Enlighten your LEDs
This page is dedicated to give some insights on how to light up your LED's. We will explain how to connect them and figure out what kind of resistor (if needed) you need to connect with it. The later is important cause a too big resistor makes your LED not illuminating sufficient and a too small resistor burn your LED in a matter of seconds to the garbage bin. So lets start figuring out how to make use of LED's in our projects
What is aN LED and how to connect it
An LED is in principal a diode hence the abbreviation of LED (Light Emitting Diode). This means in principle that it only allows to flow current in one direction i.e. from Anode to Kathode, we will show and explain this later one more thoroughly.
But lets start from the beginning an LED is a Light Emitting Diode this means as the words tell that it is a diode that emtis light and this only happens when a current passes thru it. And because it is a diode the current needs to pass from the Anode to the Kathode.
Below the traditional symbol for an LED:
The left side of the symbol is called the Anode and the right side therefore the Kathode. Note the symbol looks like it only can conduct current from one side to the other. If we would connect a possitive voltage to the right side of the symbol and the ground to the left side the LED would NOT emit light, and if the voltage is too high the LED will get broken.
So how to connect the LED correct such that it will emit light? If we would take a red LED which works usually between 1.7 Vdc to
2.0 Vdc we then apply a voltage between this range from Anode to Kathode, the below circuit gives an example:
The above circuit shows that the LED is connected to a voltage supply of 1.8 Vdc (dc = Direct Current, which is e.g. a battery). These type of batteries don't exist, so in order to light up our red LED we need to do some other "tricks"to be able to light is up without damaging the LED. And now we reach at the most imprtant law in electronics, called Ohm's Law, which we will discover below.
Applying Ohm's Law
Omh's law is a imple linear equation that explains the relationship between Voltage (V in Volts), Current (I in Ampére) and Resistance (R in Ohm). The standard fomula is show below:
Where again:
- V = Voltage in Volts
- I = Current in Ampére
- R = Resistance in Ohms
So now we know that the Voltage is the current that is flowing thru the led multyplied by its resistance. But now we come to an important point! If one would take a look to a datasheet of a red LED we would see a Vf i.e. the Forward Voltage so in out case the 1.8 Vdc and a If, the forward current. The If is the current which the LED needs to light up nicely. Well as a general rule all LED's need about 20 mA (i.e. 20 miliAmps) this is 0.020 A to light up nice.
Now we know the votage for a red LED and we know also the current for the led to light up nice. And we also know that a battery of 1.8Vdc does not exist we can think a bit further to make it all practically.
A standard AA battery give 1.5 Vdc this is not enought for a standard LED so lets put two in series we can sum the voltages up So 2 times 1.5 will be in total 3 Vdc.
This means that our source voltage will be 3 Vdc, but we can not connect this directly to the LED as in our previous circuit, this will damage the LED. We need to figure out a way that only 1.8 Vdc is across the LED and the rest is dissipated but the total current thru the circuit is 20 mA.
Now Omh's law comes in handy, we need to determine a correct value for a resistance which we could connect into the circuit. Across this resistance we should have the the total voltage minus the voltage of that is needed for the LED. Well that's easy that is the 3V - 1.8V = 1.2V.
Now we know a desired current and desired voltage. The previous given Ohm's law we can manipulate easily with basic algebra to figure out what would be the need resistance. So lets do that:
To get the resistance we need we need to divide the voltage across it by the current flowing thru it. That's easy that will be dividing the 1,2 Vdc by 20 mA:
The above equation shows what value of resistance in Ohms we need to make the circuit working well. The complete circuit is shown below.:
Some practical information
Well now comes one important question what is the Anode and what is the Kathode on a LED??? If one looks at an LED there is always a long pin and a short pin. The Kathode (which is connected to ground) is always the short pin.
The above three different LED's have all a short pin and a long pin. And for all three the negative is the short pin and the positive is the long pin. Now if one would buy LED's that are allready pre-wired they come usually with a Red and Black wire and in this case the Black wire would be the negative and the Red wire would be the positive.
Conclusion
To conclude this small tutorial, connecting a LED is no rocket science. Using Ohm's law, knowing the voltage for the particular LED (remember that the voltage needed for the LED is depending on the color one is using) and almost all LED's need 20 mA we can easily create a nice circuit for them to the illuminate perfectly.
There are some special LED's such as RGB LED's or LED-strips which need to be connected a bit different but that is maybe for another tutorial for now have fun with just connecting a standard LED.
All the best StarWarsScalemodels!