ULN2803 Relay Driver Working

In this Tutorial we are going to see ULN2803 Relay Driver Working Function. Let’s Start.

Introduction of ULN2803

The ULN2803 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 50v/500 mA. The Darlington pairs may be connected in parallel for higher current capability. This article brings out the working of ULN2803 IC and how to use it in a circuit.

Applications include stepper motor, relay drivers, hammer drivers, lamp drivers, display drivers (LED and gas discharge), line drivers, and logic buffers. The ULN2803 has a 2.7-k series base resistor for each Darlington pair for operation directly with TTL or 5-V CMOS devices.

Purpose of ULN2803

Most of the Chips operates with low level signals such as TTL, CMOS, PMOS, NMOS which operates at the range of (0-5)v and are incapable to drive high power inductive loads. However this chip takes low level input signals (TTL) and use that to switch/turn off the higher voltage loads that is connected to the output side.


  • Internally frequency compensated.

  • Large DC voltage gain: 100 dB.

  • Very low supply current per operator essentially independent of supply voltage.

  • Low input bias current: 20 nA (temperature compensated).

  • Low input offset voltage: 2 mV.

  • Differential input voltage range equal to the power supply voltage.

  • Large output voltage swing 0 V to (VCC – 1.5 V).

ULN2803 Relay Driver Working

The ULN2803 IC consists of eight NPN Darlington pair which provides the proper current amplification required by the loads. We all know that the transistors are used to amplify the current  but here Darlington transistor pairs are used inside the IC to make the required amplification.

Darlington Working

A Darlington pair is two transistors that act as a single transistor providing high current gain. In this pair the current amplified by the first transistor is further amplified by the next transistor providing high current to the output terminal. When no base voltage is applied that when is no signal is given to the input pins of the IC , there will be no base current and transistor remains in off state.

When high logic is fed to the input both the transistors begin to conduct providing a path to ground for the external load that the output is connected. Thus when an input is applied corresponding output pin drops down to zero there by enabling the load connected to complete its path.

Pin Diagram

ULN2803 Relay Driver Working

Using this single IC we can able to connect 8 Relays. You can see the Datasheet of ULN2803A Here.

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