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General provisions
Let’s immediately make a reservation that the article is related to inverters for CCFL lamps. At this time, instead of CCFL backlighting, LED backlighting is actively used, where LEDs of the brands LATWT470RELZK SBWVT120E PT30W45 V1 and others are considered the best.
For the operation of an LCD panel, the light source is of paramount importance, the luminous flux of which, passed through the structure of the liquid crystal, forms an image on the monitor screen. To create a luminous flux, cold cathode fluorescent lamps (CCFLs) are used, which are located at the edges of the monitor (usually top and bottom) and, using frosted diffusing glass, evenly illuminate the entire surface of the LCD matrix. The “ignition” of the lamps, as well as their power supply in operating mode, is provided by inverters. The inverter must ensure reliable starting of lamps with voltages above 1500 V and their stable operation for a long time at operating voltages from 600 to 1000 V. The lamps in LCD monitors are connected using a capacitive circuit (see Fig. 1). The operating point of stable glow (PT - on the graph) is located on the line of intersection of the load straight line with the graph of the dependence of the discharge current on the voltage applied to the lamps. The inverter in the monitor creates conditions for a controlled glow discharge, and the operating point of the lamps is on the flat part of the curve, which makes it possible to achieve a constant glow for a long time and ensure effective brightness control. You can buy inverters for LCD TVs and monitors in the Dalincom online store.
Rice. 1. Graph of the position of the operating current of the stable glow of the lamps
The inverter performs following functions:
converts DC voltage (usually +12 V) into high-voltage AC;
stabilizes the lamp current and, if necessary, regulates it;
provides brightness adjustment;
matches the output stage of the inverter with the input resistance of the lamps;
Provides short circuit and overload protection.
No matter how diverse the market for modern inverters is, the principles of their construction and operation are almost the same, which simplifies their repair.
Structural scheme inverter is shown in Fig. 2. The block for standby mode and turning on the inverter is made in this case on keys Q1, Q2. The LCD monitor takes some time to turn on, so the inverter also turns on 2...3 s after the monitor is switched to operating mode. ON/OFF voltage is supplied from the main board and the inverter enters operating mode. The same block ensures that the inverter is turned off when the monitor switches to one of the energy saving modes. When a positive ON voltage (3...5 V) is supplied to the base of transistor Q1, +12V voltage is supplied to the main circuit of the inverter - the brightness control unit and the PWM regulator.
Rice. 2. Block diagram of the inverter
The unit for monitoring and controlling the brightness of lamps and PWM (3 in Fig. 2) is made according to the circuit of an error amplifier (EA) and a PWM pulse shaper. It receives the dimmer voltage from the main board of the monitor, after which this voltage is compared with the voltage feedback, and then an error signal is generated that controls the frequency of the PWM pulses. These pulses are used to control the DC/DC converter (1 in Fig. 2) and synchronize the operation of the inverter converter. The amplitude of the pulses is constant and is determined by the supply voltage (+12 V), and their frequency depends on the brightness voltage and the threshold voltage level.
The DC/DC converter (1) provides constant (high) voltage, which is supplied to the autogenerator. This generator is turned on and controlled by PWM pulses from the control unit (3).
The level of the inverter's AC output voltage is determined by the parameters of the circuit elements, and its frequency is determined by the brightness control and the characteristics of the backlight lamps. The inverter converter is usually a self-excited generator. Both single-cycle and push-pull circuits can be used.
The protection unit (5 and 6) analyzes the level of voltage or current at the inverter output and generates feedback (OS) and overload voltages, which are supplied to the control unit (2) and PWM (3). If the value of one of these voltages (in the event of a short circuit, converter overload, low supply voltage) exceeds the threshold value, the autogenerator stops operating.
As a rule, on the screen the control unit, PWM and brightness control unit are combined in one chip. The converter is made on discrete elements with a load in the form of a pulse transformer, the additional winding of which is used to switch the trigger voltage.
All main inverter components are housed in SMD component housings.
There are a large number of modifications of inverters. The use of one type or another is determined by the type of LCD panel used in a given monitor, so inverters of the same type can be found from different manufacturers.
Let's look at the most commonly used types of inverters, as well as their typical faults.
Inverter type PLCD2125207A from EMAX
This inverter is used in LCD monitors from Proview, Acer, AOC, BENQ and LG with a screen diagonal of no more than 15 inches. It is built according to a single-channel circuit with a minimum number of elements (Fig. 3). At an operating voltage of 700 V and a load current of 7 mA using two lamps, the maximum screen brightness is about 250 cd/m2. The starting output voltage of the inverter is 1650V, the protection response time is from 1 to 1.3 s. At idle, the output voltage is 1350V. The greatest depth of brightness is achieved by changing the control voltage DIM (pin 4 of connector CON1) from 0 (maximum brightness) to 5 V (minimum brightness). The inverter from SAMPO is made according to the same scheme.
Rice. 3. Schematic diagram of the inverter PLCD2125207A
Description of the circuit diagram
+12 V voltage is supplied to the pin. 1 connector CON1 and through fuse F1 - to pin. 1-3 assemblies Q3 (source field effect transistor ). The boost DC/DC converter is assembled using elements Q3-Q5, D1, D2, Q6. In operating mode, the resistance between the source and drain of transistor Q3 does not exceed 40 mOhm, while a current of up to 5 A is passed into the load. The converter is controlled by a brightness and PWM controller, which is made on a U1 chip of the TL5001 type (analogous to FP5001) from Feeling Tech. The main element of the controller is a comparator, in which the voltage of the sawtooth voltage generator (pin 7) is compared with the voltage of the control device, which in turn is determined by the relationship between the reference voltage of 1 V and the total feedback and brightness voltage (pin 4). The frequency of the sawtooth voltage of the internal generator (about 300 kHz) is determined by the value of resistor R6 (connected to pin 7 of U1). PWM pulses are taken from the output of the comparator (pin 1), which are supplied to the DC/DC converter circuit. The controller also provides short circuit and overload protection. If there is a short circuit at the inverter output, the voltage at the divider R17 R18 increases, it is rectified and supplied to the pin. 4 U1. If the voltage becomes 1.6 V, the controller protection circuit is activated. The protection response threshold is determined by the value of resistor R8. Capacitor C8 provides a “soft” start when starting the inverter or after the end of a short circuit. If the short circuit lasts less than 1s (the time is determined by the capacitance of capacitor C7), then normal operation of the inverter continues. Otherwise, the inverter operation stops. To reliably start the converter, the protection response time is selected so as to be 10...15 times longer than the start and “ignition” time of the lamps. When the output stage is overloaded, the voltage at the right terminal of inductor L1 increases, the zener diode D2 begins to pass current, transistor Q6 opens and the response threshold of the protection circuit decreases. The converter is made according to the circuit of a half-bridge generator with self-excitation using transistors Q7, Q8 and transformer PT1. When the ON/OFF power-on voltage (3 V) is received from the main monitor board, transistor Q2 opens and power is supplied to controller U1 (+12 V to pin 2). PWM pulses with pin. 1 U1 through transistors Q3, Q4 goes to the gate of Q3, thereby starting the DC/DC converter. In turn, power is supplied from it to the autogenerator. After this, a high-voltage alternating voltage appears on the secondary winding of transformer PT1, which is supplied to the backlight lamps. Winding 1-2 PT1 acts as a feedback generator. While the lamps are not turned on, the output voltage of the inverter rises to the starting voltage (1650V), and then the inverter goes into operating mode. If the lamps cannot be ignited (due to a break, “exhaustion”), spontaneous generation failure occurs.
Malfunctions of the PLCD2125207A inverter and how to eliminate them
Backlights do not turn on
Check the +12 V supply voltage at the pin. 2 U1. If it is not there, check fuse F1, transistors Q1, Q2. If fuse F1 is faulty, before replacing it, check transistors Q3, Q4, Q5 for a short circuit.
Then check the ENB or ON/OFF signal (pin 3 of CON1 connector) - its absence may be due to a malfunction of the monitor’s main board. This is checked in the following way: a control voltage of 3...5 V is supplied to the ON/OFF input from an independent power source or through a divider from a 12V source. If the lamps turn on, then the main board is faulty, otherwise the inverter is faulty.
If there is supply voltage and a turn-on signal, but the lamps do not light, then carry out an external inspection of the transformer PT1, capacitors C10, C11 and lamp connectors CON2, CON3, and replace the darkened and melted parts. If at the moment of switching on the pin. 11 of transformer PT1, voltage pulses appear for a short time (the oscilloscope probe is connected through a divider in advance, before turning on the monitor), and the lamps do not light, then check the condition of the lamp contacts and the absence of mechanical damage on them. The lamps are removed from their seats, having first unscrewed the screw securing their housing to the matrix body, and, together with the metal housing in which they are installed, are removed evenly and without distortions. In some monitor models (Acer AL1513 and BENQ), the lamps are L-shaped and cover the LCD panel around the perimeter, and careless actions during dismantling can damage them. If the lamps are damaged or darkened (which indicates a loss of their properties), they are replaced. Lamps can only be replaced with ones of similar power and parameters, otherwise either the inverter will not be able to “ignite” them, or an arc discharge will occur, which will quickly damage the lamps.
The lamps turn on for a short time (about 1 second) and then turn off immediately
In this case, protection against short circuit or overload in the secondary circuits of the inverter is most likely triggered. Eliminate the reasons for the protection to operate, check the serviceability of the transformer PT1, capacitors C10 and C11 and the feedback circuit R17, R18, D3. They check the zener diode D2 and the transistor Q6, as well as the capacitor C8 and the divider R8 R9. If the voltage at the pin. 5 is less than 1 V, then replace the capacitor C7 (preferably with a tantalum one). If all of the above steps do not produce results, replace the U1 chip.
Turning off the lamps may also be due to a failure of the converter generation. To diagnose this malfunction, instead of lamps, an equivalent load is connected to connectors CON2, CON3 - a resistor with a nominal value of 100 kOhm and a power of at least 10 W. A 10 ohm measuring resistor is connected in series with it. Devices are connected to it and the oscillation frequency is measured, which should be in the range from 54 kHz (at maximum brightness) to 46 kHz (at minimum brightness) and the load current from 6.8 to 7.8 mA. To monitor the output voltage, connect a voltmeter between pin 11 of transformer PT1 and the terminal of the load resistor. If the measured parameters do not correspond to the nominal, control the magnitude and stability of the supply voltage at inductor L1, and also check transistors Q7, Q8, C9. If, when the right (according to the diagram) diode of assembly D3 is disconnected from resistor R5, the screen lights up, then one of the lamps is faulty. Even with one working lamp, the image brightness is enough for the operator to work comfortably.
The screen flickers periodically and the brightness is unstable
Check the stability of the brightness voltage (DIM) on pin. 4 connectors CON1 and after resistor R3, having previously disabled feedback (resistor R5). If the control voltage at the connector is unstable, then the main board of the monitor is faulty (the test is carried out in all available modes of operation of the monitor and across the entire brightness range). If the voltage is unstable at the pin. 4 controller U1, then check its DC mode in accordance with table. 1, while the inverter must be in operating mode. The faulty microcircuit is replaced.
Table 1
They check the stability and amplitude of oscillations of their own sawtooth pulse generator (pin 7), the signal swing should be from 0.7 to 1.3 V, and the frequency should be about 300 kHz. If the voltage is not stable, replace R6 or U1.
Instability of the inverter may be due to aging of the lamps or their damage (periodic loss of contact between the supply wires and the lamp terminals). To check this, as in the previous case, connect an equivalent load. If the inverter operates stably, then it is necessary to replace the lamps.
After some time (from several seconds to several minutes) the image disappears
The protection circuit is not working correctly. Check and, if necessary, replace capacitor C7 connected to the pin. 5 controllers, control the DC mode of controller U1 (see previous fault). Check the stability of the lamps by measuring the level of sawtooth pulses at the output of the feedback circuit, on the right anode D3 (swing about 5 V) with medium brightness set (50 units). If voltage surges occur, check the serviceability of the transformer and capacitors C9, C11. Finally, check the stability of the PWM controller circuit U1.
Inverter type DIVTL0144-D21 from SAMPO
The schematic diagram of this inverter is shown in Fig. 4. It is used to power the backlight lamps of 15-inch matrices from SUNGWUN, SAMSUNG, LG-PHILIPS, HITACHI, which are used in PROVIEW, ACER, BENQ, SAMSUNG, LG monitors. Operating voltage is 650 V at a load current of 7.5 mA (at maximum brightness) and 4.5 mA at minimum. The starting voltage (“ignition”) is 1900 V, the frequency of the lamp supply voltage is 55 kHz (at average brightness). The brightness control signal level ranges from 0 (maximum) to 5 V (minimum). The protection response time is 1…4 s.
Rice. 4.
A U201 microcircuit of type BA9741 from ROHM (its analogue TL1451) is used as a controller and PWM. It is a two-channel controller, but in this case only one channel is used.
When the monitor is turned on, +12 V voltage is supplied to pin 1-3 of the Q203 transistor assembly (source of the field-effect transistor). When the monitor is turned on, the inverter ON/OFF start signal (+3 V) comes from the main board and opens transistors Q201, Q202. Thus, +12 V voltage is supplied to the pin. 9 controllers U201. After this, the internal sawtooth voltage generator begins to operate, the frequency of which is determined by the ratings of the elements R204 and C208 connected to the pin. 1 and 2 microcircuits. PWM pulses appear on pin 10 of the microcircuit, which are supplied to the gate of Q203 through an amplifier on transistors Q205, Q207. On the pin. 5-8 Q203 a constant voltage is generated, which is supplied to the autogenerator (on elements Q209, Q210, PT201). A sinusoidal voltage with a swing of 650 V and a frequency of 55 kHz (at the moment the lamps are “ignited” it reaches 1900 V) from the output of the converter through connectors CN201, CN202 is supplied to the backlight lamps. Elements D203, R220, R222 are used to generate a protection signal and a “soft” start. When the lamps are turned on, the energy consumption in the primary circuit of the inverter increases and the voltage at the output of the DC/DC converter (Q203, Q205, Q207) increases, the zener diode D203 begins to conduct current, and part of the voltage from the divider R220 R222 goes to pin 11 of the controller, thereby increasing the same threshold for the protection circuit to operate during startup.
The stability and brightness of the lamps, as well as short-circuit protection, is ensured by a feedback circuit on elements D209, D205, R234, D207, C221. The feedback voltage is supplied to the pin. 14 microcircuits (direct input of the error amplifier), and the brightness voltage from the main monitor board (DIM) - to the inverse input of the control unit (pin 13), determining the frequency of PWM pulses at the controller output, and hence the output voltage level. At minimum brightness (DIM voltage is 5 V) it is 50 kHz, and at maximum (DIM voltage is zero) it is 60 kHz.
If the feedback voltage exceeds 1.6 V (pin 14 of the U201 chip), the protection circuit is turned on. If a short circuit in the load lasts less than 2 s (this is the time of charging capacitor C207 from the reference voltage +2.5 V - pin 15 of the microcircuit), the functionality of the inverter is restored, which ensures reliable starting of the lamps. If there is a long-term short circuit, the inverter turns off.
Malfunctions of the DIVTL0144-D21 inverter and methods for their elimination
Lamps don't light up
Check the presence of +12 V voltage on the pin. 1-3 Q203, serviceability of fuse F1 (installed on the main board of the monitor). If the fuse is faulty, then before installing a new one, check transistors Q201, Q202, as well as capacitors C201, C202, C225 for a short circuit.
Check the presence of ON/OFF voltage: when turning on the operating mode, it should be equal to 3V, and when turning off or switching to standby mode, it should be zero. If there is no control voltage, check the main board (the LCD monitor microcontroller controls the inverter switching on). If all of the above voltages are normal, and the PWM pulses are on the pin. 10 there is no V201 microcircuit, check zener diodes D203 and D201, transformer RT201 (can be determined by visual inspection by a darkened or melted case), capacitors C215, C216 and transistors Q209, Q210. If there is no short circuit, then check the serviceability and rating of capacitors C205 and C207. If the above elements are in good condition, replace the U201 controller. Note that the absence of illumination of the backlight lamps may be due to their breakage or mechanical failure.
Lamps turn on and off briefly
If the illumination persists for 2 s, then the feedback circuit is faulty. If, when disconnecting elements L201 and D207 from the circuit, pin. 7 of the U201 chip, PWM pulses appear, then either one of the backlight lamps or the feedback circuit is faulty. In this case, check the zener diode D203, diodes D205, D209, D207, capacitors C221, C219, and inductor L202. Monitor the voltage at the pin. 13 and 14 U201. In operating mode, the voltage at these pins should be the same (about 1 V - at average brightness). If the voltage at the pin. 14 is significantly lower than on pin. 13, then check diodes D205, D209 and lamps for open circuits. With a sharp increase in voltage at the pin. 14 U201 microcircuits (above the 1.6V level) check elements PT1, L202, C215, C216. If they are working, replace the U201 chip. When replacing it with an analogue (TL1451), check the threshold voltage at the pin. 11 (1.6 V) and, if necessary, select the value of elements C205, R222. By selecting the values of elements R204, C208, the frequency of the sawtooth pulses is set: on the pin. 2 chips should be around 200 kHz.
The backlight turns off after some time (from several seconds to several minutes) after turning on the monitor
First, check capacitor C207 and resistor R207. Then check the serviceability of the contacts of the inverter and backlight lamps, capacitors C215, C216 (by replacement), transformer RT201, transistors Q209, Q210. Monitor the threshold voltage at the pin. 16 V201 (2.5V), if it is low or missing, replace the microcircuit. If the voltage at the pin. 12 is above 1.6V, check capacitor C208, otherwise also replace U201.
The brightness spontaneously changes (flashes) throughout the entire range or in individual monitor operating modes
If the malfunction appears only in certain resolution modes and in a certain brightness range, then the malfunction is related to the main board of the monitor (memory or LCD controller). If the brightness changes spontaneously in all modes, then the inverter is faulty. Check the brightness adjustment voltage (at pin 13 U201 - 1.3 V (at average brightness), but not higher than 1.6 V). If the voltage at the DIM contact is stable, and at the pin. 13 - no, replace the U201 chip. If the voltage at the pin. 14 is unstable or low (less than 0.3 V at minimum brightness), then instead of the lamps, connect an equivalent load - a resistor with a nominal value of 80 kOhm. If the defect persists, replace the U201 chip. If this replacement does not help, replace the lamps and also check the serviceability of their contacts. The voltage is measured at pin 12 of the U201 microcircuit; in operating mode it should be about 1.5V. If it is below this limit, check elements C209, R208.
Note.In inverters from other manufacturers (EMAX, TDK), made according to a similar circuit, but in which other components are used (except for the controller), instead of SI443 -> D9435, 2SC5706 -> 2SD2190, the voltage at the terminals of the U201 microcircuit can vary within ±0, 3 V.
This inverter (its circuit diagram shown in Fig. 5) is used in 17-inch ACER and ROVER SCAN monitors with SAMSUNG matrices, and its simplified version ( rice. 6 ) - in 15-inch LG monitors with LG-PHILIPS matrix. The circuit is implemented on the basis of a 2-channel PWM controller from OZ960 O2MICRO with 4 control signal outputs. Transistor assemblies such as FDS4435 (two field-effect transistors with a p-channel) and FDS4410 (two field-effect transistors with an n-channel) are used as power switches. The circuit allows you to connect 4 lamps, which provides increased brightness of the LCD panel backlight.
Rice. 5
The inverter has the following characteristics:
– supply voltage - 12 V;
– rated current in the load of each channel - 8 mA;
– output voltage frequency - from 30 kHz (at minimum brightness) to 60 kHz (at maximum brightness). The maximum brightness of the screen with this inverter is 350 cd/m2;
– protection response time - 1…2 s.
When the monitor is turned on, +12 V is supplied to the inverter connector - to power the Q904-Q908 keys and +6 V - to power the U901 controller (in the version for the LG monitor, this voltage is formed from the +12 V voltage, see diagram in Fig. 6) . In this case, the inverter is in standby mode. The ENV controller turn-on voltage is supplied to the pin. 3 microcircuits from the microcontroller of the main monitor board. The PWM controller has two identical outputs for powering two inverter channels: pin. 11, 12 and pin. 19, 20 (Fig. 5 and 6). The operating frequency of the generator and PWM are determined by the values of resistor R908 and capacitor C912 connected to the pin. 17 and 18 microcircuits ( rice. 5 ). Resistor divider R908 R909 determines the initial threshold of the sawtooth voltage generator (0.3 V). On capacitor C906 (pin 7 U901) the threshold voltage of the comparator and protection circuit is formed, the response time of which is determined by the rating of capacitor C902 (pin 1). The protection voltage against short circuit and overload (if the backlight lamps break) is supplied to the pin. 2 microcircuits. The U901 controller has built-in soft start circuitry and an internal stabilizer. The start of the soft start circuit is determined by the voltage at the pin. 4 (5 V) controllers.
Rice. 6
Voltage transformer direct current The high-voltage supply voltage of the lamps is made on two pairs of p-type FDS4435 and n-type FDS4410 transistor assemblies and is forcedly triggered by pulses with PWM. A pulsating current flows in the primary winding of the transformer, and the supply voltage for the backlight lamps connected to connectors J904-J906 appears on the secondary windings of T901. To stabilize the inverter output voltages, the feedback voltage is supplied through full-wave rectifiers Q911-Q914 and the integrating circuit R938 C907 C908 and is supplied to the pin in the form of sawtooth pulses. 9 controllers U901. If one of the backlight lamps breaks, the current increases through the divider R930 R932 or R931 R933, and then the rectified voltage is supplied to the pin. 2 controllers exceeding the set threshold. Thus, the formation of PWM pulses on the pin. 11, 12 and 19, 20 U901 is blocked. In the event of a short circuit in the circuits C933 C934 T901 (winding 5-4) and C930 C931 T901 (winding 1-8), “spikes” of voltage occur, which are rectified by Q907-Q910 and also supplied to the pin. 2 controllers - in this case the protection is triggered and the inverter is turned off. If the short circuit time does not exceed the charging time of capacitor C902, then the inverter continues to operate in normal mode.
The fundamental difference between the circuits in Fig. 5 and 6 is that in the first case more complex circuit soft start (the signal is sent to pin 4 of the microcircuit) on transistors Q902, Q903. In the diagram in Fig. 6 it is implemented on capacitor C10. It also uses field-effect transistor assemblies U2, U3 (p- and n-type), which simplifies their power matching and ensures high reliability in circuits with two lamps. In the diagram in Fig. 5 field-effect transistors Q904-Q907 are used, connected in a bridge circuit, which increases output power circuits and reliability of operation in starting modes and at high currents.
Inverter malfunctions and ways to eliminate them
Lamps do not turn on
Check the presence of supply voltage +12 and +6 V per pin. Vinv, Vdd of inverter connector respectively ( rice. 5 ). If they are absent, check the serviceability of the main monitor board, assemblies Q904, Q905, zener diodes Q903-Q906 and capacitor C901.
Check the supply of +5 V inverter switch-on voltage to the pin. Ven when switching the monitor to operating mode. You can check the serviceability of the inverter using an external power source by applying a voltage of 5 V to the pin. 3 U901 chips. If the lamps turn on, then the cause of the malfunction is in the main board. Otherwise, they check the inverter elements and monitor the presence of PWM signals on the pin. 11, 12 and 19, 20 U901 and, in case of their absence, replace this microcircuit. They also check the serviceability of the windings of the T901 transformer for open circuits and short circuits of the turns. If a short circuit is detected in the secondary circuits of the transformer, first of all, check the serviceability of capacitors C931, C930, C933 and C934. If these capacitors are working properly (you can simply unsolder them from the circuit), and a short circuit occurs, open the installation location of the lamps and check their contacts. Burnt contacts are restored.
The backlights flash for a short time and then go out immediately
Check the serviceability of all lamps, as well as their connection circuits with connectors J903-J906. You can check the serviceability of this circuit without disassembling the lamp unit. To do this, turn off the feedback circuit for a short time, sequentially soldering diodes D911, D913. If the second pair of lamps turns on, then one of the lamps of the first pair is faulty. Otherwise, the PWM controller is faulty or all the lamps are damaged. You can also check the performance of the inverter by using an equivalent load instead of lamps - a 100 kOhm resistor connected between pins. 1, 2 connectors J903, J906. If in this case the inverter does not work and there are no PWM pulses on the pin. 19, 20 and 11, 12 U901, then check the voltage level at the pin. 9 and 10 microcircuits (1.24 and 1.33 V, respectively. In the absence of the specified voltages, check elements C907, C908, D901 and R910. Before replacing the controller microcircuit, check the rating and serviceability of capacitors C902, C904 and C906.
The inverter turns off spontaneously after a while (from a few seconds to a few minutes)
Check the voltage at the pin. 1 (about 0 V) and 2 (0.85 V) U901 in operating mode, change capacitor C902 if necessary. If there is a significant difference in voltage at the pin. 2 from the nominal value, check the elements in the short circuit and overload protection circuit (D907-D910, C930-C935, R930-R933) and, if they are working, replace the controller chip. Check the voltage ratio on the pin. 9 and 10 microcircuits: on pin. 9 voltage should be lower. If this is not the case, check the capacitive divider C907 C908 and feedback elements D911-D914, R938.
Most often, the cause of such a malfunction is caused by a defect in the capacitor C902.
The inverter is unstable, the backlight lamps are blinking
Check the performance of the inverter in all operating modes of the monitor and in the entire brightness range. If instability is observed only in some modes, then the main board of the monitor (circuit for generating brightness voltage) is faulty. As in the previous case, an equivalent load is connected and a milliammeter is installed in the open circuit. If the current is stable and equal to 7.5 mA (at minimum brightness) and 8.5 mA (at maximum brightness), then the backlight lamps are faulty and must be replaced. They also check the secondary circuit elements: T901, C930-C934. Then check the stability of rectangular pulses (average frequency - 45 kHz) on the pin. 11, 12 and 19, 20 U901 microcircuits. The DC component on them should be 2.7 V at the P-outputs and 2.5 V at the N-outputs). Check the stability of the sawtooth voltage at the pin. 17 microcircuits and, if necessary, replace C912, R908.
The schematic diagram of the SAMPO inverter is shown in Fig. 7. It is used in 17 inch SAMSUNG monitors, AOC with SANYO matrices, in “Proview SH 770” and “MAG HD772” monitors. There are several modifications of this scheme. The inverter produces an output voltage of 810V at rated current through each of the four fluorescent lamps (about 6.8mA). The starting output voltage of the circuit is 1750V. The operating frequency of the converter at average brightness is 57 kHz, while the brightness of the monitor screen is achieved up to 300 cd/m 2 . The response time of the inverter protection circuit is from 0.4 to 1 s.
Rice. 7
The basis of the inverter is the TL1451AC microcircuit (analogues - TI1451, BA9741). The microcircuit has two control channels, which makes it possible to implement a power supply circuit for four lamps. When the monitor is turned on, +12 V voltage is supplied to the inputs of the +12 V voltage converters (sources of field-effect transistors Q203, Q204). The DIM brightness control voltage is supplied to the pin. 4 and 13 microcircuits (inverse inputs of error amplifiers). When a turn-on voltage of 3 V (ON/OFF pin) is received from the main monitor board, transistors Q201 and Q202 open and pin. 9 (VCC) of the U201 chip, +12 V is supplied. 7 and 10, rectangular PWM pulses appear, which arrive at the bases of transistors Q205, Q207 (Q206, Q208), and from them to Q203 (Q204). As a result, voltages appear on the right-hand terminals of the chokes L201 and L202, the value of which depends on the duty cycle of the PWM signals. These voltages power oscillator circuits made on transistors Q209, Q210 (Q211, Q212). On the primary windings of 2-5 transformers RT201 and RT202, respectively, appears impulse voltage, the frequency of which is determined by the capacitance of capacitors C213, C214, the inductance of the windings of 2-5 transformers RT201, RT202, as well as the level of the supply voltage. When adjusting the brightness, the voltage at the outputs of the converters changes and, as a result, the frequency of the generators. The amplitude of the inverter output pulses is determined by the supply voltage and load condition.
Autogenerators are made according to a half-bridge circuit, which provides protection against high currents in the load and breakage in the secondary circuit (turning off lamps, breaking capacitors C215-C218). The basis of the protection circuit is located in the U201 controller. In addition, the protection circuit includes elements D203, R220, R222 (D204, R221, R223), as well as a feedback circuit D205 D207 R240 C221 (D206 D208 R241 C222). When the voltage at the output of the converter increases, the zener diode D203 (D204) breaks through and the voltage from the divider R220, R222 (R221, R223) goes to the input of the overload protection circuit of the controller U201 (pins 6 and 11), increasing the protection threshold for the time the lamps are started. Feedback circuits rectify the voltage at the output of the lamps and it goes to the direct inputs of the controller error amplifiers (pin 3, 13), where it is compared with the brightness control voltage. As a result, the frequency of the PWM pulses changes and the brightness of the lamps is maintained at a constant level. If this voltage exceeds 1.6 V, a short circuit protection circuit will be activated, which will operate while capacitor C207 is charging (about 1 s). If the short circuit lasts less than this time, the inverter will continue to operate normally.
Malfunctions of the SAMPO inverter and ways to eliminate them
The inverter does not turn on, the lamps do not light up
Check the presence of +12V voltages and the active state of the ON/OFF signal. If +12V is missing, check its presence on the main board, as well as the serviceability of transistors Q201, Q202, Q205, Q207, Q206, Q208) and Q203, Q204. If there is no ONN/OFF inverter switch-on voltage, it is supplied from an external source: +3...5V through a 1 kOhm resistor to the base of transistor Q201. If the lamps turn on, then the malfunction is associated with the formation of the inverter turn-on voltage on the main board. Otherwise, check the voltage at the pin. 7 and 10 U201. It should be equal to 3.8V. If the voltage at these pins is 12V, then the U201 controller is faulty and must be replaced. Check the reference voltage at the pin. 16 U201 (2.5 V). If it is zero, check capacitors C206, C205 and, if they are working, replace controller U201.
Check the presence of generation on the pin. 1 (sawtooth voltage with a swing of 1 V) and, in its absence, capacitor C208 and resistor R204.
The lamps light up, but immediately go out (within a period of time less than 1 s)
Check the serviceability of zener diodes D201, D202 and transistors Q209, Q210 (Q211, Q212). In this case, one of the pairs of transistors may be faulty. Check the overload protection circuit and the serviceability of zener diodes D203, D204, as well as the values of resistors R220, R222 (R221, R223) and capacitors C205, C206. Check the voltage at the pin. 6 (11) controller chips (2.3 V). If it is underestimated or equal to zero, check elements C205, R222 (C206, R223). If there are no PWM signals on the pin. 7 and 10 microcircuits U201 measure the voltage at the pin. 3 (14). It should be 0.1...0.2V more than the pin. 4 (13), or the same. If this condition is not met, check elements D206, D208, R241. When performing the above measurements, it is better to use an oscilloscope. The inverter shutdown may be due to a break or mechanical damage to one of the lamps. To check this assumption (in order not to disassemble the lamp assembly), the +12V voltage of one of the channels is turned off. If the monitor screen starts to light up, then the disconnected channel is faulty. They also check the serviceability of transformers RT201, RT202 and capacitors C215-C218.
The lamps turn off spontaneously after some time (from a few seconds to minutes)
As in previous cases, the elements of the protection circuit are checked: capacitors C205, C206, resistors R222, R223, as well as the voltage level at the pin. 6 and 11 U201 chips. In most cases, the cause of the defect is caused by a malfunction of capacitor C207 (which determines the protection response time) or controller U201. Measure the voltage at the chokes L201, L202. If the voltage rises steadily during the operating cycle, check transistors Q209, Q210 (Q211, Q212), capacitors C213, C214 and zener diodes D203, D204.
The screen flickers periodically and the screen backlight brightness is unstable
Check the serviceability of the feedback circuit and the operation of the error amplifier of the U201 controller. Measure the voltage at the pin. 3, 4, 12, 13 microcircuits. If the voltage at these pins is below 0.7V, and at the pin. 16 below 2.5V, then replace the controller. Check the serviceability of the elements in the feedback circuit: diodes D205, D207 and D206, D208. Connect load resistors with a nominal value of 120 kOhm to connectors CON201-CON204, check the level and stability of the voltages on the pin. 14 (13), 3 (4), 6 (11). If the inverter operates stably with the load resistors connected, replace the backlight lamps.
Hi all!
Sometimes, during renovation LCD backlight , difficulties arise in acquiring the necessary luminescent (CCFL ) lamps . In such cases, you can convert the lamp backlight to LED. Such a conversion is not so difficult, and there are no special problems with spare parts.
In this article I offer you the principle of such a reconstruction in the form of some instructions.
Replacement steps LCD backlight to LED:
Disassemble the monitor or TV. Taking off plastic case, carefully disconnect the wires from the board, remove the metal frame from the LCD module and remove the matrix. You need to be especially careful with the matrix so as not to damage the fragile connecting cables. If everything is done correctly, then full access to electronic board, the power inverter and backlight elements will be open.
2. Disconnect the pencil cases from lamps from the matrix or the lamps themselves, if they are installed without canisters.
3. Disconnect old lamps and recycle them. With elements CCFL You also need to be extremely careful, because they contain mercury.
4. We proceed to the replacement stage. First you need to purchase an LED strip, preferably with a reserve so that it is enough to replace all the lamps (measure the length of the lamp and multiply by their number). It should be as narrow as possible and have at least 120 LEDs per meter. So that the backlight is more pleasing to the eye, it is better to take LEDs with a white glow.
5. The strip with LEDs must be glued with double-sided tape to where the lamps were. Next, wires from old lamps are soldered onto the contact terminals of the strips and insulated with hot-melt adhesive. You can immediately check the functionality of this design by connecting the wires to an external power source.
6. Now you need to connect the backlight to the power board of the monitor or TV. To do this, you need to find jumpers marked “12 V” and solder the backlight wires there, observing the polarity accordingly. Reassemble the monitor in reverse order and enjoy your invention.
Backlight in this case it will work when the device is connected to the network.
To control the backlight and bring it into normal mode, you will have to work hard. The wires leading to the LEDs must be powered in such a way that it is possible to turn on the backlight when you press the on/off buttons and adjust its brightness. There are 2 options for this:
1.We independently create a power supply circuit and adjust the backlight brightness:
On the monitor or TV power chip we look for a plastic box (connector) with wires coming out of it, where each socket is labeled on the board.
Here we are interested in the “DIM” output. It will be responsible for sending a signal to turn it on/off and adjust the brightness by changing the duty cycle in the PWM controller. The duty cycle of the pulses changes until the desired brightness level is established, and the limit values will correspond to turning on and off.
Now we need any N-channel field-effect transistor (field transistor). Wires from LED strip with a minus, the common wire from the backlight is also connected to the source, and the gate is connected through a 100-200 ohm resistor and any wire is connected to the “DIM” terminal.
We still have wires from the backlight with a plus, we bring them to the +12V power supply on the microcircuit and solder them.
Now we install the backlight in its rightful place and assemble the monitor in reverse order. Do not forget about caution and accuracy when handling the matrix and filters so that dust does not get in and the cables are not damaged. That's it, you can use it.
The second way, more expensive but convenient, is to buy a ready-made LED backlight with your own inverter :
Again, pay attention to the plastic connector and DIM pin (brightness) and the on/of pin (it’s better to use the pinout).
Using a multimeter, we determine the places on the control unit of old lamps from which the signal for brightness and on/of comes.
Now solder the wires to the found places inverter new LED backlight .
Also, it is better to unsolder the jumpers from the inverter power supply of the old lamps so that the backlight can be regulated by the new inverter.
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Until 2004-2005, CRT monitors and televisions, or, in other words, those containing a kinescope, were widely used. They, like televisions, are also called monitors and CRT televisions (electronic - ray tube) type. But progress does not stand still, and at one time LCD TVs were released that included an LCD (liquid crystal) matrix. Such a matrix must be well illuminated by 4 CCFL lamps located on both sides, top and bottom.
CCFL lamps
This applies to 17 - 19 inch monitors and TVs. On larger TVs and monitors, there may be six or more lamps. Such lamps in appearance resemble ordinary fluorescent lamps, but, unlike them, they are much smaller in size. Among the differences, such lamps will not have 4 contacts, like fluorescent lamps, but only two, and their operation requires high voltage - over a kilovolt.
Monitor backlight connector
So, after 5-7 years of operation, these lamps often become unusable; faults appear typical of conventional fluorescent lamps. . First, reddish tints appear in the image, the start is slow, in order for the lamp to light up, it needs to blink several times. In particularly severe cases, the lamp does not light up at all. A question may arise: well, one lamp has gone out, they are located above and below the matrix, usually two of them installed parallel to each other, even if only three of them are lit and the image will only be dimmer. But not everything is so simple...
PWM inverter controller
The fact is that when one of the lamps goes out, the protection on the inverter PWM controller will be triggered, and the backlight, and most often the entire monitor, will turn off. Therefore, when repairing LCD monitors and TVs, if there is a suspicion of an inverter or lamps, it is necessary to check each of the lamps with a test inverter. I purchased a test inverter from Aliexpress, as in the photo below:
Test inverter with Ali Express
This test inverter has a connector for connecting an external power supply, wires with alligator clips at the output, and connectors for connecting plugs and monitor lamps. There is information on the Internet that such lamps can be checked for functionality using electronic ballast from energy-saving lamps, with a burnt-out lamp filament, but with working electronics.
Electronic ballast from energy saving lamp
What should you do if, using a test inverter or an electronic ballast from an energy-saving lamp, you discovered that one of the lamps has become unusable and does not light up at all when connected? You can, of course, order lamps on Aliexpress, individually, but given that these lamps are very fragile, and knowing the Russian Post, you can easily assume that the lamp will arrive broken.
Monitor with a broken LCD matrix
You can also remove the lamp from a donor, for example from a monitor, with a broken matrix. But it is not a fact that such lamps will last a long time, since they have already partially exhausted their service life. But there is another option, a non-standard solution to the problem. You can load one of the outputs from the transformers, and there are usually 4 of them, according to the number of lamps on 17-inch monitors, with a resistive or capacitive load.
Monitor power supply and inverter board
If everything is clear with a resistive one, it can be an ordinary powerful resistor, or several connected in series or parallel, in order to gain the required rating and power. But this solution has a significant drawback - the resistors will generate heat when the monitor is operating, and given that it is usually hot inside the monitor case, the additional heating may not be to the liking of electrolytic capacitors, which, as is known, do not like prolonged overheating and swell.
Swollen capacitors monitor power supply
As a result, if it were, for example, a network electrolytic capacitor at 400 Volts, that same big barrel known to everyone from the photo - we could get a burnt-out mosfet or a PWM controller chip with a built-in power element. So, there is another way out: to extinguish the required power using a capacitive load, a capacitor of 27 - 68 PicoFarads and an operating voltage of 3 KiloVolts.
Capacitors 3 kV 47 pF
This solution has some advantages: there is no need to place bulky heating resistors in the case, but it is enough to solder this capacitor, which has small sizes. When choosing a capacitor value, be careful and do not solder just any values, but strictly according to the list given at the end of the article, in accordance with the diagonal of your monitor.
We solder a capacitor instead of a backlight lamp
If you solder a capacitor of a lower value, your monitor will turn off as the inverter will still go into protection due to the fact that the load is small. If you solder a capacitor of a larger value, the inverter will operate with overload, which will negatively affect the service life of the mosfets located at the output of the PWM controller.
If the mosfets are broken, the backlight, and possibly the entire monitor, will also not be able to turn on, since the inverter will go into protection. One of the signs of an inverter overload will be extraneous sounds emanating from the inverter board, such as hissing. But when the VGA cable is disconnected, a slight hiss sometimes appears coming from the inverter board - this is the norm.
Selecting capacitor values for the monitor
The photo above shows imported capacitors; there are also their domestic analogues, which are usually slightly larger in size. I once soldered our domestic ones at 6 KiloVolt - everything worked. If your radio shop does not have capacitors for the required operating voltage, but there are, for example, 2 KiloVolts, you can solder 2 capacitors of 2 times the nominal value connected in series, and their total operating voltage will increase, and will allow us to use them for our purposes.
CCFL lamp device
Similarly, if you have capacitors of 2 times smaller rating, 3 Kilovolts, but not the required rating, you can solder them in parallel. Everyone knows that series and parallel connections of capacitors are calculated using the inverse formula for series and parallel connections of resistors.
Parallel connection of capacitors
In other words, when connecting capacitors in parallel, we use the formula for connecting resistors in series or their capacitance simply adds up; when connecting in series, the total capacitance is calculated using a similar formula parallel connection resistors. Both formulas can be seen in the figure.
Many monitors have already been directed in a similar way, the brightness of the backlight dropped slightly, due to the fact that the second lamp on top or bottom of the monitor or TV matrix still functions and provides, although less, sufficient illumination for the image to remain quite bright.
Capacitors in the online store
A similar solution for home use may well suit a novice radio amateur as a way out of this situation, if the alternative is repair at a service center costing one and a half to two thousand, or buying a new monitor. These capacitors cost individually only 5-15 rubles in radio stores in your city, and any person who knows how to hold a soldering iron can perform such repairs. Happy repairs everyone! Especially for - AKV.
Discuss the article NON-STANDARD REPAIR OF MONITOR BACKLIGHT
I also wanted to ask you about the “PMS” contact, which goes from the main board to the power supply or vice versa, from the power supply to the main board. Can't define his role?
I'm interested in this because I also want to turn it off. I will hang the monitor on a swivel bracket and want to power it from a standard TFX power supply from the mini case in which it will be assembled new computer for parents (with not very new components, with DDR3L memory and Intel processor 3rd generation :). Today I conducted an experiment, supplied 5V, 12V and minus from the floppy drive connector from the computer power supply. The monitor worked fine and surprisingly even turned on and off with the power button (I assumed that the PMS sends a signal to the power supply to turn off the power to the inverter or the inverter and the main board at the same time). It’s just that the monitor will hang above the bedside table and there’s not enough space there, so it’s much easier for me to power it from the power supply, especially since I built a two-phase switch into the power supply that turns off zero and phase at the same time (that is, the computer is no longer needed unplug). And if you run a separate 220V cord to the monitor, then there are more wires, plus more hassle with turning it on/off, and the efficiency of the power supply will be slightly lower (the total energy consumption when powered from the computer power supply will decrease by ~5-10 watts). Power supply with "GOLD" certificate, Sea Sonic Electronics SSP-300TGS Active PFC 300W. Therefore, I need to know what the “PMS” signal does, wouldn’t its absence on the monitor’s power supply be critical?I also conducted an experiment with "PMS" today. 2.794 volts are supplied to this contact and only when the monitor is running. If the monitor goes to sleep or is turned off via the button on the front panel, then “PMS” immediately drops to zero. It also turned out that the first coil produces 5 volts 1.5 amperes, and the second produces simultaneously 12 volts 1.2 amperes (to power the main board) and 12 volts 3 amperes (to power the inverter). That is, whenever the monitor is turned off or asleep, 12 volts disappear from both lines, and 5 volts are supplied all the time while the monitor is plugged in and the main switch supplies 220 volts to the power supply (apparently 5 volts also goes as power to the main board and at the same time they are needed to wake up the monitor from standby mode).
So most likely “PMS” still comes from the main board to the power supply and is needed to launch a high-power coil, but I still want to know the opinion of an expert, since I judge only from practice and logical guesses.And if possible, I have three more requests for you.
1) You cannot look at the 12 volt circuit that comes from the power supply to the main board; it’s okay that 12 volts will be supplied constantly during sleep or turning off the monitor through the button on the main panel. As I wrote above, 5 volts work constantly from the built-in power supply, but 12 volts are supplied only when the monitor is running. Just want to be sure that 12 volts will not damage the main board while sleeping or turning off the monitor.2) In addition to power supply from system unit, I want to implement LED backlighting with adjustable brightness using a variable resistance to avoid PWM diodes at low brightness (flicker). I understand that the diodes will heat up more, the efficiency will drop (energy consumption will increase slightly), but eye health is more important. I myself don’t know how to correctly calculate what power variable resistor should be placed in the circuit. According to the manufacturer, the energy consumption of the tape is 9.6 watts per meter. The tapes are cut at a distance of 5 cm, and my matrix needs two strips of 45 cm, that is, a total of 90 cm. And according to the manufacturer (which I don’t really trust), the consumption at 12 volts is 800 milliamps per meter of tape, minus 10% = 720 milliamps. But it is better to take a resistance with a good power reserve, at least 2-3 amperes. I would also like to add an additional ordinary resistance to the circuit, so that at maximum brightness (where the variable resistance supplies power directly), not 12 volts go to the diodes, but 10.5 - 11 volts, no more. This is necessary so that the diodes do not overheat at maximum brightness, and also to increase their service life, since completely disassembling the monitor and matrix box once again is a pleasure.
If it’s not difficult, then write the number or model (I don’t know how to correctly) of the variable resistance (you need it with a handle, like a volume speaker systems, since there is a good place in the back of the monitor where it can be brought out) and by how many Ohms (even more likely kOhms) and Watt should we take the “simple” resistance, which will further reduce the voltage from 12 volts to 10-11 volts.
3) You also need to find a place in the power circuit of the main board, from where you can get 12 volts to power the LED backlight, where the power will be lost when you turn off the monitor from its power button and sleep mode. I myself can use a tester to find 12 volts, which disappear when the monitor is turned off and asleep, but I’m afraid that they suddenly pass through some kind of resistor or transistor, which can burn out from an additional load of 0.7-.08 amperes.
For several weeks now I have been assembling the most compact computer with standard components (that is, a standard power supply, standard motherboard, processor, OP memory, even the presence of a laptop DVD drive There is). I brought out the missing "RESET" button and the missing indicators, replaced the terrible blue computer operation indicator with a warm orange one, installed a switch for the DVD drive (so that it would not make unnecessary noise when turning on the computer) and the amplifier and speakers, and also attached the amplifier itself to the face and volume control. All that remained was to wait for the arrival of dust filters on the case and power supply and a 6-pin connector to remove the speakers from the case and indicate their operation. I plan to screw the speakers to the bottom of the monitor case, and display the indication of their operation on the bottom of the case of the speakers themselves (the lower plexiglass of both will glow during operation). I was already glad that there was a little hemorrhoids left before the assembly of this Frankenstein was completed, and then they called me and said that the monitor had stopped working. It was a strong ambush :(
That’s why I want to do everything as reliably as possible, so that it works for a long time and doesn’t cause any more trouble for at least 10 years o_O.P.S.
Sorry for the abundance of questions, I’m just afraid of unknowingly burning the main board of the monitor. Considering that this model has not been produced for more than 10 years (and as I already wrote, there are no alternatives to it, of the modern ones there are only two models on IPS matrices; they have been making them on VA for a long time, especially on PVA), but to buy the same used In good condition it is practically impossible (in Moscow and St. Petersburg they occasionally appear on sale). But if you buy it remotely, you will get darkening or scratches on the matrix, as well as broken or burnt-out pixels. When I bought the second 2190UXp through Avito, the seller from St. Petersburg assured me that the matrix was ideal, and when the monitor arrived, it turned out that the lamps had gone to zero (apparently that’s why I sold it, so that they wouldn’t completely fail) and as a bonus on top, I got two dead pixels(fortunately, at least the pixels are not in the center of the screen and on the VA matrix they are not so noticeable, parents do not notice them at all).The most common cause of failures in the operation of LCD monitors and matrices is the failure of the backlight lamps. If LED strips are used for phones and small displays in tablets, CCFL lamps are installed for these purposes in matrices with large diagonals. Essentially it's the same Fluorescent Lamp daylight, but with a cold cathode.
They have a nasty habit of breaking down without much notice. visible reasons, and even the failure of one lamp causes the protection unit to trip and turn off the power to the monitor.
Above is a burnt out CCFL lamp in the backlight module.
Getting rid of old CCFL
The most obvious way to solve the problem is to replace the lamp, but repairs also have some pitfalls. For example, to replace exactly the same lamp is needed. The inverter does not want to accept sources with slightly different power parameters, and finding a complete analogue for a model released 5-6 years ago is sometimes problematic.
In light of this, the idea of converting the monitor to LED backlighting is very attractive.
To switch to LED, you will have to deal with the inverter for CCFL lamps. It will no longer be useful to us, since its output generates a high-voltage, high-frequency signal that is lethal to the LED.
Simply disconnect the inverter connector cable from the main board. For the future we need a “dim” connector to control brightness LED strip.
To replace the lamps in the monitor with an LED strip, you will need a dimmable power driver.
The replacement is carried out in two stages. The first is removing the CCFL lamps and power inverter, the second is installing the LED strip, power driver and connecting them. As an LED driver, you can use 220V and 12V models, the main thing is that they fit in size.
As a CCFL equivalent, tapes with 120 diodes per meter are best suited. If you cannot find such a tape of a suitable width, you can use 90 diodes per meter.
The tape must be neutral white, otherwise color distortion is guaranteed. When choosing an LED strip for your monitor, pay special attention to this. Read more about the color of the lamps.
When replacing a lamp, do not get carried away with achieving too high brightness; powerful LEDs significant heat generation, which will not have the best effect on the matrix itself.
How to replace the monitor backlight with LED
The most difficult and painstaking part of the work for us will be dismantling the housing.
Any careless movement can cause the cable to break or even damage the matrix. There is no point in disassembling the case while the power is on; a voltage of the order of a kilovolt is generated at the inverter output. Breaking it into a scanner or matrix is guaranteed to burn these blocks.
But by and large, replacing the monitor backlight with an LED one yourself is quite simple.
The electronic filling consists of three blocks:
- Power unit;
- image scanning unit;
- lamp inverter unit.
Typically, the inverter unit is covered with a protective casing.
The LED strip installed instead of the monitor backlight lamps must match the width of the lamp grooves as much as possible, otherwise the backlight will be uneven.If you decide to use a 12V LED backlight driver, make sure that the power supply has an output with this voltage. You can, of course, find a point on the board with a 12V supply voltage, but connecting a tape driver to it can cause a voltage drop and unstable work electronics.
Dimmable LED strip driver circuit
As already mentioned, to replace CCFL with LED, you will have to install an LED strip power driver in the monitor.
You can assemble a simple PWM controller for dimming the backlight brightness with your own hands using the N555 chip.
Monitor LED backlight circuit with built-in dimmer
The dimming signal generator is assembled on the NE555 pulse generator; a feature of this microcircuit is the ability to change both the frequency and duty cycle of the pulses. The variable resistor in this circuit affects the duty cycle.
The advantages of such a backlight brightness control scheme are low heat generation and a wide signal range, the disadvantage is mechanical adjustment. This circuit will be needed if there is a software dimmer on the lamp power inverter board. This LED backlight circuit is universal and suitable for screens of any manufacturer.
Circuit for external dimming
This is a copy of the output stage of the previous circuit. If the signal level from the dimming output is insufficient for the field-effect transistor to operate correctly, it is possible to install an additional low-power transistor switch in front of the gate, which will act as a voltage switch.
And this circuit will allow you to control the brightness of the tape through a standard channel. Please note that the dimming depth for ccfl lamps is less than that of LEDs, so in this scheme the brightness range will be less than with the first option.
On many Toshiba, JVS, BenQ devices, PWM is software, when the inverter receives a signal to increase or decrease the duty cycle, and the dimming signal is generated by the controller of the inverter itself. In Samsung and LG, all models have a “dim” output, which is suitable for controlling the brightness of the monitor’s LED backlight.
Replacing ccfl with LED in a monitor can significantly reduce costs compared to installing a new lamp. Even at minimum prices, four lamps will cost 3-5 dollars, and half a meter of LED strip along with a driver will cost you less than a dollar.
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