A current transformer detects a sine-wave alternating current having a maximum effective current value Imax(Arms) and a frequency f(Hz) and a half-sine-wave rectified current having a maximum peak value Ipeak(Aop) and a frequency f(Hz) in a primary winding, consisting of at least one magnetic core with one turn of primary winding and at least one multiple turns of secondary winding to which a detecting resistor is connected.