LEDs and their applications. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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LEDs, or light emitting diodes (LED, in the English version LED - light emitting diode) - a semiconductor device that emits incoherent light when an electric current is passed through it. The work is based on the physical phenomenon of the appearance of light radiation when an electric current passes through a pn-junction. The color of the glow (the wavelength of the emission spectrum maximum) is determined by the type of semiconductor materials used that form the pn junction.

Advantages

1. LEDs do not have any glass bulbs and filaments, which ensures high mechanical strength and reliability (shock and vibration resistance)
2. The absence of heating and high voltages guarantees a high level of electrical and fire safety
3. Inertialess makes LEDs indispensable when high speed is required
4. Miniature
5. Long service life (durability)
6. High efficiency,
7. Relatively low supply voltages and current consumption, low power consumption
8. A large number of different colors of the glow, the directivity of the radiation
9. Adjustable intensity

Disadvantages

1. relatively high cost. The money/lumen ratio for a conventional incandescent lamp compared to LEDs is approximately 100 times
2. small luminous flux from one element
3. degradation of LED parameters over time
4. increased requirements for the power source

Appearance and main parameters

LEDs have several basic parameters.

1. Case type
2. Typical (working) current
3. Drop (working) voltage
4. Luminescence color (wavelength, nm)
5. Beam angle

Basically, the type of housing is understood as the diameter and color of the bulb (lens). As you know, the LED is a semiconductor device that must be powered by current. So the current that should power a particular LED is called typical. In this case, a certain voltage drops across the LED. The color of the emission is determined both by the semiconductor materials used and by the dopants. The most important elements used in LEDs are: Aluminum (Al), Gallium (Ga), Indium (In), Phosphorus (P), which cause a glow in the range from red to yellow. Indium (In), Gallium (Ga), Nitrogen (N) are used to produce blue and green glows. In addition, if a phosphor is added to a crystal that causes a blue (blue) glow, then we get a white LED. The angle of emission is also determined by the production characteristics of the materials, as well as the bulb (lens) of the LED.

Currently, LEDs have found application in a wide variety of areas: LED lights, automotive lighting, advertising signs, LED panels and indicators, tickers and traffic lights, etc.

Switching scheme and calculation of the necessary parameters:

Since the LED is a semiconductor device, the polarity must be observed when connected to the circuit. The LED has two outputs, one of which is the cathode ("minus"), and the other is the anode ("plus").

The LED will be on only when directly connected, as shown in the figure

When turned back on, the LED will not light up. Moreover, the failure of the LED is possible at low allowable values ​​of the reverse voltage.

The dependences of current on voltage for direct (blue curve) and reverse (red curve) connections are shown in the following figure. It is not difficult to determine that each voltage value corresponds to its own amount of current flowing through the diode. The higher the voltage, the higher the current value (and the higher the brightness). For each LED, there are permissible values ​​of the supply voltage Umax and Umaxrev (respectively for direct and reverse switching). When voltages above these values ​​are applied, an electrical breakdown occurs, as a result of which the LED fails. There is also a minimum value of the supply voltage Umin, at which the LED glows. The range of supply voltages between Umin and Umax is called the "working" zone, since this is where the operation of the LED is ensured.

\

1. There is one LED, how to connect it correctly in the simplest case?

In order to properly connect the LED in the simplest case, you need to connect it through a current-limiting resistor.

Example 1

There is an LED with an operating voltage of 3 volts and an operating current of 20 mA. It must be connected to a 5 volt source.

Calculate the resistance of the current limiting resistor

R = Uquenching / ILED
Uquenching = Upower - ULED
Usupply = 5 V
ULED = 3V
ILED = 20mA = 0.02A
R \u5d (3-0.02) / 100 \u0.1d XNUMX Ohm \uXNUMXd XNUMX kOhm

That is, you need to take a resistor with a resistance of 100 ohms

2. How to connect multiple LEDs?

We connect several LEDs in series or in parallel, calculating the required resistance.

Example 1.

There are LEDs with an operating voltage of 3 volts and an operating current of 20 mA. It is necessary to connect 3 LEDs to a source of 15 volts.

We make the calculation: 3 LEDs for 3 volts \u9d 15 volts, that is, a XNUMX volt source is enough to turn on the LEDs in series

The calculation is similar to the previous example.

R = Uquenching / ILED
Uquenching = Upower - N * ULED
Usupply = 15 V
ULED = 3V
ILED = 20mA = 0.02A
R \u15d (3-3 * 0.02) / 300 \u0.3d XNUMX Ohm \uXNUMXd XNUMX kOhm

Example 2

Let there be LEDs with an operating voltage of 3 volts and an operating current of 20 mA. It is necessary to connect 4 LEDs to a source of 7 volts

We make the calculation: 4 LEDs for 3 volts \u12d 2 volts, which means we do not have enough voltage to connect the LEDs in series, so we will connect them in series-parallel. Let's divide them into two groups of XNUMX LEDs. Now we need to calculate the current-limiting resistors. Similarly to the previous paragraphs, we calculate the current-limiting resistors for each branch.

R = Uquenching / ILED
Uquenching = Upower - N * ULED
Usupply = 7 V
ULED = 3V
ILED = 20mA = 0.02A
R \u7d (2-3 * 0.02) / 50 \u0.05d XNUMX Ohm \uXNUMXd XNUMX kOhm

Since the LEDs in the branches have the same parameters, the resistances in the branches are the same.

Example 3

If there are LEDs of different brands, then we combine them in such a way that each branch has LEDs of only ONE type (or with the same operating current). In this case, it is not necessary to observe the same voltages, because we calculate our own resistance for each branch.

For example, there are 5 different LEDs:
1st red voltage 3 volts 20 mA
2nd green voltage 2.5 volts 20 mA
3rd blue voltage 3 volts 50 mA
4th white voltage 2.7 volts 50 mA
5th yellow voltage 3.5 volts 30 mA

Since we divide the LEDs into groups by current
1) 1st and 2nd
2) 3rd and 4th
3) 5th

we calculate resistors for each branch

R = Uquenching / ILED
Uquenching = Upower - (ULEDY + ULEDX + ...)
Usupply = 7 V
ULED1 = 3 V
ULED2 = 2.5 V
ILED = 20mA = 0.02A
R1 = (7-(3+2.5))/0.02 = 75 Ohm = 0.075 kOhm

like
R2 = 26 Ohm
R3 = 117 Ohm

Similarly, you can arrange any number of LEDs

Important notice!

When calculating the current-limiting resistance, numerical values ​​\uXNUMXb\uXNUMXbof which are not in the standard series of resistances are obtained, therefore we select a resistor with a resistance slightly larger than calculated.

3. What happens if there is a voltage source with a voltage of 3 volts (or less) and an LED with an operating voltage of 3 volts?

It is acceptable (BUT NOT DESIRABLE) to include an LED in a circuit without current-limiting resistance. The disadvantages are obvious - the brightness depends on the supply voltage. It is better to use dc-dc converters (voltage boost converters).

4. Is it possible to turn on several LEDs with the same operating voltage of 3 volts in parallel to each other to a source of 3 volts (or less)? In the "Chinese" lanterns, this is how it is done.

Again, this is acceptable in amateur radio practice. The disadvantages of this inclusion: since the LEDs have a certain spread in parameters, the following picture will be observed, some will glow brighter, while others will be dimmer, which is not aesthetic, which is what we observe in the flashlights above. It is better to use dc-dc converters (voltage boost converters).

Important notice!

The circuits presented above do not differ in the high accuracy of the calculated parameters, this is due to the fact that when current flows through the LED, heat is generated in it, which leads to heating of the pn junction, the presence of current-limiting resistance reduces this effect, but the balance is established at a slightly increased current through the LED . Therefore, it is advisable to use current stabilizers rather than voltage stabilizers to ensure stability. When using current stabilizers, you can only connect one branch of LEDs.

Author: Sivent; Publication: cxem.net

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