lcd screen backlight voltage factory

It is clearly seen from (13) that the switching frequency must be varied to regulate the output voltage. The highest switching frequency appears at the highest input voltage and the lightest load. On the other hand, the lowest switching frequency happens at the lowest input voltage and the heaviest load. For the SRC to operate in the zero-voltage-switching (ZVS) region, the lowest switching frequency must be higher than the resonant frequency as expressed in (5). Moreover, due to the switching speed limitations of the power devices, the highest switching frequency is below a specified value. In other words, the variations of the input DC voltage and the load variations must be confined to a small range. Usually a power factor corrector (PFC) is added in front of the DC-DC converter to raise the input power factor and reduce the input current harmonics. A phase-shift pulse width modulation (PSPWM) dimming control can effectively confine the load variation of the DC-DC SRC. Consequently, the output voltage variation of the PFC can be limited to a smaller extent. This results in a better operating condition for the SRC. For the PSPWM dimming strategy, the working durations of the shunt LED arrays are properly phase-shifted to confine the variation of the output current of the SRC. Figure 6 illustrates the circuit arrangement for N shunt single-colored LED arrays with PSPWM dimming method. It is almost the same as the conventional one, except that the dimming signals are applied with a specified phase difference. With the PSPWM dimming, there are always overlaps between the LED driving currents. The maximum duty cycle, or the overlap, is 100 %, corresponding to the highest brightness. To prevent the DC-DC SRC from operating at no load, the minimum duty cycle of the PSPWM dimming signal is 1/N, where N is the number of the shunt LED arrays. Under this circumstance, the overlap is zero, corresponding to the lowest brightness. Compared with the conventional dimming scheme, it is apparently recognized that the load variation of the SRC is less with the proposed PSPWM dimming function. To further investigate the operating principle of the PSPWM dimming, a more general case with N shunt LED arrays is discussed as follows. Figure 7 shows the waveforms of the N driving currents and the output current of the SRC. As stated earlier, the duty cycle range of the dimming signal is from 1/N to 100 %. In terms of the phase angle, if a complete period is 360º, the duty cycle range is from 360º/N to 360º. Assuming that the dimming signal for the LED array 1 starts at 0º, then the dimming signal for the k-th LED array would start at

I ran into this old cockpit/pilot room for heavy mining machinery in which there was an LCD display that"s not lit properly, that is, the image was there but the backlight clearly was gone.

Upon careful disassembling the display, I found out that the backlight was indeed not properly working. However I couldn"t tell if it was the fault of the fluorescent tube, or the power supply.

This led me to believe it probably was the fault of the converter board instead of the tube, because no way you could light up a fluorescent tube with a meager 7 or so volts, which sometimes requires as much as 10,000 volts! However I have no specialty in this and relied solely on my common sense, I could be wrong, as there could be for example circuitry that brings up the voltage in the LCD assembly?

But that is unlikely as I should probably point out that the display was, and I say it with great confidence, a run-of-the-mill 17 inch commerical lcd display, nothing special about it, other than it was embedded in the control panel of the pilot room for mining machinery.

We are often asked how to drive a backlight in our displays. There are three main ways to do so: with an LED driver, a current generating resistor, or a current generating resistor and a transistor. We will explore the benefits and drawbacks of these choices.

It’s important to remember that display backlights are typically LED arrays and, like all diodes, LEDs are all about current. Every datasheet will list the necessary current for the backlight, typically called IF (forward current) or ILED.

Using an LED driver is the ideal way to drive an LED backlight. An LED driver is current controlled so the backlight brightness is consistent. If a constant voltage is used instead of a constant current, the backlight brightness can change with the LED temperature. The brightness can also be controlled via pulse width modulation (PWM), which allows designs to be usable in a wider variety of lighting situations.

It’s important to note that an LED driver will not work in all situations. Some display backlights include an internal diode with a forward voltage of 3.3v. If this is the case, the drive will be unable to turn off the backlight. Additionally, some backlights have a forward voltage below 3.3v. In this case, a different method will have to be used or a series resistor will be needed so the voltage is above the forward voltage.

This method is less efficient than an LED driver, but is flexible and uses common parts. Plus, any forward voltage on the LEDs can use this method. However, this method leaves the brightness open to change with LED temperature and does not allow for brightness adjustment..

To calculate the value of resistor to use, find the supply or forward voltage and current specifications for the backlight from the datasheet. Determine what your voltage source and voltage value will be. Calculate the voltage drop between the voltage source and the forward voltage spec of the LED. Then use Ohm’s Law to calculate the resistor value (V=IR so R=V/I).

The final method is to use a resistor in series with the backlight and a transistor with the emitter tied to ground. This method is similar to method 2, but with the added bonus of being able to control the backlight brightness. Connect a fairly large resistor between a PWM signal and the base of the transistor. This makes the voltage drop between the collector and emitter close to zero.

As on charts, LED driving Voltage is always higher than controller"s supply voltage (usually 12V) , that way we need driver/inverter, whatever we call step-up converter, it make from regular Vcc (12V) voltage, voltage able to supply and light up LED strings.

Most problem become, because for newer bigger screens are not available any datasheets, so many times I have to test LED backlight parameter by myself.

That is the reason, we must tread LED backlit screens very carefully, normally we must to test each particular screen separately and be sure to NOT overCurrent or overVoltage LED strings, as LEDs will simply burn off.

Some of those universal drivers have on-board places(pads) for I-Sense resistors mount/remove, and/or short-pads to regulate Current for particular LED backlight, this made possible to match LED driver"s Current parameter with particular screen. However such work need some experience and knowledge about DC/DC step-up converters, to avoid any LED burning disaster.

In practice, is not so difficult, because in monitor"s screens are usually 2 - 6 strings, in fact all strings practically have almost equal Vf , so in all those "universal" drivers, Cathodes of strings are connected together.

So there is only one I-Sense Resistor for total current, so if we know the driver"s chip reference voltage and proper Current value for particular screen backlight, we can count Sense Resistor for particular current, as from Ohm"s law R=U/I , but we have to know exactly the Current of particular LCD display we use, and reference (feedback threshold) voltage of IC used in driver (usually 150mV to 500mV depend on IC type and manufacturer) .

Therefore, 3 LEDs in series will require a forward voltage of 9 volts (3 volts x 3 LEDs), and 6 LEDs in series will require a forward voltage of 18 volts (3 volts x 6 LEDs).

What happens to LEDs in parallel? The voltage remains the same but the current is split equally among each of the parallel circuits. Therefore, if you have 3 parallel groups that each draws 50 mA at 24 volts, the total power draw is 150 mA, also at 24 volts.

You will see that at 3.0V, for example, this particular LED will draw about 120 mA. If we decrease the voltage to 2.9V, the LED will draw a bit less, only about 80 mA. If we increase the voltage to 3.1V, the LED will draw more, about 160 mA.

Because in a 12V LED strip there are 3 LEDs and a resistor in series, supplying 11V instead of 12V is a bit like reducing the voltage for each LED by 0.25V.

What if we supplied just 10V to a 12V LED strip? In this case, we are reducing the voltage per LED by 0.5V each. If we reference the chart, at 2.5V, the LEDs will barely draw any current.

All voltages less than the LED strip rating are safe, as you will always be drawing less current and therefore avoiding any possibility for damage or overheating. But what about voltage levels over 12V?

And keep in mind that each LED will have a different rating, and inherent variation in manufacturing can affect the actual voltage ranges that are acceptable for a particular LED strip.

While it is possible to supply a voltage that is slightly different from the rated voltage, you will have to be careful and precise to ensure that you do not cause any damage to the LEDs.

One way to dim an LED strip is to adjust the input voltage to below is rated level, as we saw above. In reality, however, power electronics are not very good at reducing the voltage output in this way.

One of the significant advantages of LED strip light products is simple yet versatile they are given compatibility with simple constant voltage power supply devices.

It can sometimes be useful to understand the inner workings of such devices as it can help us understand some of the more nuanced aspects of its performance, such as dimming and voltage input changes.

LED backlighting is the most commonly used backlight for small, LCD panels. Light-emitting diodes, or LEDs, are practical components for a light source because of their small size. LED backlighting is popular due to its overall low cost, long life, variety of colors and high brightness.

LED backlights are housed in a light box that has a diffuser to evenly distribute the LED light. The light box is then mounted behind the LCD’s viewing area. The LED backlight comes in two configurations: array and edge lit. The array configuration has the LEDs mounted in a uniform, grid layout within the light box. This configuration gives off a very bright, even light. The disadvantage of an array configuration is that it requires a thick light box design to accommodate the number of LEDs required. The high number of LEDs in this configuration also means it consumes more power.

The other configuration for LED backlights is edge lit. An edge lit configuration is the most commonly used construction for LED backlights. This configuration mounts the LEDs along one edge of the light box. The layout results in a thin design. Edge lit also uses less LEDs overall and therefore consumes less power than an array configuration.

Another type of backlight options is the use of fiber optic technology. Fiber optic backlights use sheets of fiber optic woven cloth and are bundled by a ferrule (metal cap) to an LED or halogen light source. Advantages for the fiber optic technology includes low voltage, low power, and a very uniform brightness. This type of backlighting is ideal for custom display shapes or sizes however it is priced at a higher cost compared to other technologies available.

A third type of backlight option available uses an electroluminescent (EL) panel. The EL backlight is constructed of a series of different material layers that work together to create the light. The EL panel generates light when an electric current (AC power) is applied to its conductive surfaces. The advantage with EL backlighting is its low power consumption, no heat emission, and overall thin composition. EL backlighting is limiting in that it requires an invertor to generate the VAC needed to emit the light.

The last common backlight option available are cold cathode fluorescent lamps (CCFLs). CCFL backlights are a cost effective option typically found in graphic displays. The CCFL backlight for LCDs is usually configured with the lamp on the edge of a diffuser to distribute the light. An inverter is required to supply the voltage required by the fluorescent lamp. CCFLs offer a bright white light with low power consumption. This backlight option is not ideal for cold-temperature applications (less than 15°C) as the light output decreases with decreased ambient temperature.

There are many different backlight options available for your LCD. The most common types are LED, fiber optic, EL, and CCFL backlights. Cost and application of your product will have the highest influences on which backlight technology is best for your LCD.

LCD displays don’t emit light by themselves. They need a light source, and LED backlights are now dominating the market. In this article, Orient Display’s Bill Cheung provides a complete overview of LED backlight technology, discussing different types, driver technologies, color deviation, brightness options and more.

LCD (liquid crystal display) has long been the dominant technology in the display world. Certainly, there are some emerging competing display technologies—such as OLED (Organic Light Emitting Diode) [1] and micro-LED—that have the potential to threaten LCD’s position in the market. But both are currently only used for niche and high-end markets.

An LCD display can’t emit light by itself. In order to have an LCD display [2] used in a dim environment, a backlight has to be used as the light source. There are a few different technologies that are able to produce backlight ranging from EL (electroluminescent), CCFL (cold cathode fluorescent lamps) and LED (light emitting diode). However, a breakthrough in blue LED technology by Shuji Nakamura [3] led to LED backlights dominating the market.

One of the greatest benefits of LED backlighting is its long lifetime. Normally, LED lifetime can be measured with half-life when the original brightness decreases by 50%. With different LED chip manufacturing materials, technologies and environment used, the LED life can vary from 20,000 hours to well over 100,000 hours.

LED backlights have low power consumption and produce much less heat than other backlight technologies, which extends the durability and performance of the other display components. Furthermore, this reduces the risk of fire and explosion. LED backlights are also driven with DC (direct current) and low voltage (can be as low as 1.5V), which are good for battery drive and emit no interference to the circuitry. With the development of LED technology, the LED chips become small. So, it is possible to produce very thin backlight (0.5mm thick or thinner).

Although white LED is the most popular color, LED backlight can be made into different single colors, bi-colors and tri-colors [4] (Figure 1) (Figure 2). With RGB LED backlight color mixing, normal 8 color LED backlight can be produced (Figure 3).

LED backlight can be classified as bottom (array) lit and side (edge) lit backlights, and each have their plusses and minuses. The advantages of the bottom lit (array) backlight are that it is uniform and bright. Its disadvantage is high current draw, thickness, heat dissipation and cost. Meanwhile, the advantages of the side lit backlight are its thinness, flexibility in design, low current and lower cost. The main disadvantage of the side lit backlight is its non-uniformity—hot spots can be seen from most of the side lit backlight from certain angle. Figure 4 compares the bottom lit and side (edge) lit backlight LCD types.

Now let’s look at LED backlight structures. An LED backlight can be simplified into layers starting with a LED chip, light guide, diffusor and reflector (Figure 5). This is the lowest cost structure. Except for some very low current efficiency LCD displays—such as utility meters, battery-powered clock, watch, GPS and so on—most LCD displays need backlights to be visible in the dim lighting. Most often the backlight is actually at the back of the LCD. In rare cases, this light can be done as front light. The traditional LCD structure with LED backlight shown in Figure 6.

Direct current driving: This is the simple and low-cost way to drive a LED backlight, however, be mindful of the current limit otherwise the LED life can deteriorate quickly. The solution is simply to add a current limiting resistor in the circuit. Current limitation resistors value calculation formula: R = (V0– Vf)/If.Also be mindful of reverse drive, otherwise, the LED chip can break down easily.

LED driver with constant voltage: Using a constant voltage LED driver makes sense when using an LED or array that has been specified to take a certain voltage. This is helpful because constant voltage is a much more familiar technology for design and installation engineers. Moreover, the cost of these systems can be lower, especially in larger scale applications.

There are a variety of ways to connect a backlight and LCD module electrically. It can be done with wires that are soldered on the LCD or LCD module. It can be connected using pins, which can be soldered onto the LCD or LCD module. A third way is to use a FPC (flexible printed circuit), which can be soldered or plugged in a ZIF (zero insertion force) connector. And finally, there is the connector method. With this method you use connectors which can be plugged into mating connectors.

As the LED is manufactured via the semiconductor process, there are some color deviations that can be a quality control issue. One way to solve the issue is through a process of selection and sorting after manufacturing the LEDs. The LEDs are sorted into different categories or bins. How this sorting is done and what each bin actually contains is defined differently by each LED manufacturer. The backlight manufacturer can choose from which bin they take the LEDs for backlight color hue.

In actual LED backlight production, most customers will accept the LED color for two big categories: white with yellowish (warm) and white with bluish (cold). Of course, the LED brightness will also need to be defined. For general application, most customers will accept a brightness tolerance of 70 percent.

It is extremely hard to estimate the LED backlight lifetime or MTBF (mean time between failures) because there are so many variable factors. However, the most important is the temperature on the LED chip. The factors that can affect the LED chip temperature include: surrounding temperature, humidity, driving current, voltage, backlight design (how many LED chips to be used, how close to each other, heatsink design), backlight manufacturing process (type and thickness of adhesive), quality of the LED chip and so forth.

To test the LED life is also very time consuming, requiring at least 1,000 hours. That’s the reason why no LED manufacturers can guarantee LED backlight life and most backlight manufacturers also are reluctant to provide lifespan data. As for LCD manufacturers, they need to discuss it with the customer to understand the applications and provide suggestions. It is normal that the LCD datasheet lists the typical life time and avoids providing a minimum lifetime. From Figure 10, we can see that over room temperature, the current needs to decrease as the temperature increases. At over 85°C, the LED is not usable.

To estimate LED backlight lifetime, you can use ballpark estimation or theoretical calculation. Let’s first examine the ballpark method. To take white LED as example, the nominal biasing current is 20mA. If we use a safe lifetime estimation, we can estimate using Table 1.

Now let’s use the theoretical calculation approach. As we previously mentioned, LED life is affected by a lot of factors: surrounding temperature, humidity, driving current, voltage, backlight design (how many LED chips to be used, how close to each other, heatsink designed), backlight manufacturing process (type and thickness of adhesive), quality of the LED chip and so on. LED chip manufacturers are not willing to give absolute values of LED chip lifetimes, but there is a theoretical calculation that we can use.

Finally, let’s look at ways to increase LED backlight brightness. There are many ways to increase LED backlight brightness, but all these measures are balanced with performance and cost. Here are some of the methods:

For the LCD module side, using better aperture opening ratio, anti-reflection coating on surface, optical bonding. This results in higher cost. Actually, this measure is not to increase LED backlight brightness directly but to increase to the visibility to users.

Bill Cheung is an engineering lead and marketing manager at Orient Display, an LCD and display technology provider with over two decades of industry experience in delivering cutting edge display solutions. You can browse Orient Display"s knowledge base [7] to learn more about LCDs.

Most hardware operates at voltages between 1.8 V - 5.2 V. However, the backlight circuit operates at about 15 - 20 V. At this higher voltage, the backlight components are more prone to damage when a short circuit occurs. The high voltage backlight circuit is also prone to corrosion from water damage.

The backlight diode - Like the backlight filters, the backlight diode is a fragile component. In cases where a backlight filter is particularly burned up, you’ll often find that the diode has failed as well. Diode failure in the absence of filter damage is rare, but it can happen.

The most common cause of a self-induced short occurs from working on the device with the battery still connected. Even when the screen is dark, there is voltage in the backlight circuit. A slipped pair of tweezers or misalignment of the LCD connector can short the backlight circuit to ground. Avoid self-induced shorts by always disconnecting the battery before working on a device.

Another cause of backlight shorts is faulty assembly procedures. During device fabrication, these solder joints are protected by piece of black tape—however, during the screen refurbishing process some manufacturers neglect to replace the tape, apply it misaligned, or fail to apply it securely. As a result, the screen initially works during testing—but once the metal LCD shield is installed, the exposed solder joints touch the grounded frame, shorting the backlight circuit.

Backlight shorts can occur when the latch for the ZIF connector securing the LCD flex is missing. The LCD flex slides out an angle and the high voltage backlight pin contacts the ground pin, causing a short circuit.

Water damage is a frequent source of backlight problems. Water will corrode the LCD connector pin/pad junction, which breaks the electrical path to the connector and can damage the filter.

Backlight circuit failure can also occur from damage to the electrical traces on the circuit board. If the electrical traces buried in the board are inadvertently severed—for example, from trying to fasten the board with too large a screw—the backlight circuit will not conduct power to the backlight LEDs.

To diagnose whether your device is “dead” or just has a malfunctioning screen, try connecting it to your computer. If the computer recognizes the device, then the problem probably resides with the LCD screen or backlight circuit.

The good news is that nearly all backlight failures are repairable. Once the damaged component is identified it can simply be replaced. Follow this Samsung Television Backlight Replacement guide for more details.

This content is almost entirely sourced from Reed Danis and his iPhone/iPad Backlight Troubleshooting page. Users were finding this content when searching for TV related backlight issues so we repurposed it for TV.

My TV recently turned off the backlight. Sound works and with the flashlight I can see the picture. I pulled out the power cord, after a few hours I tried again, worked fine, after about 5 minutes the backlight turned off again, the sound is fine and the flashlight test too. I noticed that if I let it be without power cord around 24 hours, the light will work for about 10 or 15 minutes before it turns off. When the light is working then the LED switch has 111.3-114.6 volts, but when it turns off, it is 144.6 volts. I also tried factory reset, not help.

Visually does not see the traces of burning on the power supply. And when the backlight is on, I don"t see dark spots on the screen, all the LEDs seem to work.

Article today is about a way to revive some old units with faulty display backlight to former glory. It takes huge amount of man-hours and grey-beards to develop advanced and high performance test instruments. Also it is not trivial to maintain equipment in good health, especially for that extended lifespan over 25+ years. Reason is rather generic – many components from the past often not available on the market today. This means difficult and time consuming workarounds and repairs with modern equivalents if any of the obsoleted parts go bad.

In this article we will look at finding replacement for key component of user interface such as graphical LCD screen. Back in the 90’s vacuum fluorescent displays were the king of the market for high end instrumentation. We can name just few well known examples – HP 3458A, Fluke 5700A, Datron 1281 or even daily bench meter like HP 34401A. LCD tech still was quite new and developing then. Here we will be patching up not just any instrument, but the crown jewel of the resistance metrology field – Measurements International 6010B 9½-digit DCC bridge. 6010B is a predecessor from 1997 of today’s modern Model 6010D improved version.

Metrology lab targeted bridge system like 6010 in 99.9% of target usage cases is controlled remotely via control program over GPIB bus. But in rare case when operator needs to run quick check or verify good connections to resistance standards user can find onboard LCD handy. However, here is a weak point in such case if factory-made backlight got faulty due to age of some older instruments.

Older 6010 bridges using miniature incandescent light bulb for providing backlight. This backlight is quite fragile and with 25-year old obsoleted instrument like 6010B chances for dead lamp or near failing one are high. It would be hard or next to impossible to find exact to-spec replacement for original bulb today. But not all is lost and we can rectify the situation with new LED-backlit high contrast display instead.

This would give more user enjoyment out of older but still perfectly capable instruments. Keep in mind that LCD replacement procedure and steps in this guide are shown only as simple example on what can be done. It is not the only possible solution, but perhaps rather easy one.

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MIL 6010B build is very modular and mechanically well designed. Front panel MIL 6010B can be detached from chassis by removing four screws on the sides. 6010 units have LCD, backlight and keypad are mounted on front panel metalwork.

This display does not have integrated backlight but has optional special option for electroluminescent backlight (-EO suffix). Measurements International didn’t go for electrically noisy high-voltage electroluminescent backlight. Instead they went for linear low voltage incandescent bulb in aluminum reflective/heatsink focused on very cool looking multi-fiber optical cable to spread light into LCD light diffuser.

Each strand on fiber cable is edged and polished to perfectly flat surface to have good light transmission from light source to backlight module in LCD.

Lamp is driven with power from separate low voltage winding. Lamp loaded voltage is around 4.2 VAC. Measured current with working lab shown on oscilloscope capture Image 8.

Despite reasonably low power that lamp use it gets to blazing hot temperatures. Infrared camera thermograph with temperature markers are shown below. Hottest center element is actually glass infrared filter to prevent lamp heat reaching fragile fiber-optic cable. It is transparent to visible light allowing very warm yellowish backlight to operate. Temperatures of excess +195 °C were present close to lamp.

We spent some time trying different LEDs to act as a light source for backlight as lamp replacement. But brightness of used LED was either too low or light source required some non-standard chip-type LED that is quite difficult to position into perfect spot in reflector body to aim focused light on fiber input.

To match style of light-blue Measurements International instruments and latest 6010D negative white display used. We got few mechanically and most important, electrically compatible RA6963-based EastRising ERM24064DNS-1 LCDs. These modules available for inexpensive $28 USD and have very close dimensions to AND original LCD used by factory.

MIL 6010 use 20-conductor flat ribbon cable to interface with LCD. Pins 21 and 22 used for backlight power. These need to be connected to +5V power source by user.

First we need to provide power to white LED backlight. Power measurement on my display sample revealed current draw around +110 mADC at supplied +5.00 VDC.

This is in line with datasheet specification, but I feel like bit smaller backlight power might be actually better. Jumping ahead, 6010B with new display is brightest instrument in the lab at night now. :)

For backlight I’ve just jump-wired pin 21 to +5V positive terminal of input power tantalum capacitor CE1. And backlight cathode from pin 22 connected to negative terminal of the same capacitor. This supplied +5VDC across integrated LED backlight just fine.

Next modification is LCD contrast adjustment. Measurements International 6010B bridge does not allow user programmable LCD contrast change. We have to do it the hardware way. Contrast is controlled by bias from small negative supply and simple R3/R4 divider network. One way is to change values of R3 or R4 resistors on LCD PCB. Or alternative path, also recommended in interfacing document supplied with LCD is to connect multi-turn 10 kΩ trim potentiometer between negative supply (pin 20, marked orange on Image 12) and ground (marked blue on Image 12). Trim-pot was soldered directly to -15V and GND pads available at R3, R4 and all needed is good fixed contrast setting.

After modifications on LCD completed, install it into unit. Make sure to insulate exposed 4.2 VAC pin header and wire used for old lamp power. I didn’t remove original lamp assembly and holder, you can still see it on Image 12.

Such LCD replacement was easy and quick improvement of expensive and rare instrument. Hopefully this brings an example on how obsolete old parts can be replaced with modern technology, bringing second life to instruments we all love.