ULN2803

Same Darlington architecture and same input structure, different channel count and pinout. The ULN2803A has eight channels in an 18-pin package with COM on pin 10 and ground on pin 9; the ULN2003A has seven channels in a 16-pin package with COM on pin 9 and ground on pin 8. They are not pin-compatible and cannot be swapped on the same footprint. Both are rated 500 mA and 50 V per output. There is also the ULN2804A for 6 V to 15 V CMOS inputs, the ULN2802A for 14 V to 25 V PMOS and the ULN2805A for 6 V to 15 V logic — always match the input series resistor to your logic family.

Yes, and it is the classic choice for unipolar motors. Connect the centre taps of a five- or six-wire unipolar stepper to the motor supply, tie COM to the same supply so the clamp diodes catch the winding flyback, and connect the four coil ends to four ULN2803 outputs. Then sequence those four inputs from a microcontroller, a CD4017 decade counter or an L297 controller. This is a unipolar topology. A four-wire bipolar motor is a poor fit — it needs an H-bridge per winding, so use an L298N, TB6612FNG, DRV8833 or, for microstepping with an on-chip indexer, the A4988 or DRV8825.

Marginal, and often not at high current. The 2.7 kΩ input resistor plus two Darlington base-emitter junctions means the datasheet requires 2.4 V to turn on at 200 mA, 2.7 V at 250 mA and 3.0 V at 300 mA. A 3.3 V CMOS output clears 2.4 V but leaves almost no margin at 300 mA, and a low-tolerance or loaded GPIO may fall below it. For 3.3 V logic either keep the load under about 200 mA, or use a variant designed for lower-voltage logic, or insert a 74HCT buffer, or switch to a logic-level MOSFET array such as the TPS22929 or discrete AO3400 switches.

COM is the common cathode of the eight internal suppression diodes, one from each output. When you switch an inductive load the collapsing field drives the output above the supply; the diode conducts that energy into COM, clamping the transistor. Tie COM to the positive rail of the inductive load (typically the same supply as the relay coil). If you leave COM floating, inductive loads will generate hundreds of volts on the output and destroy the Darlington pair — this is the most common field failure mode in ULN2803 designs. For purely resistive loads such as LEDs, COM can be left open.

Total package dissipation, not the per-channel 500 mA, is the limit. At VCE(sat) of about 1.3 V and 350 mA, one channel dissipates roughly 0.45 W, which already approaches the budget of a small SOP-18 on a modest copper pour. The datasheet derating note is explicit: at 70 °C ambient, VCE(sat) of 1.6 V and a 20 ms pulse, the continuous current per channel is 350 mA. If you need several channels on simultaneously at full current, use the PDIP with a large copper area, reduce the duty cycle, or split the load across two devices. Always calculate worst-case total dissipation for your actual on-pattern.

They complement each other rather than compete. A 74HC595 gives you eight latched outputs from three MCU pins but only ±25 mA per pin and a 50 mA package limit — far too weak for relays. The standard pairing is 74HC595 driving ULN2803A, which gives eight protected 500 mA sink outputs from three pins. If you need to source current rather than sink it, or need high-side switching to a rail above the logic supply, use a P-channel MOSFET array or a high-side driver instead. If you need speed, note the 1 µs maximum turn-on and turn-off delay, which rules out PWM above a few kilohertz.

It is electrically easy. Drive one output with 500 µA of base current and sink 350 mA: a genuine part reads about 1.3 V of VCE(sat) and stays under 1.6 V, while remarked or smaller-die clones exceed 1.8 V and heat quickly. Verify the clamp diode conducts from the output to COM at about 1.7 V, check the input current at VI = 3.85 V (0.93 mA typical, 1.35 mA maximum — clones with the wrong series resistor read 300 µA or 3 mA), and inspect the laser marking and date code. Confirm all eight channels and the correct input-to-output pairing, since relabelled seven-channel ULN2003A parts appear in this stream.

MOQ is one tube of 20 pcs for the 18-lead DIP, one reel (typically 1000 or 2000 pcs) for the SOP-18, or 100 pcs loose for prototyping. Because the part is multi-sourced by TI, ST, onsemi, Diodes and Toshiba with compatible Asian equivalents, availability is excellent and pricing is stable. Shenzhen stock ships in one to two business days; TI authorized-channel lead time is eight to sixteen weeks. List two vendors on your AVL so procurement is never single-sourced on this commodity part.