LEDs are driven by DC, so a power adapter, ie LED drive power, needs to be added between the mains and the LED. Its function is to convert AC mains power into DC power suitable for LEDs. Usually the LED is driven by a dedicated constant current source or driver chip, which is easily limited by factors such as volume and cost. The most economical and practical method is to use a capacitive step-down power supply. Using it to drive low-power LEDs has the advantages of not being afraid of load short-circuits and simple circuits, and a circuit can drive 1 to 70 low-power LEDs (however, the current impact when the power supply circuit starts, especially frequent starts, will give LEDs Cause damage. Of course, with appropriate protection can avoid this impact).

The typical circuit of a capacitive step-down power supply is shown in Figure 1. C1 is a step-down capacitor (using a metalized polypropylene capacitor), and R1 provides a discharge circuit for C1. Capacitor C1 provides a constant operating current for the entire circuit. Capacitor C2 is an electrolytic capacitor, and its withstand voltage value depends on the number of LEDs connected in series (about 1.5 times or more than its total voltage). Its main function is to suppress voltage mutations caused by energized moments, thereby reducing voltage shock Impact on LED life. R4 is the bleeder resistor of capacitor C2, and its resistance value should increase appropriately with the increase of the number of LEDs.

It should be noted that the circuit must select the appropriate capacitor according to the current of the load, rather than the voltage and power of the load. The relationship between the capacity C of the buck capacitor C1 and the load current Io can be approximated as: C=14.5Io, The capacity unit of C is uF, and the unit of Io is A. The current-limiting capacitor must be a non-polar capacitor, and the withstand voltage of the capacitor must be above 630V.

Since the capacitor step-down power supply is a non-isolated power supply, a large current will be generated at the moment of power-on, which is called the inrush current. In addition, due to the influence of the external environment (such as lightning strikes), the power grid system will invade various surge signals, and some surges will cause damage to the LED. The LED's anti-surge current and anti-reverse voltage capabilities are relatively poor, it is also very important to strengthen the protection in this area, especially some LED lights are installed outdoors (such as LED street lights). Therefore, the LED drive power must have the ability to suppress the intrusion of the surge and protect the LED from being damaged. This circuit uses a negative temperature coefficient thermistor to limit the abrupt change of current, and uses a positive temperature coefficient thermistor to automatically adjust the current size to make it tend to a certain change range. At the same time, there is a transient voltage suppressor at the power input To avoid voltage overload.