White LED parameters current and voltage. Features of white LED power supply. Scheme, description

The LED is a semiconductor device, therefore, it must be turned on strictly observing the polarity. For this purpose, its conclusions have corresponding names: Anode - “plus” and cathode - “minus”.

The LED will only light up when turned on directly, as shown in the figure. When turned on in reverse, in most cases, it fails irrevocably.

Since the LED will only operate at certain voltage values ​​and the strength of the current passing through it, an additional limiting resistance is introduced into the connection diagram, which is calculated based on Ohm’s law for the circuit section:

R=U extinguishing/ I LED,

Where R– resistance of the current-limiting resistor in ohms,

I LED - current strength at which the LED will operate normally,

U quenching – voltage that needs to be quenched by a resistor. It is calculated using the formula:

U quenching= U power supply - U LED, where

U power supply – voltage of the power source to which the LED needs to be connected,

U LED – operating voltage of the LED (at which it will work normally).

Now let's look directly at the various LED connection schemes.

How to connect one LED?

Let's say we have an LED with an operating voltage of 3 V and an operating current of 20 mA. We need to connect it to a 12V power source.

Let's convert the data units to those used in the formula:

20mA = 0.02A.

Now let's find the required values:

Uquenching = 12 – 3 = 9 V – “extra” voltage that must be extinguished with a resistor.

R = 9V/0.02A = 450 Ohm.

Thus, one LED with an operating voltage of 3 V and an operating current of 20 mA must be connected according to Figure 1 through a resistance of 450 Ohms. If a non-stabilized source is used as a power source (the voltage value may fluctuate), then the resistance can be taken at a slightly higher value, for example, 490 Ohms.

How to connect multiple LEDs?

Let's consider the connection diagram of several LEDs shown in Figure 2. From a school physics course it is known that with a series connection, which is observed in Figure 2, the total operating voltage of the LEDs will be equal to the sum of the operating voltages of each, and the current flowing through the resulting chain will be the same at any of its points. From the latter we can conclude: LEDs can only be switched on according to this scheme with the same operating current, otherwise their brightness will be different. For example, a current of 20 mA will flow through the chain, and the operating current of the LED is 30 mA, which means it will shine dimmer than during normal operation.

Let's move on to the calculations. Since the total operating voltage of the chain is equal to the sum of the operating voltages of each LED in it, then

Uquenching = Upower supply – (ULED 1 + ULED 2).

Let's connect two LEDs with an operating voltage of 3V and an operating current of 20mA to a 12V power source according to the circuit in Figure 2. Again, you need to convert milliamps to amperes: 20mA = 0.02A

R=6/0.02=300 Ohm

Thus, two LEDs with an operating voltage of 3 V and an operating current of 20 mA must be connected according to Figure 2 through a resistance of 300 Ohms. Do not forget that if an unstabilized source is used as a power source (the voltage value may fluctuate), then the resistance can be taken at a slightly higher value, for example, 330 Ohms.

How to connect different LEDs to the same power source?

There are a large number of different LEDs, which can differ in both glow color and emission power. luminous flux, and, consequently, the operating parameters will also differ from each other. If you need to connect different LEDs to the same power source, you need to sort them according to the same operating current, and then connect them according to the diagram shown in Figure 3.

For example, we need to connect 2 red LEDs with an operating voltage of 2.5V and an operating current of 20mA, 2 yellow LEDs with an operating voltage of 3V and an operating current of 25mA, and 1 blue LED with an operating voltage of 3.5V and an operating current of 50mA. We sort them according to the same parameters. In our case, we will get three groups: red, yellow and blue. Next, for each group separately we calculate the resistance using the method described above.

For reds:

Uquenching=12- (2.5+2.5)=7V

R=7V/0.02A=350 Ohm.

For yellow ones:

Uquenching=12- (3+3)=6V

R=6V/0.025A=240 Ohm.

For blue:

Uquenching = 12 - 3.5 = 8.5 V

R=8.5V/0.05A=170Ohm.

The limiting resistances have been calculated; all that remains is to connect them according to diagram 3.

Is it possible to connect an LED with an operating voltage of 3V to a power supply of 3V (or less)?

Such connections are allowed, but not recommended, since the brightness will depend directly on the power source.

Is it possible to connect LEDs with the same operating voltage in parallel?

Such inclusion is also acceptable, but the parameters of diodes, sometimes even from the same batch, may differ, which will directly affect their brightness - one is brighter, the other is dimmer.

RGB LEDs

There are semiconductor devices in which the housing can immediately contain red (R-RED), green (G-GREEN) and blue (B-BLUE) LEDs. By changing their brightness, you can achieve a general emission of any color, similar to mixing colors in a palette. For example, if you light all three LEDs at full power, it will turn out white. If you light only red and green, you get yellow. By changing the brightness of the LEDs, you can change the shades of the resulting colors.

Please note that the diagrams shown are simple and approximate. Therefore, in order to increase the life of the LED, it is necessary to use stabilized power supplies. Since the brightness of the LED, and therefore its operation, depends directly on the strength of the current flowing through it, stabilizers must be used for current, not voltage.

An LED only allows electrical current to flow in one direction, which means that in order for an LED to emit light, it must be connected correctly. The LED has two contacts: anode (plus) and cathode (minus). Usually, the long contact of an LED is the anode, but there are exceptions, so it is better to clarify this fact in technical specifications specific LED.

LEDs belong to this type of electronic components, for which, for long and stable operation, it is important not only the correct voltage, but also the optimal current strength - so always, when connecting an LED, you need to connect them through the appropriate resistor. Sometimes this rule is neglected, but the result is most often the same - the LED either burns out immediately, or its resource is greatly reduced. Some LEDs have a resistor built into them “from the factory” and can immediately be connected to a 12 or 5 volt source, but such LEDs are quite rare on sale and most often it is necessary to connect an external resistor to the LED.

It is worth remembering that resistors also differ in their characteristics and, to connect them to LEDs, you need to choose a resistor of the correct value. In order to calculate the required resistor value, you should use Ohm's law - this is one of the most important physical laws related to electricity. This law Everyone taught it at school, but almost no one remembers it.

Ohm's law is a physical law with which you can determine the interdependence of voltage (U), current (I) and resistance (R). The essence of the ego is simple: the current strength in a conductor is directly proportional to the voltage between the ends of the conductor, if the properties of the conductor do not change when the current passes.

This law is visually displayed using the formula: U= I*R
Once you know the voltage and resistance, using this law you can find the current using the formula: I = U/R
Once you know the voltage and current, you can find the resistance: R = U/I
Once you know the current and resistance, you can calculate the voltage: U = I*R

Now let's look at an example. You have an LED with an operating voltage of 3 V and a current of 20 mA, you want to connect it to a 5V voltage source from a USB connector or power supply so that it does not burn out. This means we have a voltage of 5 V, but the LED only needs 3 V, which means we need to get rid of 2 V (5V - 3V = 2V). To get rid of the extra 2 V, we need to select a resistor with the correct resistance, which is calculated as follows: we know the voltage that needs to be eliminated and we know the current required by the LED - we will use the formula stated above R = U/I. Accordingly, 2V/0.02 A= 100 Ohm. This means you need a 100 ohm resistor.

Sometimes, depending on the characteristics of the LED, the required resistor is obtained with a non-standard value that cannot be found on sale, for example 129 or 111.7 Ohms. In this case, you just need to take a resistor with a slightly higher resistance than the calculated one - the LED will not operate at 100 percent of its power, but at about 90-95%. In this mode, the LED will work more reliably, and the decrease in brightness will not be visually noticeable.

You can also calculate how much power of resistor you need - to do this, multiply the voltage that will be delayed across the resistor by the current that will be in the circuit. In our case it is 2V x 0.02 A = 0.04 W. This means that a resistor of this power or greater will suit you.

LEDs are sometimes connected several times in parallel or in series using one resistor. For correct connection It should be remembered that with a parallel connection the current is summed up, and with a series connection the required voltage is summed up. You can connect only identical LEDs in parallel and in series using one resistor, and if you use different LEDs with different characteristics, then it is better to calculate each LED its own resistor - this will be more reliable. LEDs of even the same model have a slight discrepancy in parameters, and when connecting a large number of LEDs in parallel or in series, this small discrepancy in parameters can result in many burnt-out LEDs. Another pitfall may be the fact that the seller or manufacturer (much less often) may give slightly incorrect data on the LEDs, and the LEDs themselves may not have a clear operating voltage, but a set of minimum/optimal and maximum voltage parameters. This factor will not have much effect when connecting a small number of LEDs, but if a large number is connected, the result may be the same burnt-out LEDs. So you shouldn’t get too carried away with parallel and serial connections; it will be safer to connect a separate resistor to each LED or a small group of LEDs (3-5 pieces). Let's look at a few connection examples.

Example 1. You want to connect three LEDs in series, each rated at 3 V and 20 mA, to a 12 V current source (for example, from a molex connector). Three LEDs of 3 volts each will draw 9 volts together (3V x 3=9V). Our current source has a voltage of 12 volts, so we will need to get rid of 3 volts (12 V - 9 V = 3 V). Since the connection is serial, the current will be 20 mA, respectively, 3 volts (the voltage that needs to be eliminated) divided by 0.02 A (the current required by each LED) and we get the value of the required resistance - 150 Ohms. This means you need a 150 ohm resistor.

Example 2. You have four LEDs, each rated at 3 volts, and a 12 V power supply. In this situation, you might think that a resistor is not needed, but this is not the case - LEDs are very sensitive to current and it is better to add a resistor to the circuit at 1 ohm. A resistor of this value will not affect the brightness of the glow, but will be something like a “fuse” - the LEDs will work much more reliably. Without using a resistor, in in this case, LEDs can simply burn out, quickly or not very quickly.

Example 3. You want to connect three LEDs, each rated at 3V and 20mA, in parallel to a 12V current source. Because paralleling adds current rather than voltage, the three LEDs will require 60mA of current (20mA x 3 = 60 mA). Our current source has a voltage of 12 volts, and the LEDs need a voltage of 3 volts, so we need to get rid of 9 volts (12 V - 3 V = 9 V). Since the connection is parallel, the current will be 60mA, respectively, 9 volts (the voltage that needs to be eliminated) divided by 0.06 A (the current required by all LEDs) and we get the value of the required resistance - 150 Ohms. This means you need a 150 ohm resistor.

There are also a large number of different “calculators for LEDs” on the Internet that you can use. Just go to the appropriate website, indicate the characteristics of the LED and the current source, and you will receive all the necessary data on the resistor, as well as its color marking.

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LEDs. Power features of white LEDs

Let's take a closer look power features of white LEDs. As is known, an LED has a nonlinear current-voltage characteristic with a characteristic “heel” in the initial section (Fig. 4.21).

As we can see, the LED begins to glow if a voltage greater than 2.7 V is applied to it.

Attention! When the threshold voltage is exceeded (above 3 V), the current through the LED begins to grow rapidly and here it is necessary to limit the current and stabilize it at a certain level.

Rice. 4.21. Volt-ampere characteristics white LED

The simplest current limiter via an LED is resistor. There are several options for circuitry of LEDs. They are divided into circuits with parallel, serial and mixed connection. At sequential connection LEDs (as shown in Fig. 4.22), the current I flowing through the LEDs will be equal to

Sequential switching aims to either increase the radiation power or increase the radiated surface.

Rice. 4.22. LED sequential circuit diagram

The disadvantages of sequential connection are:

• firstly, as the number of LEDs increases, the supply voltage also increases, because in order for current to pass through series-connected LEDs, the condition Upit > Uvd1 + Uvd4 + Uvd3 must be met;
• secondly, an increase in the number of LEDs reduces the reliability of the system; if one of the LEDs fails, all LEDs connected in series stop working.

When connected in parallel LEDs, a separate current flows through each emitter, set by a separate current-setting resistor.

In Fig. Figure 4.23 shows a parallel circuit of emitting diodes. The total current consumed from the power source in this case is equal to

Rice. 4.23. LED parallel circuit diagram

Advantage parallel connection is highly reliable, since if one of the emitters fails, the others continue to work.

Flaws:

• each LED consumes a separate current and energy consumption increases;
• losses on current-setting resistors increase.

The most effective is mixed (combined) series-parallel connection, shown in Fig. 4.24. In this case, the number of series-connected emitters is limited by the supply voltage, and the number of parallel branches is selected depending on the required power.

Rice. 4.24. Scheme of series-parallel connection of LEDs

where n is the number of LEDs connected in series in one branch; N is the number of parallel branches.

Mixed connection includes the positive properties of parallel and series connection options.

Due to the fact that the human visual apparatus is inertial, quite often when powering LEDs they use pulse current. The value of the average pulse current flowing through the LED is determined from the expression

In Fig. Figure 4.25 shows timing diagrams of pulsed current.

Rice. 4.25. Pulse current timing diagrams

If the pulse duration and pause duration are specified, then the maximum permissible value of the pulse current can be determined:

where In is the rated current of the LED.

As already mentioned, a resistor is the element that limits the current flowing through the LED. But it is convenient to use a resistor if the supply voltage is constant. In practice, it often happens that the voltage is not stable, for example, the battery voltage decreases over a fairly wide range when it is discharged. In this case, it is widely used linear current stabilizers.

The simplest linear current stabilizer can be assembled on widely used microcircuits such as KR142EN12(A), LM317 (and their numerous analogues), as shown in Fig. 4.26.

Rice. 4.26. Circuit of the simplest linear current stabilizer

Resistor R is selected within the range of 0.25-125 Ohms, while the current through the LED is determined by the expression

The design of such current stabilizers is simple (a microcircuit and one resistor), compact and reliable. Reliability is additionally due to a developed overload and overheating protection system built into the stabilizer chip.

To stabilize currents from 350 mA and above, you can use more powerful microcircuits of linear regulators with low voltage drop of the 1083, 1084, 1085 series from various manufacturers or domestic analogues KR142EH22A/24A/26A.

But Linear current stabilizers have significant disadvantages:

• low efficiency;
• large losses, strong heating when regulating large currents.

Therefore in this moment Pulse converters and stabilizers are increasingly used to power LEDs and LED modules. In Fig. Figure 4.27 shows the appearance of the LED module and secondary optics.

Rice. 4.27. Appearance LED module and secondary optics

It should be noted that the LEDs and the power converter are structurally designed on a single board.

See other articles section.