Today LED has become an integral part of consumer electronics.
LED TV, LED Display, LED Lights and so on. These are becoming very popular because of there low power consumption.
What is LED?
LED stands for Light emitting diode.
A light emitting diode is essentially a PN junction semiconductor diode that emits a monochromatic(single) colour light when operated in a forward biased direction.
For detail in technical evolution refer the following url
http://en.wikipedia.org/wiki/Light-emitting_diode
Early LEDs were only bright enough to be used as indicators, or in the displays of early calculators and digital watches. More recently they have been starting to appear in higher brightness applications.
LED Basics – Characteristics voltage drop
When a LED is connected around the correct way in a circuit it develops a voltage across
it called the CHARACTERISTIC VOLTAGE DROP. A LED must be supplied with a voltage that is higher than its “CHARACTERISTIC VOLTAGE” via a resistor – called a VOLTAGE DROPPING RESISTOR or CURRENT LIMITING RESISTOR
How LED works?
LED and resistor are placed in series and connected to a voltage.As the voltage rises from 0v, nothing happens until the voltage reaches about 1.7v. At this voltage a red LED just starts to glow. As the voltage increases, the voltage across the LED remains at 1.7v but the current through the LED increases and it gets brighter. As the current increases to 5mA, 10mA, 15mA, 20mA the brightness will increase and at 25mA, it will be a maximum.
This is just a simple example as each LED has a different CHARACTERISTIC VOLTAGE DROP and a different maximum current.
In the diagram below we see a LED on a 3v supply, 9v supply and 12v supply. The current-limiting resistors are different and the first circuit takes 6mA, the second takes 15mA and the third takes 31mA. But the voltage across the red LED is the same in all cases.
LED Basics – Head Voltage
As the supply-voltage increases, the voltage across the LED will be constant at 1.7v (for a red LED) and the excess voltage will be dropped across the resistor. The supply can be any voltage from 2v to 12 or more. The resistor will drop 0.3v to 10.3v. This is called HEAD VOLTAGE.
The voltage dropped across this resistor, combined with the current, constitutes wasted energy and should be kept to a minimum.
Most supplies are derived from batteries and the voltage will drop as the cells are used.
Here is an example of a problem:
Supply voltage: 12v
7 red LEDs in series = 11.9v
Dropper resistor = 0.1v
As soon as the supply drops to 11.8v, no LEDs will be illuminated.
Example 2:
Supply voltage 12v
5 green LEDs in series @ 2.1v = 10.5v
Dropper resistor = 1.5v
The battery voltage can drop to 10.5v
Suppose the current @ 12v = 25mA.
As the voltage drops, the current will drop.
At 11.5v, the current will be 17mA
At 11v, the current will be 9mA
At 10.5v, the current will be zero
Many batteries drop 1v and still have over 80% of their energy remaining. That’s why you should design your circuit to have a large HEAD VOLTAGE.
Some Basic circuits using LED
1. Polarity Tester
2. Continuity Tester
3. USB Reading Lamp
https://digitalab.org/2012/08/usb-reading-lamp/
I am interested in changing the incandescent bulb in a 18 vdc work light to using a LED as the light source. Your statements about head voltage seem to contradict each other or as least confuse me. You said the circuit should be designed to have a large head voltage but earlier you said the head voltage is wasted and should be kept to a minimum. So what’s a good compromise? The battery is lion 18 volts and a single led rated 3.6-18. volts Thanks Mickey
Why I said HEAD Voltage should be high, is that when battery discharges after some usage, your LED might not light up if current becomes zero. Having a High head voltage be ensure the circuit works longer with the specified battery. But yes, too high HEAD voltage will be wastage of energy. You need to select optimum head voltage – not too high and not too low.
Thanks for your reply. I understood the point you made, I don’t have control over the voltage of the battery or what the LED requires, I thought there may be some other thing, circuity wise, that you would suggest. Thanks again, Mickey