ac current regulation using each half cycle?

Question

how to sense ac current at each half cycle and to regulate it.

Answer 1

The AC Current would look like a sine wave. Do you want to regulate the instantaneous current? Probably not that would look more like a clamp with the current rising to a point and then flat topping. You probably want to regulate the average current over at least a few cycles maybe 10 or more. So you would need to measure the average current not the instantaneous current and modulate the driving voltage accordingly.

How you perform all this will depend on whether your circuit is resistive, inductive or capacitive.

Answer 2

I assume that by "each half cycle" you mean the positive and negative wave.
You can do it by regulating the positive and negative wave separately by the use of zenor diodes and current limiting resisters to a common ground. Or, you could get a little more complex with the use of transistors which would switch at predetermined voltage peaks and conduct the excessive voltage to ground. Either way, the supply voltage would always have to be somewhat higher than the regulated voltage to assure a constant, steady supply. There are also a variety of ic chips available that can be used. The method and circuitry will depend on how critical the regulation needs to be.