The 1N6003A is a 13 V Zener diode in a DO-35 axial-leaded package, rated for 500 mW continuous power dissipation. That 500 mW ceiling sets the thermal budget: in free air at 25°C the part can pass about 38 mA before junction temperature becomes the limiter; at 175°C the allowable current drops to near zero. For a 13 V rail clamp or a simple shunt regulator, the load current plus the Zener current must stay under that dissipation curve. The ±10% tolerance on Vz means the actual Zener voltage for any given unit falls between 11.7 V and 14.3 V. That spread is fine for transient suppression or general-purpose voltage reference, but for a precision bias circuit you would reach for a tighter-tolerance Zener or a band-gap reference instead.
Board integration — DO-35 axial leaded
The DO-35 (DO-204AH) body is 0.166 inches long with 0.020-inch diameter leads — a standard through-hole footprint that hand-solders or wave-solders without special profile. The axial leads bend to 0.300-inch or 0.400-inch hole spacing; the cathode band marks the negative side. The junction-to-ambient thermal resistance for a DO-35 in still air is roughly 200°C/W — at 500 mW the junction sits 100°C above ambient, so at 175°C ambient the part is at its absolute limit.
Leakage and forward drop — the secondary specs that matter
Reverse leakage is 100 nA maximum at 8.4 V reverse voltage — that is 65% of the nominal Zener voltage. For a high-impedance bias string drawing microamps, this leakage is negligible; for a 10 kΩ pull-up it adds less than 1 mV of error. Forward voltage is 1.1 V maximum at 200 mA forward current — the part conducts as a standard silicon diode in the forward quadrant, useful if the circuit needs a low-cost clamp in both polarities. Maximum Zener impedance (Zzt) is 36 Ohms at the test current — this is the dynamic resistance at the Zener knee. A 36 Ω impedance means the Zener voltage shifts about 360 mV per 10 mA of current change, which matters for line regulation in a shunt regulator.