Why Driver ICs Exist
Microcontrollers output a few milliamps at 3.3 V or 5 V, but relays, solenoids, stepper coils, and LED strings need far more current and often a higher voltage. A driver IC bridges that gap, and the Darlington array is the most common solution: a chip containing several high-gain transistor pairs, each with an integrated clamp diode to absorb the kickback from inductive loads.
How a Darlington Pair Works
A Darlington pair stacks two bipolar transistors so that the first amplifies the current for the second, producing a very high current gain. That high gain means a small logic-level input current can switch a substantial load current – the essence of a good driver. The cost is a higher saturation voltage (roughly 0.9 to 1.2 V), which increases on-state power loss and requires attention to heating at high currents.
The ULN2003 and ULN2803 Families
The ULN2003 packs seven Darlington channels, and the ULN2803 packs eight – both classics for relay and stepper driving. Each channel includes a base resistor for direct logic drive and a suppression diode for inductive loads, which simplifies board design enormously. The output is open-collector (sinking), so the load connects between the positive supply and the output pin, and the common flyback pin ties the internal diodes to the load supply.
Current and voltage limits
These arrays handle on the order of 500 mA per channel, with a total package current limit that is lower than the sum of the channels – so you cannot run every channel at maximum simultaneously. Respect both the per-channel and total current ratings, and derate for ambient temperature.
Input compatibility
The internal series base resistor makes the inputs compatible with 5 V and, for many variants, 3.3 V logic, but check the actual datasheet thresholds. For very low-voltage logic, a dedicated level shifter or a low-threshold variant may be required.
Relay and Inductive-Load Driving
Driving a relay coil illustrates the essentials. Pick the driver’s current capability above the coil’s steady current with margin; rely on the internal clamp diode for flyback where present, but add an external diode if you parallel multiple coils or use a driver without built-in clamps. Keep the logic and the load grounds sensible, and route the load return to avoid injecting switching noise into sensitive analog traces. For relay selection itself, see our reed relay guide and the relay technology comparison.
When to Choose a Darlington Array vs Alternatives
Darlington array
Best when you need many simple, low-to-moderate-current sinking channels with integrated clamps and minimal external parts – relay banks and stepper drives are the classic cases.
MOSFET driver
When efficiency matters, a low-side or high-side MOSFET with a proper gate driver beats a Darlington on conduction loss, at the cost of more design work. Our MOSFET selection guide covers those parameters.
Dedicated motor or relay driver ICs
For H-bridge or PWM motor control, dedicated drivers add current sensing, protection, and dead-time control that a bare Darlington array does not provide. For simple logic-level switching of small loads, a digital transistor (resistor-equipped transistor) can replace a whole array channel.
Thermal and Layout Notes
Darlington arrays dissipate real power, so provide copper area for the package and consider a heatsink where many channels run near their limit. Place the clamp diodes close to the load’s return path, and keep inductive switching traces short to limit radiated noise.
By Technology supplies Darlington arrays, driver ICs, MOSFETs, and relay accessories from traceable channels with cross-references to alternates. Send your channel count and load current to sales@by-techs.com for matched devices, pricing, and lead time.