The **MF-R400** is a specific model of **Resettable PTC (Positive Temperature Coefficient) Fuse**, manufactured primarily by **Bourns**. It is part of the Multifuse® polymer-based product line. --- ## 1. Core Technical Specifications The MF-R400 is designed to protect electrical circuits from overcurrent and short-circuit conditions. | Parameter | Specification | | :--- | :--- | | **Hold Current ($I_{hold}$)** | 4.0 Amperes | | **Trip Current ($I_{trip}$)** | 8.0 Amperes | | **Maximum Voltage ($V_{max}$)** | 30 Volts (DC or AC) | | **Maximum Current ($I_{max}$)** | 100 Amperes | | **Power Dissipation ($P_d$ typical)** | 2.5 Watts | | **Package Type** | Radial Leaded (Through-hole) | --- ## 2. How it Works Unlike a traditional glass fuse that "blows" and must be replaced, the MF-R400 is **resettable**. 1. **Normal Operation:** Under normal current ($I_{hold}$), the device maintains a very low resistance. 2. **Fault Condition:** When current exceeds the Trip Current ($I_{trip}$), the internal polymer heats up rapidly. 3. **Tripping:** The heat causes the polymer to expand, breaking the conductive chain and increasing resistance significantly. This limits the current to a safe leakage level. 4. **Resetting:** Once the power is removed and the fault is cleared, the device cools down, the polymer contracts, and the conductive paths are restored. --- ## 3. Physical Dimensions & Material * **Form Factor:** It typically looks like a yellow or orange "dipped" ceramic capacitor. * **Material:** It is composed of a non-conductive polymer mixed with carbon black particles. * **Lead Wire:** Tin-plated copper. --- ## 4. Typical Applications The MF-R400 is used in circuits where high current protection is needed but physical access for fuse replacement is difficult. * **Rechargeable Battery Packs:** To prevent fire during a short circuit. * **Power Supplies:** Protecting the output rails of AC/DC adapters. * **Industrial Controls:** Protecting sensors and motors. * **Computer Peripherals:** USB ports and mainboard power protection. --- ## 5. Comparison: Resettable vs. Standard Fuse | Feature | MF-R400 (PTC) | Standard Fuse | | :--- | :--- | :--- | | **Serviceability** | Resets automatically | One-time use; must be replaced | | **Resistance** | Slightly higher | Very low | | **Speed** | Slower (Thermal based) | Fast (Melting based) | | **Cost** | Higher per unit | Lower per unit |