800 V blocking, 1.25 A conduction — SOT-223 thyristor for logic-level gate drive
The STMicroelectronics X0202NN 5BA4 is a sensitive-gate SCR rated for 800 V off-state voltage and 1.25 A RMS on-state current in a SOT-223 surface-mount package. This is a current-production part (active lifecycle status) from STMicroelectronics, fully ROHS3 compliant.
Surge capability and gate sensitivity — what the ratings mean for the circuit
The 25 A non-repetitive surge current (60 Hz half-sine) defines the inrush handling for capacitive, lamp, or motor-start loads. A 100 µF capacitor bank charged to 400 V through this SCR sees a peak current well above the 1.25 A steady-state rating; the surge rating tells you whether the die survives that first cycle without fusing. Maximum on-state voltage is 1.45 V at the rated average current — this sets the conduction loss at about 1.8 W at full 1.25 A RMS. The 5 mA holding current means the SCR latches on once triggered and stays conducting until the load current drops below that threshold; a resistive load below ~4 W at 800 V may not self-commutate, requiring a forced-commutation circuit or a snubber to turn off. Off-state leakage is 5 µA maximum — negligible in most circuits, but relevant when the SCR is in a high-impedance node like a capacitive-discharge ignition or a sample-and-hold crowbar where microamps affect the timing.
Package and board-fit — SOT-223 footprint for reflow assembly
The SOT-223 package (TO-261-4, TO-261AA) has a large tab on the drain/collector side that carries the main current and provides the primary thermal path. This matters at continuous currents above 0.5 A RMS. The surface-mount body is compatible with standard lead-free reflow profiles (260°C peak per J-STD-020).