconductor devices
Single-phase and three-phase induction motors, which are used in many electronic devices including industrial equipment, incorporate protection devices against overcurrent and overheating. In particular, PTC thermistors are used as motor protection sensors to protect the motors from overheating. Highly reliable protection can be realized by thermally bonding three motor protection sensors to a winding of a motor (stator side) as shown in the diagram below. When the PTC thermistor heats up to a certain temperature, a tripping unit (automatic circuit breaker) is activated to shut off the power supply.
Figure 3&#;Overtemperature protection for three-phase induction motors
The name "thermistor" is created by combining the words "thermal" and "resistor". As a passive component with temperature-sensitive properties, it is primarily employed as a temperature sensor.
A thermistor is a type of resistor that undergoes a significant change in resistance in response to variations in temperature.
Every type of resistor exhibits a certain level of temperature dependency, a characteristic encapsulated by their temperature coefficient. Thermistors, specifically, can achieve a high current coefficient. In thermistors, resistance decreases as temperature rises, leading to what is commonly known as a negative temperature coefficient (NTC). Conversely, thermistors with a positive temperature coefficient are referred to as positive temperature coefficient thermistors (PTC).
Thermistors are usually ceramic semiconductors. They are composed mainly of metal oxides that are dried to obtain the desired form factor. The characteristic behavior of a thermistor is determined by the types of oxides.
Two types of thermistors are as follows:
NTC Thermistors: cobalt, nickel, iron, copper, and manganese are the primarily used materials.
PTC Thermistors: barium, strontium, and lead titanates are the primarily used materials.
A NTC thermometer is used when a change in resistance over a large temperature range is required. It is used as a temperature sensor in the range of -55 °C to °C. Even though they can be produced for measuring either a much lower or higher temperature, they have quick response, reliability, robustness, and a low price.
When a sudden change in resistance at a specific temperature is required. They exhibit a sudden change or increase in resistance above a defined temperature (Curie temperature). The typical switching temperature is in the range of 60 °C to 120 °C. They are used for self-regulating heating elements, self-resetting, and over current protection.
The fundamental working principle of a thermistor is that resistance is dependent on temperature. The resistance of a thermistor is measured by an ohm meter (a device used for measuring electrical resistance). It should be noted that the thermistors do not read values, but the resistance varies with respect to temperature. Depending on the substance applied to a device, the amount of resistance is calculated.
Thermistors are non-linear sensors. For thermistors, a significant change in temperature coefficient resistance is required for temperature measurement. Thermistor has to be placed in the device body for which temperature measurement needs to be done and will be connected to an electrical circuit. When device temperature changes, the thermistor's resistance will also be changed, which will be recorded by the connected circuit and calibrated against the temperature that is set. Thermistors have two wires. One of the wires is connected to the excitation source, which is used for measuring the voltage of the thermistor. The primary advantage of thermistors is their capability to deliver a great change in resistance, providing us with a more sensitive and precise reading.
Several types of thermistor packages and sizes are available. The most common thermistor package is the radial-leaded type, which is mainly constructed from epoxy. In more challenging environments,glass-encapsulated packages prove to be a better choice. Integrated packages, such as lugs, probes, and threaded housings for easy monitoring, are available.
Radial leaded
Axial leaded
Glass
Threaded
Probe
Each component serves a unique function in electronic circuits:
Primarily used for temperature sensing.
Found in thermostats, climate control systems, and medical equipment.
Broad applications in detecting light, motion, temperature, pressure, etc.
Used for tuning circuits, adjusting volume, and controlling voltage.
Specifically designed for controlling current in circuits.
There is no manual adjustment; resistance changes with temperature.
No manual adjustment; response based on physical parameters.
Manual adjustment by the user.
Manual adjustment for precise current control.
Thermistors change resistance with temperature.
Variable resistors allow manual adjustment.
Rheostats provide manual control over current.
Thermistors are used in temperature-sensitive circuits.
Variable resistors adjust resistance in electronic applications.
Rheostats control current flow in electrical circuits.
The most important reason for the popularity of thermistors is that they are lower in cost. At a minimal cost, it has the ability to provide an accurate date for a small temperature range.
It is a compact design, and thermistors are fabricated in various forms.
Beads
Discs
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Rods
Though their size is small, they are extremely durable and long-lasting.
Inrush is nothing, but when you switch on the device, it is changed with an exceptionally high amount of current. Without proper protection, harmful results will occur to the device. To protect sensitive circuits, NTC thermistors are used as inrush current limiters (ICL). Inrush currents can damage the capacitors, harm power switch contacts, and destroy rectifier diodes. PTC thermistors are also used as ICLs and for protecting overcurrent. Inductive electrical equipment such as transformers, motors, and ballast lighting experience. Inrush can be controlled by connecting thermistors in series, which limit the initial current to a safe value. NTC thermsitors are used because of their low values of cold resistance.
In an electrical circuit, the current flow produces heat, which is dissipated. This resultant heat increases the temperature of the resistor. By using a thermistor, definite resistance is reached, and hence the heat is reduced.
The main use of a thermistor is temperature measurement, yet it can also be used as a means of measuring pressure, power, and liquid levels. They are used as overload protectors and can provide malfunction-related warnings.
Thermistors will be installed at a specific and definite distance from the circuit. This arrangement eradicates errors in readings because of the resistance in lead. The readings of thermistors are more precise as they are operated over a small temperature range. For small temperature changes, they respond quickly.
Since thermistors can respond to slight incremental temperature changes, they provide instant data with a small amount of delay. This kind of property is due to the small temperature ranges they monitor.
There are several varieties of thermistors that can be changed, adapted, or configured for any type of temperature application. Their various types and sizes allow them to be utilized in any kind of operation, condition, or situation.
Thermistors, or thermal resistors, are temperature-sensitive resistors that find applications in various electronic and industrial systems due to their ability to change resistance with temperature. Here are some common applications of thermistors:
Thermistors are widely used as temperature sensors in electronic devices, such as thermostats, temperature controllers, and climate control systems.
They are employed in medical equipment to monitor and regulate body temperature.
Thermistors are utilized in circuits to provide temperature compensation. They can help maintain stable operating conditions by adjusting other components' characteristics based on temperature changes.
Thermistors with a high resistance at low temperatures can be employed in power supply circuits to limit inrush current when a device is powered on. As the device warms up, the thermistor's resistance decreases.
Thermistors can be integrated into heating elements to control the temperature, preventing overheating and improving energy efficiency.
In certain applications, thermistors can be used to measure the level of liquids by detecting temperature variations due to changes in the thermal conductivity of the liquid.
Thermistors are found in refrigerators and HVAC systems for temperature monitoring and control, ensuring that the systems operate within specified temperature ranges.
Thermistors are used in automotive systems for temperature monitoring of engine components, coolant, and intake air. They contribute to the efficient operation of the engine and control systems.
In battery management systems, thermistors are used to monitor the temperature of batteries. This helps prevent overheating and ensures safe charging and discharging.
Certain thermistors are sensitive to changes in gas composition. They can be used in gas detection systems by monitoring the temperature changes associated with specific gas reactions.
In industrial settings, thermistors play a crucial role in monitoring and controlling temperature in processes such as manufacturing, chemical reactions, and material processing.
Thermistors are used in various consumer electronics, such as laptops and smartphones, for thermal management. They help prevent overheating by adjusting fan speeds or triggering thermal shutdowns.
Thermistors are employed in weather stations to measure ambient temperatures accurately.
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See Also -
Resistors & Its Resistance
Light Intensity Measurement using LDR sensor and Arduino on TinkerCAD
Interfacing LDR Sensor with Arduino
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