Positive temperature coefficient (PTC) thermistors are devices that act as resistors, that increase their resistance with temperature. They can be utilized in cases where the use of a variable resistor is required to regulate the flow of electricity flowing through the circuit.
Their usage in these types of circuits is largely replaced with field-effect transistors using metal oxide (MOSFETs). The MOSFET type offers more power-handling capabilities and usually requires lower voltage drop to achieve sufficient current flow than what would be needed for a similar PTC device. But, PTC thermistors are still often used in applications like battery protection circuits as they are simpler to design as compared to MOSFETs.
Working Principle of Different Types of PTC Thermistor
PTC is a PTC characteristic is caused by an increased quantity of charge carriers that are present at higher temperatures. The number of charge carriers may be affected very significantly by the structure and type of the material. This is based upon which we can divide them into two categories are Polymer PTC thermistor and silicon PTC thermistor.
Polymer PTC Thermistor
The basic principle that drives the polymer PTC thermistor is founded in the nature that electrical resists it about of temperature. This is due to the number of charge carriers (electrons or lattice sites that are vacant dependent on the materials used in semiconductors) which contribute to conduction are activated by heat. In other words, at low temperatures, the carriers slow down and do not contribute much to the process of electrical conduction however, at higher temperatures they are moving so fast that their contribution is dramatically increased. This is shown in Fig. 1. (blue lines) for an imaginary semiconductor material, however, the same behavior (although using different quantities) could be observed for real materials. Polyswitch (blue line), semi-fuse, and multi-fuse are only some of the many names that are used to advertise these materials.
Silicon PTC Thermistor
Contrary to the Polymer PTC thermistor that is made of polymer, it is a silicon PTC thermistor that is well-known for its linear character that makes them suitable for use in the field of temperature sensing. Most of the time it is the case that the resistivity curve in the silicon PTC thermistor is a straight line (refer to the blue line in Figure. 1.) with a single intersection. This allows you to utilize the thermistor as a temperature sensor. You can determine an analog reading that is proportional to temperature.
What are PTC Thermistors Primarily used to do?
PTC thermistors are used in numerous applications, such as:
- The management of thermal energy in microprocessors as well as CPUs.
- Protecting power supplies that are high-voltage against voltage spikes and overcurrents.
- Ensuring safety features are available for the industry of aviation
- As the sensor used to detect static electricity that has built up on turbine blades of jet engines
- Detecting conditions of over-temperature in electric motors.
- The measurement of the temperature of exhaust gases released by combustion engines
PTC thermistors can also be used as a resettable fuse in devices like power supplies as well as in the auto industry.
What is the best way to select the best PTC thermistor?
There are numerous kinds of PTC thermistors and each one has distinct features that make them more suitable for specific applications than others.
- When the circuit protected is functioning normally, determine the temperature of the ambient maximum as well as the current working in this circuit as well as the necessary action time, as well as other details.
- Select different types of mounting. If, for instance, you're in search of a high-temperature device that is low in power consumption, an axial lead model is the best choice. If you're looking for a device that has higher current ratings or better sensors for temperature, the chip type might be the best choice for your requirements.
- Choose the specification of the product that has the most "maintenance current" based on the ambient temperature maximum and the circuit's working current.
- Be sure that the time for action for the thermistor you select is shorter than the time it takes to safeguard the circuit.
Circuit Examples of PTC Thermistor
In the normal temperature of the ambient at normal ambient temperature, the resistance value is that of PTC thermistor Rp is smaller than the Rs output voltage. When ambient temperatures exceed that setpoint PTC thermistor Rp's resistivity is rapidly increasing until it is at the point of reaching Rs when Vo rises.
Figure (1) illustrates the fundamental circuit for temperature compensation. When you are biasing the transistor, utilize the resistor of the PTC thermistor. If the transistor gets too hot it will cause, likely, the PTC thermistor is also likely to get hot. If you turn on the PTC thermistor passes through the switching temperature, it turns into high resistance, and doesn't influence the circuit, and switches off the transistor. In the instance, of figures (2) (3) and (3) it could also be utilized as an overheat detector.
Utilizing multiple PTC thermistors can provide coverage for multiple hot spots when used together with the comparator. The following diagram shows the fundamental circuit that includes the two PTC thermistor sensors that are connected to measure the limit temperature and then turn on the fan if the limit temperature is over. If the PTC thermistor at least detects an overheated state, the comparator can work by utilizing high-temperature resistance. It can be easily substituted for multiple PTC thermistors as well as induction temperature with the same circuit.
In this post, we discussed the fundamentals of how the PTC thermistor works, its applications, and the best way to select the right one to enhance your work. In contrast to NTC thermistors, they do not alter the input impedance. Therefore, they're ideal for circuits that don't require a change in input impedance. This includes the pulse circuits of area amplifiers and measurement equipment.