| Symptom | Likely Cause | Solution | | :--- | :--- | :--- | | No output, VIN present | Short between pins 1-2 or 2-3 | Replace IC; check D1 and L1 for shorts | | Output voltage low or pulsing | Current limit triggering (overload or inductor saturation) | Reduce load or replace L1 with higher saturation rating | | Excessive ripple voltage | Bad output capacitor (high ESR) | Replace C2 with low-ESR type | | IC hot even at light load | Oscillator dead or catch diode open | Replace D1; check compensation pin | | Output voltage incorrect | Resistor divider damaged or FB pin leakage | Clean PCB; replace R1/R2 |

| Parameter | Value / Range | Remarks | |-----------|---------------|---------| | | DIP-6 (0.3" lead spacing) | Through-hole, also available in SMD (SN51DP-S) | | Input Diode Forward Current (IF) | 50 mA (max) | Typically driven at 5–20 mA | | Reverse Input Voltage | 6 V (max) | | | Output Blocking Voltage (VDRM) | 600 V (min) | Suitable for 110/230 VAC mains | | On-State Current (ITRMS) | 100 mA | Direct control only for tiny loads; usually drives a power triac | | Peak Surge Current (ITSM) | 1.2 A (non-repetitive) | | | Zero-Crossing Voltage | ±20 V (typical) | Activates within ~20V of AC zero crossing | | Isolation Voltage | 5000 Vrms (min) | Reinforced isolation | | Operating Temperature | -30°C to +100°C | |