# Relay driver circuit using uln2003

### What is relay?

A relay is electro mechanical switch which is used in industrial application to provide isolation between high voltage and low voltage circuits. These two circuits have different voltage rating. One might be a low voltage side and other high voltage side. A relay is electrical mechanical switch which is used for switching between 5  volt circuits and 220/ 120 Volt AC circuits. For example, in microcontrollers based circuits, relay is used to isolate microcontrollers from 220 volt AC supply.

### Components of electro mechanical relay?

Electrical mechanical relay have three main components:

1. Coil
2. spring
3. contacts

A five volt signal at the coil can turn on relay. When coil is energize with 5 volt signal, current flows through the coil. When current flows through the coil, a magnetic field is created in the coil which attract causes the armature ( contacts) to attract towards spring. When no current flows through coil, spring causes the contacts to pull towards normal position. Circuit diagram of electro mechanical relay with components are given below:

how to use relay

In above circuit diagram 5 volt signal is given to coil of electro mechanical relay, when switch is closed, current flows through the coil and energize the coil. When coil energize through 5 volt signal, spring attracts contact form normally open position to closed position. On Left hand side of coil is normally open position of contact. Because circuit is no complete or in other words current flow path is not complete. When coil is energized with the help of 5 volt signal, internal spring of electro mechanical relay attract or pulls contact towards other point and complete the circuit.  A electro mechanical relay can be either normally open or normally closed. In above circuit diagram, Normally open electro mechanical relay is used. A voltage required to energize the coil, vary from relay to relay. Voltage may vary from 5 volt to 50 volt and current may very from mA to 20mA. The relays also have minimum voltage rating. Its means below voltage rating relay will not operate. One can check minimum operating voltage o relay in its data sheet. But current requirements are not specified in date sheets. Coil resistance is usually provided in data sheet of relay which can be used to find current requirement of relay using Ohm’s law formula V= IR. After getting basics understanding of relay, now you can understand what is relay driver and how to use relay driver circuit IC ULN2003?

### What is relay driver circuits IC?

Relays are used mostly interfaced with microcontrollers and digital systems. But microcontroller usually can not provide enough current to drive relays. Microcontrollers pins usually  provide maximum current of 1-2 mA per pin which is not enough o operate relay. The circuits which are used to derive relays are called relay driver circuits. Therefore Relay driver circuits using ULN2003 is used to drive relays. There are many other ways to operate relays. I will also discuss other methods of relay driver circuits in later part of this article.

### Why we need to use relay driver circuits?

While working on electronics projects which used microcontroller, we need to use relays to control AC loads or high voltage loads. Relays are used to provide isolation between microcontrollers circuits and high voltage operating loads. Microcontrollers are only used to provide on/off signals to relays.  Microcontrollers don’t have enough current sourcing ability to derive relays. Therefore relay driver circuits IC is used to derive relays properly.  There are many ways to design relay driver circuits. But dedicated relay driver circuits integrated circuits are available which serve the purpose of relay driver IC. Relay driver circuit IC ULN2003 is one of popular realy driver circuits. I will disucss relay driver IC ULN2003 in this article.  Following types of relay driver circuit IC’s are also available in market:

• ULN2003
• CS1107
• MAX4896
• FAN3240
• A2550

### Relay driver circuit using Transistor

Relay driver circuit using NPN transistor is given below. Transistor is used as a switch in this circuit. Microcontroller provides High or low input signals to NPN transistor. NPN transistor provide high driving current to electro mechanical relay through 12 volt external power supply.

relay driver cricuit using transistor

Following  components are used in relay driver circuit using transistor:

• Base current limiter resistor R1 = 1K
• zener diode of 12 volt used as a free wheeling diode
• NPN transistor
• 12 volt dc power supply

### Relay driver circuit using ULN2003

ULN2003 is very famous relay driver integrated circuit.  Relay driver IC uln2003 is high voltage and high current integrated IC which used Darlington array. Its contains seven Darlington pair of transistor which have high voltage and high current carrying capability. Its mean ULN3002 can drive up to seven relay at a time. Diode is used with each pair of NPN Darlington transistor. Diode makes it easily useable with inductive loads.

pin configuration of ULN2003

#### Applications of relay driver IC using ULN2003

There are many applications of relay driver circuit using uln2003, but some of famous applications are given below:

• relay drivers circuits
• motor drivers circuits
• lamp drivers
• line drivers
• hammer drivers
• logic buffers and many others

#### Diagram of relay driver circuit using uln2003:

Relay driver circuit using uln2003 is given below. In this circuit diagram, pic microcontroller is providing signal to 4 relays through relay driver IC uln2003.

relay driver circuit using uln2003

if we make above circuit diagram using transistors, it will be very difficult to design this circuit. Single ULN2003 can be easily used int this circuit to drive upto five relays. Microcontrollers can easily provide high or low signal to relays through its output port D. Simple programming instructions tells microcontroller to provide either high or low signal to relays. According to signals provided by microcontroller, ULN2003 IC drive relays.

I have posted a project on green house system in which relay driver circuit using uln2003 is used to drive four relays. Four relays are connected with four different relays. Check following link to to know more about practical use of relay driver circuit:

Intelligent green house control system

### Stepper motor interfacing using Uln2003

While interfacing stepper motor with microcontrollers, relay driver circuit using uln2003 is also used. Its circuit diagram using pic microcontroller is given below:

stepper motor interfacing with pic microcontroller

### Schematic of relay driver circuit using uln2003:

Internal circuit diagram of relay driver ic Darlington pairs is given below. Clamped diode is used as a free wheeling diode to avoid back emf effect in case of inductive loads.

schamtic of darlington pair of transistor in uln2003

Each output pin of ULN2003 provide upto 500mA current and peak current of 600mA. So this is all about relay driver circuit  using uln2003. If you feel any issue while using uln2003 IC in your project, can you write in comments. Thanks 🙂

Photo credits: ULN2003 data sheet