The ULN2003A is an array of seven NPN Darlington transistors, each rated 500 mA with outputs that tolerate 50 V, plus clamp diodes for inductive loads. It is the chip on the small blue board that drives a 28BYJ-48 stepper, and it is a cheap way to switch relays, solenoids and small lamps from logic pins. It sinks current only: the load connects between the supply and the output. Figures are from the Texas Instruments ULx200x datasheet SLRS027T.
Key specifications
| Channels | 7 NPN Darlington |
|---|---|
| Collector current | 500 mA per output |
| Output voltage | up to 50 V |
| Clamp diodes | built in |
| Inputs | TTL / 5 V CMOS (ULx2003A) |
| Packages | SOIC, PDIP, SOP, TSSOP, SOT (16 pin) |
What the datasheet promises
| Item | Datasheet value |
|---|---|
| Channels | Seven NPN Darlington pairs |
| Collector current, single output | 500 mA rated |
| Output voltage | Up to 50 V |
| Clamp diodes | Built in, common cathode, for switching inductive loads |
| Parallel outputs | The datasheet says pairs can be paralleled for more current |
| Inputs | ULx2003A: for TTL or 5 V CMOS. ULx2004A: 10.5 kΩ base resistor for 6 V to 15 V CMOS |
| Packages | SOIC, PDIP, SOP, TSSOP and SOT, all 16 pin |
The 500 mA rating is per output. The datasheet’s total-package limits depend on how many outputs are on at once, so do not assume seven loads of 500 mA each are safe together; check the power dissipation section for your package.
How to wire a load
- Connect the load between its positive supply and the output pin (1C to 7C).
- Drive the matching input pin (1B to 7B) from a logic pin; high turns the output on.
- Connect COM to the positive supply of an inductive load so the clamp diodes can absorb the spike when it switches off.
- Join the ground of the load supply to the chip ground and the controller ground.
Because it is a Darlington pair, the output does not pull all the way to ground; it leaves a saturation voltage across the chip, so a motor on a low supply sees noticeably less than the supply voltage. For efficient motor drive use a MOSFET driver such as the TB6612FNG. The relay module guide and flyback diode guide cover the common mistakes.
Sources and further reading
- Texas Instruments ULN200x and ULQ200x datasheet (SLRS027T) — every figure in the table above.
Last reviewed: November 2026. Limits are from the Texas Instruments datasheet; modules sold with this chip vary. No price is quoted because it has not been checked.
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