In celebration days of Spring Festival, Christmas day and so on, people hang generally various decorative light strings to decorate a room, an aisle, Christmas trees and so on, in which the more often used is a bunchy LED decorative light string.
A LED decorative light string in the prior art is shown in FIG. 1, wherein each of the LEDs is simply connected in parallel between each other, and is connected to an operational power supply. As a result of parallel connection between each LED, the failure of any one of LEDs will not affect the operation of the other LEDs. However, since the operating voltage of the LEDs is lower, the LED decorative light string in such connecting relation may not be directly connected to AC power supply, and then a corresponding step-down transformer is required, such that the cost of the whole production would be increased.
Another LED decorative light string in the prior art is shown in FIG. 2, wherein each of LEDs is generally simple connected in series, then the LED decorative light string is connected to an operational power supply (generally is AC power supply). One disadvantage of such decorative light string is that the whole decorative light string will be led to disconnection and can not be in operation when a certain LED is in failure, thereby its reliability is rather poor and its life-span is shorter.
For the decorative light string shown in FIG. 2, only half a period of Sin AC (alternating current) can be effective working-time when AC is used as its operational power supply, while each LED will not emit light in the other half period, such that the luminous efficiency thereof is not high. Moreover, referring to FIG. 3, a noise current will be resulted from the inherent characteristic of LEDs, and the noise current will generate an adverse effect to the quality of the whole AC power supply.