The **FSMD035-R** is a surface-mount **PPTC (Polymeric Positive Temperature Coefficient)** device, commonly known as a resettable fuse. It is manufactured by companies like Littelfuse or Bourns (Polyfuse series) and is used for overcurrent protection in electronic circuits. --- ### ## Technical Specifications The following table summarizes the key electrical characteristics of the FSMD035-R: | Parameter | Specification | Description | | :--- | :--- | :--- | | **Hold Current ($I_{hold}$)** | 0.35 A | The maximum current the device can carry without tripping. | | **Trip Current ($I_{trip}$)** | 0.75 A | The minimum current at which the device will switch to high resistance. | | **Max Voltage ($V_{max}$)** | 6.0 V | Maximum voltage the device can withstand without damage. | | **Max Current ($I_{max}$)** | 40 A | Maximum fault current the device can safely interrupt. | | **Package Size** | 1210 (EIA) | Surface mount dimensions (approx. 3.2 x 2.5 mm). | | **Resistance ($R_{min}/R_{1max}$)** | 0.30 - 1.30 $\Omega$ | Resistance range before and one hour after tripping. | --- ### ### How it Works Unlike a standard fuse that melts and must be replaced, the FSMD035-R operates on the principle of thermal expansion: 1. **Normal State:** The conductive particles in the polymer matrix form a low-resistance path, allowing current to flow. 2. **Fault State:** When current exceeds the limit, I²R heating causes the polymer to expand. 3. **Tripped State:** The expansion breaks the conductive paths, increasing resistance drastically and "shutting off" the circuit. 4. **Reset:** Once the fault is removed and the device cools down, the polymer contracts, and the device returns to a low-resistance state. --- ### ### Common Applications Due to its small 1210 form factor and low voltage rating, it is typically found in: * **USB Ports:** Protecting against shorts in peripherals. * **HDMI/Display Ports:** Overcurrent protection for plug-and-play interfaces. * **Motherboards:** Protecting sensitive logic rails. * **Battery Packs:** Protecting cells from external short circuits. --- ### ### Component Identification In a schematic or on a PCB, you can identify it by: * **Reference Designator:** Usually marked as `F`, `PTC`, or `PS`. * **Marking Code:** The top of the component usually has the numeral **035** to indicate the 0.35A hold current.