The **MF-R800** (commonly referred to as the MF-R8 in electronic catalogs) is a **Radial Leaded Resettable Fuse** based on Polymeric Positive Temperature Coefficient (**PPTC**) technology. It is manufactured by Bourns and other electronic component providers. --- ### 1. Key Technical Specifications The "800" in the part number typically signifies a holding current of **8.0 Amperes**. | Parameter | Value (Typical) | | :--- | :--- | | **Hold Current ($I_{hold}$)** | 8.0 A | | **Trip Current ($I_{trip}$)** | 16.0 A | | **Max Voltage ($V_{max}$)** | 30V (DC/AC) | | **Max Current ($I_{max}$)** | 100 A | | **Power Dissipation** | 4.0 W | | **Termination Style** | Radial Leaded (Through-hole) | --- ### 2. How it Works (PPTC Technology) Unlike a standard glass fuse that blows and must be replaced, the MF-R800 is **resettable**. 1. **Normal State:** The conductive particles in the polymer matrix form low-resistance paths, allowing current to flow to the circuit. 2. **Fault State (Overcurrent):** When current exceeds the trip limit, $I^2R$ heating causes the polymer to expand. This breaks the conductive paths. 3. **Tripped State:** The resistance increases dramatically, reducing the current to a small leakage level to protect the load. 4. **Reset:** Once the fault is removed and the device cools down, the polymer contracts, the paths reconnect, and the fuse returns to a low-resistance state. --- ### 3. Component Construction * **Encapsulation:** Cured, flame-retardant epoxy polymer (meeting UL 94V-0). * **Lead Material:** Tin-plated copper. * **Marking:** Usually marked with the manufacturer logo, "R800", and a date code. --- ### 4. Typical Applications The MF-R800 is used in high-current environments where accessibility for replacement is difficult: * **Industrial Controls:** Protection for motors and heavy-duty actuators. * **Power Supplies:** Secondary side protection for transformers. * **Battery Packs:** Protecting against short circuits in high-capacity Li-ion or Lead-acid banks. * **Computing:** Protection for high-wattage peripheral ports. --- ### 5. Design Considerations * **Ambient Temperature:** PPTC devices are sensitive to heat. As the surrounding temperature rises, the $I_{hold}$ decreases (thermal derating). * **Time to Trip:** At 40A (5x the hold current), the device typically trips in roughly 18.8 seconds. It is not an "instant" blow fuse. * **Voltage Rating:** Never exceed the $V_{max}$ (30V), or the device may arc and fail permanently.