June 29, 2015 1:38 pm
LED Lighting Basics Infographic
What is a LED?
How does a LED work?
What is the difference between constant current and constant Voltage?
Magnitude’s new LED Lighting Basics Infographic contains the questions and the answers related to LED lighting.
What is a L.E.D?
LED stands for Light Emitting Diode. A diode is a unique electronic component with two electrodes called the anode and the cathode.
How does a LED work?
Electric current can pass in both directions in a copper wire, however, LEDs are semiconductors, meaning they will always conduct in one direction. When electric current passes through this type of diode, it generates light.
Now you can understand why LEDs run on direct current (DC) and not alternating current (AC).
- In order for current to pass from one side to the other side of the LED, Voltage must “build up” in the LED. Also, it will require between 2.8V to 3.4V to make the LED Emit light that is proportional to the amount of current passing through it.
This is called FORWARD VOLTAGE which varies according to each LED’s specification.
- Each LED holds a maximal Forward Current that can pass through it. Try to drive47 current higher than that and you are risking over-heating the LED and eventually burning it. Also, if you try to increase the voltage potential you will over-drive the LED.
- Let’s build a small lighting fixture!
All we need to do now is connect 3 LEDs set to a power source. For this example, we will use 3 LEDs having a typical forward voltage of 3.4V. And When we connect them to a power source, the industry’s standard of 12V, it will look something like this!
That’s it! We’re ready to test our small lighting fixture! let there be li….. STOP. There’s a problem.
Here’s our problem: we have three LEDs of 3.4 Volts each which together sums to 10.2 Volts, and the industry’s standard is 12V which means we will be over-driving our 3 LEDs with an excess of 1.8 Volts! which may cause our LEDs to overheat.
Can we add a 4th LED?
Well the good news is we are no longer at risk of blowing up our LEDs. The Bad news are that now there isn’t enough voltage to drive the current due to the 4th LED. Which simply means: the light will not turn on.
So how do we solve this problem?
Using a component that is designed to resist the passage of an electrical input, or in a word – a Resistor.
The resistor acts as a safety device which controls the voltage across each of the LEDs. When a slight current increase is sensed, the voltage across the resistor will also increase and will bring the current back down, keeping the voltage constant over the string of LEDs.
We will connect the resistor to our small lighting fixture so the resistor will “absorb” the remaining 1.8V. and that’s it. Problem solved!
Ok let’s test it once more. It’s working perfectly now!
We need more light!
In order to increase the lighting of our 3 LED lighting fixture we need to connect more strings in parallel. Each string of 3 LEDs & resistor experience the same voltage over them.
This type of layout is called “constant voltage layout”. The current that is running through each of the strings will be regulated by each resistor. However, each 3 LED string + resistor is wired to the 12V power supply.
The best example for this type of a constant voltage lighting fixture is in the form of an LED tape light.
Most LED Tape Lights come with an input of 12VDC or 24VDC.
As you can see the constant voltage array allows you to expand or narrow the length of you lighting fixture by wiring one piece of LED tape light to another in a column or the current that flows through the LED is the same.
We looked into the constant voltage fixture, now let’s build a constant current one. Our building block will be 3 LEDs rated at forward voltage of 3.4V and 50mAmps each, and multiply that block by 7 for this example. So here we have a new current source of 350mA because we have 7 strings of 50mA each, and our output voltage will be 10.2V DC, why? Because we have 3 LEDs in our building block and each is 3.4V so each string of LEDs will need 10.2V DC in order to operate correctly.
What if we changed our building block a bit and added two LEDs to each string but kept everything else the same, what would happen then?
Here’s you can tell that the only thing that changed is the Input voltage, since we have 5 LEDs on each string and each are 3.4V the new input voltage is now 17V DC. But the main point is that current stays the same (constant)!
More to come in our next infographic: Dimming for dummies