White Light Emitting Diodes recently available can be used as a powerful alternative to incandescent lamps in lighting applications. Today’s White LEDs are less expensive, draw much less current, and project a fairly well focused beam.
Described here is a simple circuit of an efficient White LED Night Light which can be directly powered from AC230V supply. First of all, note that a “capacitive transformerless power supply” is used in this circuit.
Capacitive Transformerless Power Supply
When a capacitor and resistor are connected in series to an AC source, as shown here, a constant current can be maintained through the resistor, so long as the reactance of the capacitors is much greater than the resistance. The current flow is dependent upon the value of the capacitor and assuming that V1 is much greater than V2, the value of the current can be assumed to be: IRMS = V1/XC where XC is the reactance of the capacitor. According to this, if we are using a 1uF capacitor , the current value is near 70mA at 230VAC/50Hz input. In order to get a DC voltage using this method, as usual, rectifiers and filter capacitors should be added.
Here, a 12VDC /70mA (max) capacitive transformerless power supply is used to power three 5mm white straw-hat LEDs. One advantage of this circuit is that the output voltage will not climb to an unsafe level if the output load is absented by a fault in the white LED string. This safety mechanism is realized by adding two zener diodes (ZD1 & ZD2) as an integral part of the bridge rectifier unit. In an orthodox capacitive transformerless power supply based LED circuit, there is a risk of filter capacitor explosion in an open-load condition.
The circuit is straight forward and self-explanatory. Resistor R1 limits inrush current. The value of R1 is chosen so that it does not dissipate too much power, yet is large enough to limit inrush current. Needless to say, C1 is the important component. The maximum current flowing through the circuit components is controlled – almost exclusively – by the value of C1. Electrolytic capacitor C2 simply stores current for release to the load, and is rated at twice the voltage expected at output. Resistor R3 limits the operating current of the white LED string, comprising three white LEDs (LED 1 – 3) in series. This circuit is suitable for staircase, corridor, wardrobe, bedroom and bathroom lighting applications.