Effect of constant current drive and non-constant current drive on LED digital tube

LED digital tube should be displayed normally, it is necessary to use the drive circuit to drive the various segments of the digital tube, thus showing the number we want, so according to the different driving methods of the LED digital tube, it can be divided into two types: static and dynamic. .

A, static display driver:

Static drives are also called DC drives. Static driving means that each segment code of each digital tube is driven by the I/O terminal of a single chip microcomputer or by decoding with a BCD code two-decimal decoder. The advantage of static driving is that the programming is simple, the display brightness is high, and the disadvantage is that it occupies more I/O ends. For example, driving 5 digital tube static displays requires 5×8=40 I/O terminals to drive, to know a 89S51 The I/O terminals available for the microcontroller are only 32. Therefore, the actual application must increase the decoding driver for driving, which increases the complexity of the hardware circuit.

B, dynamic display driver:

The digital tube dynamic display interface is one of the most widely used display modes in the single-chip microcomputer. The dynamic drive is to display the same names of the eight display strokes of all digital tubes "a, b, c, d, e, f, g, dp". The terminals are connected together, and the bit gate control circuit is additionally added for the common pole COM of each digital tube. The bit gates are controlled by independent I/O lines. When the single chip outputs the font code, all the digital tubes are received. The same glyph code, but which digital tube will display the glyph, depending on the control of the single-chip strobe COM terminal circuit of the single-chip microcomputer, so we only need to open the gating control of the digital tube that needs to be displayed, and the bit is displayed. Out of the glyph, there will be no bright digital tube without strobing.

By controlling the COM end of each LED digital tube by means of time-sharing, the digital tubes are controlled to display in turn, which is dynamic driving. In the process of rotating display, the lighting time of each bit digital tube is 1~2ms. Due to the persistence of human vision and the afterglow effect of the LED, although the digital tubes are not lit at the same time, as long as the scanning speed Fast enough, giving the impression that a stable set of display data, no flickering, dynamic display and static display are the same, can save a lot of I / O, and lower power consumption.

The main effects of constant current drive and non-constant current drive on LED digital tube are as follows :

1, display effect :
Since LEDs are basically current-sensitive components, their forward voltage drop is highly dispersive and also related to temperature. In order to ensure good brightness uniformity of digital tubes, it is necessary to have a constant operating current and cannot be affected. The influence of temperature and other factors. In addition, when the temperature changes, the wafer is driven to automatically adjust the magnitude of the output current to achieve color difference balance temperature compensation.
2. Security :
Even a short-time current overload may cause permanent damage to the LED. After using a constant current drive circuit, it can prevent large-area damage of the digital tube caused by current failure. In addition, the ultra-large integrated circuit we use also has a cascaded delay switching feature to prevent damage to the LED from the reverse spike voltage. The ultra-large integrated circuit also has a thermal protection function that automatically turns off when any one of the temperatures exceeds a certain value, and can see the fault display in the control room.

Why is the brightness of the digital tube uneven?

There are two big factors that affect brightness consistency. One is the selection of raw material wafers, and the other is the control method used when using digital tubes.
1. Raw materials - The ventricular fibrillation and brightness and wavelength of the grains are a normal distribution. Even if the LED grains are screened, the VF, brightness and wavelength are already in a small range, and the products produced are still in a range. The result is that the brightness is inconsistent.
2, to ensure that the LED digital tube brightness is the same, there are differences in the choice of control methods. The best way is constant current control, the current flowing through each LED is the same, so the LED looks the same brightness. For example, the constant voltage control causes the currents of the LEDs with different VFs to be different, so the brightness is also different. Of course, these two conditions are complementary.

How to measure the digital tube pins, sub-yin and common?

Looking for public sympathy and public yang, first, we find a power supply (3 to 5 volts) and different specifications of resistance, VCC connected to a resistor and GND connected to any 2 feet, there are many combinations, but there are always One LED will illuminate to find one, then use GND to move, VCC (string resistance) touch the remaining feet one by one. If there are multiple LEDs (usually 8), then it is common. Instead, use VCC to move, GND to touch the remaining feet one by one. If there are multiple LEDs (generally 8), then it is common. You can also use a digital multimeter directly. The red pen is the positive pole of the power supply, and the black pen is the negative pole of the power supply.

(Editor: Wen Jing)