The **FSMD035-0805-R** is a surface-mount Resettable PTC (Positive Temperature Coefficient) Fuse, commonly referred to as a **PolySwitch** or **Multifuse**. It is designed to protect electronic circuits from overcurrent conditions. --- ### 1. Key Technical Specifications The part number follows a specific nomenclature used by manufacturers like **Formosa MS** or similar generic standards. | Parameter | Value / Description | | :--- | :--- | | **Package Size** | 0805 (Metric 2012) | | **Hold Current ($I_{hold}$)** | 0.35 Amps (350mA) | | **Trip Current ($I_{trip}$)** | 0.75 Amps (750mA) | | **Maximum Voltage ($V_{max}$)** | 6V to 15V (Varies by specific series) | | **Maximum Current ($I_{max}$)** | 40A to 100A | | **Mounting Type** | Surface Mount (SMD) | | **RoHS Status** | Compliant (indicated by -R) | --- ### 2. How it Functions The device is made of a composite of semi-conductive polymers. 1. **Normal State:** The fuse has low resistance, allowing current to flow to the circuit. 2. **Fault State (Overcurrent):** When current exceeds the limit, internal heating causes the polymer to expand. The resistance increases dramatically (trips), effectively cutting off the majority of the current. 3. **Reset State:** Once the fault is removed and the device cools down, the polymer contracts, and the fuse returns to a low-resistance state. --- ### 3. Application Use Cases This specific 0.35A fuse is typically used in low-power consumer electronics: * **USB Port Protection:** Protecting peripherals from drawing too much current. * **Battery Packs:** Safeguarding Li-ion cells against short circuits. * **Motherboards:** Protecting I/O ports (HDMI, Keyboard, Mouse). * **IoT Devices:** General circuit protection for small sensors. --- ### 4. Comparison: PTC Fuse vs. Traditional Fuse | Feature | PTC Resettable Fuse (FSMD035) | Traditional One-Shot Fuse | | :--- | :--- | :--- | | **Longevity** | Reusable multiple times | Must be replaced after tripping | | **Resistance** | Slightly higher | Very low | | **Speed** | Slower (Thermal based) | Very fast (Melting wire) | | **Cost** | Slightly higher | Lower | --- ### 5. Part Number Breakdown * **FSMD**: Series Prefix (Surface Mount Device). * **035**: Hold Current rating (0.35 Amps). * **0805**: Physical package size (0.08 x 0.05 inches). * **-R**: RoHS Compliant / Tape & Reel packaging.