Transformer step-down scheme to achieve 220 V to DC 12 v
This scheme is commonly used in switching power supply and charger, and its core components are transformer, rectifier bridge and buck chip. As shown in the figure below, it is a typical circuit diagram of this scheme.
AC 220 V is changed into AC 12 V through voltage transformation, and then converted into pulsating DC through rectifier bridge. After filtering by electrolytic capacitor C1, it is connected to LDO step-down chip. The 7812 scheme is used in the figure above. The chip can output a maximum current of 1.5 A, but the conversion efficiency is relatively low, so heat sink needs to be installed. Lm2596 and other power chips with high conversion efficiency can also be used. At present, most mobile phone chargers and switching power supply are transformer solutions. Mobile phone chargers use high-frequency transformer, which greatly reduces the size of products and makes them smaller and smaller.
This solution is a chip level solution, which is small in volume but small in output current, so it is widely used in small power household appliances, such as electric rice cooker, washing machine, coffee machine, etc. There are many chips that can realize this function, such as fsl336, lnk304, ucc28881 and so on.
The circuit in the figure above can provide a maximum current of 120mA at 12V output, and the power is relatively small, which is suitable for small power household appliances.
3. Using RC resistance and capacitance to reduce voltage to realize the conversion from 220 V to 12 V DC
Among these schemes, RC resistance capacitance buck scheme has the lowest cost and the simplest circuit, which is common in fish tank heating rod, LED lighting and other products.
In the figure above, the capacitor and resistor are connected in parallel to form a resistance capacitance step-down circuit, and then enter the rectifier bridge for rectification. Finally, the 12V voltage regulator is used to obtain a stable voltage value, but the output power is relatively small. The larger the capacitance, the smaller the capacitive reactance, and the larger the output current. This scheme is not suitable for high-power load equipment, and the input and output are not isolated, so it needs to be used carefully.
