Monday, November 18, 2013

100 Watt sub woofer amplifier.

This is the circuit diagram of a fully transistorized sub woofer amplifier that can produce an output of 100W.There are seven transistors including four in the output stage. The transistors Q1 and Q2 form the preamplifier stage. Transistors Q4 to Q7 form the output stage. Since no ICs are used the circuit is very robust and can be easily assembled on a general purpose PCB.
Circuit diagram with Parts list.
  • The circuit can be powered from a +35V/-35V, 5A dual power supply.
  • Use a 100W, 12 inch sub woofer at the output.
  • All electrolytic capacitors must be rated 100V.
  • The transistor Q4 to Q7 must be fitted with heat sinks.


Working of Relays

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Working of Relays

In this article, the basics of a relay like energized relay and de-energized relay are explained in detail. Also, the design, construction, working, applications, and also relay selection is explained in detail.

What is a relay?

We know that most of the high end industrial application devices have relays for their effective working. Relays are simple switches which are operated both electrically and mechanically. Relays consist of a n electromagnet and also a set of contacts. The switching mechanism is carried out with the help of the electromagnet. There are also other operating principles for its working. But they differ according to their applications. Most of the devices have the application of relays.

Why is a relay used?

The main operation of a relay comes in places where only a low-power signal can be used to control a circuit. It is also used in places where only one signal can be used to control a lot of circuits. The application of relays started during the invention of telephones. They played an important role in switching calls in telephone exchanges. They were also used in long distance telegraphy. They were used to switch the signal coming from one source to another destination. After the invention of computers they were also used to perform Boolean and other logical operations. The high end applications of relays require high power to be driven by electric motors and so on. Such relays are called contactors.

Relay Design

There are only four main parts in a relay. They are
  • Electromagnet
  • Movable Armature
  • Switch point contacts
  • Spring
The figures given below show the actual design of a simple relay.
Relay Construction
Relay Construction
It is an electro-magnetic relay with a wire coil, surrounded by an iron core. A path of very low reluctance for the magnetic flux is provided for the movable armature and also the switch point contacts.  The movable armature is connected to the yoke which is mechanically connected to the switch point contacts. These parts are safely held with the help of a spring. The spring is used so as to produce an air gap in the circuit when the relay becomes de-energized.

How relay works?

The working of a relay can be better understood by explaining the following diagram given below.
Relay Design
Relay Design
The diagram shows an inner section diagram of a relay. An iron core is surrounded by a control coil. As shown, the power source is given to the electromagnet through a control switch and through contacts to the load. When current starts flowing through the control coil, the electromagnet starts energizing and thus intensifies the magnetic field. Thus the upper contact arm starts to be attracted to the lower fixed arm and thus closes the contacts causing a short circuit for the power to the load. On the other hand, if the relay was already de-energized when the contacts were closed, then the contact move oppositely and make an open circuit.
As soon as the coil current is off, the movable armature will be returned by a force back to its initial position. This force will be almost equal to half the strength of the magnetic force. This force is mainly provided by two factors. They are the spring and also gravity.
Relays are mainly made for two basic operations. One is low voltage application and the other is high voltage. For low voltage applications, more preference will be given to reduce the noise of the whole circuit. For high voltage applications, they are mainly designed to reduce a phenomenon called arcing.

Relay Basics

The basics for all the relays are the same. Take a look at a 4 – pin relay shown below. There are two colours shown. The green colour represents the control circuit and the red colour represents the load circuit. A small control coil is connected onto the control circuit. A switch is connected to the load. This switch is controlled by the coil in the control circuit. Now let us take the different steps that occour in a relay.
relay operation
relay operation
  • Energized Relay (ON)
As shown in the circuit, the current flowing through the coils represented by pins 1 and 3 causes a magnetic field to be aroused. This magnetic field causes the closing of the pins 2 and 4. Thus the switch plays an important role in the relay working. As it is a part of the load circuit, it is used to control an electrical circuit that is connected to it. Thus, when the relay in energized the current flow will be through the pins 2 and 4.
Energized Relay (ON)
Energized Relay (ON)
  • De – Energized Relay (OFF)
As soon as the current flow stops through pins 1 and 3, the switch opens and thus the open circuit prevents the current flow through pins 2 and 4. Thus the relay becomes de-energized and thus in off position.
De-Energized Relay (OFF)
De-Energized Relay (OFF)
In simple, when a voltage is applied to pin 1, the electromagnet activates, causing a magnetic field to be developed, which goes on to close the pins 2 and 4 causing a closed circuit. When there is no voltage on pin 1, there will be no electromagnetic force and thus no magnetic field. Thus the switches remain open.

Pole and Throw

Relays have the exact working of a switch. So, the same concept is also applied. A relay is said to switch one or more poles. Each pole has contacts that can be thrown in mainly three ways. They are
  • Normally Open Contact (NO) – NO contact is also called a make contact. It closes the circuit when the relay is activated. It disconnects the circuit when the relay is inactive.
  • Normally Closed Contact (NC) – NC contact is also known as break contact. This is opposite to the NO contact. When the relay is activated, the circuit disconnects. When the relay is deactivated, the circuit connects.
  • Change-over (CO) / Double-throw (DT) Contacts – This type of contacts are used to control two types of circuits. They are used to control a NO contact and also a NC contact with a common terminal. According to their type they are called by the names break before make and make before breakcontacts.
Relays are also named with designations like
  • Single Pole Single Throw (SPST) – This type of relay has a total of four terminals. Out of these two terminals can be connected or disconnected. The other two terminals are needed for the coil.
  • Single Pole Double Throw (SPDT) – This type of a relay has a total of five terminals. Out f these two are the coil terminals. A common terminal is also included which connects to either of two others.
  • Double Pole Single Throw (DPST) – This relay has a total of six terminals. These terminals are further divided into two pairs. Thus they can act as two SPST’s which are actuated by a single coil. Out of the six terminals two of them are coil terminals.
  • Double Pole Double Throw (DPDT) – This is the biggest of all. It has mainly eight relay terminals. Out of these two rows are designed to be change over terminals. They are designed to act as two SPDT relays which are actuated by a single coil.

Relay Applications

  • Relays are used to realize logic functions. They play a very important role in providing safety critical logic.
  • Relays are used to provide time delay functions. They are used to time the delay open and delay close of contacts.
  • Relays are used to control high voltage circuits with the help of low voltage signals. Similarly they are used to control high current circuits with the help of low current signals.
  • They are also used as protective relays. By this function all the faults during transmission and reception can be detected and isolated.

Relay Selection

You must note some factors while selecting a particular relay. They are
  • Protection – Different protections like contact protection and coil protection must be noted. Contact protection helps in reducing arcing in circuits using inductors.  Coil protection helps in reducing surge voltage produced during switching.
  • Look for a standard relay with all regulatory approvals.
  • Switching time – Ask for high speed switching relays if you want one.
  • Ratings – There are current as well as voltage ratings. The current ratings vary from a few amperes to about 3000 amperes.  In case of voltage ratings, they vary from 300 Volt AC to 600 Volt AC. There are also high voltage relays of about 15,000 Volts.
  • Type of contact used – Whether it is a NC or NO or closed contact.
  • Select Make before Break or Break before Make contacts wisely.
  • Isolation between coil circuit and contacts

Simple UPS

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This is the circuit diagram of a simple UPS that can deliver 12V unregulated and 5V regulated DC. The transformer T1 steps down the mains voltage to 12V AC and then the bridge B1 rectifies it. The rectified signal is smoothed by the capacitor C1.When the mains supply is available the battery will be charged via diode D3 and the regulator IC gets supply via diode D5. 12V and 5V DC will be available at the output terminals. When mains supply is not available the battery supplies current to the regulator IC and to the 12V DC terminal through diode D4.Also, the diode D3 blocks reverse flow of current during battery mode. Capacitors C2 and C3 acts as filters.
Circuit diagram with Parts list.
  • Assemble the circuit on a good quality PCB.
  • The transformer T1 can be a 230V AC primary, 12V secondary,3A step-down transformer.
  • The bridge B1 can be a 2A bridge. If such a bridge is not available, make one using four 1N4007 diodes.
  • The capacitor C1 must be rated at least 25V.

******Related Topic ******

Interfacing dot matrix LED display to 8051

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Interfacing dot matrix LED display to 8051 microcontroller.

LED dot matrix display.
An LED dot matrix display consists of a matrix of  LED’s arranged in a rectangular configuration. The desired character or graphics can be displayed by switching ON /OFF  a desired configuration of LED’s. Common display configurations available are 7×5, 8×8, 7×15, etc. LED dot matrix can be used in simple display applications where the resolution is not a big concern.  The figure below shows the arrangement of LEDs in a typical 7×5 dot matrix display.

7x5 led dot matrix display
7x5 Led dot matrix display
Any individual LED or a group of LEDs in the matrix can be activated by switching the required number of rows and columns. For example, in the above figure if  Row1 is made high and Column1 is made low, the top left LED (address R1C1) will glow. As a demonstration, lets see how we can display letter “A” using the display. The tables given below shows the logic levels at each pin  for displaying A.
led dot matrix displaydot matrix led display
In the above diagram you can see that only one LED in a row will be ON at a time but any number of  LEDs in a column can be ON at a time.  That means the microcontroller’s port pin can directly drive a row but it requires  additional circuit for driving the column lines.  The circuit diagram for interfacing dot matrix display and 8051 microcontroller is shown below.
Circuit diagram.
interfacing dot matrix led display to 8051
ULN2003A driver IC.
The purpose of ULN2003A here is to drive the column lines of the display. ULN2003A is a high voltage (50V), high current (500mA per channel) darlington transistor array. Each IC has 7 channels with individual output clamp diodes. ULN2003A  an active high device, which means a logic high must be applied to the input  to make the corresponding output high. The input pins are designated as 1B, 2B, 3B, 4B, 5B, 6B, 7B while corresponding output pins are designated as 1C, 2C, 3C, 4C, 5C, 6C, 7C.  The pin configuration and simplified internal logic of ULN2003A is shown in the figure below.
uln2003 logic diagram
MOV P3,#00000000B        // initializes port 3 as output port
MOV P1,#00000000B        // initializes port 1 as output port
MAIN: MOV P3,#01111110B
      MOV P1,#11111110B
      MOV P3,#00010001B
      MOV P1,#11111101B
      MOV P3,#00010001B
      MOV P1,#11111011B
      MOV P3,#00010001B
      MOV P1,#11110111B
      MOV P3,#01111110B
      MOV P1,#11101111B
      SJMP MAIN         // jumps back to the main loop
DELAY: MOV R6,#255D     // 1ms delay subroutine