ULN2803A Darlington Transistor Arrays

Pandi June 10, 2016 2 min read

Bring in some muscle to your output pins with 8 mighty Darlington’s! Tenet Technetronics introduces This DIP chip which contains 8 drivers that can sink 500mA from a 50V supply and has kickback diodes included inside for driving coils. This will let your little micro-controller or microcomputer power solenoids, DC motors (in one direction) and uni-polar stepper motors.

The ULN2803A is a high-voltage, high-current Darlington transistor array. The device consists of eight NPN Darlington pairs that feature high-voltage outputs with common-cathode clamp diodes for switching inductive loads. The collector-current rating of each Darlington pair is 500 mA. The Darlington pairs may be connected in parallel for higher current capability.

Applications include

Relay drivers
Hammer drivers
Lamp drivers
Display drivers (LED and gas discharge)
Line drivers, and logic buffers.

The ULN2803A comprises seven high voltage, high current NPN Darlington transistor pairs. All units feature a common emitter and open collector outputs. To maximise their effectiveness, these units contain suppression diodes for inductive loads. The ULN2803A has a series base resistor to each Darlington pair, thus allowing operation directly with TTL or CMOS operating at supply voltages of 5.0 V or 3.3 V. The ULN2803A offers solutions to a great many interface needs, including solenoids, relays, lamps, small motors, and LED’s. Applications requiring sink currents beyond the capability of a single output may be accommodated by paralleling the outputs. Tenet Technetronics introduces an easy to interface breakout board with ULN2803.

Functional Block diagram:

The GPIO voltage is converted to base current via the 2.7 kΩ resistor connected between the input and base of the pre-driver Darlington NPN. The 7.2 kΩ & 3.0 kΩ resistors connected between the base and emitter of each respective NPN act as pull-downs and suppress the amount of leakage that may occur from the input. The diodes connected between the output and COM pin is used to suppress the kick-back voltage from an inductive load that is excited when the NPN drivers are turned off (stop sinking) and the stored energy in the coils causes a reverse current to flow into the coil supply via the kick-back diode. In normal operation the diodes on base and collector pins to emitter will be reversed biased. If these diode are forward biased, internal parasitic NPN transistors will draw (a nearly equal) current from other (nearby) device pins.

Interfacing:

This breakout board can be interfaced with any of controller and even with the raspberry pi and also with ARM. This ULN2803 breakout consists of 8 Darlington inputs and 8 outputs which can drive any motors. Here we have shown an interface of stepper motor with Arduino Uno using the ULN2803 breakout board.