There are several luminous sources to choose from based on the desired applications. >>The incandescent lamps >>The discharge lamps >>The electroluminescent sources LED, OLED.
In this chapter, we shall study the main characteristics of incandescent and discharge light sources.
The current that traverses the filament heats it due to the Joule effect p=R*I2. This results in production [...]
High-power LEDs generate a lot of heat. Therefore it is not advisable to place high power LEDs and resistors on the same circuit for fear of exceeding the working temperature of LEDs.
For this reason, the current for high performance LEDs is stabilized in the source and not in the circuit as is the case for [...]
3.1.1 Simple mounting of LEDs in series without current stabilization (?massive??circuit)
LEDs have very low resistance; the current changes exponentially in relation to the voltage (see figures 11 and 12). LEDs are always placed in series with a current limiting resistor and powered by a stabilized voltage source. Nevertheless, when several LEDs are connected in series [...]
ADVANTAGES
DISADVANTAGES
Good energy output for monochromatic light emission in relation to the incandescent light sources that need filters
High purchase price especially for white and blue
Less heating,even with high performance diodes
Low light efficiency in relation to the fluorescent sources
Long lifespan,less maintenance
Not yet adapted to general illumination:low flux/LED,behavior of luminous characteristics with time
High resistance to impacts and vibrations
Strong [...]
In absence of standardized test procedures, the LED manufacturers give the duration as 100000h, which is 11 years of continuous working with a loss of luminous flux less than 50% . We must recall that the high power diodes were introduced in the market less than 5 years ago.
The manufacturers naturally select the optimal working [...]
2.4.4 Light output The light output of a LED is being constantly improved. For the high power diodes marked these days, the following values are obtained.
Lm/w
Red
Amber
Green
Blue
White
Efficiency
45-50
35-40
38-42
10-12
15-25
The luminous efficiency depends on the source capacity to convey energy into light and also on the ocular perception of colors. Red colors are always better perceived than blue colors [...]
The junction temperature of the semi-conductor has a direct influence on the electrical and luminous characteristics of the LED and on its life span. The junction temperature is a function of the current intensity traversing the diode and of the ambient temperature. Evolution of luminous flux with the temperature
Figure 16: Evolution of luminous flux with [...]
Continuous supply
To prevent the flickering effect and also because reverse voltage acceptable by a LED is relatively weak, the LED is powered with direct current. Chapter 3 of this file is dedicated to power supply of LEDs and to their mounting in series or in parallel. Voltages and intensities, characteristic powers The technical characteristics are [...]
The optical and electrical characteristics of the LED are statistical data whose components taken individually may deviate. When the project groups the LED to get a uniform effect for signalling or lighting, it is advisable to get LEDs from the same production lot.
2.4.1 Type of semi-conductor, color and threshold voltage
The emitted light depends on the [...]
2.3.1 The mineral LED The LEDs is in the form of a component to be mounted on the surface (SMD) or a component to be inserted.
Figure 8: Diagram of the LED to be mounted on the surface (Source LUMIILEDS) Figure 9: Diagram of the LED to be inserted (Source LUMILEDS)
The diode chip is connected to [...]