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Jan 2 2018

LED lighting circuit protection

Warm hints: The word in this article is about 4000 and reading time is about 20 minutes.


LED circuit is a kind of semiconductor that can convert electric energy into visible light. It changes the principle of incandescent tungsten light and tri-color emitting light of energy-saving lamp. And it adopts electric field to emit light. If you need to use more LED products and connect all of the LEDs in series. It will require the LED driver to output a higher voltage. If all of the LEDs are connected in parallel, you will need a larger LED driver output current. All of the LED series or parallel, not only limits the amount of LED usage, and parallel LED load current is larger, the cost of the drive will increase, the solution is to use hybrid approach. Arrange average number of LEDs in a series and parallel, so that the voltages assigned to one LED series branch are the same and the currents in each LED in the same series branch are also substantially the same with the same brightness and the current through each series branch Similar. This article is mainly talking about three aspects of the LED circuit including what is LED circuit,How to protect the LED circuit; Circuit protection of series LED lighting and etc.

Article Core

LED lighting circuit protectionCategoryoptolectronics/LED/circuit

English name

LED circuit



I.What is LED circuit3.1 LED lighting system protection3.7 Silicon controlled rectifier
II.How to protect the LED circuit3.2 Single LED protection3.8 Zener diode
2.1 Using the fuse in protection circuit (tube)3.3 LED control and failure mode3.9 Polymer Polymers ESD Protector
2.2 Using transient voltage suppression diodes (referred to as TVS)3.4 Circuit protection parameters selected by the device3.10 LED open circuit protector
2.3 Select self-recovery fuse3.5 Potential LED Bypass Protector Analysis3.11 LED open circuit protector operation
III.Circuit protection of series LED lighting3.6 Varistor



I.What is LED circuit

LED circuit is a kind of semiconductor that can convert electric energy into visible light. It changes the principle of incandescent tungsten light and tri-color emitting light of energy-saving lamp. And it adopts electric field to emit light. If you need to use more LED products and connect all of the LEDs in series. It will require the LED driver to output a higher voltage. If all of the LEDs are connected in parallel, you will need a larger LED driver output current. All of the LED series or parallel, not only limits the amount of LED usage, and parallel LED load current is larger, the cost of the drive will increase, the solution is to use hybrid approach. Arrange average number of LEDs in a series and parallel, so that the voltages assigned to one LED series branch are the same and the currents in each LED in the same series branch are also substantially the same with the same brightness and the current through each series branch Similar.

White LED is a voltage-sensitive device, in the actual work, we should limit current to 20mA. but due to various reasons in use,  current increases often occur. If there is no protection measures, over a certain period of time, , this increased current wi;ll damage amplitude LED. 

LED lighting circuit protection

LED damage caused by the main reasons are:

  • ① sudden increase in supply voltage.

  • ② a component of the circuit line or printed lines or other wires short circuit cause the formation of short LED power supply circuit, so that the voltage increases.

  • ③Because of  LED’s own quality damage, thus form a short circuit. It is the original voltage drop passed on to other LED.

  • ④ lamp temperature is too high, so that the characteristics of the LED deterioration.

  • ⑤ lamps into the water inside, water is conductive.

  • ⑥You did not do a good job of anti-static work in the assembly , so that the internal LED has been hurt by the static electricity. Although the application of the normal voltage and current values, but also easily lead to LED damage.

  • light source


    Power (Lumens / watt)

            Heat loss(%)




    Incandescent lamp






    Fluorescent light






    High-intensity gas discharge light source












Table 1:Comparison of the thermal conductivity of various light sources

The optical properties of LEDs vary significantly with temperature. The amount of light emitted by an LED decreases as the junction temperature increases. For some technologies, the emission wavelength also varies with temperature. Without proper control of the drive current and junction temperature, the LED efficiency drops rapidly, resulting in diminished brightness and shortened lifetime. Another LED characteristic associated with the junction temperature is the forward voltage (Figure 1), VF is the forward voltage drop across the LED. VF is about 2 volts (for the red LED) and 3.5 volts (for the blue LED). TF stands for the temperature inside the LED when the PN junction. If this temperature is too high, the LED will be damaged. Ij is the forward current through the LED. If only one simple bias resistor is used to control the drive current, the VF will decrease with increasing temperature and the drive current will increase. This is especially true for high-power LEDs, which can lead to thermal breakdown and component failure. It is common practice to control the junction temperature by accelerating heat transfer by mounting the LED on a metal-core printed circuit board (PCB). Wire-coupled transients and inrush currents also reduce LED life, and many LED drivers are susceptible to DC voltage and polarity errors, LED drivers output can also be damaged or destroyed by short circuits. Most LED drivers include built-in safety features, including thermal shutdown and LED open and short circuit detection. However, to protect ICs and other sensitive electronic components, additional overcurrent protection components may be required. 

LED lighting circuit protection

II.How to protect the LED circuit

2.1 Using the fuse in protection circuit (tube)

Because the fuse is disposable, and the reaction speed is slow, the effect is poor, so the fuse is not suitable for use in the finished LED light. Because the LED light is now mainly in the city of luster engineering and lighting engineering. It requires LED protection circuit to be very harsh: in the normal use of current can immediately start the protection, so that the LED power supply circuit is disconnected, so that the LED and power can be protected in the normal light and then automatically restore power supply, Does not affect the LED work. The circuit can not be too complicated, the volume can not be too big, the cost is even lower. Therefore, the use of fuses is very difficult.

2.2 Using transient voltage suppression diodes (referred to as TVS)

Transient voltage suppressor diode is a diode form of high-performance protection device. When its poles are subjected to the reverse transient high-energy impact, the high resistance between the two poles can be immediately reduced to low resistance at a very short negative time of 10 seconds 12, absorbing the surge power of up to several kilowatts , Clamps the voltage between the two poles at 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, consistent breakdown voltage deviation, easy to control the clamping voltage, no damage limit, small size and so on. However, in actual use, it is not easy to find a TVS device that meets the required voltage value. LED light bead damage is mainly due to over current caused by overheating the chip internal. TVS can only detect over-voltage can not detect over-current. To choose the right voltage protection point is difficult to grasp, this device can not be produced is difficult to use in practice.

2.3 Select self-recovery fuse

Self-recovery fuse, also known as polymer positive temperature PTC thermistor, is composed of polymer and conductive particles. After special processing, the conductive particles form a chain-like conductive path in the polymer. When the normal operating current travel through (or components in the normal ambient temperature), PTC self-recovery fuse shows low resistance state. When abnormal over-current go through the circuit (or ambient temperature increases), the high current (or ambient temperature) heat generated by the rapid expansion of the polymer, also cut off the conductive path formed by the conductive particles, PTC self-recovery fuse shows high impedance state. When the circuit over-current (over-temperature state) disappears, the polymer becomes cooling and volume recovers to normal, in which the conductive particles re-constitute the conductive path. PTC self-recovery fuse shows the initial state of low resistance. In normal working condition, the heat of self-recovery fuse is very small. In the abnormal working condition, its heat resistance is very high, which limits the current passing through it and thus plays a protective role.

In the specific circuit, you can choose:

  • ① shunt protection. General LED lights are divided into many tandem branches. We can add a PTC component in front of each branch separately for protection. The benefits of this approach are high accuracy and good reliability of protection.

  • ② overall protection. In front of all light beads plus a PTC component, the entire lamp to protect. The benefits of this approach is simple, does not account for the volume. For the civilian product, the result of this protection in actual use is still satisfactory.


III.Circuit protection of series LED lighting

Light-emitting diodes (LEDs) are fragile devices that are exposed to heat, mechanical shock, electrostatic discharge and lightning, especially when used outdoors. Due to the increased use of LED strings for lighting and backlighting, researchers are required to pay more attention to the reliability of LED strings. High-brightness LEDs, because of their sapphire substrates, are very sensitive to voltage transients caused by nearby lightning strikes. Even in home applications, LED strings still require electrostatic discharge (ESD) protection to ensure long-term, reliable operation of the entire assembly. In the absence of such protection, if one of the LEDs in the series fails and the circuit is opened, all other LEDs will be off.

3.1 LED lighting system protection

A lot of protection devices can be used for power and LED driver, and many publications can guide the choice. The circuit in Figure 1 is an example of switching power protection in an LED street lighting system. In this circuit, the AC fuse provides basic fire protection against major system faults that may cause overcurrent conditions, tolerate surges from 3KA to over 6KA without breaking . In the event of downstream component failure, the DC fuses can be used for fast overcurrent protection in DC-DC converters or LED driver circuits.  

Examples of LED Street Lighting Circuits--LED lighting circuit protection

Figure 1: Examples of LED Street Lighting Circuits

In the circuit AC input side, we alsoneed to deal with overvoltage events and voltage transients. They are usually caused by nearby lightning strikes, but they can also occur through transient changes in power lines. A typical protection for overvoltage conditions is a metal oxide varistor (MOV) that can be combined with a transient voltage suppressor (TVS). Circuits for power protection also need to be isolated from the ground to prevent possible electrical shock hazards (these specifications are included in IEC / UL 60950-1, UL 1449 and IEC / UL 6500). Figure 2 shows the solution to meet these requirements. The design incorporates a metal oxide varistor with a transient voltage suppressor.  

Example of a power input overvoltage protection scheme for LED lighting--LED lighting circuit protection

Figure 2: Example of a power input overvoltage protection scheme for LED lighting

In addition, the overvoltage protection should also be considered into LED driver chips. The appropriate decoupling capacitor, along with the TVS device rated for the line driver's supply voltage, will provide a very stable design. Some LED driver manufacturers include a circuit that senses an open LED string, but the circuit should not be confused with the circuit that protects the string. Keep the LED string running If an LED failure, 

3.2 Single LED protection

Bypass protection inside the LED string (Figure 3) allows the string to continue operating when a single LED fails open. Help protect the LED driver by limiting any excess current or voltage across the LED string.  

 Using a suitable bypass protector to protect a single LED in a LED string can not only keep the LED string still able to illuminate, but also help protect the LED driver from overcurrent and overvoltage (possibly due to LED fault)

Figure 3: Using a suitable bypass protector to protect a single LED in a LED string can not only keep the LED string still 

able to illuminate, but also help protect the LED driver from overcurrent and overvoltage (possibly due to LED fault)

When referring to the protection of a single LED, the need to install a series circuit branch within the LED string, select the correct protection device is essential. This requires understanding the underlying mechanism of LED failure and how different types of protection devices work. This understanding helps the circuit designer choose the right device, including a device that keeps the LED strings still running when there is a fault in the LED due to an open circuit.

3.3 LED control and failure mode

The series LED strings are driven by a constant current generated by a switching power supply that drives the series LED strings to full brightness, color and intensity. The constant current source provides better control of the brightness of the LED group and a more uniform brightness of the LED group. LED is a fragile solid state device, essentially a diode, configured as a P-N junction that emits light when forward biased. The main LED failure mechanism is essentially mechanical and thermal, including thermal cycling, thermal shock and LEDs that operate at high temperature (which can cause wire solder aging and malfunctioning). As the metal oxidizes and becomes brittle over time, the likelihood of the LED malfunctioning increases. A surge caused by an ESD event or nearby lightning strike is another common cause of LED failure.

3.4 Circuit protection parameters selected by the device

The main parameters of the LED protection device selection is the rated current and rated power of a single LED, the forward voltage and the LED driver constant current output voltage. Typical high-brightness LED rated at 1 watt and 3 watts. High-brightness LED at rated power The maximum current consumption can be easily determined by the following formula:


Where I is the current, P is the LED rated power, VF is the LED forward voltage. The LEDs are available in different power ratings, so these values vary accordingly. At the same time, LEDs emitting different wavelengths (colors) have different voltage drops. For example, a red LED usually has a lower VF than a white LED, consuming more current. The main reliability issue is if a high-brightness LED fails, forming an open, high-brightness LED string operating continuously. This problem is crucial in applications that require high-reliability light sources.

Many outdoor applications are located off the ground, so a small accident can become a big problem. In series, the failure of a single high-brightness LED open-circuit can result in significant expense and inconvenience as the entire assembly must be repaired.

3.5 Potential LED Bypass Protector Analysis

In order to protect a single high-brightness LED and prevent the entire series of high-brightness LEDs from being extinguished when a single high-brightness LED fails, a bypass protector must be installed on the high-brightness LED terminal. There are many devices to consider, including varistors, thyristors, Zener diodes, electrostatic discharge protectors and LED open-circuit protectors.

3.6 Varistor

These devices are best suited for relatively high power line transients, especially those caused by lightning strikes and large inductive load conversions. Unfortunately, these devices do not respond quickly and can not protect the LEDs from low-level transients that could cause LEDs to malfunction. In addition to this drawback, if the LED fails to open, the varistor can not provide a path for the current, so the entire string of LEDs will turn off. The heat generated by varistors can also be a problem for light-emitting diodes.

3.7 Silicon controlled rectifier

In the vicinity of a failed LED, the thyristor can route current to keep the rest of the LED string from emitting light. However, these devices are large devices that typically require resistive divider to set the trigger voltage. At different temperatures, the thyristor trigger voltage changes can be very large. In addition, the reverse blocking voltage is too high, so thyristors can not provide reverse polarity protection.

3.8 Zener diode

Although Zener diodes are typically much smaller than SCRs, other problems persist. When one LED fails to form an open circuit, the Zener diode must direct all the current in the string of strings. Most Zener diodes have a relatively low current rating, so their lifetime is relatively short for this type of application. Zener diodes can also cause micro-environmental thermal events, which can lead to further LED failures.

3.9 Polymer Polymers ESD Protector

These devices are often dedicated to high-speed digital circuits and are not used for dc line protection, as is the case with LED strings. They have higher dynamic resistance than silicon devices, so their clamping voltage is too high to protect fragile LEDs. In addition, they do not provide overload protection or reverse polarity protection.

3.10 LED open circuit protector

These devices are specifically designed to hold the rest of the string of LED strings and still function properly in the event of a LED failure. They are compact silicon-based devices that are mounted on each LED terminal. As bypass devices, they are able to route current around open-circuit LEDs, keeping the rest of the LED strings still illuminated. Some LED open circuit protectors also provide electrostatic discharge / lighting and reverse polarity protection, which reduces lighting circuit costs and eliminates the need for additional protection components.

3.11 LED open circuit protector operation

The LED open-circuit protector is a two-terminal device that is internally triggered by an LED and automatically resets if the LED itself is repaired or replaced. This protector is a voltage-triggered switch with low leakage and similar to a microampere switch. When it is triggered (Figure 3), it becomes a low impedance microampere switch that minimizes power consumption. The via status LED drops about 0.7V, which is not enough to turn on the protector device. Once there is an open LED fault, there is enough circuit voltage to trigger the protector to the via state (constant current output voltage provided by the LED driver circuit). The PLED6 series, described in Figure 3, is also built-in surge resistant to help protect the LED from surges caused by nearby lightning strikes or electrostatic discharge events.  

Typical Open LED Protector V-I Features

Figure 4: Typical Open LED Protector V-I Features

Take the Littelfuse PLED6 family of devices as a typical LED open circuit protector. Figure 4 shows the key parameters of VBR, IS, IH, IT, and VT. As shown by the V-I curve, VBR defines the area between the device's open-circuit voltage rating and the breakdown voltage rating. In the off state, VBR is a continuous peak combination of DC and AC voltages that can be applied to the device, which results in currents through the device of less than 5 μA (various minimum VBR ratings from 6 to 33 V DC are feasible ). IS is the current value that can cause the device to switch from the off state to the via state when the lowest VBR is applied. In general, the maximum IS is 100mA. The holding current (IH) is the minimum current requirement (typically 5mA) to maintain the device in the on-state. Path state voltage (VT) is the maximum voltage across the device during full conduction. IT is the maximum current rating that the device can pass within two seconds (maximum is 1A) while in the path. In general, the LED string current is much lower than this value, so that the LED open circuit protector can be maintained.

Ideally, each LED has a protection device in the open-LED protection scheme. However, "budget" protection programs may play a role. For example, a PLED can be mounted on two LEDs in a series when selecting the appropriate LED open-circuit device. One LED fault will cause both LEDs to darken, but this cuts the protection cost by half.



Lighting manufacturers are making the transition to producing LEDs because of their low cost, high performance, and low maintenance, and their lifespan extends to tens of thousands of hours. However, they still require antistatic protection, especially in high-reliability applications. Such as safety-critical lighting deployed in harsh environments. In fact, outdoor LED lighting has unreliability over traditional lighting unless designers add the appropriate circuit protection to prevent the most severe overvoltage conditions from occurring.

The first line of defense is a good circuit protection scheme that extends the input power to each LED. Open-LED protectors handle major overvoltage transients. When one LED fails to form an open circuit, the remaining LEDs in series can still be illuminated. Although such protection is beneficial, it is still only part of a total protection solution that includes fuses that protect the switched-mode power supplies and LED drivers, MOVs and TVS installations.

LED over-current circuit proteciton video is adding: 

Book Recommendation

  • Practical Lighting Design with LEDs

This book covers all of the information needed to design LEDs into end-products.  It is a practical guide, primarily explaining how things are done by practicing engineers.  Equations are used only for practical calculations, and are kept to the level of high-school algebra.  There are numerous drawings and schematics showing how things such as measurements are actually made, and showing curcuits that actually work.  There are practical notes and examples embedded in the text that give pointers and how-to guides on many of the book's topics.  After reading each chapter of the book, readers will have the knowledge to implement practical designs.  This book will be kept as a reference tool for years to come.

--Ron Lenk  (Author),‎ Carol Lenk (Author)

  • Make It Glow: LED Projects for the Whole Family

Everyone loves to play with light and this collection of kid-ready LED projects will have young Makers exploring electricity and electronics while opening up a world of endless fun! Makers, tinkerers, hobbyists, and parents will be drawn to the decorative and exciting possibilities of the projects in Make It Glow.  Filled with full-color photographs and step-by-step instructions that anyone can follow, this beautiful book features 21 exciting projects that can be completed by even the youngest Maker. You'll start with ultra simple projects and then tackle increasingly complicated ones. Building upon lessons learned in earlier projects ensures that kids learn and succeed. Everyone will be thrilled by the fun, decorative designs that result as you develop new ways to unleash your creativity!

With Make It Glow, you'll learn to make: Blue-light greeting cardsEyes in the dark FlickerbugsLight-up fairy wingsAn illuminated tote bagA bouquet of electric rosesand more!Featuring beautifully photographed inspirational projects for kids and adults, Make It Glow helps you learn the basics of electronics and soft circuits to create costumes, home decorations, clothing, jewelry, and more.

--Emily Coker  (Author),‎ Kelli Townley (Author)

  • LED Lighting: A Primer to Lighting the Future

We’re on the brink of a lighting revolution with light-emitting diodes—the tiny LEDs you’ve seen in electronic devices for years. With this practical guide, you’ll go behind the scenes to see how and why manufacturers are now designing LED devices to light everything from homes and offices to streets and warehouses.Author Sal Cangeloso shows you the working parts of a “simple” LED bulb and explains the challenges electronics companies face as they push LED lighting into the mainstream. You’ll learn how you can use LEDs now, and why solid state lighting will bring dramatic changes in the near future. Explore the drivers, phosphors, and integrated circuits in a typical LED bulb

  1. Understand the challenges in producing LED bulbs with acceptable brightness, color temperature, and power consumption

  2. Learn about non-bulb LED applications, including lamps, street lights, and signage

  3. Discover the market forces driving—and impeding—the adoption of LED lighting

  4. Compare LEDs to compact fluorescent lamps (CFLs) and electron-stimulated luminescence (ESL) bulbs

  5. Gaze into the future of intelligent lighting, including networked lighting systems

--Sal Cangeloso  (Author)

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