hello
I want to make a circuit similar to attached circuit.(30 led 1W in series)
Is the circuit before bridge necessary?and how it could be useful?(how it is works?)
This circled with red parts, decrease voltage to lower value, graetz (bridge rectifier) rectify current. 10uF and 100R reduces flicking they sayed that on circuit.
tpetar is correct, these kind of passive AC-DC converter circuits are only suitable for outputting a few watts at most. At higher currents, the value of the AC line capacitor needs to be excessively large (tens of uF). For 30W, you should really use a proper AC-DC converter, like a SMPS.