PTC Basics

PTC Thermistor

PTC is the abbreviation of Positive Temperature Coefficient, which means positive temperature coefficient. It generally refers to semiconductor materials or components with a large positive temperature coefficient. Usually, the PTC we refer to is a positive temperature coefficient thermistor, or PTC thermistor for short. PTC thermistor is a typical semiconductor resistor with temperature sensitivity. When the temperature exceeds a certain value (Curie temperature), its resistance value increases stepwise with the increase of temperature.

PTC thermistor structure and functional principle

Ceramic materials are usually used as high-resistance insulators, and ceramic PTC thermistors are made of barium titanate as a base and doped with other polycrystalline ceramic materials, with low resistance and semiconductor properties. This is achieved by purposefully doping a material with a higher chemical valence as a lattice element of the crystal: a portion of the barium ions or titanate ions in the lattice are replaced by ions with a higher valence, thereby obtaining a certain number of free electrons that generate conductivity. The reason for the PTC thermistor effect, that is, the reason for the step increase in resistance value, is that the material structure is composed of many small microcrystals, and a potential barrier is formed on the interface of the grains, that is, the so-called grain boundary (grain boundary), which prevents electrons from crossing the boundary and entering the adjacent area, thereby generating high resistance. This effect is offset at low temperatures: the high dielectric constant and spontaneous polarization strength on the grain boundary hinder the formation of the potential barrier at low temperatures and allow electrons to flow freely. At high temperatures, this effect greatly reduces the dielectric constant and polarization strength, resulting in a significant increase in the potential barrier and resistance, showing a strong PTC effect.

PTC Thermistor Manufacturing Process

The mixture (barium carbonate and titanium dioxide and other materials) that can meet the requirements of electrical and thermal properties is weighed, mixed, wet-ground, dehydrated, dried, and dry-pressed to form disc-shaped, rectangular, annular, and honeycomb-shaped blanks. These pressed blanks are sintered into ceramics at a higher temperature (about 1400°C), and then electrodes are applied to metalize the surface, and the resistance value is tested in grades. According to the structural form of the finished product, the package is brazed or the shell is assembled, and then the final comprehensive test is carried out.

Weighing >> Ball Milling >> Pre-Sintering >> Granulation

>> Molding >> Sintering >> Upper electrode >> Resistance sorting

>> Brazing >> Packaging and assembly >> Marking >> Withstand voltage test

>> Resistance test >> Final test >> Packing >> Warehousing

PTC thermistor temperature dependence (RT characteristics)

The resistance-temperature characteristic is usually referred to as the resistance-temperature characteristic, which refers to the dependence between the zero-power resistance of a PTC thermistor and the temperature of the resistor body under a specified voltage.
Zero-power resistance means that when measuring the PTC thermistor value at a certain temperature, the power consumption added to the PTC thermistor is extremely low, so low that the change in the resistance of the PTC thermistor caused by its power consumption can be ignored. The rated zero-power resistance refers to the zero-power resistance value measured under an ambient temperature of 25°C.

Rmin : Minimum resistance

Tmin : Temperature at Rmin

RTc: 2 times Rmin

Tc : Curie temperature

表征阻温特性好坏的重要参数是温度系数α，反映的是阻温特性曲线的陡峭程度。温度系数α越大，PTC热敏电阻对温度变化的反应就越灵敏，即PTC效应越显著，其相应的PTC热敏电阻的性能也就越好，使用寿命就越长。PTC热敏电阻的温度系数定义为温度变化导致的电阻的相对变化 。 α = (lgR2-lgR1)/(T2-T1) 一般情况下，T1取Tc+15℃、T2取Tc+25℃来计算温度系数。

Relationship between voltage and current (VI characteristics)

The voltage-current characteristic is referred to as the volt-ampere characteristic. It shows the mutual dependence of voltage and current when the PTC thermistor reaches thermal equilibrium with an electrical load.

Ik Operating current
Ir at applied voltage Vk Residual current Vmax at applied voltage
Vmax Maximum operating voltage
VN Rated voltage
VD Breakdown voltage

The volt-ampere characteristics of PTC thermistors can be roughly divided into three regions:

The area between 0-Vk is called the linear area. The relationship between voltage and current here basically conforms to Ohm's law and does not produce obvious nonlinear changes. It is also called the non-action area. The area between Vk-Vmax is called the jump area. At this time, due to the self-heating of the PTC thermistor, the resistance value jumps. The current decreases with the increase of voltage, so this area is also called the action area. The area above VD is called the breakdown area. At this time, the current increases with the increase of voltage, and the resistance of the PTC thermistor decreases exponentially. Therefore, the higher the voltage, the greater the current, the higher the temperature of the PTC thermistor, and the lower the resistance, which quickly leads to thermal breakdown of the PTC thermistor. The volt-ampere characteristic is an important reference characteristic of the overload protection PTC thermistor.

Relationship between current and time (It characteristic)

The current-time characteristic refers to the characteristic of the current changing with time when the voltage is applied to the PTC thermistor. The current at the moment of power-on is called the starting current, and the current when thermal equilibrium is reached is called the residual current.

At a certain ambient temperature, add a starting current to the PTC thermistor (make sure it is the operating current). The time it takes for the current through the PTC thermistor to drop to 50% of the starting current is the operating time. The current-time characteristic is an important reference characteristic for automatic demagnetization PTC thermistors, delayed start PTC thermistors, and overload protection PTC thermistors.