White LED is a voltage-sensitive device, in the actual work is a 20mA of the current limit, but often due to the use of various reasons for the current increase, if not to take protective measures, this increase in the current more than a certain amount of time and magnitude of the LED will be damaged.
The cause of the LED damage is mainly:
① the sudden rise of the supply voltage.
② a component in the line or a printed line or other wires short-circuit and form the LED power supply path local short-circuit, so that the voltage increase in this place.
③ a LED because of its own quality damage to form a short-circuit, its original voltage drop on the other led on the transfer.
④ lamps in the temperature is too high, so that the characteristics of the led to deteriorate.
⑤ inside the lamp into the water, the water is conductive.
⑥ in the assembly time did not do the work of anti-static, led to the internal has been harmed by electrostatic. Although the normal voltage and current value is imposed, it is also very easy to cause damage to the LED.
So how do I protect the LED circuits?
1. Use fuse (tube) in protection circuit
Because the fuse is disposable, and the reaction speed is slow, the effect is poor, the use of trouble, so the fuse is not suitable for the LED lamp finished product, because the LED lamp is now mainly in the city Guangcai project and lighting project. It requires the LED protection circuits to be very harsh: in excess of the normal use of the current to start protection immediately, so that the LED power supply path is disconnected, so that the LED and power can be protected, the entire lamp will be able to automatically resume power, without affecting the LED work. The circuit can not be too complex volume can not be too large, the cost is low. So it is difficult to implement the fuse.
2. Use of transient voltage suppression diode (TVS)
The transient voltage suppression diode is a kind of high-performance protection device in diode form. When its poles are subjected to a reverse transient high energy shock, it can take a 10 minus 12 times a very short time of the speed of the second, so that the high impedance of the two poles immediately reduced to low resistance, absorbing up to thousands of watts of surge power, the bipolar voltage clamp at a predetermined voltage value, effectively protects the electronic circuits of precision components. The transient voltage suppression diode has the advantages of fast response time, large transient power, low leakage current, consistency of breakdown voltage, easy control of clamping voltage, no damage limit, small volume and so on. However, it is not easy to find the TVs devices that satisfy the required voltage value in actual use. The damage of LED beads is mainly due to the overheating of the current ambassador Chip. TVS can only detect overvoltage and cannot detect current. To choose the right voltage protection point is difficult to grasp, this device can not be produced and it is difficult to use in practice.
3. Choose the self recovery fuse
The self-recovery fuse is also known as polymer positive temperature thermistor PTC, which is composed of polymer and conductive particles. After special processing, conductive particles constitute a chain conductive pathway in the polymer. When the normal working current passes (or the component is in the normal ambient temperature), the PTC Self recovery fuse is low-resistance state; the heat generated by the large current (or the ambient temperature rises) makes the polymer swell rapidly, and it cuts the conductive pathway formed by the conductive particles, and the PTC fuse is a high-impedance state; When the circuit is over the current (over-temperature) disappears, the polymer cools, the volume is restored to normal. In which the conductive particles are again formed, the PTC self-recovery fuse is initially low-resistance state. In the normal working state self recovery Insurance tube fever is very small, in abnormal working state its fever very high resistance is very large, also limited through its current, thus played a protective role.
In a specific circuit, you can choose:
① tap Protection. The general LED lamp is divided into many strings. We can add a PTC component to the front of each branch to protect it separately. The advantages of this approach are high accuracy and good reliability.
② overall protection. A PTC component is added to the front of all light beads to protect the whole lamp. The advantage of this approach is simplicity, not the volume. For civilian products, the actual use of this protection in the results is satisfactory.
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