PTC Thermistors for RS232 and RS485

PTC thermistors are widely used in RS232/RS485 communication interface protection. They primarily provide dual protection against overcurrent and overvoltage, while also possessing self-recovery properties. The following are their core applications and technical highlights:
I. Core Functions and Principles
‌Overcurrent Protection‌
During normal operation, the PTC’s resistance is low (30-60Ω), minimally impacting signal transmission.

When a short circuit or lightning surge causes a sudden increase in current, the PTC’s resistance rises sharply (up to several thousand ohms), limiting the current and protecting the interface chip.

‌Overvoltage Protection‌
When used in conjunction with a varistor (VDR), it can absorb overvoltage energy generated by improper connection to 220V/380V mains or lightning strikes. Thermal coupling triggers a sudden change in the PTC’s resistance, blocking the abnormal voltage.

PTC Thermistor SPMZ11-10A300-600RM Application and Circuit Diagram

II. Typical Application Schemes
‌Basic Protection Circuit‌
‌Series PTC‌: Directly connected in series to the RS485 A/B lines, the PTC has a steady-state resistance of ≤60Ω and a response time of <1s.
Parallel TVS: Combined with a transient suppressor diode (TVS), it quickly clamps ESD and surges, while the PTC provides subsequent current limiting.

Enhanced Protection

Triple-Level Protection: PTC + TVS + Gas Discharge Tube (GDT) protects against overcurrent, rapid ESD, and high-energy lightning strikes, respectively.
Combined Design: For example, the WMZ13A series integrates a PTC and overvoltage protection module, making it suitable for harsh environments such as smart meters.

The PTCR SPMZB-6 30-60Ω (SPMZ11-06A300-600RM) and SPMZB-10 30-60Ω (SPMZ11-10A300-600RM) PTC thermistors, developed from our company’s programmable security units for programmable switches, are protection components for RS232 and RS485 communication interfaces. Connected in series with the RS232/RS485 communication interface, these products effectively prevent damage to the interface caused by 220V or 380V mains power introduced due to accidental or human damage to the communication line. They also minimize the impact of lightning-induced damage to the communication line. These products feature low steady-state resistance, low rated current, high withstand voltage, and fast response speed, making them an economical and practical choice for RS232/RS485 communication interface protection.

III. Key Selection Parameters

IV. Application Scenario Comparison

V. Advantages and Limitations
Advantages: Self-recovery, no replacement required, compact size (surface-mount package), and low cost.
Limitations: Requires a cool-down period (in seconds) after activation, making it unsuitable for high-frequency pulse applications.

Through appropriate selection and circuit design, PTC thermistors can effectively improve the reliability of RS232/RS485 interfaces. They perform particularly well in industrial control and smart metering applications.