It is well known that a light-emitting diode is an electronic component having a unidirectional conductive property, which is often encountered in hardware design. The most common function of LEDs is to allow current to pass only in a single direction (called forward bias) and reverse in reverse (called reverse bias). Light-emitting diodes function as a rectifier circuit, a detection circuit, a voltage stabilization circuit, various modulation circuits, and the like. There are many types of light-emitting diodes, which can be classified into a germanium diode (Ge tube) and a silicon diode (Si tube) depending on the semiconductor material used. Light-emitting diodes seem simple, but there are many minefields buried, which requires our special attention.
Minefield 1: The tube voltage drop Vf of the LED. The silicon diode (non-illuminated type) has a forward voltage drop of 0.7V, and the forward tube voltage drop of the neon tube is 0.3V. The forward voltage drop of the LED will vary with different illumination colors. The general experience is estimated to be around 1.7V~2.2V. Therefore, the tube drop before and after the LED cannot be ignored in the normal design. To put it bluntly, the one-way pass is at the expense of a part of the voltage. Therefore, if some light-emitting diodes are used in some circuits with relatively sensitive power supply voltages, special attention should be paid to the voltage changes to prevent the voltage after the light-emitting diodes from passing the requirements. The tube voltage drop of ordinary LEDs is higher. If you want to drive, don't use a power supply like 1.8V.
Minefield 2: The highest reverse operating voltage Udrm is added to the reverse voltage of the LEDs to a certain value, which will break the tube and lose the unidirectional conductivity. In order to ensure safe use, the highest reverse operating voltage value is specified in the specification manual of the LED. Reverse blocking is limited, especially in the AC rectifier circuit and boost circuit, not only to consider the normal reverse voltage, but also to take into account the inductance of the inductor and other factors, leaving enough margin.
Minefield 3: Reverse current Idrm Reverse current refers to the reverse current flowing through the LED under normal temperature (25 ° C) and the highest reverse voltage. The smaller the reverse current, the better the unidirectional conductivity of the tube. The reverse current is closely related to the temperature, and the reverse current is doubled for every 10 °C increase in temperature. For example, the 2AP1 type germanium diode has a reverse current of 250uA at 25°C, a rise in temperature to 35°C, a reverse current of 500uA, and so on. At 75°C, its reverse current has reached 8mA. Not only does it lose its unidirectional conductivity, it also overheats and damages the tube. Silicon diodes have better stability at high temperatures than germanium diodes. Many designs pay special attention to the fact that the reverse current of the LED is multiplied in a high temperature environment. When selecting the type, attention should also be paid to the influence of the reverse current parameter of the tube on the circuit design.
USB5V is powered by USB. When USB is not plugged in, it is powered by external 12V. The work works as follows. After USB is plugged in, the 3904 transistor is turned on, giving the CPU a low-level interrupt, allowing the CPU to make the corresponding action. Later, it is not When plugged into the USB, it was found that the voltage of 3V was connected, and the CPU could detect the low level interrupt of the USB, which caused a malfunction. Before R13 posted the 47K, this phenomenon was discovered and it was determined that the LED was leaking backward. The current is passed through the 47K resistor of R13, and the resulting voltage causes the transistor to conduct. Later, the base of the transistor does have a voltage of 0.68V.
The parameter characteristics of 1N4148, IR is 2.5uA, the characteristic curve is as follows: