We can get cheap LEDs of 20watts, 10watts , etc at some local electronics shop. But the real problem connecting these LEDs with power supply is not knowing their characteristics. For eg i bought a 12 Volt LED chip from market for just 30 Rs and the shopkeeper just told about the voltage specification of the LED which was 12 V. We can easily connect it with a battery of 12 Volts but if we need to connect it to some different source we will need its characteristics.

Though sometimes it wattage of led may be mentioned but we cannot use this for the calculations as these generic LEDs are not very standard in characteristics.

Now coming straight to the calculations:

I had a variac where i could vary the AC from 1 Volt to 240 Volts. I connected a bridge rectifier to convert in into DC. I made a chart varying the DC voltage from 6 Volts to 12 Volts and also connected an ammeter in series with the LED to measure the current it was drawing. The LED showed almost no response till 9 Volts and the current increased non linearly at a high rate between 9 to 12 volts.

Since LED shows non linear relation between current and voltage it doesnt have constant resistance , its Resistance is called Dynamic Resistance. One can use anywhere between 10 Volt to 12 Volt characteristic of LED. At higher voltage we will get greater lumens but it was rightly said in spiderman that with great power comes great responsibility. With higher lumens or higher volatge heat produced is also large an we may need forced air cooling alongside heat sink. Anyways its upto you to decide what operating voltage you want.

If you don’t want to complicate things simply provide the rated voltage to LED and measure the current it is drawing.

for eg your 12 VOLT LED draws .8 Amps when connected with a 12 V supply, then,

Its dynamic resistance will be 12/0.8=15 Ohms.

Now if you want to light it with your household 220 Volts, simply draw a circuit as below:

The value can be calculated as below:

Vs= voltage source=220 V

R1 = External Resistor

Rd= Dynamic Resistance of LED at 12 V=15 Ohms

Note : Vs is DC voltage, If supply is AC it can be easily converted into DC by a bridge rectifier.

I=V/R

therefore 0.8=220/(R1+15)

By above equation R1= 260 Ohms