4.7 V zener at 5 W — the junction temp tells the story
The Microchip Technology 1N6636 is a 4.7 V zener diode rated for 5 W continuous power dissipation in the E axial through-hole package. That 5 W rating is only valid if you keep the lead temperature below the 175 °C absolute maximum junction temperature — in free air at 25 °C ambient, derate it aggressively. The -65 to 175 °C operating range puts it in the military-grade temperature band, so it handles the thermal cycling you see in avionics or downhole gear.
Regulation tolerance and leakage
Zener voltage tolerance is ±5 %, so a 4.7 V part regulates between roughly 4.465 V and 4.935 V at the test current. The 450 Ohm maximum dynamic impedance (Zzt) at the zener test point means regulation stiffens up as current increases — don't run it near the knee if you need a stable reference. Reverse leakage is 20 µA at 1 V, which is low enough for most bias and clamp circuits.
Forward drop and surge handling
Maximum forward voltage is 1.5 V at 1 A. That number matters if you use the diode in a bidirectional clamp or as a rectifier — the 5 W rating applies in the zener region, not forward conduction, so don't count on the full 5 W dissipation in forward bias.