LED electronic display maintenance basics

1. Circuit Basics and Uses of Common Components

What is a circuit???
A circuit is a network composed of interconnected electronic and electrical devices, such as resistors, capacitors, inductors, diodes, transistors, and switches. The size of a circuit can vary greatly, from an integrated circuit on a silicon chip to a transmission network. Depending on the signal being processed, electronic circuits can be divided into analog circuits and digital circuits.
Analog circuits process the current and voltage of signals. The most typical analog circuit applications include: amplifier circuits, oscillation circuits, and linear operation circuits (addition, subtraction, multiplication, division, differentiation, and integration circuits).
In digital circuits, the size of the signal only represents a limited number of states, and most use Boolean algebraic logic to process the signal. Typical digital circuits include oscillators, registers, adders, subtractors, etc.

The CMOS gate circuit has a high output level VOH and a low output level VOL. The theoretical value of the CMOS gate circuit VOH is the power supply voltage VDD, VOH (min) = 0.9VDD; the theoretical value of VOL is 0V, VOL (max) = 0.01VDD. Therefore, the logic swing (i.e. the difference between the high and low levels) of the CMOS gate circuit is large and close to the power supply voltage VDD.

TTL gate circuit level:
output high level>2.4V, output low level<0.4V. At room temperature, the general output high level is 3.5V, and the output low level is 0.2V. Minimum input high level and low level: input high level>=2.0V, input low level<=0.8V, and the noise margin is 0.4V.

Broken circuit/open circuit: The current does not form a loop in the circuit.

Short circuit: The current does not pass directly through the load normally, but passes through an object with a very small resistance in parallel with the load, and the object is not within the electrical range of the designed circuit. The connection phenomenon caused by other reasons is called a short circuit. Intentional short circuit will not affect the normal operation of the circuit, but unintentional short circuit will damage the circuit and make it unable to work normally.

Direct current (voltage/current): The phase of the voltage/current does not change over time.

AC (voltage/current): The phase of the voltage/current varies with time.

Constant current: The current does not change with changes in load.

Constant voltage: The voltage does not change with changes in load.

Digital signal: only high/low levels appear. Computers process digital signals, and so do our LED displays. Generally, high levels are represented by "1" or "H", and low levels are represented by "0" or "L". Data is represented by binary, octal, and hexadecimal. Octal is less commonly used. We use decimal in daily life.

Example: Binary (01010101) = Octal (125) = Hexadecimal (55H) = Decimal (85)

Binary (00000001) = Decimal (1), Binary (00000010) = Decimal (2)

Binary (00000011) = Decimal (3),,,,,,,,,,,,,,,,,,,,,,,,,

Current I: 0.001kA = 1A = 1000mA = 1000000uA

Voltage U: 0.001Kv=1V=1000mV=1000000uV

Resistance R: 1MΩ=1000KΩ=1000000Ω

Capacitance C: 0.001F = 1uF = 1000nF = 1000000pF

Ohm's law: I = U / R

1. Resistors: They play the role of current limiting and voltage dividing in the circuit. They are represented by R and the unit is ohm (Ω). In pixel products, resistors are mostly used to limit the current.

For example: When a certain LED is required to be lit with 5V, a resistor must be connected in series with the LED to prevent overcurrent from burning out. Resistance Rled = (5-Uled)/Iled, Uled is the forward voltage drop of the LED, Iled is the current passing through the LED, and generally the current is not allowed to be greater than 20mA.

2. Capacitor: It blocks direct current and passes alternating current. It is mostly used for filtering in pixel products. It is represented by C and the unit is (F).

For example, the 104PF capacitor in the common 0805 package is used to filter out the higher frequency voltage ripple in the circuit, and the electrolytic capacitor 470UF/16V is used to filter out the lower frequency voltage ripple. They all play the role of filtering out interference signals, improving the anti-interference ability of the circuit, and making the circuit work stably.

3. IC: Integrated circuit, as the name suggests, is to integrate some circuits into a small substrate to complete certain circuit functions, reduce the size for easy installation, and improve circuit stability.

Example: Our 74HC595, TB62726, etc. are all ICs.

The function of 74HC245: signal power amplification.

Pin 1, DIR, is used for input/output port conversion. When DIR=“1” and high level, the signal is input from the “A” end and output from the “B” end. When DIR=“0” and low level, the signal is input from the “B” end and output from the “A” end.

Pins 2 to 9 are “A” signal input and output terminals, A1=B1,,,,,,,A8=B8, A1 and B1 are a group, if DIR="1"G="0" then A1 is input and B1 is output, the same as above. If DIR="0"G="0" then B1 is input and A1 is output, the same as above.

Pins 11 to 18 are the "B" signal input and output terminals, and their functions are the same as those of the "A" terminals, so they will not be described here.

Pin 19 G is the enable pin. If this pin is "1", the signal at the A/B terminal will not be conductive. Only when it is "0", the A/B terminal will be enabled. This pin acts as a switch.

Pin 10 GND, power ground.

Pin 20 VCC, positive power supply.

Function of 74HC04: 6-bit inverter.

Pin 7 GND, power ground.

Pin 14 VCC, positive power supply.

The signal is input from the A terminal and output from the Y terminal in reverse phase. A1 and Y1 are a group, and the others are analogous. For example: A1 = "1" then Y1 = "0", A1 = "0" then Y1 = "1", and the functions of other groups are the same.

Function of 74HC138: 8-bit binary to decimal decoder.

Pin 8 GND, power ground.

Pin 15 VCC, positive power supply

Pins 1 to 3, A, B, and C, are binary input pins.

The chip select signal control of pins 4 to 6 will only be selected when pins 4 and 5 are "0" and pin 6 is "1", and the output is controlled by signals A, B, and C. Any other combination will not be selected, and the outputs of Y0~Y7 are all "1".

By controlling the select pin to cascade, it can be expanded to sixteen bits.

Example: G2A=0, G2B=0, G1=1, A=1, B=0, C=0, then Y0 is "0" and Y1~Y7 are "1", see the truth table for details.

The function of 74HC595: LED driver chip, 8-bit shift latch.

Pin 8 GND, power ground.

Pin 16 VCC, positive power supply

Pin 14 is DATA, the serial data input port. Display data enters from here and must be coordinated with a clock signal to be moved in.

Pin 13 EN is the enable port. When the pin is "1", all QA~QH ports are "1". When it is "0", the output of QA~QH is controlled by the input data.

Pin 12, STB, is the latch port. After the input data is transferred to the register, only by providing a latch signal can the shifted data be sent to the QA~QH ports for output.

Pin 11 CLK is the clock port. Each clock signal will shift one bit of data into the register.

Pin 10 SCLR is the reset port. As long as there is a reset signal, the data shifted into the register will be cleared. The display screen does not use this pin and is generally connected to VCC.

Pin 9, DOUT, is the serial data output terminal, which transmits data to the next one.

Pins 15, 1~7 are the parallel output ports, which are also the drive output ports, driving the LED.

Function of 4953: row driver tube, power tube.

There are two CMOS tubes inside, pins 1 and 3 are VCC, pins 2 and 4 are control pins, pin 2 controls the output of pins 7 and 8, and pin 4 controls the output of pins 5 and 6. Only when pins 2 and 4 are "0" will pins 7, 8, 5, and 6 output, otherwise the output is in a high-impedance state.

Function of TB62726: LED driver chip, 16-bit shift latch.

Pin 1 GND, power ground.

Pin 24 VCC, positive power supply

Pin 2 DATA, serial data input

Pin 3 CLK, clock input

Pin 4 STB, latch input

Pin 23 is the output current adjustment terminal, connected to a resistor for adjustment

Pin 22 DOUT, serial data output

Pin 21 EN, enable input

Other functions are similar to 74HC595, except that TB62726 is a 16-bit shift latch with output current adjustment function, but no high level will appear on the parallel output port, only high impedance state and low level state. 74HC595 parallel output port has high level and low level output. TB62726 has the same pin function as 5026 and similar structure.

2. Understanding of common signals of LED display screens

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CLK clock signal: The shift pulse provided to the shift register. Each pulse will cause the data to shift in or out one bit. The data on the data port must be coordinated with the clock signal to transmit the data normally. The frequency of the data signal must be 1/2 times the frequency of the clock signal. In any case, when the clock signal is abnormal, the entire board will display in a disorderly manner.

STB latch signal: Send the data in the shift register to the latch, and display the data content by lighting up the LED through the drive circuit. However, since the drive circuit is controlled by the EN enable signal, the premise of its lighting must be that the enable is in the on state. The latch signal also needs to be coordinated with the clock signal to display a complete image. In any case, when the latch signal is abnormal, the entire board will be displayed in a disorderly manner.

EN enable signal: the whole screen brightness control signal, also used for display screen blanking. The brightness change can be controlled by adjusting its duty cycle. When the enable signal is abnormal, the whole screen will appear dark, dim or tailing.

Data signal: Provides the data needed to display images. It must be coordinated with the clock signal to transmit data to any display point. Generally, the red, green and blue data signals are separated in the display screen. If a data signal is short-circuited to the positive or negative pole, the corresponding color will appear fully lit or not. When the data signal is suspended, the corresponding color display is uncertain.

ABCD line signal: only exists in dynamic scanning display. ABCD is actually a binary number, A is the lowest bit. If the binary number is used to represent the maximum range of ABCD signal control, it is 16 lines (1111). In 1/4 scanning, only AB signal is needed because the range of AB signal is 4 lines (11). When the line control signal is abnormal, there will be display dislocation, highlight or image overlap.

3. Common troubleshooting methods (tools: multimeter, soldering iron, blade, screwdriver, tweezers, etc.)

* When judging problems, we must deal with them in order of priority and secondary issues, dealing with obvious and serious issues first and minor issues later.

Short circuiting should be the highest priority.

1. Resistance detection method: adjust the multimeter to the resistance range, detect the resistance value to ground of a certain point on a normal circuit board, and then test the same point on another identical circuit board to see if the resistance value is different from the normal one. If they are different, the scope of the problem is determined.

2. Voltage detection method: adjust the multimeter to the voltage range, detect the voltage to ground at a certain point in the circuit suspected of having problems, and compare whether it is similar to the normal value. Otherwise, determine the scope of the problem.

3. Short circuit detection method: adjust the multimeter to the short circuit detection range (some are diode voltage drop range or resistance range, generally with alarm function), detect whether there is a short circuit phenomenon, and solve it first after finding a short circuit so as not to burn other components. This method must be operated when the circuit is powered off to avoid damaging the meter.

4. Voltage drop detection method, adjust the multimeter to the diode voltage drop detection mode, because all ICs.