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Seven-channel Darlington driver

ULN2003A Internal Circuitry

Explore the internal circuitry and functional stages of ULN2003A.

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Inside ULN2003A

Seven input resistors, seven Darlington pairs and seven clamp diodes.

One representative channel has a 2.7 kΩ input resistor feeding a Darlington pair. Both collectors join the output, emitter returns to E, and a diode leads from collector anode to COM cathode. The channel repeats seven times.
Representative functional channel. Internal bias resistors are simplified; the PDF contains the detailed equivalent.

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Current gain

The first NPN transistor drives the second, increasing current gain.

Open collector

Both collectors connect to the output. The driver provides a current sink, not a positive voltage source.

Clamp diode

The diode anode is at the collector and its cathode is at COM, returning inductive current to the load supply.

Component view