Active BOM line — no last-time-buy pressure
The ULN2801A: ROHS3 compliance is confirmed, so the part clears EU material declarations without exemption paperwork.
The ULN2801A packs eight NPN Darlington pairs in a single 18-pin DIP, each rated for 500 mA continuous collector current and a 50 V collector-emitter breakdown. That is enough to drive a bank of 24 VDC relays, small solenoids, or indicator lamps from a logic-level signal — the Darlington gain (hFE 1000 minimum at 350 mA) means the microcontroller or PLC output only needs to source about 350 µA per channel to switch the load. The catch: 2.25 W total package dissipation. At 350 mA per channel with a 1.6 V Vce sat, each channel dissipates roughly 0.56 W. Running all eight channels simultaneously at that current would exceed the package limit — the thermal budget forces derating. In practice, most relay-driver applications cycle channels, so the average dissipation stays under the ceiling.
Through-hole 18-DIP — field-swappable without a hot-air station
The 18-DIP (0.300-inch row spacing) through-hole package is the socket-friendly form factor. No hot-air rework station needed — a manual solder sucker and iron swap it in minutes on a customer site. For a control cabinet or conveyor-side panel, the temperature margin is fine.
Sourcing and second-source reality
The closest functional second-source is the ULN2803A, which is pin-compatible and shares the same package, power dissipation, and per-channel ratings. The difference is the input resistor value — the ULN2803A has a higher-value base resistor, making it a direct match for 5 V logic, while the ULN2801A is optimized for a slightly different input threshold. In most relay-driver sockets, they swap without a board change.