|
|
|
|
.:: Menu ::.
|
|
|
|
  LED technically |
|
|
|
|
|
|
|
|
.:: Calendar ::.
|
|
|
|
|
September 2010
|
|
|
|
|
|
|
|
| Sun |
Mon |
Tue |
Wed |
Thu |
Fri |
Sat |
|
|
| 29 |
30 |
31 |
1 |
2 |
3 |
4 |
| 5 |
6 |
7 |
8 |
9 |
10 |
11 |
| 12 |
13 |
14 |
15 |
16 |
17 |
18 |
| 19 |
20 |
21 |
22 |
23 |
24 |
25 |
| 26 |
27 |
28 |
29 |
30 |
1 |
2 |
|
|
|
|
|
|
|
|
|
|
|
|
.:: LED technically spoken ::.
|
|
|
|
Let’s get right down to the nitty-gritty!
This page is a manual for people who want to do something with LEDs, but know absolutely nothing about electronics.
“LEDs for Dummies” would be a good title for this page, but that sounds very condescending...
The goal of this page is to answer some of the questions that are sent via e-mail, and to ensure that you can perform your planned hobby project successfully.
It is important to know something about LEDs before connecting them to a power source.
Additionally we hope to shed some light on a few of the practical qualities of LEDs that are not mentioned a lot on the internet.
Polarity of a diode
To start... A LED is a diode; it has a polarity and thus only lets current pass through it in one direction.
If you connect it the wrong way round it will not allow current to pass through and the LED will not emit any light!
It will also not break as long as the voltage is below the threshold Voltage. 
The schematically symbol for a LED is an arrow against a line, that looks like a coffee filter.
Thanks to the arrow it is also easy to see in which direction the coffee... err... current will pass through the LED.
The arrow is the ‘anode’ and the line is the ‘cathode’. The current goes from the anode to the cathode.
On (new) LEDs the longer of the connection wires is the anode (+) and the slightly shorter wire is the cathode (-). What’s that?
You already cut the wires to the right length? Well, the round 3 and 5 mm LEDs have a flat edge on the cathode side of the housing.
Threshold Voltage
In opposed to light bulbs with LEDs not the Voltage but the current is the important factor.
It is very important that the current that is running though the LED is limited.
The current determines the amount of light that is emitted, while the Voltage is determined by the ‘bandgap’
of the semiconductor material. The bandgap is determined by the composition (pollution) of the semiconductor material.
The larger the bandgap, the shorter the wavelength, and the higher the so called threshold Voltage will be.
The threshold Voltage is important to calculate how much power is generated in the LED, and how much Voltage will be on the resistors in the circuit.
That Threshold Voltage is also dependent on the temperature an the LED current, and below the threshold voltage the current drop so much that the LED
will hardly be emitting any light, or even none at all, but that depends on the current leakage of the circuit.
Measured Voltages at 20 mA:
 |
Deep red | 660 nm | Gallium-Aluminum-Arsenide (AlGaAs) | 1.8 Volt |
|
Red | 630 nm | Gallium-Indium-Phosphide (AlGaInP) | 1.9 Volt |
|
Orange | 610 nm | Gallium-Arseen-Fosfide (GaAsP) | 2.0 Volt |
|
Amber | 592 nm | Gallium-Arseen-Fosfide (GaAsP) | 2.0 Volt |
|
Yellow | 585 nm | Gallium-Arseen-Fosfide (GaAsP) | 2.0 Volt |
|
Yellow/Groen | 565 nm | Gallium-Nitride (GaN) | 2.1 Volt |
|
Green | 525 nm | Gallium-Fosfide (GaP) | 2.2 Volt |
|
Turquoise | 505 nm | Zink-Selenide (ZnSe) | 3.0 Volt |
|
Blue | 470...430 nm | Zink-Selenide (ZnSe)
Silicium-Carbide (SiC)
Indium-Gallium-Nitride (InGaN) | 3.3 to 3.5 Volt |
|
Violet | 420 nm | Indium-Gallium-Nitride (InGaN) | 3.2 Volt |
|
Ultra Violet | 400...380 nm | Diamant (C) | 3.5 Volt |
|
White | 400...380 nm | Silicium-Carbide (SiC) +Phosphor
Indium-Gallium-Nitride (InGaN) +Phosphor | 3.3 Volt |
Efficiency
I would like to give a short indication of the efficiency of various light sources.
Efficiency (June 2005):
|
Candle | - | efficiency < 1% |
|
Bulb 15W | 6 lumen/watt | efficiency 2,5 to 3% |
|
Bulb 100W | 12 lumen/watt | efficiency 5% |
|
Halogen | 25 lumen/watt | efficiency 10 to 11% |
|
Fluorescent light | 60 lumen/watt | efficiency 25% |
|
HID lights (Intensity Discharge) | 100 tot 120 lumen/watt | efficiency 40 to 50% |
|
Sodium lights 400 to 600W
(Sodium, oranje licht) | 140 tot 150 lumen/watt | efficiency 50 to 55% |
|
5mm white LED | 15 to 20 lumen/watt | efficiency 6 to 8% |
|
1W Golden Dragon (OSRAM) White | 22 lumen/watt | efficiency 9% |
|
5W Luxeon Star White | 24 lumen/watt | efficiency 10% |
|
1W Luxeon Star White | 30 lumen/watt | efficiency 12 to 13% |
|
Luxeon K2 White | 60 lumen/watt | efficiency 25% |
Ageing
LEDs get older. However the lifetime of a LED can be as much as 100.000 hours as long as the LED is not used in extreme conditions; extreme conditions being high temperatures,
a damp environment, Voltage spikes and electrostatic electricity. Just Like with Bulbs there is a spread in the LEDs quality, which means one LED will last longer or shorter then the next...
At high temperatures and a relatively high LED current the LED chip will get very hot. Therefore calculations must be done with the sum of the temperature resistances (Rth JL (Junction to Lead Frame)
+ Rth LA (Lead Frame to Ambiant)). The LED current must be set to a value where the LED chip will not get to hot, or one needs to provide some type of cooling of the LEDs.
Due to the fact that the expansion coefficient of the LED chip is different to that of the LED casing there will be mechanical stresses around the bonding wire (the this wire in the middle of the chip).
This can cause a fracture, a circuit break, which could cause the LED to blink on and off or even to not go on at all any more.
Some LEDs are used in a Damp environment, such as traffic signaling signs above the roads, traffic lights, cars, swimming pools or simply outside of buildings (illuminated commercials) or in gardens.
This would seem ok, since the housing of the LED appears to be waterproof. Unfortunately the plastic housing (epoxy) is hydroscopic, meaning it does absorb water but in minute amounts.
In the long run the LED chip is sensitive to this water, and special purpose LEDs are made that consist of a different composition and therefore are not hydroscopic to water.
Aligent is one of the manufacturers of these special purpose LEDs, but they come at a higher price. For some branches in industry the price difference is too much in comparison to the unreliability
of the normal LEDs, and they prefer to just replace them and pay for new LEDs when they break.
Voltage spikes and electrostatic discharges (ESD) can really ?mess up? a LED without you even noticing. However the lifetime of the LED could be shortened significantly.
For this reason you should work ESD safe. Your work area should include as a minimum, a grounded dissipative work surface of sufficient size and a grounded skin contact wrist strap.
Both the wrist strap and the work surface should be connected separately to the electrical ground through a 1 Mega-ohm resistor for personnel safety.
It is recommended that all soldering irons have the tip connected to ground, also through a 1 Mega-ohm resistor. The recommended relative humidity in your work area is within 35-50%.
Even under ideal circumstances the LEDs emitted light will diminish over time. When the LEDs emitted light has diminished to half its original value the so called ?End of Life? has been reached.
Around that time green, blue, violet and white LEDs also show a shift in color (or color temperature in white LEDs).
Temperature Influence
When the temperature changes the bandgap also changes, which leads to a change in wavelength as well. When the LED gets hotter the bandgap get smaller and therefore the threshold Voltage also will be slightly lower.
The wavelength of the light will then be slightly longer, typically from .1nm/°C - .2nm/°C depending on the type of LED used.
Try heating up a green LED (with a solder iron agains the connection wires for example), you will then see the color shifting from green to yellow to orange!
In white LEDs considerable heating prevents the fluorescent material from emitting warm white light but makes it emit a bluer color instead.
(A white LED is basically a blue LED that mixes wamr white fluorescence with the blue color).
Current limiting
Since the LED is a diode, it has also a diode characteristic.
When you connect a LED to a small power source, with a slightly smaller Voltage then the threshold voltage, hardly any current will be running through the LED and the LED will hardly be emitting any light.
In A Red LED for example that is at 1.6 Volts. When we increase the Voltage to for example 1.75 Volts there will be a current of 5mA, at 1.8 Volts 20mA and at 2 Volts the current will already be more then 100mA.
This means that a LED that is specified to function at 30mA Max will not last very long. Additionally because it?s getting hotter the threshold Voltage will drop, which leads to an even bigger current and the LED will get even hotter!
This way with a power source you can hardly keep the current running though the LED stable at a safe value. We need to limit the current!
In simple circuits with low Voltages this can be done by adding a resistor in series with the LED, the current limiting resistors which we will just call ‘the resistor’ for ease.
Of course there are circumstances where the Voltage is much higher or even lower then the threshold Voltage. There are solutions for this as well, but they will be discussed later.
A resistor is a relatively small component that can be bought in any old electronics store.
The Specifications
Some more about the maximum LED current according to the specification, usually about 30mA. Meaning that at that current the LED is already working as hard as it can.
In an environment like a car, the LED has to endure some less then enjoyable circumstances, such as small Voltage spikes on the cars electric circuits as a result of switching relays, windscreen wipers and dynamo glitches.
The temperatures also vary from below 0 degrees in the winter to over 60 degrees in the summer. Then considering there are hundreds of LEDs in the instrument panel a few will always die on you after just a few months; annoying...
because all LEDs in series with the broken ones will also not work any more. And we prefer not to reopen the dash to replace a few LEDs.
It is therefore recommended to stay below the specified maximum current, usually about 20mA in stead of 30mA. But even then you have no guarantee that after about a year at least one of the LEDs will die on you after a few resolving blinks.
The Resistor and Ohms Law
Now about that resistor... To limit the current through the LED at a certain value, this resistance must be calculated using the voltage that is over the resistor, and the current we want to have running through the LED.
The formula used for this is probably the most well known in Electronics: Ohms Law. The formula goes: ‘U=IxR’, which mean the Voltage over that resistor is determined by the current running through the resistor multiplied by the value of the resistor.
In laymen’s terms: ‘voltage = current x resistance’. Voltage is given in ‘Volts’, the current in ‘Ampere’ and the resistance in ‘Ohms’ (Yes, those are names of big-time scientists of the past).
The scheme below shows several circuits where LEDs are in series with resistors. We will do some calculations in each of the examples to determine what the value of the resistors must be in in order to have the LED light up decently.
Some enlightening examples:
Example 1
We have one red LED. The threshold voltage is 1.8 Volt. We will assume the power supply is 12 Volts, so the voltage over the resistor will he 12-1.8 = 10.2 Volt.
We want to have 20mA running through the LED, to do this we use Ohms Law: We devide the voltage that is over the resistor by the current, and we know the value of the resistor.
So 10.2 Volt devided by 0.02 Ampere (btw, 1 Ampere = 1000 mili-Ampere), and then the value of the resistor will be 510 Ω.
That value is part of the famous E-24 range, but a hobby shop sometimes will only have resistors from the E-12 range.
Safely rounding off the value upwards to the next E-12 range will make the value 560 Ω.
Example 2
We have two red LEDs in series. Deducting two times the 1.8 Volts from the 12 Volt Supply leaves us with 8.4 Volt over the resistor.
This 8.4 Volts we divide by 20mA, and as a result get a value of 420 Ω, which we round off up to either 430 Ω (E24 range) or 470 Ω (E12 range).
Example 3
We have four red LEDs in series. Deducting four times the 1.8 Volts from the 12 Volt Supply leaves us with 4.8 Volt over the resistor.
This 4.8 Volts we divide by 20mA, and as a result get a value of 240 Ω, which is a value that already falls in the E24 range.
Example 4
We now have one blue LED. Deducting 3.3 Volts from the 12 Volt Supply leaves us with 8.7 Volt over the resistor.
This 8.7 Volts we divide by 20mA, and as a result get a value of 435 Ω, which can be rounded off to 420 Ω (E24 range).
Example 5
We now have two blue LEDs in series. Deducting 6.6 Volts from the 12 Volt Supply leaves us with 5.4 Volt over the resistor.
This 5.4 Volts we divide by 20mA, and as a result get a value of 270 Ω, which is already a value in the E12 range.
Example 6
We now have two red LEDs and one blue LED in series. Deducting 2x1.8 and 1x3.3 Volts from the 12 Volt Supply leaves us with 5.1 Volt over the resistor.
This 5.1 Volts we divide by 20mA, and as a result get a value of 255 Ω, which we can round off to 270 Ω (E12 range).
Example 7
This example is the most fun for at home. White LEDs are between the 3.3 and 3.6 Volt, depending on if it is a blue or an ultra violet LED that is fitted with a phosphor layer.
Here were using 4 white LEDs in series. A small calculation tells us that the voltage will be somewhere between 4x3.3 = 13.2 and 14.4 Volt.
So you cannot connect this to 12 Volts, but with the 3.3 Volt LEDs you will have a chance to get some light (diode curve), but not a lot.
If we build this into a car environment the LEDs will not go on until you start the engine and the voltage rises to above 13.2 Volt.
But if you have white or UV LEDs of 3.6 Volt, then you can’t connect four LEDs in serial in your car!
What about if we use three LEDs?
Then you will have 3x3.3 Volt = 9.9 Volt, or 3x3.6Volt = 10.8 Volt. Say we have the latter, then at a 12 Volt supply we will need the following resistor: 12V ? 10.8V = 1.2 Volt.
This 1.2 Volt we divide by 20mA, and as a result get a value of 60 Ω, which we can round off to 62 Ω (E24 range). When we build this into a car and when we turn on the party lights there will be a current of 20 mA running though the LEDs.
However when we start up the engine the battery voltage will raise to 14.4 Volt and then over that 62 Ω resistor there will be a voltage of 14.4V ? 10.8V = 3.6 Volt. Yes, that is three times as high when compared with a non-running engine!
3.6 Volt divided by 62 Ω = 58mA, and with that current the LEDs will not last very long!
In short, the closer you get to the supply voltage with your (in series) LEDs, the higher the efficiency of your circuit (you will generate less heat in the pre-circuit resistor), but you circuit will also be more sensitive to voltage changes.
A solution for this would be to build a smart switch mode power supply with a MOSfet and an inductor, that will not generate a lot of heat no matter what voltage is over it, due to it being very efficient.
Also this power supply can stabilize that LED current with the correct value, so that voltage changes have no effect on it, even if your total LED voltage is very close to the supply voltage.
The circuit will send the current, even when the Voltage changes, driven by pulse width control.
Below you can find a LED resistor calculator:
Resistor Color Guide.
In the chart below you can click on the color fields to update the resistor colors and values.
|
|
|
|
|
|
|
|
|
English *
Nederlands
LEDs Citroen Saxo
