In the climate-controlled silence of the Advanced Cryptography Lab at MIT, Dr. Elara Vance stared at a brick of gold-plated ceramic and silicon. It was the ZD10-100.
Without proper heat sinking (e.g., PCB copper area ≥ 25mm² per lead), actual power capability drops to ~0.5W. zd10-100 datasheet
The power dissipation exceeds 0.5W. Increase the series resistance or use a higher wattage Zener (e.g., 1N5346B – 5W). Check if the load current flows through the Zener instead of the load. zd10-100 datasheet