1. Advantages of SMD devices compared to plug-in devices
- small volume
- light weight
- Strong anti-vibration ability
- High reliability
- Low defect rate of solder joints
- Good high frequency characteristics
- Easy to automate
- Welding is cheap
2. What is a thyristor?
Also called thyristor, it is a rectifier device with unidirectional conductivity.
A forward voltage is applied between GK to turn on the thyristor, but the thyristor cannot be turned off, and the current between AK is less than the threshold value and it is automatically turned off
3. Power Module
Regulates and Provides Power
4. Linear step-down power supply, if the pressure difference is too large, the module will become hot, so it is better to use multiple step-down modules to reduce the voltage evenly.
5. After AC rectification becomes DC, the output voltage is the input voltage multiplied by the square root of 2
6. Selection of resistors
Consider temperature characteristics, package, and resistance accuracy (generally 5% and 1% specifications)
Generally, the current limiting resistor is 5% accurate, and the voltage dividing resistor is 1% accurate.
7. From the design purpose, reverse the structure of the circuit
8. Selection of capacitors
Main considerations: capacity, allowable deviation, rated voltage, leakage current, dissipation factor, temperature coefficient, frequency characteristics
When the capacitor is rectified and filtered, the larger the capacity is, the more stable the subsequent output voltage will be. The selection of the capacity is also related to the line current.
First find the limit ripple, then select the capacitor according to the frequency, C=Iout/ △V*f (△V is the maximum allowable ripple voltage)
The ripple in the project is generally 3%~8%
Capacitor withstand voltage is at least 1.5 times the input voltage
9. The difference between a current source and a voltage source:
A voltage source is an ordinary power supply with extremely low internal resistance. The resistance of the load must be much larger than the internal resistance of the power supply when it changes in a large range, so the terminal voltage of the voltage source is stable, and it can be seen that all the electromotive force is dropped on the load. It has a very high internal resistance, which acts as a current limiter. Usually, the load resistance value is much smaller than its internal resistance, so the output current is constant.
Current source: no matter whether it is used or not, there must be current flowing. The current source has internal resistance, so it will definitely waste electricity, and the voltage source can be disconnected when the circuit is not used. Therefore, the voltage source is generally used in household appliances.
10. When the power supply provides current to the rear circuit, a capacitor filter is generally connected in parallel
11. When actually choosing a capacitor, a margin should be left. The calculation result can be selected as 68uf, but in practice, a larger capacitor will be selected.
**Drive capability
To drive a load, there are 2 requirements
1. The voltage is sufficient
2. The output current of the I/O port is sufficient
The driving ability is the maximum output current. If the driving ability is insufficient, the output voltage will drop. For the logic circuit, it is impossible to maintain its high level, resulting in logic confusion and the expected effect cannot be achieved.