Because of their inertia, power consumption, contact wear, and often size, relays are not really suitable for switching alternating voltages. Moreover, they cannot be used in phase control circuits. Replacing them by triacs may give problems owing to the forward voltage drop across these devices.
A much better solution is the use of anti-series-connected SIPMOS transistors. Unfortunately, these need a control voltage to be switched. This difficulty is, however, overcomes by making use of the inverse diodes of the transistors as shown in the diagram.
In the off condition, when the optoisolator does not conduct. C1 is charged during the negative half period (A positive, B negative) via D3 and one of the inverse SIPMOS diodes. If the output is terminated, the capacitor will also be charged during the positive half period either via D4 and the inverse diode of T2 (load between A2 and B2 ) or via D4 and the inverse diode of T1 (load between A1 and B1).
The circuit is switched on by making the optoisolator conduct, whereupon the voltage across C1 is connected to the gates of T1 and T2 via R1 and the transistor in the optoisolator. Diode D5 prevents the gate voltage becoming too high. When the circuit is on, C1 continuous to be charged during the negative half periods via D3 and one of the SIPMOS transistors.
Diode D2, in conjunction with D1 and D4. suppresses current peaks caused by the switching of inductive loads.
The circuit can switch alternating voltages of up to 45 V. Without a heat sink for the SIPMOS transistors, the maximum current should not exceed 3 A. For larger currents or when large inductive loads are switched regularly, a small heatsink should be used.