## Overview of the RJS400-R The **RJS400-R** is a high-performance **radial leaded resettable fuse**, belonging to the Polymeric Positive Temperature Coefficient (PPTC) family. These components are primarily used for overcurrent protection in electronic circuits. ### Key Technical Specifications | Parameter | Value / Specification | | :--- | :--- | | **Hold Current ($I_{hold}$)** | 400mA (0.4A) | | **Trip Current ($I_{trip}$)** | 800mA (0.8A) | | **Maximum Voltage ($V_{max}$)** | 600V (AC/DC) | | **Maximum Current ($I_{max}$)** | 3.0A | | **Package Type** | Radial Leaded (Through-hole) | | **Operating Temperature** | -40°C to +85°C | --- ### Internal Electronic Composition The RJS400-R is not a traditional mechanical fuse; it is a solid-state device composed of a specialized **conductive polymer matrix**. 1. **Conductive Particles:** The device contains carbon black particles dispersed throughout a non-conductive crystalline polymer. 2. **Conductive State:** In normal operation, these particles form dense chains, allowing current to flow with very low resistance. 3. **Tripped State:** When an overcurrent occurs, the polymer heats up and expands. This expansion breaks the conductive chains, causing the resistance to increase exponentially, effectively "opening" the circuit. 4. **Reset Mechanism:** Once the fault is removed and the device cools down, the polymer shrinks, the chains reconnect, and the fuse returns to a low-resistance state. --- ### Comparison: RJS400-R vs. Standard Fuses | Feature | RJS400-R (PPTC) | Standard Glass Fuse | | :--- | :--- | :--- | | **Reusability** | Resettable multiple times | Single-use (Must be replaced) | | **Response Time** | Slower (Thermal based) | Fast (Melting based) | | **Resistance** | Increases when tripped | Becomes infinite (Open circuit) | | **Maintenance** | Low (Maintenance-free) | High (Requires physical swap) | --- ### Common Applications Due to its high voltage rating (600V), the RJS400-R is frequently found in: * **Telecommunications:** Protection for tip-and-ring interfaces. * **Power Supplies:** Secondary side protection for transformers. * **Modems/Routers:** Protection against lightning surges and power cross events. * **Industrial Controls:** Protecting sensors and I/O ports from wiring errors. ---