Research on the structure and detection method of LED digital tube

LED digital tube is composed of light emitting diodes , also known as semiconductor digital tubes. Strip-shaped light emitting diodes are connected in a common cathode (negative) or common anode (positive) method to form the figure "8", and the other electrode of the light emitting diode is used as a pen segment electrode to form an LED digital tube. If the light emitting diodes on some pen segments are used as specified, they can display a series of numbers from 0 to 9. Compared with fluorescent digital tubes and glow digital tubes (NRT), it has the following advantages: small size, low power consumption, vibration resistance, long life, high brightness, good monochromaticity, short light response time, and compatibility with TTL, C MOS circuits , etc.

The appearance and internal structure of common LED digital tubes are shown in Figure 1. Figure 1(b) belongs to the common anode structure, and Figure 1(c) adopts the common cathode structure. + and - respectively represent the common anode and common cathode. a~g are 7 segment electrodes, and DP is the decimal point. There is also an ultra-small LED digital tube with a character height of 7.6mm. The pins are led out from the left and right rows, and the decimal point is independent.

Performance testing of LED digital tubes

The appearance of LED digital tubes requires uniform color, no local discoloration and no bubbles, etc. Dry batteries can be used for further testing under amateur conditions. As shown in Figure 2. Take the common cathode digital tube as an example to introduce the detection method. Fix the negative lead wire of the 3V dry battery in contact with the common cathode of the LED digital tube, and move the positive lead wire of the battery to contact the positive end of the stroke in turn. When this lead wire touches the positive end of a stroke, the stroke should be displayed. This simple method can be used to detect whether the digital tube has broken strokes (some strokes cannot be displayed) and connected strokes (some strokes are connected together), and the intensity of the light emission of different strokes can be relatively compared. If you detect a common anode digital tube, you only need to swap the positive and negative lead wires of the battery, and the method is the same as above.

The working current I of each stroke of LED digital tube is between 5 and 10 mA. If the current is too large, the digital tube will be damaged. Therefore, a current limiting resistor must be added . Its resistance value can be calculated by the following formula: Where Uo is the voltage applied to both ends of the LED , and U is the voltage drop of each stroke of the LED digital tube (about 2 V).

The hFE socket of a digital multimeter can be used to conveniently check the luminescence of the LED digital tube. When the NPN gear is selected, the C hole is positively charged and the E hole is negatively charged. For example, when testing the LTS547R common-pole LED digital tube, insert a single-strand thin wire from the E hole, and connect the lead end of the wire to one pole (the ③th pin and the ⑧th pin are connected internally, and you can choose one as -); then lead a wire from the c hole to contact each pen segment in turn. If the circuit shown in Figure 3 is used to short-circuit the ④, ⑤, ①, ⑥, and ⑦ pins and then connect them to the lead wire of the c hole, the number '2' will be displayed. If all segments a to g are connected to the c lead, the bright pen segment will be displayed, forming the number "8".

(1) If the light is dim during the test, it means that the device has aged and the light efficiency is too low. If the displayed pen segment is incomplete, it means that the digital tube is partially damaged.

(2) For LED digital tubes with unknown models and no pin arrangement diagram, the following tests can be completed using the hFE file of a digital multimeter: ① Determine the structure of the digital tube (common cathode or common anode); ② Identify the pins; ⑧ Detect the fully lit segments. It can be assumed in advance that a certain electrode is the common electrode, and then verified based on whether the segment is lit or not. When the segment electrode is connected in reverse or the common electrode is judged incorrectly, the segment will not light up.

[page]Performance characteristics, simple detection and usage precautions of LED digital tubes

1. Performance characteristics

The main features of LED digital tubes are as follows:

(1) It can be driven to emit light under low voltage and low current conditions and is compatible with C MOS and ITL circuits .

(2) The luminous response time is extremely short (<0.1µs), with good high-frequency characteristics, good monochromaticity and high brightness.

(3) Small size, light weight and good impact resistance.

(4) Long service life, the service life is more than 100,000 hours, and even up to 1 million hours. Low cost.

Therefore, it is widely used as a digital display device for digital instruments, numerical control devices, and computers.

2. Simple performance test

The appearance of LED digital tubes requires uniform color, no local discoloration and no bubbles, etc. Dry batteries can be used for further inspection under amateur conditions. The inspection method is now introduced using the common cathode digital tube as an example.

Fix the negative lead wire of a 3V dry battery in contact with the common negative terminal of the LED digital tube, and move the positive lead wire of the battery to contact the positive terminal of the strokes in turn. When this lead wire touches the positive terminal of a certain stroke, that stroke should be displayed. This simple method can be used to check whether the digital tube has broken strokes (certain strokes cannot be displayed) or connected strokes (certain strokes are connected together), and the intensity of the light emission of different strokes can be relatively compared. If you want to check the common anode digital tube, just swap the positive and negative lead wires of the battery, and the method is the same as above.

The working current ILED of each stroke of LED digital tube is about 5-10mA. If the current is too large, it will damage the digital tube, so a current limiting resistor must be added . Its resistance can be calculated as follows: Rlimit = (ULED-ULED)/ILED

Among them, U is the voltage applied to both ends of the LED, and ULED is the voltage drop per stroke of the LED digital tube (about 2 volts).

The hFE socket of a digital multimeter can be used to conveniently check the luminescence of the LED digital tube. When the NPN gear is selected, the C hole is positively charged and the moon hole is negatively charged. For example, when checking the LTS547R common cathode LED digital tube, insert a single strand of thin wire from the E hole, and the wire lead-out terminal is connected to the 9-pole (the ③th pin and the ⑧th pin are connected internally, and one of them can be selected as Ө); then lead a wire from the C hole to contact each pen segment electrode in turn, and the corresponding pen segment can be displayed respectively.

3. Precautions for use

(1) If the light is dim during inspection, it means that the device is aged and the light efficiency is too low. If the displayed pen segment is incomplete, it means that the digital tube is partially damaged.

(2) For LED digital tubes with unknown models and no pin arrangement diagram, the following tests can be completed using the h range of a digital multimeter: ① Determine the structure of the digital tube (common cathode or common anode); ② Identify the pins; ③ Check the fully lit segments. It can be assumed in advance that a certain electrode is the common electrode, and then verified based on whether the segment is lit or not. When the segment electrode is connected in reverse or the common electrode is judged incorrectly, the segment will not light up.