The part number **S28A5T175BN** typically refers to a specialized high-power, surface-mount **NTC (Negative Temperature Coefficient) Thermistor** or a specific **Current Limiter** component, often manufactured by companies like TDK/EPCOS or similar power electronics specialists. Below is a breakdown of its electronic characteristics and technical specifications. --- ## 1. Technical Specifications This component is designed to manage "Inrush Current" in power supplies and motor controllers. As the temperature of the device increases, its resistance decreases, allowing current to flow more freely after the initial power-up. | Feature | Specification | | :--- | :--- | | **Component Type** | NTC Thermistor (Inrush Current Limiter) | | **Resistance at 25°C (R25)** | 5.0 $\Omega$ | | **Tolerance** | ±20% | | **Max. Steady State Current** | ~17.5 Amperes | | **Body Diameter** | 28 mm (approx.) | | **Mounting Type** | Through-hole (Radial Leads) | --- ## 2. Key Electronic Functions ### A. Inrush Current Limiting When a device (like a Switch Mode Power Supply) is first turned on, the filter capacitors act as a short circuit, drawing a massive spike of current. The **5 $\Omega$** cold resistance of this part limits that spike to prevent blown fuses or tripped breakers. ### B. Thermal Operation As current flows through the S28A5T175BN, it heats up due to Joule heating ($P = I^2R$). * **Cold state:** High resistance (5 $\Omega$). * **Hot state:** Resistance drops significantly (often to <0.1 $\Omega$), minimizing power loss during normal operation. ### C. Power Handling The "175" in the part number usually signifies its ability to handle a continuous steady-state current of **17.5A**, making it suitable for high-wattage industrial equipment. --- ## 3. Common Applications * **AC-DC Power Supplies:** Protecting bridge rectifiers and input capacitors. * **Variable Frequency Drives (VFDs):** Managing the initial charging of the DC bus. * **Electric Motors:** Soft-starting heavy induction loads. * **Uninterruptible Power Supplies (UPS):** Handling battery charging surges. --- ## 4. Design Considerations 1. **Cool-down Time:** After the device is switched off, the NTC must cool down to return to its high-resistance state. If the power is toggled "ON-OFF-ON" too quickly, the NTC remains hot (low resistance) and will not protect the circuit from the second surge. 2. **Heat Dissipation:** These components get very hot (often >150°C) during normal operation. They should be mounted away from heat-sensitive components like electrolytic capacitors.