# AC Controller using MOSFETs Schematic Circuit Diagram

An AC motor controller is known as the device that controls the speed of the AC motor. An AC controller can also be referred to as a variable frequency drive, adjustable speed drive, frequency converter, and more. The AC motor receives power, which is converted by the AC motor controller into an adjustable frequency.

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.

MOSFET stands for a metal-oxide-semiconductor field-effect transistor. It is a field-effect transistor with a MOS structure. Typically, the MOSFET is a three-terminal device with gate (G), drain (D), and source (S) terminals.
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