The **FSMD110-R** is a specific model of a **Polymeric Positive Temperature Coefficient (PPTC)** device, commonly known as a resettable fuse. It is manufactured by companies like **Formosa MS** (and often cross-referenced with Littelfuse or Bourns equivalents). --- ## 1. Technical Specifications The FSMD110-R is designed for surface-mount applications where space and circuit protection are critical. | Parameter | Value | Description | | :--- | :--- | :--- | | **Hold Current ($I_{hold}$)** | 1.10 A | The maximum current the device will pass without tripping. | | **Trip Current ($I_{trip}$)** | 2.20 A | The minimum current at which the device will switch to high resistance. | | **Max Voltage ($V_{max}$)** | 6.0 V - 16.0 V | The maximum voltage the device can withstand (varies by sub-series). | | **Max Current ($I_{max}$)** | 100 A | The maximum fault current the device can safely interrupt. | | **Package Size** | 1812 (EIA) | Physical dimensions: 4.5mm x 3.2mm. | | **Resistance ($R_{min}/R_{max}$)** | 0.040 - 0.210 $\Omega$ | Initial resistance range at $25^\circ C$. | --- ## 2. Operating Principle Unlike a traditional fuse that "blows" and must be replaced, the FSMD110-R is **resettable**: 1. **Normal State:** The conductive polymer is in a crystalline state, allowing electrons to flow easily (low resistance). 2. **Fault State:** If an overcurrent or over-temperature event occurs, the polymer heats up and expands into an amorphous state. This breaks the conductive paths, causing resistance to spike and limiting the current to a safe leakage level. 3. **Reset:** Once the power is disconnected and the fault is cleared, the device cools down, the polymer shrinks, and it returns to a low-resistance state. --- ## 3. Physical Characteristics The "1812" footprint is standard in the industry, making it compatible with automated SMT (Surface Mount Technology) assembly lines. * **Terminal Material:** Solder-plated copper. * **Marking:** Typically marked with "110" to indicate the 1.1A hold current rating. * **RoHS Compliant:** Usually lead-free and halogen-free. --- ## 4. Common Applications The FSMD110-R is frequently used in low-voltage electronics where users might accidentally cause short circuits: * **USB Ports:** Protecting hubs and peripherals from drawing too much power. * **HDMI/DisplayPort:** Safeguarding signal lines. * **Motherboards:** Protecting logic circuits and I/O pins. * **Battery Packs:** Preventing over-discharge heating.