Figure AN1003.17 shows a very simple variable resistance half-wave circuit. It provides phase retard from essentially zero (SCR full on) to 90 electrical degrees of the anode voltage wave (SCR half on). Diode CR1 blocks reverse gate voltage on the negative half-cycle of anode supply voltage. This protects the reverse gate junction of sensitive SCRs and keeps power dissipation low for gate resistors on the negative half cycle. The diode is rated to block at least the peak value of the AC supply voltage. The retard angle cannot be extended beyond the 90-degree point because the trigger circuit supply voltage and the trigger voltage producing the gate current to fire are in phase. At the peak of the AC supply voltage, the SCR can still be triggered with the maximum value of resistance between anode and gate. Since the SCR will trigger and latch into conduction the first time IGT is reached, its conduction cannot be delayed beyond 90 electrical degrees with this circuit.