Eight-channel sink driver for relay and solenoid banks
The Texas Instruments ULN2803CDWR is an 8-channel NPN Darlington transistor array in a 20-SOIC surface-mount package. Each channel sinks up to 500 mA with a 50 V collector-emitter breakdown, making it the standard choice for driving relay coils, solenoid valves, stepper motor windings, and lamp loads from a PLC digital output or microcontroller I/O pin. The Darlington pair per channel provides high current gain, so the 500 mA load current requires only microamps of base drive from the upstream logic.
Saturation voltage and thermal budget at full load
At 350 mA collector current with 500 µA base drive, the Vce saturation is 1.6 V maximum. This 1.6 V drop at 350 mA dissipates 0.56 W per channel. With all eight channels conducting simultaneously, the total dissipation reaches 4.5 W — the 20-SOIC package on a standard FR-4 board with minimal copper area will exceed the 150°C junction limit without forced airflow or a thermal relief plane. The 50 µA maximum collector cutoff current at 50 V is the leakage when the channel is off — relevant for high-impedance loads where off-state leakage could cause a false trigger. In practice, the 50 µA is negligible for relay and solenoid loads drawing 50–500 mA.
The base product number ULN2803 is a long-running TI family with multiple package variants, so the supply channel is mature and the die is not expected to be discontinued without a long PCN window.