Particularly with low voltages, triacs are usually used as control elements for ac voltages. The disadvantage, as so often is the case, is in the power dissipated in the control element, which is quite evident for currents greater than 1 ampère. In such cases, it is essential to use a heatsink for the triac. If you want to control the brightness of a halogen lamp using such an arrangement, for example, the voltage drop across the triac also results in a significant reduction in the maximum brightness of the lamp. This disadvantage can be avoided by using two MOSFETs for the control element, in place of a triac. The trick here is to connect the two MOSFETs in series with opposite polarity, with the gates connected in parallel to the control circuit.
The junction of the two gate leads represents the virtual ground of the circuit, which forms the reference for all other potentials. Modern MOSFETs, such as the 20N03 from On Semiconductor (www.on-semi.com) with an RDS(ON) of 0.035 Ω, can be used in this circuit for controlling a 50-W halogen lamp without any supplementary heatsink.
The loss in brightness is negligible since the voltage drop is only (0.035 Ω × 4.2 A) = 0.147 V. Of course, you do not necessarily have to use the 20N03; in principle, any n-channel MOSFET with a low gate voltage (preferably a ‘logic FET’) can be used. Another benefit of this circuit is its ‘zero-power’ gate drive, in contrast to triacs, which require drive currents of 10 mA or more. This means that any microcontroller, TTL gate or 555 timer IC can be used as the controller.