LED Light Sources

3 Reasons why are LEDs being used as a source of light compared to HID lamps

• LEDs don’t have a filament to burn out or break; therefore, they last much longer than conventional bulbs. Given that a very small semiconductor chip runs an LED, they are very durable and tend to last many thousands of hours.
• LEDs are “instant on,” much like halogen lamps, and thus convenient for use in applications that are subject to frequent or potential on-off cycling. Conversely, HID lamps are more fragile and have to warm up (15 – 25 seconds) during ignition
• LED technology is changing rapidly and it is foreseeable that LEDs will eventually surpass the performance of HID lighting.

How is LED used as a source of light?

Light-Emitting Diodes (LED) are light sources utilizing diodes that emit light when connected in a circuit. The effect is a form of electro luminescence where LEDs release a large number of photons outward; the LED is housed in a plastic bulb, which concentrates the light source.The most important part of an LED is the semi-conductor chip located in the center of the light source. It consists of p and n regions with a junction between them.The p region is dominated by positive electric charges and the n region is dominated by negative electric charges. The junction is a kind of wall between the two regions, blocking the passage of charge carriers between the two regions.

When sufficient voltage is applied to the semi-conductor chip, electrons can move easily across the junction where they are immediately attracted to the positive forces in the p region. When an electron moves sufficiently close to a positive charge in the p region, the two charges “re-combine”.

When an electron combines with a positive ion, the electric potential energy gets converted to electromagnetic energy and this occurs in form of emission of a photon of light.

This photon has a frequency determined by the characteristics of the semiconductor material (usually a combination of the chemical elements gallium, arsenic, and phosphorus).

LEDs that emit different colors are made of different semiconductor materials. In simple words, LEDs are small electrical circuit components which act as a bulb without any filament.LEDs are illuminated solely by the movement of electrons in a semiconductor material, making them energy efficient and extremely resilient over long periods of time.

What is the type of LED used in lightening applications?

The White LED is the most exciting new lighting technology since the introduction of the fluorescent lamps. High intensity LEDs are now popular because of their high low current requirements. High bright White LEDs are increasingly used in lighting application as a measure to conserve energy. Applications of white LED includes back light illumination in mobile phones, LCD back lights, home and vehicle lighting, display boards etc. The LEDs are extremely small in size and consume little energy and efficiently converts electricity into light.

Why is the White LED preferred?

A high watt LED usually produces around 75-100 lumens per watt at the expense of 350 milli ampere current. Energy loss through heat is practically nil in LEDs and they have very long life of thousands of hours. LEDs are eco friendly devices since they contain no lead or mercury. Unlike fluorescent lamps, LEDs do not emit UV rays.

A1 watt White LED gives around100 lm luminous flux which is sufficient to light a confined area. The amount of light emitted by a light source is measured in terms of lumen. For instance, a 60 Watt bulb emits 730 lumen and that by a 50 Watt halogen lamp is 900 lumen Each LED chip has an area of only one square millimeter, which makes for very concentrated overall luminosity

The forward voltage drop of the 1 watt white LED is 3.3 volts and consumes 350 milli amperes current. Thus Ordinary white LED requires 3 volts and around 40 mA current to give maximum brightness.

High power White LEDs can be driven at high current of hundreds of milli ampere to an Ampere to produce high intensity light. Some makes can produce over a thousand lumens. The HPLEDs must be mounted on a heat sink to allow for heat dissipation otherwise the device will damage easily.

2 ways to construct white LED

• The method involves coating an LED chip (mostly blue made of Indium Gallium Nitride, InGaN) with phosphor of different colors to produce the white light. The part of the light spectrum corresponding to the blue light gets transferred to the longer wavelength, through a process known as Stokes’ shift. The White light of white LEDs comes from the narrow-band blue naturally emitted by GaN LEDs, plus a broad spectrum yellow generated by a phosphor coating on the die which absorbs a proportion of the blue and converts it to yellow. InGaN can produce operational wavelengths from green to ultra-violet by varying the relative amounts of indium and gallium during production.The commonly used phosphor material is Cerium- doped Yttrium aluminum Garnet or Ce3+ YAG.
• White LEDs can also be made by coating near UV emitting LEDs with a mixture of high efficiency Europium based red and blue emitting phosphors plus green emitting copper and aluminum doped zinc sulfide . This is a method similar to the working of Fluorescent lamps. This method is less efficient than the blue LED with phosphor, since the Stokes shift is larger and more energy is therefore converted to heat. Another drawback is that, UV light may leak from a malfunctioning LED and cause harm to human eyes or skin.

3 Applications involving LED as a source of light

• LED based Emergency lamp

It can be plugged into the AC socket and it lights at the moment of power failure. It emits  cool white light in the room which is sufficient for reading purpose also. It is powered by a 4.5 volt cordless phone battery and uses 1 watt White LED.

Working of the system

The circuit uses a small 4.5 volts 300 mA stepdown transformer, a bridge rectifier comprising D1 through D4 and the smoothing capacitor C1 as the power supply to charge the 4.5 volts rechargeable battery. The 4.5 volt transformer is readily available and is generally used in LED emergency lights. It drops the 230 volts AC to low volt AC which is then rectified by the full wave rectifier. Capacitor C1 makes the DC ripple free for charging. Resistor R1 provides around 80 milli ampere current for charging. Since the circuit is always plugged in, low current is ideal for charging. The rechargeable battery used in the circuit is a 4.5 volts cordless phone battery.
When the mains power is available, battery charges via R1 and D5. At the same time, the PNP transistor gets positive bias through R1 and it remains off. When the power fails, D5 reverse biases and the base of T1 will become negative. It then conducts and light the LED. When the power resumes, LED automatically turns off.

How to Set?

The circuit can be assembled on a small piece of perf board. Enclose the circuit, transformer and the battery in a plug in type adapter box. Fix LED outside the box preferably with a reflective back ground. You can paste Foil paper for this purpose.It will also act as a heat sink for the LED. Since high current flows through T1 when the LED lights, T1 requires a heat sink. 1 Watt white LED cost around Rs.50 and the battery Rs.150.

•  Automatic Lamp using 1 W LED

Here is an Automatic Lamp using high power White LED that turns on in the evening and stays on till morning. White LEDs are being used these days as they need a simple DC power supply to power and control them, in contrast to incandescent gas discharge lamps. The LED used has a rating of 1 W and regulated power supply without transformer is used to power the device.

Working of the System

The capacitors c1 and c2 drop the AC voltage to a lower value. Normally high value capacitors are used. The bridge rectifier section produces a pulsating DC output from the ac signal. The capacitor removes the ac ripples present in the dc signal. This filtered signal is applied across the zener diode, which provides a regulated DC output across its terminal. The capacitor c3 generally allows ac signal to pass through it and blocks the DC signal. It acts a bypass element. LDR and T1 form a light sensitive switch to turn on / off the LED. LDR offers high resistance in dark and low resistance in light. So during day time, LDR conducts and the Base of T1 will be pulled to ground potential and it remains off to switch off the White LED. During night time, when the day light ceases, resistance of LDR increases and T1 gets sufficient base bias and conducts. The LED connected to the collector T1 then lights. BD 139 is a medium power NPN transistor and its pins connections are Base – Collector – Emitter from front side. Heat sink is necessary for T1 since the LED passes around 100milliamperes current. LED used in the circuit is the 1 watt white LED that has a forward voltage drop of 3.3 volts and requires 350milliampere current to get full brightness.

• Portable Emergency Lamp cum Mobile charger

Here is Portable Emergency Lamp cum Mobile charger. It uses a high bright White LED that can give sufficient light in the room when the mains power fails. A white LED can be an Indium Gallium Nitride LED which emits a bluish white light, which can be filtered using a phosphorus filter in the lens or using a mixture of multi colored LEDs. It can be also used as a Mobile charger in the plug in mode as well during journey.

Working of the System

Power supply for the circuit is derived from a 0-6 volt 300 mA step-down transformers, a full wave rectifier comprising D1 through D4 and the smoothing capacitor C1. The Emergency light circuit consists of diode D5, resistor R2 and the Transistor T1.When the mains power is available, diode D5 forward biases and the 6 volt battery charges through resistor R2.At the same time, the PNP transistor T1 will be out of conduction since its base is kept high through R1. The white LED connected to the collector of T1 remains off. When the mains power fails, D5 reverse biases and the base of T1 gets a negative bias and it conducts. White LED then turns on using the power from the battery.The Mobile charger section of the circuit consists of a Zener based regulated power supply. Charging current can be tapped from the points A and B using suitable connector.

Now i hope you have got an idea about the concept of the LED light sources and if you have any queries from the article and from the electrical electronic projects leave the comments section below.