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

Light Emitting Diode Part II

The voltage dropped across this resistor, combined with the current, constitutes wasted energy and should be kept to a minimum, but a small HEAD VOLTAGE is not advisable (such as 0.5v). The head voltage should be a minimum of 1.5v - and this only applies if the supply is fixed.
The head voltage depends on the supply voltage. If the supply is fixed and guaranteed not to increase or fall, the head voltage can be small (1.5v minimum).
But 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
But let's look at the situation more closely.
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
You can see the workable supply drop is only about 1v.
Many batteries drop 1v and still have over 80% of their energy remaining. That's why you need to design your circuit to have a large HEAD VOLTAGE.
TESTING A LED 
If the cathode lead of a LED cannot be identified, place 3 cells in series with a 220R resistor and illuminate the LED.  4.5v allows all types of LEDs to be tested as white LEDs require up to 3.6v.  Do not use a multimeter as some only have one or two cells and this will not illuminate all types of LEDs. In addition, the negative lead of a multimeter is connected to the positive of the cells (inside the meter) for resistance measurements - so you will get an incorrect determination of the cathode lead. 

CIRCUIT TO TEST ALL TYPES OF LEDs

IDENTIFYING A LED
 A LED does not have a "Positive" or "Negative" lead. It has a lead identified as the "Cathode" or Kathode" or "k". This is identified by a flat on the side of the LED and/or by the shortest lead.
This lead goes to the 0v rail of the circuit or near the 0v rail (if the LED is connected to other components).
Many LEDs have a "flat" on one side and this identifies the cathode. Some surface-mount LEDs have a dot or shape to identify the cathode lead and some have a cut-out on one end.
Here are some of the identification marks:  

 

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

tags:  voltage, battery voltage

Light Emitting Diode

CONNECTING A LED
A LED must be connected around the correct way in a circuit and it must have a resistor to limit the current.
The LED in the first diagram does not illuminate because a red LED requires 1.7v and the cell only supplies 1.5v. The LED in the second diagram is damaged because it requires 1.7v and the two cells supply 3v. A resistor is needed to limit the current to about 25mA and also the voltage to 1.7v, as shown in the third diagram.  The fourth diagram is the circuit for layout #3 showing the symbol for the LED, resistor and battery and how the three are connected. The LED in the fifth diagram does not work because it is around the wrong way.  

CHARACTERISTIC 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 - so the LED will operate correctly and provide at least 10,000 to 50,000 hours of illumination.
A LED works like this:  A 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.
We now turn our attention to the current though the LED.  As the current increases to 5mA, 10mA, 15mA, 20mA the brightness will increase and at 25mA, it will be a maximum. Increasing the supply voltage will simply change the colour of the LED slightly but the crystal inside the LED will start to overheat and this will reduce the life considerably.
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. This is because the LED creates the CHARACTERISTIC VOLTAGE DROP and this does not change. 

It does not matter if the resistor is connected above or below the LED. The circuits are the SAME in operation:

HEAD VOLTAGE
Now we turn our attention to the resistor.
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 12v or more.
In this case, the resistor will drop 0.3v to 10.3v.
This is called HEAD VOLTAGE - or HEAD-ROOM or OVERHEAD-VOLTAGE.
The following diagram shows HEAD VOLTAGE:

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

tags:  Light Emitting Diode, diode, led power supplies

circuit produces 12 different sequences including flashing, chasing, police lights and flicker.

This circuit uses the latest  TE555-5 LED FX chip from Talking Electronics. This 8-pin chip and drives 3 LEDs. The circuit can be assembled on matrix board.
The circuit produces 12 different sequences including flashing, chasing, police lights and flicker.
It also has a feature where you can create your own sequence and it will show each time the chip is turned on.

LED Torch

A common problem with small torches is the short life-span both of the batteries and the bulb. The average incandescent torch, for instance, consumes around 2 Watts. The LED Torch in Fig. 1 consumes just 24 mW, giving it more than 80 times longer service from 4 AA alkaline batteries (that is, up to one month's continuous service). Although the torchs light output is modest, it is nonetheless quite sufficient to illuminate a pathway for walking.
The LED Torch is based on a 7555 timer running in astable mode (do not use an ordinary 555). A white LED (Maplin order code NR73) produces 400 mcd light output, which, when focussed, can illuminate objects at 30 metres. Try Conrad Electronic for what appears to be a stronger white LED (order code 15 37 45-11).
A convex lens with short focal length is placed in front of the LED to focus the beam. If banding occurs at the beams perimeter, use another very short focal length lens directly in front of the LED to smooth the beam.
If a different supply voltage is preferred, the value of resistor R3 is modified as follows:
9V - 470 Ohm
12V - 560 Ohm

Simple two-transistor circuit lights LEDs

A previous Design Idea describes a circuit that uses an astable multivibrator to drive an LED (Reference). The circuit in Figure 1 uses a simpler alternative approach. The circuit uses a 2N3904 NPN transistor and a 2N3906 PNP transistor, which operate as a high-gain amplifier.



Figure 1. This simple astable multivibrator provides a low-cost way to drive an LED from a single cell.

The 1-MΩ resistor supplies bias current. The 1-kΩ resistor helps linearize the oscillator waveform into one that is close to a square wave with about a 50-to-50 duty cycle. The capacitor supplies positive feedback from the output of the amplifier to the noninverting input. The frequency of oscillation depends mostly on the RC constant of the feedback capacitor and the input-stage impedance. The circuit oscillates at 91 kHz with a 48% duty cycle. You can use almost any common NPN or PNP transistors, as long as they have moderate forward-current gain of 50 or more and can handle 100-mA collector currents.

The LED connects across the output transistor because this approach lets the inductive kickback voltage add to the battery-supply voltage and makes the LED brighter. This circuit operates well from approximately 0.8 to 1.6 V, which is the useful range of an alkaline battery. The LED-light output decreases as the supply voltage decreases from 1.6 to 0.8 V.

Reference

1. Bruno, Luca, “Astable multivibrator lights LED from a single cell,” EDN, Aug 21, 2008, pg 53.

Regulated Dual White LED Lamp

(C) G. Forrest Cook 2008

This project can be used with a CirKits solar circuit kit.

Regulated Dual White LED Lamp

Introduction

This is an ultra-simple LED lamp made with two white LEDs. It is suitable for use in both 12V solar powered and automotive applications. Only seven components are used in this circuit. It produces regulated light output from 11V to 20V. The circuit board can be potted in silicone to make the lamp completely water proof.

Specifications

Nominal Operating Voltage: 12V DC Regulated Light Voltage Range:

11-20V Operating Current: 20ma 

Theory

The input power is filtered through a pi filter consisting of two 100nF capacitors and a 100 ohm resistor, this removes voltage spikes from the rest of the circuitry. The LM317L and 56 ohm resistor act as a current regulator that is set to 20ma. The current regulator is wired in series between the power source and the LEDs to provide a constant current.

Construction

A small circuit board was made using press-n-peel blue film, the board was sized to fit inside of a 1/2" PVC pipe connector. The parts were soldered into the circuit board and a length of two conductor speaker wire was soldered to the board for the power lead. A knot was tied in the power cable to act as a strain relief. The power cable was fed through a hole in the PVC connector. The entire assembly was filled with clear GE Silicone II caulk and left to dry. Be sure to allow the caulk to dry for several days in a warm place before applying power. Another brand of bathtub caulk was tried, but the caulk was electrically conductive and the circuit quickly failed.

Use

Connect this circuit to a 12V battery or power supply, be sure to observe the correct polarity. The LEDs should put out a bright white light. This light can be used for a night light, a flash light, automotive interior lights and background house lighting. The low current draw allows it to run for many hours on a battery.

Parts

2x white LEDs, T1-3/4 size 1x 56 ohm 1/4 W resistor 1x 100 ohm 1/4 W resistor

2x 0.1uF capacitors 1x LM317L adjustable voltage regulator 1x 1/2" Schedule 40 PVC

 pipe junction GE Silicone II caulk Two conductor speaker wire

CAD Files
EAGLE CAD schematic  
EAGLE CAD board layout  
PostScript file of PC Board

website : http://www.solorb.com/

Fading LEDs Circuit

Two strips of LEDs fading in a complementary manner

9V Battery-operated portable unit

Circuit diagram

Parts:
R1,R2 4K7 1/4W Resistors
R3 22K 1/4W Resistor
R4 1M 1/4W Resistor (See Notes)
R5 2M2 1/4W Carbon Trimmer (See Notes)
R6,R10,R11,R14,R15 10K 1/4W Resistors
R7,R8 47K 1/4W Carbon Trimmers (See Notes)
R9,R13 27K 1/4W Resistors
R12,R16 56R 1/4W Resistors
C1 1?F 63V Polyester Capacitor
C2 100?F 25V Electrolytic Capacitor
D1-D4 etc 5 or 3mm. LEDs (any type and color) (See Notes)
IC1 LM358 Low Power Dual Op-amp
Q1,Q2,Q4 BC327 45V 800mA PNP Transistors
Q3,Q5,Q6 BC337 45V 800mA NPN Transistors
SW1 SPST miniature Slider Switch
B1 9V PP3 Battery
Clip for PP3 Battery

Device purpose:
This circuit operates two LED strips in pulsing mode, i.e. one LED strip goes from off state, lights up gradually, then dims gradually, etc. while the other LED strip do the contrary.
Each strip can be made up from 2 to 5 LEDs at 9V supply.

Circuit operation:
The two Op-Amps contained into IC1 form a triangular wave generator. The rising and falling voltage obtained at pin #7 of IC1 drives two complementary circuits formed by a 10mA constant current source (Q1, Q2 and Q5, Q6) and driver transistor (Q3 and Q6).
R4, R5 & C1 are the timing components: the total period can be varied changing their values. R7 & R8 vary the LEDs brightness.

Notes:
For those whishing to avoid the use of trimmers, suggested values for a 9V supply are:
R4=3M9, R9 & R13=47K and trimmers replaced by a short.
Whishing to use a wall-plug transformer-supply instead of a 9V battery, you can supply the circuit at 12V, allowing the use of up to 6 LEDs per strip, or at 15V, allowing the use of up to 7 LEDs per strip.
In this case, the value of the trimmers R7 & R8 should be changed to 100K.

author:RED Free Circuit Designs,
website: http://www.redcircuits.com

AC Powered LED Schematic

AC Powered LED Schematic

LEDs

Light Emitting Diodes (LEDs)

Example: Circuit symbol:

Function

LEDs emit light when an electric current passes through them.

Connecting and soldering

LEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method).
LEDs can be damaged by heat when soldering, but the risk is small unless you are very slow. No special precautions are needed for soldering most LEDs.

Testing an LED

Never connect an LED directly to a battery or power supply!
It will be destroyed almost instantly because too much current will pass through and burn it out.
LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1k resistor is suitable for most LEDs if your supply voltage is 12V or less. Remember to connect the LED the correct way round!
----------------------

Colours of LEDs

LEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours.
The colour of an LED is determined by the semiconductor material, not by the colouring of the 'package' (the plastic body). LEDs of all colours are available in uncoloured packages which may be diffused (milky) or clear (often described as 'water clear'). The coloured packages are also available as diffused (the standard type) or transparent.

Tri-colour LEDs

The most popular type of tri-colour LED has a red and a green LED combined in one package with three leads. They are called tri-colour because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on.
The diagram shows the construction of a tri-colour LED. Note the different lengths of the three leads. The centre lead (k) is the common cathode for both LEDs, the outer leads (a1 and a2) are the anodes to the LEDs allowing each one to be lit separately, or both together to give the third colour.

Bi-colour LEDs

A bi-colour LED has two LEDs wired in 'inverse parallel' (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time and they are less useful than the tri-colour LEDs described above.

---

Sizes, Shapes and Viewing angles of LEDs

LED Clip

LEDs are available in a wide variety of sizes and shapes. The 'standard' LED has a round cross-section of 5mm diameter and this is probably the best type for general use, but 3mm round LEDs are also popular.
Round cross-section LEDs are frequently used and they are very easy to install on boxes by drilling a hole of the LED diameter, adding a spot of glue will help to hold the LED if necessary. LED clips are also available to secure LEDs in holes. Other cross-section shapes include square, rectangular and triangular.
As well as a variety of colours, sizes and shapes, LEDs also vary in their viewing angle. This tells you how much the beam of light spreads out. Standard LEDs have a viewing angle of 60° but others have a narrow beam of 30° or less.

Calculating an LED resistor value

An LED must have a resistor connected in series to limit the current through the LED, otherwise it will burn out almost instantly.
The resistor value, R is given by:

R = (VS - VL) / I

V_S = supply voltage
V_L = LED voltage (usually 2V, but 4V for blue and white LEDs)
I = LED current (e.g. 20mA), this must be less than the maximum permitted
If the calculated value is not available choose the nearest standard resistor value which is greater, so that the current will be a little less than you chose. In fact you may wish to choose a greater resistor value to reduce the current (to increase battery life for example) but this will make the LED less bright.

For example

If the supply voltage V_S = 9V, and you have a red LED (V_L = 2V), requiring a current I = 20mA = 0.020A,
R = (9V - 2V) / 0.02A = 350, so choose 390 (the nearest standard value which is greater).

Working out the LED resistor formula using Ohm's law

Ohm's law says that the resistance of the resistor, R = V/I, where:
V = voltage across the resistor (= V_S - V_L in this case)
I = the current through the resistor
So R = (V_S - V_L) / I
----------------

Connecting LEDs in series

If you wish to have several LEDs on at the same time it may be possible to connect them in series. This prolongs battery life by lighting several LEDs with the same current as just one LED.
All the LEDs connected in series pass the same current so it is best if they are all the same type. The power supply must have sufficient voltage to provide about 2V for each LED (4V for blue and white) plus at least another 2V for the resistor. To work out a value for the resistor you must add up all the LED voltages and use this for V_L.
Example calculations:
A red, a yellow and a green LED in series need a supply voltage of at least 3 × 2V + 2V = 8V, so a 9V battery would be ideal.
V_L = 2V + 2V + 2V = 6V (the three LED voltages added up).
If the supply voltage V_S is 9V and the current I must be 15mA = 0.015A,
Resistor R = (V_S - V_L) / I = (9 - 6) / 0.015 = 3 / 0.015 = 200,
so choose R = 220 (the nearest standard value which is greater).

--------------------

Avoid connecting LEDs in parallel!

Connecting several LEDs in parallel with just one resistor shared between them is generally not a good idea.
If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it. Although identical LEDs can be successfully connected in parallel with one resistor this rarely offers any useful benefit because resistors are very cheap and the current used is the same as connecting the LEDs individually. If LEDs are in parallel each one should have its own resistor.

---

Reading a table of technical data for LEDs

Suppliers' catalogues usually include tables of technical data for components such as LEDs. These tables contain a good deal of useful information in a compact form but they can be difficult to understand if you are not familiar with the abbreviations used.
The table below shows typical technical data for some 5mm diameter round LEDs with diffused packages (plastic bodies). Only three columns are important and these are shown in bold. Please see below for explanations of the quantities.

Type	Colour	IF max.	VF typ.	VF max.	VR max.	Luminous intensity	Viewing angle	Wavelength
Standard	Red	30mA	1.7V	2.1V	5V	5mcd @ 10mA	60°	660nm
Standard	Bright red	30mA	2.0V	2.5V	5V	80mcd @ 10mA	60°	625nm
Standard	Yellow	30mA	2.1V	2.5V	5V	32mcd @ 10mA	60°	590nm
Standard	Green	25mA	2.2V	2.5V	5V	32mcd @ 10mA	60°	565nm
High intensity	Blue	30mA	4.5V	5.5V	5V	60mcd @ 20mA	50°	430nm
Super bright	Red	30mA	1.85V	2.5V	5V	500mcd @ 20mA	60°	660nm
Low current	Red	30mA	1.7V	2.0V	5V	5mcd @ 2mA	60°	625nm

IF max.	Maximum forward current, forward just means with the LED connected correctly.
VF typ.	Typical forward voltage, VL in the LED resistor calculation. This is about 2V, except for blue and white LEDs for which it is about 4V.
VF max.	Maximum forward voltage.
VR max.	Maximum reverse voltage You can ignore this for LEDs connected the correct way round.
Luminous intensity	Brightness of the LED at the given current, mcd = millicandela.
Viewing angle	Standard LEDs have a viewing angle of 60°, others emit a narrower beam of about 30°.
Wavelength	The peak wavelength of the light emitted, this determines the colour of the LED. nm = nanometre.

---

Flashing LEDs

Flashing LEDs look like ordinary LEDs but they contain an integrated circuit (IC) as well as the LED itself. The IC flashes the LED at a low frequency, typically 3Hz (3 flashes per second). They are designed to be connected directly to a supply, usually 9 - 12V, and no series resistor is required. Their flash frequency is fixed so their use is limited and you may prefer to build your own circuit to flash an ordinary LED, for example our Flashing LED project which uses a 555 astable circuit.

-------------------

LED Displays

LED displays are packages of many LEDs arranged in a pattern, the most familiar pattern being the 7-segment displays for showing numbers (digits 0-9). The pictures below illustrate some of the popular designs:

Bargraph	7-segment	Starburst	Dot matrix

Pin connections of LED displays

Pin connections diagram

There are many types of LED display and a supplier's catalogue should be consulted for the pin connections. The diagram on the right shows an example from the Rapid Electronics catalogue. Like many 7-segment displays, this example is available in two versions: Common Anode (SA) with all the LED anodes connected together and Common Cathode (SC) with all the cathodes connected together. Letters a-g refer to the 7 segments, A/C is the common anode or cathode as appropriate (on 2 pins). Note that some pins are not present (NP) but their position is still numbered.

LED Lamps

Being solid state devices, Light Emitting Diode (LED) Lamps have inherent characteristics assuring high reliability and a compatibility with low current electronic drive circuits.

LEDs have advantages and disadvantages when compared with other light sources such as incandescent or neon lamps. The advantages are small size, low power consumption, low self- heating, high reliability, they can be switched on and off quickly, and they are resistant to shock and vibration. The features that sometimes can be considered disadvantages are the narrow viewing angle, near monochromatic light, limited wavelength selection, and they require a limiting resistor with a voltage drive.

Principles of Operation

LEDs are formed from various doped semiconductor materials in the form of a P-N diode junction. When electrical current passes through the junction in the forward direction, the electrical carriers give up energy proportional to the forward voltage drop across the diode junction, which is emitted in the form of light. The amount of energy is relatively low for infrared or red LEDs. For green and blue LEDs which are produced from higher forward voltage materials, the amount of energy is greater.

Since the device is being used in the forward biased mode, once the voltage applied exceeds the diode forward voltage; the current through the device can rise exponentially. Very high currents would damage the device which is why a current limiting resistor must be added in series with the LED when driven from a voltage source.

The amount of light emitted by an LED is proportional to the amount of current passing through the device in the forward bias direction. As the current is varied, the output of the light will vary in a similar fashion. By modulating the current flowing through the LED, the light output can be modulated to produce an amplitude modulated optical signal which can be used to communicate information through free space (i.e. TV remote control).

If the voltage source is applied in the reverse direction, the P-N junction will block current flow until the voltage applied exceeds the devices ability to block the current. At that point, the device junction will break down, and if there is no current limit device in the circuit, the LED will be destroyed. The typical value of maximum reverse voltage is five volts.

Construction and Operation

The semiconductor material is typically a very small chip or die, which is mounted onto a lead frame and encapsulated in a clear or diffused epoxy. The shape of the epoxy and the amount of diffusing material in the epoxy control the angle of emission of the light output. Figure 1 illustrates the construction of a common LED package.

Many of our LEDs incorporate high efficiency chips mounted into T-1, T-1 3/4 and SMT (surface mount) packages. However, there are a wide variety of right angle, multi-package and custom packages available to meet your requirement.

The output of LEDs is typically expressed in millicandela (mcd). The candela is defined as the number of lumens per steradian of solid angle. It is usually measured along the projection axis of the device and gives the eye'™s response to the light. The viewing angle for LEDs is specified as 2q1/2, which gives the included angle between the 1/2 intensity points on either side of the output beam. For T-1 and T-1 3/4 devices this can be as low as 10 degrees for clear epoxy devices and as high as 60 degrees for highly diffused LEDS. Typical values of luminous intensity can exceed 1,000 mcd for the higher output devices. Peak radiation output is available at 940, 880, 700, 660, 625, 620, 610, 595, 590, 565, 555, 525, 470 and 430 nanometers ranging from the infrared through the visible to the deep blue. The efficiency of the higher output devices now exceeds the efficiency measured in lumens per watt of incandescent lamps.

LED Lamps may be operated in the pulsed mode. The absolute maximum ratings of LEDs have been determined theoretically and also by extensive reliability testing. Forward current, power dissipation, thermal resistance, and junction temperature are all interrelated in establishing absolute maximum ratings. In the pulsed mode, maximum tolerable limits should not exceed the LED junction temperature that would be reached by operating the LED at specified maximum continuous forward current. This correlation is obtained by establishing combinations of peak current and pulse width for various refresh rates and maintaining the maximum junction temperature as reached by operation at maximum continuous current.

Drive circuits

The drive circuits for LEDs must provide sufficient voltage to overcome the forward voltage drop of the diode junction, while controlling the current to the correct value for the specific device. The most common circuit to accomplish this is a voltage source which is significantly higher than the diode forward voltage drop and a series current limiting resistor. Several configurations are shown in the following diagram. Use Ohms law to calculate the resistor value depending on the LED chosen, the voltage source, and the maximum continuous current rating.

Using the Light Emitting Diode Tutorial - 20

Using the Light Emitting Diode Tutorial

The light emitting diode (LED) is commonly used as an indicator.
It can show when the power is on, act as a warning indicator, or be part of trendy jewelry etc.
It needs to be fed from a DC supply, with the anode positive and the cathode negative, as shown in the diagram.

To calculate the value of the series resistor we need to know the diode forward voltage and current and its connections.
The necessary data can be obtained from a catalogue or data book.
In our example it is 2 volts and 20mA (0.02 amps).
The cathode lead is the one nearest a "flat" on the body.

Since the voltage across the diode is 2 volts and the battery voltage is 12 volts, then the voltage across the resistor is 12-2 = 10 volts.
The diode is in series with the resistor, so the current through then both is the same, 0.02 amps.

We now know the voltage across, and the current through the resistor.
From Ohm's Law we can now calculate the value of the resistor.

Resistance = Volts divided by Amps = V/I = 10/0.02 =500 ohms.

Since this is not a standard value we can use a 470 or 560 ohm resistor as this application is not critical of values.