Once I purchased a China make AC-DC LED emergency lamp for Rs.70. I wonder ,how they are selling the device cheaply. Due to curiosity, I opened the case of thelamp. So simple circuit that cost only Rs.20, a small 6 volt battery and LEDs. This is the reason why they are selling the device at low cost. Reliability? Sometimes it will last years, otherwise few days. That depends on the use and conditions. Most of the China make devices use tranformerless power supply which can damage when spikes appear in the mains lines. Any way, simplicity and low cost can be considered. I have drawn the diagram and constructed one sample. It is still working. Let us see the circuit.
1. A 105 K 250 volt AC capacitor is dropping the 230volt AC to low volt AC.
2. Bleeder resistor R1 is used to remove the stored current from the capacitor. See without this resistor, the voltage in the capacitor can be as high as 400 volts and will be present many days in the capacitor. So it can give a shock if the plug is touched. The resistor will bleed the current immediately at power off.
3. D1 through D4 rectifies the low AC to DC and capacitor C2 makes the DC ripple free.
4. Around 20 volts DC with a current of 100 mA will be available from 105 K (1uF) capacitor. In capacitor power supply, as a rule there will be 100 mA current per uF of capacitor.
5. Remaining part is simple. Switch S1 is two way. When it is placed to one end, current passes through D5 to charge the battery and to light LEDs. If the switch is in the other position, Lamp turns off.
6. When the mains power is off, D5 reverse biases and D6 forward biases. LEDs light using the battery power.
Caution: This circuit uses high volt AC directly. So it can give a lethal shock if touched. Switch on the power only after enclosing the circuit in a plastic case. Do not trouble shoot, solder or test while it is connected to AC.
Always use footwear while doing work in AC circuits. Use a sleeved AC tester for screwing and unscrewing the screws. An ELCB connected switch box is ideal in work place.