ON-OFF controller uses two SCRs connected in antiparallel as shown in figure.

During positive half cycle of the AC input at the instant $wt =0,2\pi,4\pi$. SCR $T_1$ is triggered at $\alpha =0$.Thus SCR $T_1$ conducts and positive half cycle appears across the load.

During negative half cycle of the ac input at instants $wt =\pi,3 \pi, 5\pi$.SCR $T_2$ is triggered at $\alpha =0$.Thus SCR $T_2$ conducts and negative half cycle appears across the load.

When gate pulses are removed,both the SCRs are OFF.

$\therefore$ SCRs conducts for 'n' number of cycles and they are 'off' for'm' number of cycles.

Hence the name ON-OFF Control.

Figure b shows the input and output waveforms for resistive load.

Advantages:

1) The SCRs are switch ON at zero crossing .Hence the harmonics due to switching actions are reduced.

2) Electromagnetic interference to neighboring electronic circuit is minimized.

Disadvantages:

1) Output will discontinuous,hence the load has to sustain these variations.

2) Since the thyristors are triggered at zero crossing their conditioning is not being exploited.