Explaining solar cells

As renewable energy is becoming integrated into our everyday lives, new terms such as solar panel, photovoltaic and solar cell are more common and new devices, such as outdoor LED lighting are using this technology. The sun emits many forms of radiation. The best way to describe this is that there are ‘waves’ of energy that radiate from the sun at different frequencies.
This is only partially the truth as there is both a wave and particle nature to light.
The light spectrum is divided into different sections. It begins with the highest, gamma rays and ends with the lowest, long wave radio. Only a small portion of this is visible, called the visible spectrum and this occurs towards the middle of the range which lies between Ultraviolet and Infrared frequencies. Ultraviolet radiation is what burns the skin and can cause skin cancer. It is blocked by most types of glass and is partially reduced by the atmosphere especially the ozone layer. Infrared radiation is what provides the earth with heat and it is that which is trapped by green house gasses, carbon dioxide mainly and is causing global warming.
Infrared radiation is targeted by solar panels. This basically uses the energy generated by the radiation to heat water in pipes that flows and generates electricity. This can be used to charge a battery which could then power said LED lighting. As mentioned previously there is a dual nature to light. It consists of both a particle and a wave. It might help to think of the particles moving in a wave like pattern but the reality is more complex than that. The important thing to remember is that the light particle, the photon, is what is targeted by a solar cell.
Generally speaking the solar cell works by providing energy to a semiconducting material, most commonly silicon, so that electrons within the material are released from the bonds to their atoms in the semiconductor.
The arrangement of the cell into strips of conductor and semiconductor allow these freed electrons to move. They move in a directed manner away from the incoming energy, the photons, creating a flow of electrons more commonly known as current.
A high incoming rate of photons is required to release the electrons. This creates problems as much of the higher energy (higher frequency) waves emitted by the sun are blocked; the glass protective covering reflects light requiring anti-reflection membranes, glass blocks ultraviolet and the lower range of frequencies like infrared do not have enough energy to have much of an effect on the panels. Thus these panels only really target the visible spectrum which is only a small proportion of the sun’s energy.
Yet, with the improvements to the semiconductors, the anti reflection layers and the methods of directing the released electrons the efficiency of solar cells has dramatically improved. Huge fields of cells are being created in deserts and mountainous regions that can now produce kilowatts of energy.
Combined with the improvements of energy efficient products, such as LED lighting, this is becoming a valuable resource. In fact, the low energy of LED lighting is one of the most important improvements as it helps to alleviate the greatest weakness of solar cells – night time.
source: electronics-lab
Tags: cells, Led, Photovoltaics, Solar 

testing an unknown transistor

TESTING AN unknown TRANSISTOR

The first thing you may want to do is test an unknown transistor for COLLECTOR, BASE AND EMITTER. You also need to know if it is NPN or PNP.
You need a cheap multimeter called an ANALOGUE METER  - a multimeter with a scale and pointer (needle).
It will measure resistance values (normally used to test resistors) - (you can also test other components) and Voltage and Current. We use the resistance settings. It may have ranges such as "x10"  "x100"   "x1k"   "x10"
Look at the resistance scale on the meter. It will be the top scale.
The scale starts at zero on the right and the high values are on the left. This is opposite to all the other scales. .
When the two probes are touched together, the needle swings FULL SCALE and reads "ZERO." Adjust the pot on the side of the meter to make the pointer read exactly zero.

How to read:  "x10"  "x100"   "x1k"   "x10"
Up-scale from the zero mark is "1" 
When the needle swings to this position on the "x10" setting, the value is 10 ohms.
When the needle swings to "1" on the "x100" setting, the value is 100 ohms.
When the needle swings to "1" on the "x1k" setting, the value is 1,000 ohms = 1k.
When the needle swings to "1" on the "x10k" setting, the value is 10,000 ohms = 10k.
Use this to work out all the other values on the scale.
Resistance values get very close-together (and very inaccurate) at the high end of the scale. [This is just a point to note and does not affect testing a transistor.]
Step 1   - FINDING THE BASE  and determining NPN or PNP
Get an unknown transistor and test it with a multimeter set to "x10"
Try the 6 combinations and when you have the black probe on a pin and the red probe touches the other pins and the meter swings nearly full scale, you have an NPN transistor. The black probe is BASE
If the red probe touches a pin and the black probe produces a swing on the other two pins, you have a PNP transistor. The red probe is BASE
If the needle swings FULL SCALE or if it swings for more than 2 readings, the transistor is FAULTY. 

Step 2   - FINDING THE COLLECTOR and EMITTER
Set the meter to "x10k." 
For an NPN transistor, place the leads on the transistor and when you press hard on the two leads shown in the diagram below, the needle will swing almost full scale.   

For a PNP transistor, set the meter to "x10k"  place the leads on the transistor and when you press hard on the two leads shown in the diagram below, the needle will swing almost full scale.   

What is Solar Power?

what is solar energy?

Solar power is energy generated from the heat or light from the sun which can be used to produce heat, light, hot water, electricity, and cooling in a wide variety of applications .  When we say something is solar powered, we mean that the energy it uses was converted directly from solar energy or sunlight energy. Solar energy is often referred to as an "alternative energy" to fossil fuel energy sources such as oil and coal.

The history of photovoltaic technology

We've used the Sun for drying clothes and food for thousands of years, but only recently have we been able to use it for generating power.
The history of photovoltaic technology goes back more than one hundred years but it wasn't until the middle of the 20th century that scientists at Bell Telephone found that an element called silicon produced an electrical charge when exposed to sunlight. However, the earliest solar cells were expensive and not very efficient, converting only a fraction of the sun's light into electric current. Today, solar panel technology has vastly improved efficiency.

What is solar power offering that makes it such an appealing energy source?

•Solar energy is a completely free and inexhaustible fuel source
•No fuel, waste, or pollution is expelled in its usage.
•In remote areas, or small villages, solar power can be the saving grace. Sometimes it is the only realistic way to provide energy to a place that is not capable of drawing energy from other sources.
•It can be used for low-power purposes as well as larger ones- from battery chargers, hand-held calculators, and solar powered garden lights to air conditioning, cars, and satellites.
•Solar energy can be an integral part of any combination of clean, renewable energy sources to meet the nation's need for electricity while reducing harmful greenhouse gas emissions.
tags: solar power, solar panels, solar power systems, home solar energy, solar cell

Light Emitting Diode Part IV

THE RESISTOR
The value of the current limiting resistor can be worked out by Ohms Law.
Here are the 3 steps:
1. Add up the voltages of all the LEDs in a string.   e.g:  2.1v + 2.3v + 2.3v + 1.7v = 8.4v
2. Subtract the LED voltages from the supply voltage.  e.g:  12v - 8.4v = 3.6v
3. Divide the 3.6v (or your voltage) by the current through the string. 
for 25mA:   3.6/.025 =144 ohms
for 20mA:   3.6/.02  = 180 ohms
for 15mA:   3.6/.015 = 250 ohms
for 10mA:   3.6/.01   = 360 ohms
This is the value of the current-limiting resistor.

Here is a set of strings for a supply voltage of 3v to 12v and a single LED:

Here is a set of strings for a supply voltage of 5v to 12v and a white LED: 

Here is a set of strings for a supply voltage of 5v to 12v and two LEDs:

 Part I ... Part II ... Part III ... Part IV

Light Emitting Diode Part III

LEDs ARE CURRENT DRIVEN DEVICES
A LED is described as a CURRENT DRIVEN DEVICE.  This means the illumination is determined by the amount of current flowing through it.
The brightness of a LED can be altered by increasing or decreasing the current. The effect will not be linear and it is best to experiment to determine the best current-flow for the amount of illumination you want. High-bright LEDs and super-bright LEDs will illuminate at 1mA or less, so the quality of a LED has a lot to do with the brightness. The life of many LEDs is determined at 17mA. This seems to be the best value for many types of LEDs.

1mA to 5mA LEDs
Some LEDs will produce illumination at 1mA. These are "high Quality" or "High Brightness" LEDs and the only way to check this feature is to test them @1mA as shown below. 

THE 5v LED 
Some suppliers and some websites talk about a 5v white or blue LED. Some LEDs have a small internal resistor and can be placed on a 5v supply. This is very rate.
Some websites suggest placing a white LED on a 5v supply. These LEDs have a characteristic voltage-drop of 3.6v and should not be placed directly on a voltage above this value.
The only LED with an internal resistor is a FLASHING LED. These LEDs can be placed on a supply from 5v to 12v and flash at approx 2Hz.
NEVER assume a LED has an internal resistor. Always add a series resistor. Some high intensity LEDs are designed for 12v operation. These LEDs have a complete internal circuit to deliver the correct current to the LED. This type of device is not covered in this eBook.

LEDs IN SERIES
LEDs can be placed in series providing some features are taken into account. The main item to include is a current-limiting resistor.
A LED and resistor is called a string. A string can have 1, 2, 3 or more LEDs.
Three things must be observed:
1. MAXIMUM CURRENT through each string = 25mA.
2. The CHARACTERISTIC VOLTAGE-DROP must be known so the correct number of LEDs are used in any string.
3. A DROPPER RESISTOR must be included for each string.
The following diagrams show examples of 1-string, 2-strings and 3-strings: 

LEDs IN PARALLEL
LEDs CANNOT be placed in parallel - until you read this:
LEDs "generate" or "possess" or "create" a voltage across them called the CHARACTERISTIC VOLTAGE-DROP  (when they are correctly placed in a circuit).
This voltage is generated by the type of crystal and is different for each colour as well as the "quality" of the LED (such as high-bright, ultra high-bright etc). This characteristic cannot be altered BUT it does change a very small amount from one LED to another in the same batch. And it does increase slightly as the current increases.
For instance, it will be different by as much as 0.2v for red LEDs and 0.4v for white LEDs from the same batch and will increase by as much as 0.5v when the current is increased from a minimum to maximum.
You can test 100 white LEDs @15mA and measure the CHARACTERISTIC VOLTAGE-DROP to see this range.
If you get 2 LEDs with identical CHARACTERISTIC VOLTAGE-DROP, and place them in parallel, they will each take the same current. This means 30mA through the current-limiting resistor will be divided into 15mA for each LED.
However if one LED has a higher CHARACTERISTIC VOLTAGE-DROP, it will take less current and the other LED will take considerably more. Thus you have no way to determine the "current-sharing"  in a string of parallel LEDs.  If you put 3 or more LEDs in parallel, one LED will start to take more current and will over-heat and you will get very-rapid LED failure.  As one LED fails, the others will take more current and the rest of the LEDs will start to self-destruct.
Thus LEDs in PARALLEL should be avoided.
Diagram A below shows two green LEDs in parallel. This will work provided the Characteristic Voltage Drop across each LED is the same.
In diagram B the Characteristic Voltage Drop is slightly different for the second LED and the first green LED will glow brighter.
In diagram C the three LEDs have different Characteristic Voltage Drops and the red LED will glow very bright while the other two LEDs will not illuminate. All the current will pass through the red LED and it will be damaged.
The reason why the red LED will glow very bright is this: It has the lowest Characteristic Voltage Drop and it will create a 1.7v for the three LEDs. The green and orange LEDs will not illuminate at this voltage and thus all the current from the dropper resistor will flow in the red LED and it will be destroyed.

 Part I ... Part II ... Part III ... Part IV

tags: devices, led parallel circuit, voltage