Relay driver
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Transcript of Relay driver
Relay Driver and Power Factor Correction Concept with
Application in Micro controllers
Source: www.microcontrollerslab.com
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 TransistorRelay 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
Electro mechanical relay rating depend on your requirement of load
Relay driver circuit using ULN2003ULN2003 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 ULN2003Applications of relay driver IC using ULN2003There 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 Uln2003While 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.
What is power factor?Let’s start with basic introduction of power factor. Power factor is basically a ration of real
power absorbed by electrical circuit to total appearent power consumed by electrical circuit.
In other words, power factor is a ratio of real power and appearent power. Many inductive
loads are used in power system. Inductive loads need reactive power for their proper
operation. So main cause of power factor in electrical power system is reactance.
Capacitors provide reactive power. In other words, capacitors generate reactive power and
inductors consume reactive power.
Why we need to improve power factor?Reactive power is basically a burn on power system and transmission lines. Reactive power
travel back and forth from power generation point to load and hence a lost of electrical
energy. Now a days inductive loads are used in domestic as well as industrial applications.
Low power factor cause losses in transmission line and voltage drops in transmission lines.
Need of electrical power is increasing day by day, so can’t afford to waste power in
transmission line. Therefore we need to improve power factor of electrical power system
and loads. By providing reactive power locally at the load side, we can increase power
factor of system and power factor of machines. Hence Power factor improvement is local
issue. As I have already mentioned, capacitors are source of reactive power. So capacitors
can be used to improve power factor or to provide reactive power to load locally instead of
from power generations ends.
How to improve power factor:Before understanding what is power factor? Let me tell you what is good power factor?
Unity is ideal power factor. Resistive loads have unity power factor. But all electrical loads
are not fully resistive. Therefore we need to improve power factor of these load. Power
factor can be increased by adding power factor correction capacitor to loads. Capacitors
provide reactive power to load locally . There are many methods have been developed for
power factor improvement. But in this article, you will learn about power factor
measurement and correction using microcontroller.
Advantages of power factor improvement?Major advantages of power factor improvement are given below:
It reduces the losses in transmission lines and cables
It reduces the voltage drop in transmission lines and cables
It reduces the burden on power generation utility companies
Power factor measurement using pic microcontroller:Power factor is basically due to lag and lead concept of current and voltage. When current
waveform lags behinds the voltage by some angle, power factor will be lagging and when
current wave form leads the voltage by some angle, power factor will be leading. Inductor
causes a lagging power factor and capacitors produces leading power factor. Hence
capacitors improve power factor by providing leading VARS or reactive power to system
which cancels the lagging power factor effect of inductors.
Now the question is how to measure power factor using microcontroller? To measure power
factor we need to measure phase angle difference between voltage and current wave form.
The cosine of this phase angle is power factor. Power factor is also represented by
following formula:
Power factor = Cos (θ)
Where θ is basically phase angle difference between voltage and current wave form. So we
need to develop a method to measure phase angle difference between voltage and current
wave form. I have used zero crossing detection concept to measure time difference
between current and voltage wave form. Time difference between two signal or waves can
also be used to measure power factor. I will discuss more about it in later part of this article.
Now let’s proceed further with zero crossing detection?What is zero crossing detection?Zero crossing detection is used to detect sine wave zero crossing from positive half cycle to
negative half cycle or negative half cycle. To measure time difference between two waves
we need to detect zero crossing of two waves. There are many methods available to
measure zero crossing detection. But I have already posted an article on zero crossing
detection using pic microcontroller. I have explained zero crossing detection concept in
following article. So I recommend you to read following article first before reading further.
How to detect zero crossing using pic microcontrollerI assume that After reading an article on zero crossing detection, you got the idea about
how to detect zero crossing of sine waves using pic microcontroller. In zero power factor
measurement circuit, two zero crossing detection circuits are used. One for voltage wave
form and other for current wave form. After detecting zero crossing for voltage wave and
sine wave we need to measure time difference between zero crossing of voltage and
current wave forms.
How to measure time difference two wave for power factor measurement: Now I will discuss how to measure time difference between two waves. It is very easy to
measure time difference between two waves with the help of hardware external interrupt pin
of microcontroller. hardware interrupt is used to detect zero crossing signal. Microcontroller
internal timer is used to measure time. When external interrupt occur on microcontroller pin
time will start and after another external interrupt time will stop counting. One interrupt will
be generated with the help of current signal and other interrupt with the help voltage signal
zero crossing. Timer value will be stored in one variable. This variable value is basically a
time difference between two waves. For better results take 20-30 values and take their
average.
How to find angel using time difference:So using time difference we can easily calculate phase angle using below given formula’s :
Time difference = average values of timer /1000 ;
In above equation 1000 is used to convert time into seconds because half of the sine wave
is about 10us and there will 1000 counts per second.
θ = time difference * 2π;
Power factor = Cos ( θ ) ;
After that time difference is multiplied with 2π to convert it into radians and phase angle in
radians. After that by using power factor formula we can easily calculate power factor. All
these calculations are done in programming by writing a code in microcontroller.
Power factor measurement circuit using pic microcontroller:Circuit diagram of power factor measurement circuit using pic microcontroller is given
below. Voltage transformer and current transformer is used in power factor measurement
circuit.
power factor measurement circuit using pic microcontroller
In above circuit diagram. voltage transformer is used to step down voltage. Because
microcontroller can not read high voltage 220v AC directly. Current transformer is used to
get wave form of current wave. magnitude of current depend on the load and voltage almost
remains constant. After that these signals are fed to zero crossing detector circuit using
LM358 operational amplifier. I have already posted an article on zero crossing detection
using operational amplifier. For more information about zero crossing detector circuit check
following article:
Zero crossing detection using op ampMicrocontroller PIC16F877A read zero crossings of voltage and current waveform and start
timer according to zero crossing detection of current and voltage wave form. PC16F877A
microcontroller is main part of this project. In reads zero crossing detection. measures time
difference and calulate power factor according to above given formula’s.After calculating
power factor, PIC16F877A microcontroller displays it on 16 X2 LCD.
List of components for power measurement circuit:List of components for power measurement circuit using pic microcontroller is given below:
Category,Reference,Value,Order Code
Resistors,"R1",2k,
Resistors,"R2",4k,
Resistors,"R3",4k,
Resistors,"R4",4k,
Resistors,"R5",4k,
Integrated Circuits,"U1",PIC16F877A,
Integrated Circuits,"U2",LM358,
Miscellaneous,"AC SOURCE",,
Miscellaneous,"CURRENT TRANSFORMER" It can be wind according to requirement,
Miscellaneous,"POTENTIAL TRANSFORMER" 220V AC to 12V AC step down transformer,
Liquid crystal display: ,"LCD1",LM016L,
Variable resistor: ,"RV1",10k,
This guide has provided you all the information you need to measure power factor. I have
tried to cover a question on ” How to measure power factor using microcontroller. I
recommend you to write your own code for this project.