DO-35 axial — board-fit and thermal story
The 1N6021C comes in a DO-204AH (DO-35) axial-lead package, through-hole mount, supplied in bulk. The 0.020-inch diameter leads solder into 0.035-inch plated-through holes; the body length is about 0.166 inches, so it stands off the board when the leads are formed at the body. The 500 mW power dissipation assumes the leads are soldered into a 0.060-inch minimum copper land — any smaller land area or free-air mounting derates that ceiling.
75 V nominal, 265 Ω impedance — the regulation reality
Zener voltage is 75 V nominal at the specified test current, with a ±2% tolerance — that puts the actual Vz between 73.5 V and 76.5 V across a production lot. The maximum Zener impedance (Zzt) is 265 Ω at the test current; this is the dynamic resistance the regulator has to overcome. A load drawing 1 mA through that impedance drops an extra 265 mV on top of the nominal voltage — budget that into the rail tolerance. Reverse leakage is specified at 100 nA max at 56 V reverse bias — 75% of the nominal zener voltage. That low leakage matters in high-impedance divider circuits where the diode's own leakage competes with the bias network. Forward voltage is 1.1 V max at 200 mA forward current, typical for a silicon junction of this die size.
Temperature grade — -65°C to 175°C
Operating junction temperature range is -65°C to 175°C — the full military temperature band. The 500 mW power rating is at 25°C ambient; above that, derate linearly per the datasheet curve. At 175°C junction, the allowable dissipation drops to near zero — the device is thermally limited, not voltage-limited, in hot environments.