White LEDs are voltage-sensitive devices. In practice, the upper limit of current is 20mA, but the current often increases due to various reasons in use. If no protection measures are taken, this increased current will The LED will be damaged after a certain time and amplitude.
The main reasons for LED damage are:
1. Sudden rise in supply voltage.
2. The short circuit of a component or printed line or other wires in the line forms a partial short circuit of the LED power supply path, which increases the voltage in this place.
3. A certain LED is damaged due to its own quality and thus forms a short circuit, and its original voltage drop is transferred to other LEDs.
4. The temperature in the lamp is too high, which makes the characteristics of the LED worse.
5. Water has entered the interior of the lamp, and the water is conductive.
6. The anti-static work was not done well during assembly, so that the interior of the LED has been damaged by static electricity. Although the imposed
The normal voltage and current values are also very easy to cause damage to the LED.
So, how should I protect the LED circuit? Here are some thoughts to share:
1. Fuse (tube) is used in the protection circuit
Since the fuse is a one-time use, and the response speed is slow, the effect is poor, and it is troublesome to use, the fuse is not suitable for use in finished LED lights, because LED lights are now mainly used in urban glory projects and lighting projects. It requires the LED protection circuit to be very strict: the protection can be started immediately when the normal use current is exceeded, so that the power supply path of the LED 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. Does not affect the LED work. The circuit should not be too complicated, the volume should not be too large, and the cost should be lower. Therefore, it is very difficult to use the fuse method.
2. Use transient voltage suppression diodes (TVS for short)
A TVS 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 immediately reduce the high resistance between its two poles to low resistance at a very short speed of 10-12 seconds, absorb up to several kilowatts of surge power, and remove the two poles. The voltage clamping element between the two is at a predetermined voltage value, which effectively protects the precision components in the electronic circuit. TVS diodes have the advantages of fast response time, large transient power, low leakage current, good consistency of breakdown voltage deviation, easier control of clamping element voltage, no damage limit, and small size.
However, in actual use, it is not easy to find TVS devices that meet the required voltage value. The damage to the LED lamp is mainly caused by overheating the inside of the chip due to excessive current. TVS can only detect overvoltage but not overcurrent. To choose the appropriate voltage protection point is difficult to grasp, this kind of device cannot be produced and it is difficult to use in practice.
3. Select the resettable fuse
The resettable fuse, also known as the polymer positive temperature thermistor PTC, is composed of polymer and conductive particles. After special processing, the conductive particles form chain-like conductive paths in the polymer. When the normal operating current passes (or the component is at normal ambient temperature), the PTC resettable fuse is in a low resistance state; when there is an abnormal overcurrent in the circuit (or the ambient temperature rises), the high current (or the ambient temperature rises) The generated heat causes the polymer to expand rapidly, which cuts off the conductive path formed by the conductive particles, and the PTC self-recovery fuse is in a high resistance state; when the overcurrent (overtemperature state) in the circuit disappears, the polymer cools and the volume recovers Normally, the conductive particles re-form the conductive path, and the PTC resettable fuse is in the initial low resistance state.
In the normal working state, the self-recovery fuse has very little heat, and in the abnormal working state, its heat generation is very high, and its resistance value is very large, which limits the current passing through it, thus playing a protective role. In a specific circuit, you can choose:
1. Shunt protection. Generally, LED lights are divided into many series branches. We can add a PTC element in front of each branch for protection. The advantage of this method is high accuracy and good reliability of protection.
2. Overall protection. Add a PTC element in front of all light beads to protect the whole lamp. The advantage of this method is that it is simple and does not take up volume. For civilian products, the results of this protection in actual use are satisfactory.