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LEDs. Power features of white LEDs. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / LEDs

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Let's consider in more detail power features of white LEDs. As you know, the LED has a non-linear current-voltage characteristic with a characteristic "heel" in the initial section (Fig. 4.21).

As we can see, the LED starts to glow if a voltage of more 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 required to limit the current, stabilize it at a certain level.

Rice. 4.21. Volt-ampere characteristic of a white LED

The simplest current limiter through an LED is resistor. There are several options for circuitry switching on LEDs. They are divided into circuits with parallel, series 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 inclusion aims either to increase the radiation power or to increase the radiated surface.

Rice. 4.22. Sequential LED Scheme

The disadvantages of series connection is:

• firstly, with an increase in the number of LEDs, the supply voltage also increases, because for the current to pass through the 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.

On fig. 4.23 shows a diagram of the parallel connection of emitting diodes. The total current drawn from the power supply is, in this case, equal to

Rice. 4.23. Scheme of parallel connection of LEDs

The advantage parallel connection is high reliability, since if one of the emitters fails, the rest continue to work.

Disadvantages:

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

The most effective is mixed (combined) series-parallel connectionshown in fig. 4.24. In this case, the number of radiators connected in series 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 switching of LEDs

If we assume that each branch consumes the same current and, therefore, all circuit elements are identical, then the total current consumed from the power source in a mixed connection

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 surge current. The value of the average pulse current flowing through the LED is determined from the expression

On fig. 4.25 shows the timing diagrams of the pulsed current.

Rice. 4.25. Impulse current timing diagrams

If the pulse duration and pause duration are set, then you can determine the value of the maximum allowable value of the pulse current:

where Inom is the rated current of the LED.

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

Resistor R is selected in the range of 0,25-125 Ohm, while the current through the LED is determined by the expression

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

To stabilize currents from 350 mA and above, you can also use more powerful microcircuits of linear regulators with a 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 adjusting large currents.

Therefore, at the moment, pulse converters and stabilizers are increasingly being used to power LEDs and LED modules. On fig. 4.27 shows the appearance of the LED module and secondary optics.

Rice. 4.27. Appearance of the LED module and secondary optics

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

Author: Koryakin-Chernyak S.L.

See other articles Section LEDs.

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