Megger construction and working

Introduction:- Megger is used for measuring insulation resistance (IR) value of motors, cables, and panel mounted components and bus bar etc. Megger are available for different voltage range like  50 volt, 500 volt, 1000 volt, 2500 volt(2.5kv), 5 kv . IR value measurement is very important in electrical work to ensure safety of working personnel's and equipment.

Construction and working:- it consist of a permanent magnet cross coil dynamo meter type indicating instrument and a hand operated DC generator. There is permanent magnet as shown in fig. Two moving coil are placed within the magnetic field of magnet supported by pivots. The two coil are placed at 90 degree to each other. First coil is known as pressure coil connected in parallel to output voltage of generator. it provide torque to spindle in anti clock wise direction hence moves the pointer towards infinity. The second coil is connected in series with testing resistance, it produce torque in opposite direction which  moves pointer towards 0 ohms. So when Megger is used for measuring resistance , Megger pointer indicates the ratio of V/I that is R of the circuit.

Hand operated generator:- it is very small generator with permanent magnet field. armature is rotated with the the help of handle and gear system. A clutch is provided so that handle start slipping after the 120 rpm. So when handle is rotated with full speed the armature speed is limited to 120 rpm and the output voltage remains constant.

Uses:- LT motor are Meggered with 1000  volt Megger. HT motors are meggered with 2.5 kV Megger. Minimum IR value should not be less than voltage rating of machine into 1000 ohms that means 1000 ohms per volt of machine rating. In HT motors and HT cables R15/R60 ratio is also considered. R15 and R60 is meggering values at 15 seconds and 60 seconds respectively. R60 value must be 1.3 to 1.5 times higher than R15 value.

Main sections of LCD TV

Main sections of LCD TV

Power supply section:- power supply gives 12volt or 5 volt DC output. This 12/5 volt is given to 3.3 volt regulator IC which gives 3.3 volt DC . this 3.3 volt is given to mother board main ic , bios IC , reset button and IR sendser. This 3.3 volt supply is known as 3.3 v SB (stand by). Power supply also gives 24 volt DC for inverter board in LCD TV.

Mother board:- It consist of main microcontroller IC , bios ROM,  flash  IC RAM. A crystal of 24 MHz is used to generate clock pulses. Main IC receives audio video signals from A/V input, HDMI input, RF Signal from tuner and MPEG files from USB. Input signal is selected by mother board throught signal switching circuit. This signal is processed and sound signal is seperated, amplified and sent to speaker. Video signal is sent to analog to digital converter. It gives out red, blue, green, sync  and clock signals in the form of LVDS (low voltage differential signal) signal. LVDS signal is sent to timing controller IC (Tcon IC) on Tcon board through LVDS cable. Mother board sents Back light on/off  control  signal is to back light inverter/ driver board.

Tcon board:- It has normally four section
1- DC to DC IC :- Tcon board receives  vcc supply generally 12 volts  from LVDS connector is given to this Ic . It produce 3.3 volt supply which is given to Tcon IC. A 1.2 volt regulator is also used to supply core voltage to Tcon IC . One more regulator IC is used to supply 3.5/1.8/1.5/1.2 volt to RAM depending on  version of RAM (DDR1 3.5 volt, DDR2  1.8 volt, DDR3 1.5volt, DDR4 1.2 volt). When Tcon IC receives all input voltages and signals, it starts working and gives an enable signal to DC to DC IC . After receiving enable signal DC to DC IC start producing AVDD, HVDD, VGH and VGL voltages. In some cases enable signal is directly given to DC to DC ic. HVDD and VDD voltages are also given to gama IC.

2- Tcon IC :- It receives working voltage VDD 3.3 volt, 1.2 volt for core and 1.8 volt for RAM. It receives Signal 0-, 0+ (red color signal data), 1-,1+ (green color signal data), 2-,2+(blue color signal and horizontal and vertical sync signal data), 3-,3+(clk+, clk-),  4-,4+(RGB signal data) from LVDS connector. This IC process all signals and gives output as mini LVDS 0-, 0+, 1-,1+, 2-,2+, clk-,clk+, 3-,3+, to panel through COF flex strip and EN, CPV1 CPV2, STV signal to scan driver ic.

3- Scan driver or level shifter:- It receives 3.3 volt, VGH, VGL from DC to DC IC and , CPV1, CPV2, STV, and EN (enable signals) from Tcon IC. It gives CKV1, CKV2 for scan clk odd and CKVB1, CKVB2 for scan clk even to panel source COF.

4- Gama IC :- HVDD voltage are given to this IC. With the help of voltage divider circuit Vref and Vcom voltage is produced. And all gama voltages are produced with the help of voltages divider circuit. Some programmed IC are also uses in which voltage divider circuit is not used. Gama voltage start from 1.5 volt to AVDD voltage with increasing gradually in steps. Gama voltages are given through source COF for gama correction.

Chip on flex IC (COF):- This IC receives all voltage through flex strip and produces driving signal voltage to drive all TFT in the panel. Main voltages received by COF IC are VDD 3.3 volt, AVDD (16 volt), HVDD (8volt), VGH (27 volt), VGL (-8 to -10volt), CKV1, CKV2, CKVB1, CKVB2, VCOM and Gama voltages and all signal voltages from Tcon through mini LVDS.

Panel or screen:- All working voltage to panel is received through COF connector flexible strip cable. The light for screen working is given by back light ccfl/led. Main part of panel are
1 light source
2 horizontal poloizor
3 TFT screen:- Thin film transistorscreen:-  complete screen is filled with matrix of TFT. each pixel need three TFT. Each transtor three points are connected to source, commen and gate circuit of panel and controlled separately by multiplexing mathod.
4 vertical polorizor

Back light:- cold cathode florocent light (CCFL) are used in LCD panel for back light. LED Strips are used in Led TV panel. In LCD TV back light is given supply with a inverter board, where as in led TV led drivers are used to give supply to led strips. Back light on/off signal is received from mother board.

Control button panel:- all control button are placed in this panel to control TV locally. Otherwise remote control is used in which all control signals are received by IR senser and sent to mother board.

In this article  I have sum up my information regarding LCD/led TV. If any person like to give more information pls share in commect so I can add more information. Thanks for reading.


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