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Eight-channel Darlington driver

ULN2803A Working Principle

Learn how ULN2803A works with an illustrated explanation and practical readings.

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ULN2803A at a glance

Follow the labeled connections and functional stages

ULN2803A working principle: illustrated functional stages, basic circuit and connections
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How it works

3 stages · Explore one step at a time

Step 1 / 3

Apply a logic level

Input current passes through the internal resistor and drives a pair of transistors. A HIGH input enables its corresponding collector sink.

Step 2 / 3

Complete the load path

Current flows from the external positive supply, through the load, through the output transistor and back to pin 9.

Step 3 / 3

Account for turn-off and heat

Coil current returns through the internal diode to COM after switch-off. Add dissipation from every active channel and consider the selected package and board.

Switch 8 loads independently

Click an input. Follow its collector current and the combined power budget.

12 V · Driver explorer
Active channels0 / 8
Total coil current0 mA
Output-stage dissipation0 mW

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Illustrative DC loads: 240 Ω coils, a 12 V rail and a fixed 0.90 V driver drop. COM is tied to the coil rail. The calculation excludes input-stage power, flyback transients, relay mechanics and temperature rise; it is not a safe-current guarantee.

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