Working of Diodes

A diode is a specialised electronic component with two electrodes called the anode and the cathode.

 A diode allows current to flow in only ONE direction. If the cathode end (marked with a silver stripe) is connected to the negative and anode is connected to positive then current will flow.
A diode has a forward voltage drop. That means, when current is flowing, the voltage at the anode is always higher than the voltage at the cathode.

 The actual Forward Voltage Drop varies according to the type of diode.
For example: Silicon diode = 0.6 to 0.7v (depending on type) 
                     Schottky diode = 0.3v 
                     Germanium diode = 0.2v

In addition, the voltage drop increases slightly as the current increases so, for example, a silicon rectifier diode might have a forward voltage drop of 0.7v when 100mA is flowing but 1.0 volt when 1 Amp is flowing.

A ZENER diode does not allow current to flow until the voltage on its cathode reaches a value called the "Zener Voltage." At this voltage the diode "breaks down" and a LOT of current will flow and must be restricted by connecting a resistor in series. At this point the supply voltage can increase and the voltage across the zener will remain constant. Values of 2.4 volts to 100 volts or more are common. Zener diodes are used to "clamp" a voltage in order to prevent it rising higher than a certain value. This might be to protect a circuit from damage. Zener diodes are also used to provide a fixed "reference voltage" from a supply that varies. They are widely used in regulated power supply circuits. 


TO KNOW MORE ABOUT ZENER DIODE CLICK HERE

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Introduction to AC and DC

We know that battery or cell gives DC (Direct current) with steady voltage.It is used to power electronics application such as Televisions, mobiles etc.
DC has steady waveform, that is a straight line you can observe it in the below graphical diagram.

DC waveform

Whereas AC means Alternating Current, which means it alternately rises and fall from it highest
positive value to its lowest value. Thus it form a sinusoidal wave in the graphical diagram.

AC sinusoidal waveform

Thus it is important to understand between AC and DC, we cannot connect a 12V application directly to the to the AC that is main power supply it can be hazardous, thus depending upon the countries 110V AC or 230V AC is supplied to us for household application.

Production of AC and DC
In a power station, electricity can be made most easily by using a gas or steam turbine or water impeller to drive a generator consisting of a magnet spinning inside a set of coils. The resultant voltage is always "alternating" by virtue of the magnet's rotation you can observe it in the above AC sinusoidal waveform diagram how the voltage rises positive then goes negative. This alternating voltage can be carried around the country via cables far more effectively than direct current because AC can be passed through a transformer and a high voltage can be reduced to a low voltage, suitable for use in homes.
This DC is produced by several procedure such as Transformation, Rectification, etc.

DC Conversion

The electricity arrives at your house is alternating voltage. Electric light bulbs and toasters can operate perfectly from 230 volts AC. Other equipment such as televisions have an internal power supply which converts the 230 volts AC to a low DC voltage for the electronic circuits. How is this done? There are several ways but the simplest is to use a transformer to reduce the voltage to, say 12 volts AC (above figure of DC conversion). This lower voltage can be fed through a "Rectifier" which combines the negative and positive alternating cycles so that only positive cycles emerge.
TO KNOW MORE ABOUT RECTIFIER CLICK HERE

DC waveform of full wave rectifier

This "rectified" voltage is suitable for powering things like filament bulbs and electric trains but it is still no good for electronic circuits. What we need is "regulated DC" which truly simulates the steady voltage you get from a battery (cell). The first step is to connect a large value capacitor to the output of the rectifier. A capacitor acts as a voltage reservoir and has the effect of smoothing the "ripples".

Rectified waveform (smoothing the ripples)

But the output is still not the same as a battery but it's often good enough for charging batteries in mobile phones, but if you connect it to stereo equipment, you will hear the ripple as an annoying background hum. The final step is to pass this "rippling DC" through a regulator unit. This effectively reduces the ripple to leave almost pure "regulated DC" suitable for powering electronics equipment such as stereos.

Regulated DC power supply

A high-quality Regulated Power supply is capable of supplying 1.5 Amps (1,500 milliamps) of current. A rotary switch provides selection of 3, 4.5, 6, 7.5, 9 or 12 volts "regulated d.c." It has a multitude of uses, including speed control for a mini drill, amplifiers, CD players etc.

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Surface Mount Device (SMD)

Surface Mount Device/components are very small as compared to normal electronics components like resistor, capacitor etc.Surface Mount Device is specially designed for the integration of the circuit i.e to make the circuit as small as possible. Integrated Circuit (IC) is the example of the SMD components, if you open datasheet of any IC you can see the circuit diagram of that particular IC, thus we can see the result of application of SMD components in electronic circuit.


You must have seen the above figure of SMD components, they are so minute that you will need magnifying
glass in order to read there values.
If you are interested to know how to read SMD components like Resistor, Capacitor etc. then click on the below link.
How To Read SMD Resistor.
How To Read SMD Capacitor.

Thus we have seen the size of the SMD components, while reading you must have been thinking about how to solder them or how are they soldered on pcb. Below explained are two methods of soldering surface mount devices.

METHOD 1 : Surface Mount Device are soldered by automatic placement (by Robots) onto a specially designed board. This device are placed on the board and thus held on the board using glue. This board is then exposed to an high temperature air, thus the small amount of solder on the board is sufficient to make the solder connection on each of the pins of the surface mount device. The solder on each of the point is melted and forms a bond with the leads of the component. This method is known as re-flow.


METHOD 2 : Another method of soldering SMD components is by shallow bath of molten solder. The parts are glued in place and then the board is turned over and placed above the solder and a wave is created that just touches the board and at the same time deposits a small amount of solder on each of the points. Surface-mount components are designed to withstand a 10 second immersion during the soldering process.This point is mentioned in order to let you know that the temperature of the solder bath is strictly controlled and the solder-time is kept to a minimum.
 
While in case of an average hobbyist you will be needing three major items,
1. Fine tweezers
2. Very fine tipped soldering iron with adjustable temperature.
3.Fine solder (very very fine).
4.Magnifying glass lamp to help with placement, soldering and inspection.


If you are going to solder your first SMD components i suggest you to first practice desoldering and soldering on a scrap printed circuit board.(Nowadays many electronic items are been made using SMD components, hence you can find them easily.)

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How To Read SMD Capacitor

Surface Mount Capacitor
Surface mount capacitor are usually of brownish or yellowish colour, from the colour one can identify whether given component is capacitor to some extent.

But Most SM capacitors values are not identified in any way by visualising ,most of them think that they are identified according to there size but SIZE OF THE COMPONENTS DOES NOT INDICATE THE CAPACITANCE as each capacitor has a different number of layers according to there manufacturer.

The only way to "read" the value of these SM capacitor is via a capacitance meter.
But some of the SM capacitor has number on there surface as that of SM resistor (A three digit number or alphabets).

Similarly as SM resistor there values are indicated, i.e if the number is 107 then first two digit are the answer and the third digit is the number of zero's.
107 = 10 (two numbers) 0000000 (seven zero's)
These value of capacitor is given in pf, hence the value is 100000000pf = 100uf (100 microfarad).

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How To Read SMD Resistor

We have discussed about SMD component in our last post SURFACE MOUNT DEVICE.
So in this post we are going to discussed about SMD Resistor and to find the value of resistor from the printed numerical code on its surface.

 

Above image has six SMD Resistor, now first of all how can you find whether the given component is resistor or an capacitor.

The answer is from the color of the component, SMD resistor are in black color with resistance printed on there surface.

Now, the resistors in the above image has 3 digit code (some resistor has 4 digit code also).

The code is 103, so the first two digit represent the answer and the last digit represent the number of zero's should be added after the answer.

Example :

103 = 10 (the two number) 000 (three zero's).

Hence the final answer is 10000 Ohms (answer obtain is always in ohms, further we can convert it).

Here are some more example 

334 = 330 000Ohms = 330k

443 = 44 000Ohms = 44k

Now, you can see the above component with code 2512.i.e it has the four digit color coding, the value of this resistor is decoded in the same manner.

The resistor with the value below 10 ohms has letter "R" to denote the position of the decimal point.

Such as 5.7Ω can be written as 5R7

6.2Ω =6R2

2.2Ω =2R2

How To Read SMD Capacitor.(click here)

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