Since replacement power boards are unavailable, does anyone know the exact cross-reference or functional equivalent for this IC?

⟐ Answered August 27, 2026 at 5:04 MST

Bench measurements and trace analysis:

- Pin 2: GND (Vcc-)

- Pin 5: 10V Vcc+

- Pins 1 & 3: AC line phase monitoring (zero-crossing/heartbeat inputs)

- Pin 4: Output running through a 10k resistor to the anode of a PC817 optocoupler (cathode to GND), sending the power-available heartbeat signal across to the secondary-side processor board.

Because Pin 4 drives an optocoupler LED via a 10k resistor rather than a standard logic trace, an open-collector/open-drain output configuration is actually required (sinking current through the optocoupler). We are looking for an exact cross-reference or identification for the '21ASP' SOT-23-5 IC matching this open-collector comparator or logic function.