Composite PTC Thermistor for Transformers, Switching Power

A composite PTC thermistor is an electronic component that combines positive temperature coefficient (PTC) characteristics with overvoltage protection, primarily used for dual overcurrent and overvoltage protection. A composite PTC thermistor utilizes a thermally coupled combination, closely fitting and encapsulating a VDR varistor and a PTC thermistor. It is primarily used in switching power supplies and transformer primary circuits in power meters and other power supplies, providing comprehensive current and voltage protection. This solves the difficulties associated with using a single PTC thermistor with transformers. Instruments and equipment protected by a PTC thermistor may not function properly under overvoltage or overcurrent conditions, and low-temperature instruments may not be protected by the PTC when abnormalities occur.

The following is an analysis of its core features and applications:

IO. Structure and Principle

Material Composition: Typically made from a polyolefin resin, polyethylene, or epoxy resin matrix, conductive particles such as carbon black and vanadium oxide are incorporated. A temperatura ambiente, the conductive particles form continuous conductive chains, resulting in low resistivity. When the temperature rises to the polymer melting point, the matrix expands, breaking the conductive chains and causing a sudden increase in resistivity (PTC effect). Composite Design: Some models integrate a PTC thermistor and a varistor (VDR) into a single package, achieving dual overcurrent and overvoltage protection through thermal coupling. Per esempio, during an overvoltage event, the varistor absorbs energy and generates heat, triggering a jump in the PTC’s resistance, limiting current and reducing voltage by 4%.

PTC Thermistor Application Circuit Design

Ii. Performance Characteristics
RISE-TO-RESISTANCE RATIO: The resistance can vary by 5-10 orders of magnitude within a narrow temperature range, making it suitable as a thermal switch element.
RESPONSIBILITY: After actuation, it takes a long time to cool down before returning to its initial state, resulting in a slow response.
SELF-RECOVERY: Automatically returns to a low-resistance state after the fault is resolved, eliminating the need for replacement.

III. Applicazioni tipiche
Home Appliances and Industrial: Used for overcurrent protection in equipment such as electric water heaters, motori, e trasformatori.
Power Meters: Provides combined overvoltage and overcurrent protection in smart meters and switching power supplies.
Automotive Electronics: Used in temperature monitoring applications such as engine control and air conditioning systems.
When a varistor absorbs large amounts of energy, it will heat up. Due to thermal coupling, the temperature of the PTC thermistor also rises. Inoltre, the thermistor itself heats up due to the increased current. When the temperature reaches the PTC thermistor’s switching temperature, its resistance jumps, and the current decreases sharply. Simultaneously, the voltage drop across the thermistor increases significantly, reducing the voltage across the varistor and allowing only a small leakage current to flow. This reduces the voltage of the protected circuit to within the normal operating voltage range, allowing the power meter to operate normally.

IV. Selection Parameters
The following parameters should be considered during selection:
Operating current (It) and non-operating current (Ih);
Curie temperature (Tc, typically 115±7°C);
Varistor voltage (V) and maximum operating voltage (Vmax).

Common models of composite thermistors