The **FSMD010-1210-R** is a surface-mount Resettable Positive Temperature Coefficient (PTC) fuse, commonly referred to as a **PolySwitch** or **PPTC**. It is designed to protect electronic circuits from overcurrent and short-circuit conditions. --- ## 1. Core Specifications The following table outlines the primary electrical characteristics of the FSMD010-1210-R: | Parameter | Specification | Description | | :--- | :--- | :--- | | **Hold Current ($I_{hold}$)** | 0.10 A (100mA) | The maximum current the device will pass without tripping. | | **Trip Current ($I_{trip}$)** | 0.30 A (300mA) | The minimum current at which the device will switch to high resistance. | | **Voltage Max ($V_{max}$)** | 30 V | The maximum voltage the device can withstand without damage. | | **Current Max ($I_{max}$)** | 100 A | The maximum fault current the device can safely interrupt. | | **Package Size** | 1210 (EIA) | Physical dimensions of 3.2mm x 2.5mm. | | **Resistance ($R_{min}$ / $R_{1max}$)** | 1.60 $\Omega$ / 15.0 $\Omega$ | Initial resistance and post-trip maximum resistance. | --- ## 2. Theory of Operation Unlike a traditional glass fuse that blows and must be replaced, the FSMD010-1210-R is **resettable**. 1. **Normal State:** The part consists of a polymer matrix filled with conductive carbon black particles. At normal currents, it maintains a low resistance. 2. **Trip State:** During an overcurrent event, the internal temperature rises due to $I^2R$ heating. The polymer expands, breaking the conductive paths. The resistance increases dramatically, reducing the current flow to a "leakage" level. 3. **Reset State:** 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 Attributes & Part Number Breakdown The part number **FSMD010-1210-R** follows a specific nomenclature: * **FSMD:** Series designation (Surface Mount Device). * **010:** Rated Hold Current (0.10 Amps). * **1210:** Footprint/Package size (0.12 x 0.10 inches). * **R:** RoHS compliant / Radial/Tape & Reel packaging. ### Typical Applications * USB port protection. * PC motherboards and peripherals. * General electronics power supplies. * Battery pack protection. --- ## 4. Example Footprint (1210) ```c // Approximate 1210 Pad dimensions for PCB Design // Units in mm [ Pad 1 ] <--- 2.0 mm ---> [ Pad 2 ] | 2.5 mm | | 2.5 mm | ```