PTC Thermistor Application Example

PTC thermistors can be used in computers and their peripherals, mobile phones, batteries, telecommunications and networking equipment, transformers, industrial control equipment, automobiles and other electronic products.

To prevent damage to electrical equipment by overcurrent and overheating; as a ceramic heating element, PTC is a self-regulating temperature heating element, which is widely used in heaters, hair dryers, electric mosquito coils, electric irons, etc. to maintain a constant temperature. A set of temperature control circuits can be stored on electrical appliances.

(1) Overcurrent and overtemperature protection

The figure on right shows circuit diagram of load circuit thermistor protection.

After thermistor trips, current in circuit is greatly reduced, so it can perform two functions of overheating protection and overcurrent protection at same time.

Thermistors are also suitable for protection against short circuits and overheating of lithium-ion and nickel-metal hydride batteries in laptop computers and mobile phones.

When a mobile phone battery is recharged or short circuited, battery heats up and PTC resistance on battery's internal circuit board increases, limiting current to a safe range. Some parallel mobile phone batteries use ordinary resistors inside instead of PTCR, and in case of short circuit, protection effect is very poor.

(2) Application of PTCR in TV PTC degaussing circuit

The grid, shadow mask and other parts of color kinescope are made of metal materials, which are easily affected by earth's magnetic field or stray magnetic field inside and outside machine, which magnetizes these metal parts and makes image color abnormal. Therefore, color TVs are equipped with automatic demagnetization circuits .

The degaussing coil attached to kinescope is connected in series with PTCR to form a degaussing circuit. When starting, cold resistance of PTCR is very small (about 12~18 ohms), and 50Hz current flowing through degaussing coil is very large, which creates a strong alternating magnetic field. The current also flows through PTCR at same time, causing its temperature to rise. It increases rapidly over several seconds, current gradually decreases, showing a damped signal, and magnetic field gradually weakens, playing role of demagnetization. To reduce holding current, a low-power heating resistor can be installed near PTCR to keep PTCR at a high temperature during period of TV operation, and current flowing through degaussing coil can be kept at a low level to reduce power consumption.

PTC thermistor for TV degaussing circuit

Current PTC degaussing time curve

(3) Apply PTCR at engine start

PTC application in engine start circuit

Connect PTCR in series with motor starting coil circuit, there will be a large starting current for a few seconds of starting motor; when engine starts, current flowing through starting coil will be very small.

(4) Use of PTCR to protect motor from overheating

Immerse PTCR into motor coil, when motor winding overheats, PTCR will decrease, start motor protection circuit and turn off power to motor. In this example, no motor current flows through PTCR, which is quite different from previous examples: PTCR only works for temperature detection, not as a resettable fuse.