Wednesday, January 26, 2011

Weak current College】 【LED lights for the latest protection technology (1)】.




White led as has many advantages, is more and more into people's daily life, its usage now become very large. It is a new device, has its own use. White led is voltage-sensitive device. Each LED work do not exceed the current 20mA, over too many LED will easily destroyed. LED if normal use, their life is very long. But people in the actual use LED tend to bad, why? "is actually does not take into account the characteristics and the use of LED on it plus protection circuit.
First, why would you want to protect the LED

White led as has many advantages, is more and more into people's daily life, its usage now become very large. It is a new device, has its own use. White led is voltage-sensitive device. Each LED work do not exceed the current 20mA, over too many LED will easily destroyed. LED if normal use, their life is very long. But people in the actual use LED tend to bad, why? "is actually does not take into account the characteristics and the use of LED on it plus protection circuit.

LED is an optoelectronic semiconductor devices, the mounting process easily electrostatic wounded. This requires the Assembly process for static electricity protection. We found many manufacturers who do not have this concept, or simply do not understand that this is not acceptable.

LED in practice to 20mA current ceiling, but often because of various reasons current increases, if you do not take protective measures, which increase the current exceeds a certain amount of time and amplitude will damage after LED.

Second, LED the cause of the damage caused by

1, power supply voltage jumps. Let the power supply voltage jumps reason many, such as power quality issues, or the user's reason for improper use, and so on, may let power voltage jumps.
2. line a symbol or printed lines or other conductor short-circuit and formation LED power supply circuit, short circuit of the place of the voltage increase.
3, a LED for the quality of their own because damage resulting in short, it's original voltage drop is passed on to other LED.
4, lamps within the temperature is too high, the characteristics of the LED.
5. lamp inside into the water, the water is electrically conductive.
6. when the Assembly is not good anti-static work, LED by internal hurt by static electricity. Despite the imposition of the normal voltage and current values, it is extremely easy to cause damage to the LED.

These reasons will cause the led current apparent rise significantly, soon LED chips will cause overheating and burnt. In our experience, most LED after burning poles short-circuit, few are circuit breakers. Each LED about 3.2V pressure drop around, it burned down the conspiracy after if circuit breaker LED does not light up. If this voltage is transferred to the short circuit of the other LED, causing other more current LED, LED to faster burned, or even emergency power supply. Originally a small damage caused on extremely easy big accident. LED General installation in height when the installation is not easy, and even more difficult to repair. So LED protection is a real demand, but we currently do not have to be valued, and many people but there is no way to deal with challenges.
Third, how to protect LED to

Protection against LED our first thought was to use fuse, but the fuse is a one-time, and reaction speed is too slow, poor actually use both effects are also very cumbersome, the fuse is not suitable for now led lighting products, because of the LED lamp now mainly in the cities of light engineering and lighting project. In response to actual needs, we did a lot of experimental and engineering requirements summarized LED protection circuit should have a feature, which is very demanding: beyond the normal use of the current time can start immediately, have LED power supply pathway would be disconnected, so that the LED and power can be protected, in the light of normal and to be able to automatically restore the power supply, without prejudice to the work, the key LED is because it is the civil products, the circuit must not be too complex volume must not be too large, the cost is lower. These requirements are contradictory, the mutual dependence, it is difficult to implement.

First you should determine what kind of protection circuit selection and protection devices.

1, we can choose to use transient voltage suppression diode (TVS). Transient voltage suppression diode is a diode forms of high-performance protective device. When it is reverse-polarity of transient high energy impact, can be negative in 10 12 th seconds a very short time, make your own high resistance between the two poles to immediately reduce to low resistance, absorb up to several kilowatts of power, the bipolar surge of voltage clamp in an target voltage values, valid preserving electronic circuit in precision components. Transient voltage suppression diode has a faster response time, transient power, leakage current is low, votalge deviation uniformity is good, clamping voltage are easier to control, limit without damage, small size, etc.

But in the actual use of the discovery is not very satisfactory. First of all is to find enough voltage value of TVS devices is not easy. TVS devices primarily applied to Thunder and lightning and overvoltage 220V + absorption, and the LED lights on the power supply voltage of 12V or 24V General is, the voltage value of the finished product rarely TVS, the test is not good. At the same time we know: LED light Pearl of damage is mainly due to the current Ambassador chip overheat. TVS can only detect voltage cannot detect overcurrent. Overvoltage is certainly cause overcurrent, but to choose a suitable voltage protection is difficult to master, this device will not be able to produce also is very difficult to use in practice.

2, we can choose fromRecovery insurance. Since recovery insurance tube, also known as polymer PTC thermistor PTC, is a polymer and conductive particles, etc. After special processing, conductive particles in polymer chain conductive pathway components. When normal operating current through (or component is in a normal ambient temperature), PTC Resettable fuse is low resistance State; when the circuit have exceptions over current through (or ambient temperature rise), high-current (or the ambient temperature rises) the heat generated rapid expansion of the polymer, which breaking the conductive particles composed of conductive pathway, PTC Resettable fuse is a high-impedance state; when the circuit overcurrent (over-temperature condition) disappears, polymer cooling, the volume is back to normal, which constitute the conductive particles conductive path again, PTC Resettable fuse also assumes an initial low resistance State. In normal working state since recovery insurance tube heating is very small, at anomaly working state it very high resistance of fever is very large, which has limited the adoption of its current to play a protective role. Its small size, low cost, can be repeatedly used for the protection of auto-start automatic exit; it is a solid package impact resistance is not easy to be damaged; our actual tests found: because it is heat sensitive devices, the temperatures are high, due to the PTC encapsulated in lamps, light Pearl surely influence to fever is about to PTC's work performance. On the identified lamps can test to select the PTC, relatively reliable used method is to keep it away from heating dengzhu.

In the specific circuit, there are two ways available to choose from:

1. tap protection. General led lamp is divided into many threaded branch. For example, 24V voltage, we are 7 branch of threaded Pearl phase LED light plus a resistance, current General 17 ~ 19mA, as needed, we can choose an integer multiple of 7 light Pearl to combine into a whole lamp. We can be in front of each line with a PTC element individually protected. This approach has the advantage of high accuracy, the reliability of protection.

2. overall protection. In front of all light Pearl plus add a PTC component, on the whole lamp for protection. The benefits of this approach is simple, not the volume. We usually use this approach. As household products, such protection in the actual use of results or satisfactory.
PTC's selection of elegant, we are a long time before seeking out the fairly accurate corresponding values.

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