250V 270 OHM Positive Temperature Coefficient Resistor MZ21 PTC Thermal Resistor

Overcurrent Protection PTC Thermistor MZ21 250V 60mA 27R For Communication Equipment

I Description Of The Overcurrent Protection PTC Thermistor MZ21

MZ11 series PTC thermistors are used instead of conventional fuses to protect loads such as multimeter, transformers, intelligence-ammeter, and digital multimeter, etc. or electronic circuits against overcurrent, which is the third protective devices after "temperature fuse" and "temperature switch". They are also called "auto restore fuses" or "ten-thousand-time fuses".

II OPERATING PRINCIPLE OF THE Of The Overcurrent Protection PTC Thermistor MZ21

The operating principle is showed in the following figures, the PTC thermistor fuse connected in series with the load. When a circuit is in normal status, the current through PTC is no exceed the rated current and in normal state PTCR keep low resistance, which will not affect the normal operation of the protected circuit. In case of any trouble in the circuit and the current is greater than the rated value, the PTCR will become hot quickly and present as a high resistance state, which sets the circuit relatively "off" to protect the circuit from damage. After the trouble is removed, PTCR will automatically restore its low resistance state and the circuit will resume normal operation.

III Dimension Of The Overcurrent Protection PTC Thermistor MZ21 (mm)

IV Applications of PTC thermistors for telecom overcurrent protection

Application: Overcurrent protection in a surge protective device (SPD) used for security systems

PTC thermistors for telecom applications are also used for various security systems in factories and office buildings. For example, surge protective devices (SPD) are installed at important locations in these systems because signal cables used for fire alarm systems, surveillance camera systems, and other network systems that connect multiple sites can be damaged by lightning surges.
The diagram below shows an example of a protection circuit that uses a replaceable plug-in SPD. The plug side includes an arrestor and a varistor for overvoltage protection. The socket side includes a PTC thermistor for overcurrent protection.
TDK offers a comprehensive lineup of PTC thermistors for telecom applications. Telecom pair protectors (TPP), each of which incorporates two PTC thermistors packaged in a plastic case, are often used for SPDs.
The function of the PTC is very similar to that described in the previous section.

Figure 5 Example of a protection circuit for plug-in surge protection device (SPD)