A typical line commutated circuit is shown in Fig.1 and its associated waveforms are shown in Fig.2.

If the supply is an alternating voltages, load current will f‌low during the positive half cycle. During the negative half cycle, the SCR will turn-off due to the negative polarity across it. The duration of the half cycle must be longer than the turn-off time of the SCR. The maximum frequency at which this circuit can operate depends on the turn-off time of SCR.