The term “LED” is an acronym that stands for “Light Emitting Diode“. The LEDs are constituted by a P-N junction realized with gallium arsenide or gallium phosphide, both of materials able to emit light when they are traversed by an electric current; the value of this current is between 10 and 30 mA.
The operation of the LED is based on the said “electroluminescence” phenomenon, due to the emission of photons (in the band of visible or infrared) produced by the recombination of electrons and gaps when the junction is forward biased.
The LEDs have a positive and a negative terminal, and to function must be inserted in this circuit respecting polarity; typically the positive terminal is the longer one, but it can be identified with certainty observing the inside of the backlight LED: as seen in the figure, the positive electrode is thin, lance-shaped, while the negative has the ‘ I look like a flag.
When using a LED, it is necessary to always have a resistance in series to it, in order to limit the current passing and to prevent it being destroyed; to the voltage drop of an LED may vary from 1.1 to 1.6 V, as a function of the wavelength of the emitted radiation (at shorter wavelengths corresponds to a drop of more high voltage).
Unlike common bulbs, where the filament operates at high temperatures and is characterized by high thermal inertia, the LEDs emit cold light, and may flash at very high frequencies, greater than Mhz; if one considers that the light emitted is directly proportional to the current flowing through them, the leds are particularly suitable for the transmission of signals by means of light intensity modulation. One of the many uses of the LED is, for example, to injectors of signals in optical fiber networks.
The most common LEDs emit red, orange, yellow or green. In recent times it has failed to produce a clear LED characterized by the emission of blue light, using gallium nitride (GaN); the availability of a blue light LED is very important as it allows it to create, together with the red and green radiation, a source of white light.
How to calculate the resistance in series with the LED
We have already said that in series to the LED is necessary to insert a resistor to limit the current flow; the value of this resistance can be calculated using Ohm’s law:
- We denote by the supply voltage Vs which we want to connect our LED
- We denote by the voltage drop VI at the terminals of the LED (for example, 1.4 V)
- We denote by the value of current that we want to pass in the LED
To calculate the resistance value it will suffice to make the difference between Vs and Vl and divide the result by I (the value of which may vary, as mentioned, from 20 to 40 mA)
Example (see figure): we want to operate a LED with a voltage of 12 V, limiting the current to 20 mA (namely to 0.02 A)
R = (12 to 1.4): 0.02 = 530 ohms (since this value does not exist on the market, we will use the nearest standard value, for example 470 or 560 ohms)
A simple circuit for controlling the isolation
With a LED and two transistors we can build a simple circuit useful for checking the insulation of electrical parts or to check the state of the small-capacity capacitors (see figure below). The transistors are two small power NPN (BC547 type or equivalent); it is noted that on the basis of TR1 comes the current from the emitter of TR2: this type of connection is called a “Darlington” configuration and allows to obtain a high current gain.
In short, a very weak current on the basis of TR2 is able to turn on the LED that is located on the collector of TR1; for example, if you try to touch with your hand input IN1 and the other input IN2, you will see that the LED lights, and lights up so much more the more tighten the wires between your fingers. In fact, the LED lights up due to weak current from the positive pole, running through your body (from hand to hand) and arrives at the base of TR2 through the resistor RB2.
The capacitor C from 4700 pF to ground is used to send any interference that may be picked up from the entrance, because of its high impedance.
Similarly, if the two-wire input IN1 and IN2 touch any other material or object, you can check the degree of isolation: if the LED is completely off, isolation is total.
Similarly, you can verify the proper functioning of small capacitors, up to a few thousand pF. By connecting the capacitor to the two-wire input, the LED will light up for a brief moment, then go dark, more or less quickly depending on the capacitance of the capacitor; If the LED stays on, even faintly, it means that the capacitor is in dispersion. Just to get an idea, with a few thousand pF capacitors, the led emits only a brief flash; with 0.1 uF capacitors up the LED will remain lit a few seconds, then gradually fade.
You can operate the circuit with a 4.5 V battery; be careful to connect the LEDs so that the positive terminal corresponds to the positive supply.
|TR1||TRANS. NPN Type BC547||LED Pilot|
|TR2||TRANS. NPN Type BC547||Current Amplifier|
|RL||Resistor 100 Ohm||limits the current to the LED|
|RB1||Resistor 1kOhm||limits the current to the base of TR1|
|RB2||Resistor 1MOhm||limits the current to the base of TR2|
|C||Capacitor 4700 pF||Disorder elimination|