Darlington pair / array
A Darlington combines two bipolar transistors in series so the emitter of the first feeds the base of the second. The composite device behaves like a single high-gain transistor with current gain (β) of 1,000–20,000 — enough to drive a relay or a stepper coil directly from a microcontroller pin without an extra driver stage.
The first transistor's emitter current drives the second transistor's base, multiplying current gain in forward-active operation. A switching design must instead use the specified input drive and saturation characteristics. The extra junctions mean a higher on-state voltage than a comparable single saturated BJT, so power dissipation still matters.
In plain terms
A relay built from two relays in series: the first one only has to flick a small lever, and that small lever is enough to throw the much heavier second one.
Why designers use it
- Multiple low-side switching channels with a common emitter in an array such as ULN2003A.
- Inductive-load switching with integrated clamp diodes when correctly connected and rated.
- High current gain where its voltage drop and switching speed are acceptable.
Best for
- Relay drivers
- Stepper coils
- 7-segment sinking
Key specifications
- ULN2003A channels: 7 open-collector NPN Darlington channels
- Input resistor: 2.7 kΩ nominal per channel in ULN2003A
- Output capability: Check per-channel current, saturation voltage and total dissipation together
- Clamp connection: COM joins the clamp-diode cathodes; it is not a logic-supply pin
When not to use it
- When the on-state voltage and resulting dissipation exceed the load or thermal budget.
- When stored-charge turn-off behaviour is unsuitable for the required switching rate.
- When the available input drive does not meet the device's stated conditions.
Common mistakes
- Assuming all channels can carry their headline maximum current simultaneously without a thermal calculation.
- Leaving COM unconnected when relying on the internal diodes to clamp inductive loads.
- Using a discrete Darlington as though it includes the input resistor and clamps of an integrated array.
- Assuming any 3.3 V GPIO can drive the desired load current without checking the input and output characteristics.
Where you will find it
- A ULN2003A can switch relay coils or a suitable unipolar stepper motor. Connect its common emitter to the appropriate ground and the clamp common to the load supply when using the built-in inductive clamps.
- For several active outputs, add their dissipation and check package temperature rather than multiplying the single-channel current rating by seven.
A short history
The Darlington connection cascades two transistors to obtain high current gain. TI's ULN2003A integrates seven NPN Darlington channels, common emitters, open-collector outputs and common-cathode clamp diodes. The seven-channel part must not be described as an eight-channel array.
Good to know
- Open-collector outputs sink current; they do not actively source the load supply.
- An internal protection diode only helps when its external connections and energy limits are suitable.