C945 BJT is another component I have salvaged from my old ATX power supply circuit. Other components used are PC817 optocoupler, 2SC13001, TL431 adjustable voltage regulator, 2SC1815 small signal BJT transistor and SBL3040PT schottky barrier rectifier. The 2SC945 is an NPN bipolar junction transistor (BJT) commonly used in ATX power supply circuits. It is a small-signal transistor, typically found in the primary side of the PSU, where it is used for control and switching purposes. In many ATX supplies, the C945 can be seen in standby power sections, oscillator circuits, feedback regulation circuits, or as a driver for larger switching transistors or optocouplers. Its role is usually low-power control rather than handling the main current load.
The C945 transistor has moderate gain (hFE usually between 70–700 depending on the version), a maximum collector current of around 150 mA, and a voltage rating of about 50 V. Because of these characteristics, it is well suited for signal amplification, switching, and as a general-purpose transistor in low-power circuits inside the PSU. It works together with other components like resistors, capacitors, and ICs to help regulate voltage, control startup, and ensure stable operation of the power supply.
When salvaged from an old ATX PSU, the C945 can be reused in a variety of DIY electronics projects. Hobbyists often use it in small audio amplifiers, LED driver circuits, oscillator or 555 timer circuits, and general-purpose switching tasks. Although it cannot handle high current loads, its reliability and easy availability make it a handy part to keep in a component box for prototyping and simple circuit building.
