Hi Suresh,

The capacitor produces infinite resistance (R, at zero hertz) plus a capacitive reactance (-j(X_c)) inversely proportional to any frequency higher than zero hertz. The sum of these two components is the impedance (Z; Z = R ±j X | X = X_l - X_c).

In this example the writer states that the capacitor will limit the current to approximately 20 mA. The voltage is 120 V_rms at 60 Hz. The capacitor is 0.47 µF. Therefore Z = R -j X_c = 0 - [2*@PI*60*0.47E-6]^(-1) = 5644 OHMS. Thus, by Ohm's Law i = 120/5644 = 0.021 amps.

Now that all of that is said, for the sake of intellectual curiosity, go back and read Kai and Erik's posts again. And then when you've read them again, read them even yet again. This is a good example of a circuit that should not be built by anyone who doesn't already know the answers to the questions you've posed.

As a researcher I frequently kluge things together in ways that are not intended to be safe, usually because I need a certain circumstance just long enough to test whatever I am really working on at the time. A perfect example of this is a suicide cord (a power cord with a two prong plug on one end and alligator clips on the other). I might use it to temporarilly get power to something I am testing, but I would never give one to someone else to use.

Yet another real world example happened several years ago. I was working on an rf supply for a system I was working on. In order to test it I kluged together a class B (push pull) amp with some pretty high current transistors. By the time I finished my measurements, my bench was scorched with the imprints of the transistors and a couple of the bias resistors.

Good luck, and be careful.