000 = Jumper
6R8 = 6.8 Ohm
100 = 10 Ohm
750 = 75 Ohm
101 = 100 Ohm
Thursday, January 5, 2023
- 9:05 PM
- Enci
- >Dasar Elektronik
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- 8:53 PM
- Enci
- >Dasar Elektronik
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Currently, the most commonly
used light emitting
device is a fluorescent tube called a cold cathode tube for CCFL.
The CCFL is called a cold cathode
tube because even though the principle of illumination is the same as that of the hot cathode tubes used
by indoor lamps, this lamp does not require
preheating of the filament.
Also, the electrodes at the end of the bulb stay at a low temperature while emitting light. CCFL also enjoy a long life (approx 50,000 hours) without serious degradation. A special power supply, inverter, which generates approximately 600 to 1000 AC is required to drive a CCFL. This inverter is a small power supply used to make this CCFL illuminate, and is one of the important functional parts of a complete LCD display.
Display Flicker and Reddish
Normally a reddish display and display flicker in LCD Monitor were caused by one of the defective backlight (either top or bottom backlight). For those LCD Monitors that do not have the feedback circuit in the inverter board, even though the backlight have problem the LCD Monitor continue to work and never shutdown. This is totally different in LCD Monitors that have the feedback circuit in the inverter board because even a slight flicker in the display caused by a defective backlight, the LCD Monitor would immediately shutdown
- 8:43 PM
- Enci
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Most of the LCD Monitor has a
Start Circuit to control the voltage from the
power supply to the supply pin of Inverter IC. The main control signal comes from the Main board and the
voltage is from 0 and few Volts (2- 5 Volts).
If the signal is Zero Volts, then the inverter IC would not receive any supply voltage from the power supply and if the signal is 2 Volts (On) then the inverter IC would be “On” and the high voltage transformer would energized and the backlights will light up.
The Start Circuit is a good starting point to diagnose why the LCD Monitor has no display, display shutdown and intermittent no display problem.
- 8:33 PM
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For a newer LCD Monitor design, the inverter board is joining together with the power board as shown in the above photo. Older LCD Monitor has the inverter board separated from the power board as shown below.
There are four types of inverter designs (topologies) used in the LCD Monitors.
1. Buck Royer inverter
2. Push pull inverter (Direct Drive)
3. Half bridge inverter and (Direct Drive)
4. Full bridge inverter (Direct Drive)
Number 2, 3 and 4 are called Direct Drive because it eliminates the need for the inductor (buck choke) and resonant capacitors found in a conventional Royer Oscillator. In other words, Direct Drive architecture reduces component count, lower production cost and most importantly improved transformer designs that optimize performance.
In order to drive the Backlights
(CCFL lamps) embedded in the panel module,
an inverter circuit is required to convert the 12 volt DC up to hundreds or even a thousand plus AC
voltage output. The inverter is formed
by symmetric circuitry, in order to drive the separate lamp modules.
The input stage
(buck converter circuit)
consists of Inverter IC (PWM
IC), Buck P-channel FET, Buck Choke and Buck Diode. The Buck converter circuit converts
a DC voltage to a lower DC voltage.
The other stage consists of a tuning capacitors, high voltage transformer, and push-pull transistor pair to boost ac output to hundreds of voltage.
The ballast capacitor controls current amplitude through the lamp negative impedance by dropping an approximately equal voltage across its positive impedance. The feedback circuit is for protection purposes and will shut down the inverter IC just in case if the high voltage produced by the high voltage transformer exceeded the normal value and also it can detect bad or a flicker backlights. The inverter IC also used to control the brightness of the CCFL lamps. The AC frequency of the high voltage transformer is typically run at 30 to 70 KHz. The higher the frequency, the greater is the light output.
Note: Some LCD Monitor design has the Buck type P-channel FET integrated into an IC thus in order to successfully testing them you can use the comparison method with another known good FET (comparing the ohms value between pins) or by using the Peak Atlas Analyzer test equipment. The IC can be in Dual in Line package or SMD type.
- 7:54 PM
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The switch mode power supply used to power up LCD Monitor can be either the external or internal type. The function of the power supply is to convert the main supply AC 230 volts into DC output voltages to supply to the necessary boards in LCD Monitor.
Schema
LCD Power Supply Board
230 Volts AC supply enters the power supply and to the bridge rectifier ac pins (normally is the 2nd and the 3rd leg). The AC supply is then converted into DC output voltage (about 300 VDC-in USA about 155 VDC) where the big filter capacitor filtered off the ripple so that the power supply will have a nice constant of DC voltage. This high voltage DC supply is then given to a switching power FET Transistor. This switching FET transistor circuit is switched on and off at a very high speed by a control circuit (power IC) which generates very high frequency square wave pulses.
The power FET and power IC (UC3842BThe switching FET transistor circuit switches the given high voltage
DC, on and off at the same
high frequency and gives square wave pulses as
the output. These square wave pulses are then
given to the primary winding of
Switch Mode Power Transformer. These pulses induce a voltage at the primary winding of the transformer which will
generate voltage at the secondary
winding. This voltage at the secondary winding
is then rectified and filtered
to produce the required
output.
The build in power supply have output of usually 12 volts and 5 volts where the 12 volts enters the inverter IC and also audio power amp IC.The 5 volts will go through one or two voltage regulators to get the 3.3 and 2.5 volts to power the Scalar IC, MCU, EEprom and even the LCD driver/controller board.
The power FET transistor already
integrated into the power IC
Please take note that many latest designs of LCD Monitor power supply designs have the switching power FET transistor already integrated into the power IC thus you will not find the power FET in the power supply board.
