FAQs about Light Emitting Diodes (LEDs)

Publisher:EnchantedWishLatest update time:2012-03-22 Source: 电子发烧友Keywords:LED Reading articles on mobile phones Scan QR code
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1. Why can’t white LEDs be used at overvoltage or overcurrent?

Generally, the most commonly used 5mm white light LED has a normal working voltage of 3.0-3.5V and a normal working current of 20mA. However, many people mistakenly believe that using white light LED at overvoltage or overcurrent will make it brighter. In fact, the test results show that the luminous flux increases greatly after 15mA, but it hardly increases after 20mA. When it increases to 30mA, it is only 5% more than 20mA, but the LED has obvious heat generation. There is also a life test: after working at 20mA for a month, the attenuation is only 5%, and now there is still 95% of the luminous flux. When working at 30mA for 19 days, the luminous flux is only 50%. It can be said that a white light LED that can work for 100,000 hours under normal conditions will only have a life of 600 hours when used at high current! In general, LEDs can be used for more than 50,000 hours, and some manufacturers claim that their LEDs can operate for about 100,000 hours, but this does not guarantee that LED products can be used for so long. Wrong operation and process can easily "destroy" LEDs. LEDs will gradually degrade over time. It is predicted that high-quality LEDs can maintain more than 60% of the initial light brightness after 50,000 hours of continuous operation. To extend the service life of LEDs, it is necessary to reduce or completely dissipate the heat generated by the LED chip. Heat is the main reason why LEDs stop working.

2. Why does the color of the light emitted by white LEDs always tend to be bluish or yellowish?

This is because white light LEDs are originally made of 1nGaN base materials that emit blue light and covered with conversion material phosphors. This material emits yellow light when stimulated by blue light. The result is a mixture of blue and yellow light, which appears to be white light to the naked eye. If you look at the emission spectrum of white LEDs, you will see that it has two peaks, so there is no LED that truly emits white light. Such devices are difficult to manufacture because the characteristic of LEDs is that they only emit single-color light of one wavelength, while true white light requires the synthesis of multi-color spectra. Due to process reasons, even LEDs imported from more than ten yuan also have this problem, and there is color cast at the edge of the light spot, but the difference is the amount.

White LEDs have a yellowish to slightly purple white light. Common white LED color temperatures are usually in the range of 6500K to 8000K.

3. Is it better to connect LEDs in parallel or in series?

The parallel or series connection of LEDs should be determined mainly according to the form and requirements of the power supply box circuit. Parallel or series connection has its own advantages and disadvantages. The parallel connection method only requires a lower voltage to be applied across each LED, but a ballast resistor or current source is required to ensure the uniform brightness of each LED. If the bias current flowing through each LED is different, their brightness will also be different, resulting in uneven brightness of the entire light source. However, using a ballast resistor or current source to ensure the uniform brightness of the LED will shorten the battery life. The series connection method can essentially ensure the consistency of the current flowing through each LED, but requires a high power supply voltage. When the LED is connected in parallel, since the total current of the circuit is the sum of the currents of each LED, the power supply is required to be able to supply a sufficiently large current.

In addition, in the circuit of series connection, when one LED is disconnected, the whole string of LEDs will not light up; but when one LED is short-circuited, the other LEDs can still light up. In the circuit of parallel connection, when one LED is disconnected, the other LEDs can still light up; but when one LED is short-circuited, the power supply of the whole circuit will be short-circuited, so that not only the other LEDs cannot work normally, but also the power supply may be damaged. Therefore, the circuit of series connection is more advantageous in comparison.

In actual application, LED arrays formed by series and parallel connection are often used. This can overcome or reduce the impact of the above-mentioned single LED disconnection or short circuit causing the entire string of LEDs to not light up or on the entire circuit and power supply. The so-called series-parallel connection is to first connect a small number of LEDs in series and then connect a ballast resistor in series to form a branch, and then connect several branches in parallel to form a "branch group". In addition, the series-parallel-series form can also be used, that is, on the basis of the "branch group" that has been formed, several "branch groups" are connected in series to form the entire lamp circuit. This connection method not only reduces the impact of a LED failure, but also breaks down the ballast resistor into zero, turning several high-power resistors into dozens of low-power resistors, and changing from centralized installation to decentralized installation. This is not only conducive to the heat dissipation of the resistor, but also allows the lamp to be designed more compactly.

FAQs about LEDs

4. Can other colored LEDs be used instead of white LEDs?

Yes, it is possible. However, it must be noted that the normal operating voltages of LEDs of different colors are different and vary greatly. For example, the normal operating voltages of red and yellow LEDs are only about 2V, while the normal operating voltages of blue and green LEDs are closer to white light LEDs, which are about 3V. Therefore, when using them, the number of tubes in series or parallel must be changed accordingly according to the working voltage of various tubes, or the resistance value of the current limiting resistor in series must be changed, otherwise the LED may exceed the normal working current and shorten its service life, and even burn the LED in serious cases. When using red or yellow LEDs, the number of LEDs in series should be increased, or the current limiting resistor in series should be increased; when using blue or green LEDs, generally only the resistance value of the current limiting resistor should be adjusted.

5. Why is the light not on when the circuit is installed and powered on? How should I check it?

This needs to be analyzed according to the specific circuit. But most of the reasons are that the LED is not tested before installation or the positive and negative polarities of the LED are connected incorrectly during the installation and welding process. It may also be that the welding is not done well, resulting in a cold soldering (cold soldering means that it looks soldered on the surface but is not actually soldered firmly). Cold soldering is the main cause of this failure, especially for novices who lack welding experience. The second is that the power of the electric soldering iron is too large and overheated or the welding time is too long, which burns the tube. It may also be that the electric soldering iron leaks electricity and causes the LED to break down and short-circuit. In addition, part of the drive circuit is connected incorrectly or not soldered well. In some circuits, the value of the component that should be adjusted may not be adjusted properly.

When the circuit is installed, you should check it carefully before turning on the power. If the light does not come on, you should turn off the power immediately and then check again. Do not knock on the circuit board with power on to try to find the fault, especially for circuits using 220V power.

If the circuit is connected to a filter capacitor, you should first use a screwdriver or wire to short-circuit the two pins of the filter capacitor to discharge it before checking. This step is very important! Because the filter capacitor has a high voltage of more than 1.4 times the power supply voltage (such as up to 310V for a 220V power supply), so as to avoid the high voltage remaining on the capacitor to injure the human body or destroy the LED at the moment the circuit is connected!

To check the LED that has been soldered on the circuit board, you should first identify the negative pole mark on the notch of the LED tube body, check whether the polarity of each LED is connected incorrectly, and then use two batteries in series to lead out the positive and negative power supplies to touch the two feet of each LED respectively. It is important to pay attention to the polarity of the battery power supply to be consistent with the polarity of the LED to check whether each LED can light up.

When checking the drive circuit, you should carefully check whether the circuit is connected incorrectly according to the circuit diagram, and pay special attention to checking whether the polarity of the rectifier bridge (the longer leg is the positive output, the opposite leg is the negative output, and the other two legs are the AC input) or the rectifier diode and the voltage regulator diode is correct (the end with a black or white line is the negative pole), and also check whether the three electrodes of the transistor or the voltage regulator integrated circuit are connected incorrectly.

6. What is the difference between concentrated LED and diffuse LED? How to choose?

The luminous intensity value of the concentrated LED is very high, because its light is gathered by itself, so its luminous angle is generally small, and the light irradiation range is small. The light it emits is like the beam of light emitted by a flashlight, the brightness of the light spot is very high, but the brightness of the light spot is not too high. The luminous intensity value of the diffused light LED is lower, and its luminous angle is large, which can reach more than 120 degrees, so its illumination range is large and the light emitted is uniform, just like the light emitted by ordinary lighting. Although the brightness value of diffused light LED is generally lower, in fact, the total luminous flux (total light) they emit is generally higher than that of concentrated light LED.

Focusing LEDs are most suitable for situations that require high brightness but a small illumination range, such as spotlights, downlights, flashlights, etc., while diffuser LEDs are more suitable for general room lighting and situations that require soft and uniform light. Therefore, you should choose according to the specific situation to achieve the best lighting effect.

Keywords:LED Reference address:FAQs about Light Emitting Diodes (LEDs)

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