Positive temperature coefficient resistor, also called PTC resistor, is a type of resistor whose resistance increases with increase in temperature. The goal of this section is to understand the characteristics of positive temperature coefficient resistors and how such types of resistors are used in practical applications.
Before we discuss in detail PTC resistors, it is quite important to know why such devices are studied and manufactured. You know that temperature affects all semiconductors, conductors and resistors. These effects are almost always undesirable because the change in characteristics of materials affects the behavior of circuit. Sometimes, for sophisticated applications, we even have to design temperature compensation circuits. However there are some situations where the temperature dependence of device characteristics is desirable. One such application include temperature sensors whose characteristics dependence on temperature is the key factor behind the working of circuits. In such types of applications, the whole conceptual idea behind the circuit is that the characteristics of the device will change as a result of change in temperature. This will lead to change in voltage or current in the circuit. The magnitude of change in voltage or current will depend on the changes in temperature. Thus by sensing the changes in voltage/current, we can take necessary actions as per our requirement.
With that being said, let us study in detail positive temperature coefficient resistors. These are resistors whose resistance increases with an increase in temperature. The characteristics of such resistors depends on the type of resistor. There are basically two types of positive temperature coefficient resistors.
- Switching type PTC resistors.
Silistors are made made from thermally sensitive silicon materials. Since the construction material of silistors mainly consists of silicon, these are sometimes also referred to as silicon thermistors. The characteristics of these type of resistors is fairly linear over the entire temperature range. Take a look at the graph of resistance vs temperature for silistors.
It is clear from the graph of resistance vs temperature that the resistance increases almost linearly with temperature. The temperature coefficient of silistors is approximately 0.0077 /°C. Silistors are mostly used for temperature compensation of silicon devices.
Switching type PTC resistors.
These are also called PTC thermistors. Switching type PTC thermistors are made from polycrystalline ceramic material. Here we shall not discuss the details of manufacturing process. Let us look at the resistance vs temperature curve for the switching type PTC.
Looking at the graph of resistance vs temperature for switching type PTC resistors, we can observe the following.
- Initially the PTC resistors exhibits slightly negative temperature coefficient. This is shown in the graph for temperatures below transition temperature.
- “Transition temperature”, or “Curie temperature”, or “threshold temperature” is a temperature at which the resistance starts to increase rapidly with temperature. The resistance changes by orders of magnitude within just a few degree change in temperature. Most PTCs are designed with the temperature coefficient in the range of 60°C to 120°C.
The characteristic seen above can be very useful in certain applications. We shall see in detail how such characteristic can be used for our advantage.
Self Resettable fuse : You might be knowing that fuse are used in household wiring to protect electrical devices against excessive current. The basic working principle of such fuse is that whenever there is an excessive amount of current, the fuse wire melts due to heating, thus preventing the flow of current. Thus electrical devices are safe from excessive current. Similar kind of goal can be achieved by the use of positive temperature coefficient (PTC) thermistors.
To use PTC resistors as self resettable fuse, we put PTC in series with the load. The circuit is designed in such a way that the current through PTC does not heat it to the transition temperature. Under normal working condition, the current passes through the PTC and the load. As the current passes through the PTC resistor, heat will be dissipated and its temperature will increase. However the circuit is designed in such a way that the current does not heat the PTC resistor to the extent that it reaches transition temperature.
Now as soon as current rises above the normal working condition, it will overheat the PTC resistor. Its temperature will rise above the transition temperature. Once the transition temperature is reached, the resistance of PTC resistor will increase abruptly. The increase in resistance of PTC reduces the flow of current in the circuit thereby protecting the device.
Used in Tv sets with degaussing coil : PTC resistors are used in TV sets with degaussing coils. Lets understand what is degaussing coil and how PTC resistors are used with degaussing coils. You probably (don’t) know that there is a metal screen just inside the glass tube whose function, apart from others, is to nullify the effect of earth’s magnetic field on the electrons. If this screen is not provided, then the magnetic field of earth and the surrounding would cause distortion in focus of the electron beam and hence would result in the reduction in quality of picture. You might have seen colored (like purple )spots in certain TV sets. This is because the screen is itself magnetised.
This is where degaussing coils come into picture. You know that “gauss” is the unit magnetic field. The word “degaussing” is the process of removing or reducing magnetic field. Degaussing coil is a coil which is used to demagnetise the metal screen, also called shadow mask.
PTC (positive temperature coefficient) resistors are used in series with the degaussing coils. Initially when power is turned on, the current flows through the PTC resistor and the degaussing coil thereby energizing the coil. As soon as the current heats up the PTC resistor and transition temperature is reached, its resistance increases to a very large value. Thus the current essentially stops flowing in the circuit. Normally the circuit is designed in such a way that the transition temperature is reached in about a few seconds. During this time, the degaussing coil will perform the function it is designed for.