1. Why protect LED
White LEDs are entering more and more people's daily life due to their many advantages, and its usage is now very huge. It is a new device with its own characteristics. White LEDs are voltage sensitive devices. When each LED works, the current should not exceed 20mA, and too much LED will be easily burned. If the LED is used normally, its life is very long. However, LEDs are often easily broken in actual use. What is the reason? In fact, it does not take into account the use characteristics of LED and add protection circuit to it.
LEDs are optoelectronic semiconductor devices and are easily damaged by static electricity during assembly. This requires electrostatic protection during assembly. We have found that many manufacturers do not have this concept or do not understand it at all, which is not acceptable.
In actual work, the LED is based on a current of 20mA, but the current will often increase due to various reasons in use. If no protective measures are taken, the increased current will exceed the LED after a certain time and amplitude. Will be damaged.
Second, the cause of LED damage
1. Sudden increase in power supply voltage. There are many reasons for the sudden increase of the power supply voltage. For example, the quality of the power supply or the user's improper use may cause the power supply voltage to suddenly increase.
2. A short circuit of a component or printed line or other wire in the circuit forms a partial short circuit of the LED power supply path, which increases the voltage in this place.
3. An LED is damaged due to its own quality and thus forms a short circuit, and its original voltage drop is passed on to other LEDs.
4. The temperature in the lamp is too high, which deteriorates the characteristics of the LED.
5. Water has entered the lamp, and the water is conductive.
6. The anti-static work was not done during assembly, which caused the interior of the LED to be damaged by static electricity. Although the normal voltage and current values ​​are applied, it is also easy to cause damage to the LED.
These reasons will cause the LED current to rise significantly, and soon the LED chip will be burned due to overheating. According to our experience, most LEDs are short-circuited after being burned, and a few are open circuits. Each LED has a voltage drop of about 3.2V. If it burns out, the string of LEDs will not emit light if it is disconnected. If this voltage is short-circuited, it will be transferred to other LEDs, resulting in a larger current of other LEDs, and other LEDs will be burned faster, or even a critical power supply. Originally, a small damage can easily cause a major accident. LEDs are generally installed in high places, which is not easy to install, and even more difficult to maintain. Therefore, the protection of LEDs is a practical requirement, but it has not been valued by everyone, and it is also a problem that many people have no choice but to deal with.
3. How to protect the LED
For the protection of LEDs, the first thing we think of is to use a fuse, but the fuse is disposable, and the reaction speed is too slow, which is not only effective but also troublesome, so the fuse is not suitable for the current LED Lamp products. Because LED lights are now mainly in the city's glorious projects and lighting projects. In response to this actual demand, we have done a lot of experiments, and summarized the characteristics of the LED protection circuit according to the requirements of the project. It is very harsh: it can immediately start protection when the normal use current is exceeded, so that the LED power supply path It is disconnected, so that both the LED and the power supply can be protected, and the power supply can be automatically restored after the entire lamp is normal, which does not affect the operation of the LED. The key is because it is a civilian product, and the added circuit cannot be too complicated. The cost is lower. These requirements are contradictory and restrictive, and it is very difficult to achieve.
First of all, you should decide which protection circuit and protection device to choose.
1. We can choose to use transient voltage suppression diodes (referred to as TVS). Transient voltage suppression diode is a high-efficiency protection device in the form of a diode. When its two poles are impacted by reverse transient high energy, it can reduce the high resistance between its two poles to low resistance at a very short time of 10 negative 12th power seconds, absorbing surge power of up to several kilowatts. The voltage between the two poles is clamped to a predetermined voltage value, which effectively protects the precision components in the electronic circuit. Transient voltage suppression diodes have the advantages of fast response time, large transient power, low leakage current, good consistency of breakdown voltage deviation, easy control of clamping voltage, no damage limit, and small size.
However, it was found to be not ideal in actual use. The first is to find a TVS device that meets the required voltage value is not easy. TVS devices are mainly used for lightning protection and lightning protection, as well as overvoltage absorption above 220V, and the power supply voltage of LED lamps is generally 24V or 12V. TVS products of this voltage value are few and the test is not easy to carry out. At the same time, we know that the damage of the LED Light bead is mainly caused by the overheating inside the chip of the current ambassador. TVS can only detect overvoltage but not overcurrent. Over-voltage is definitely the cause of over-current, but it is difficult to grasp the appropriate voltage protection point, this device cannot be produced and is difficult to use in practice.
2. We can choose self-recovery insurance tube. Self-recovery fuse, also known as polymer polymer positive temperature thermistor PTC, is composed of polymer and conductive particles. After special processing, the conductive particles form a chain-shaped conductive path in the polymer. When the normal operating current passes (or the component is at normal ambient temperature), the PTC resettable fuse assumes a low resistance state; when an abnormal overcurrent passes through the circuit (or the ambient temperature increases), the large current (or the ambient temperature increases) The generated heat causes the polymer to expand rapidly, which cuts off the conductive path formed by conductive particles. The PTC self-recovery fuse is in a high-resistance state; when the overcurrent (over-temperature state) in the circuit disappears, the polymer cools and the volume recovers Normally, the conductive particles reconstruct the conductive path, and the PTC resettable fuse assumes the initial low resistance state. In normal working state, the self-recovery fuse has very little heat. In abnormal working state, its heat is very high and its resistance value is very large, which also limits the current passing through it and thus plays a protective role. It is small in size, low in cost, and can be used repeatedly. It realizes the automatic start and automatic exit of protection; it is a solid-state package that is resistant to shock and is not easily damaged; we found in actual tests: because it is a heat-sensitive device, it is affected by temperature The impact is very large, because the PTC is packaged inside the lamp, the light bead must heat to affect the performance of the PTC. The PTC can be selected by experiment for the identified lamps. The more reliable method of use is to keep it away from the hot lamp beads.
In the specific circuit, there are two methods available for selection:
1. Shunt protection. Generally, LED lights are divided into many serial branches. For example, 24V voltage, we use 7 LED light beads connected in series and add a resistor, the current is generally 17 ~ 19mA, according to need, we can choose an integer multiple of 7 light beads to form a whole lamp. We can add a PTC element in front of each branch to protect separately. The advantages of this method are high accuracy and good reliability of protection.
2. Overall protection. Add a PTC element in front of all the light beads to protect the whole lamp. The advantage of this method is that it is simple and does not occupy volume. We generally choose this method. As far as household products are concerned, the results of such protection in actual use are satisfactory.
The selection of PTC is very particular, and we have only found out more accurate corresponding values ​​after a long time of experiment.
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