The **LD25A** (often associated with the **LD25** or **LD35** series) is a popular, low-cost **Electronic Load Resistor** module used primarily for testing power supplies, chargers, cables, and batteries. Below is a breakdown of its electronic components and functional specifications. --- ### 1. Key Technical Specifications | Feature | Specification | | :--- | :--- | | **Rated Operational Voltage** | DC 4 - 25.0V | | **Max Discharge Power** | 25W | | **Rated Operational Current** | 0.25 - 4.00A | | **Cooling Method** | Hydraulic Bearing Intelligent Temperature Control Fan | | **Fan Speed** | 8000 ± 10% RPM | | **Display** | 4-digit LED Segment Display | --- ### 2. Core Electronic Components The LD25A functions by converting electrical energy into heat. Its internal circuitry manages this process precisely. #### A. Power Transistor (The "Workhorse") The heart of the device is usually a high-power **MOSFET** or **BJT** (often mounted directly under the heatsink). It acts as a variable resistor, controlled by the onboard MCU to dissipate specific amounts of current. #### B. Control Unit (MCU) The module uses a small **Microcontroller** to: * Read the user input via buttons/potentiometers. * Monitor the current via a shunt resistor. * Drive the 4-digit LED display to show Voltage (V), Current (A), and Power (W). #### C. Cooling System * **Aluminum Heatsink:** Transfers heat away from the power transistor. * **Intelligent Fan:** Uses a thermistor (temperature sensor) to trigger the fan only when the temperature exceeds a certain threshold (usually ~40°C), extending the fan's lifespan and reducing noise. #### D. Inputs and Interfaces The LD25A is versatile because it includes multiple input types: 1. **USB-A (Male):** For testing standard chargers. 2. **Micro-USB (Female):** For testing mobile cables. 3. **USB Type-C (Female):** For modern high-speed chargers. 4. **Terminal Blocks (External):** For raw wire connections from DIY power supplies. --- ### 3. Safety & Protection Features The electronic logic includes several "OVP/OCP" (Over-Protection) features: * **Over-Voltage Protection (OVP):** If the input exceeds ~25.5V, the load shuts down. * **Over-Power Protection (OPP):** If the product of $V \times I$ exceeds 25W, it will cease operation to prevent the transistor from melting. * **Over-Temperature Protection (OTP):** Shuts down if the heatsink exceeds safe operating limits (usually ~80°C). --- ### 4. How to Operate ```python # Logic Flow for Testing a Charger: 1. Connect the LD25A to a USB Charger. 2. The LED displays the current voltage of the charger. 3. Rotate the potentiometer to increase the Current (Amps). 4. Observe if the voltage drops significantly (Voltage Sag). 5. If the LD25A reaches 25W, the "OPP" warning will trigger. ``` ---