lcd panel led backlight voltage quotation

You should have basic electrical test equipment such as a Digital Multimeter and a basic working knowledge of electronics. Soldering is a must if you plan on doing more than testing a screen.

We will be gaining access under the dreaded “DO NOT TOUCH” plastic tape. What they really mean to say is.. “Do not touch the multiple ribbons along the top or bottom edge of the board.” These ribbons interface with the LCD screen and pass information to the pixel matrix. They are fragile!

This guide applies to Laptop LCD panels more so than your desktop"s 24" LCD screen. Some LED backlights are fed with up to 96 volts, but not usually… and not on laptops.

Finally, if you break something.. it"s on you! There is a lot of variance between LCD manufacturers.. they are all different! Hold only yourself responsible if something goes wrong... that"s an order, not a suggestion

I"m not following. How would constanst current source work in this case? Let"s say for 1 LED vs 20 LEDs, the first one draws 20mA for If and 1.2VDC for Vf, but the second set there, the 20 LEDs, draw 20 mA for If but 3VDC for Vf. The 20 LEDs would be wired in series. So what do they do? Provide a constant source of 20mA, but how do they measure the needed voltage, especially without knowing what it needs? I am still struggling a little with understanding this.

The forward voltage drop of an LED is current dependant. The correct way to drive an LED is with a constant current source. You can use constant voltage, but it"s tricky, since the resistance of an LED drops, both with increasing temperature and current. Suppose an LED has a normal forward voltage of 3V at 10mA, so you set the power supply"s voltage to 3V, but don"t limit the current, the LED might pass 10mA, but if it gets warm, its resistance will fall and it"ll pass more than 10mA, causing it to heat up more and pass more current. Using a constant current source, which passes the same current, irrespective of the terminal voltage solves this problem. If the same LED is connected to a 10mA constant current source and it warms up, the resistance will fall, but the power supply will limit the current to 10mA, so the voltage across it will fall, in accordance with Ohm"s law and it will heat up less, as P = V*I.

QuoteIf I set my PSU to 20mA and the output voltage to 30VDC, I"d think I"d still somehow damage the LED, wouldn"t it? I realize diodes are current driven, not voltage driven. Thanks!

Now you"re referring to a constant current, constant voltage power supply. The PSU has two modes: constant voltage and constant current. It can"t regulate both the current and voltage at the same time, because it would violate Ohm"s law.

When the power supply has a high resistance connected to it, it will be in constant voltage mode and when a low resistance is connected to it, it will be in constant current mode.

So when the resistance connected to your power supply is above 15k, it will regulate the voltage and when the resistance is lower, it will regulate the current.

If your LED will have a forward voltage of 3V, at 10mA, it will increase a little at 20mA, but not much, say 3.2V, so its resistance will be R = V/I = 3.2/0.02 = 160R.

Assuming the LED can handle 20mA and power supply can switch between constant voltage and constant current modes instantaneously, switching it on and connecting an LED with a forward voltage of 3V to its output, will cause the voltage to rapidly drop from 30V to 3V and the current to increase from zero to 20mA, so LED won"t be damaged.

In reality, the power supply might take a few milliseconds to switch from constant voltage, to constant current mode, briefly allowing a much higher current to flow through the LED, which could damage it. The avoid this, connect the LED to the output of the power supply, before switching it on, then it should start in constant current mode, limiting the current through the LED to 20mA.

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.

Figure 5(a) shows a half-bridge DC-DC Series-Resonant Converter (SRC) topology for driving the RGB LEDs. The soft-switched DC-DC resonant converter includes power switches Q1 and Q2, resonant inductor Lr, resonant capacitor Cr, transformer T1, rectifier diodes Df1 and Df2, filter capacitor Cf, and the LED arrays represented by an equivalent resistance Ro. The characteristic impedance and the resonant frequency are respectively [18-21].

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

It can be observed from Figure 7 that if the end of the dimming signal for LED array 1 is at d, where d is between k and k+1 and k 1, then the output current of the SRC in the range of k to k+1 is

A favored feature is that the load variation of the SRC is always within one step change of Ip, no matter what the load level is. Therefore, by carefully designing the duty cycle and the amplitude of the driving current for each LED array, the no load operation of the DC-DC SRC may be precluded. Moreover, the output transient of the SRC is improved due to the confined load change. The number of the LED array for one color, and the peak driving current of each LED array are first determined according to the specifications of the LED and the spectrum of the white color. Then a suitable duty cycle is chosen allowing a reasonable span of variation for dimming control.

Ever had your TV showing nothing but a black screen even if the audio was working? Unfortunately, that’s a common issue with low/middle-end LCD/LED TVs these days… Even more frustrating, this issue often comes from a rather tiny and cheap component that can be easily replaced. Most common issues are:

The first step into repair is to find the root cause of the issue. As backlight failure is a very common issue, this is the first thing to test. To do so, the easiest way is to power on your screen, put a flashlight very close to it and check if you can see the image through. The image would be very dark, like turning the brightness of the screen very very low.

That implies disassembling the TV to access the backlight which is between the LCD screen in the front and the boards in the rear. In my case, with a Samsung F5000, I had to process as follows:

First we have to remove the back housing to reveal the boards (from left to right: main board, T-CON, power supply) and disconnect the LCD panel from the T-CON board.

Note: Older TVs have neon tubes for backlight, which is thicker and less exposed to this kind of failure. LED backlight is the most common thing these days, but do not mistake an LED TV with an OLED TV. The first one is a classic LCD panel with a LED backlight, whereas the second is an OLED panel that doesn’t need any backlight as it is integrated in each pixels (making the spare parts much more expensive by the way).

As we can see, the backlight system is made of 5 LED strips. First thing to do is look for burnt LEDs. Most LED backlight systems have strips set in series, meaning that if one of the them fails, all the system goes dark…

Using a multimeter, we can confirm that the strips are indeed set in series, so now we have to test each strip individually. Professionals use LED testers such as this one (about 40$ on amazon) but as I didn’t had one at the time, I decided to make one, McGyver style!