Lights and Display Board CircuitsLow Voltage Circuit

AC Controller using MOSFETs Schematic Circuit Diagram

AC Motor Controller: Introduction and Terminology

An AC motor controller, also known as a variable frequency drive, adjustable speed drive, or frequency converter, is a device designed to regulate the speed of an AC motor. This controller transforms the power received by the AC motor into an adjustable frequency, enabling precise control over the motor’s operation.

Triacs and Power Dissipation Challenges

In applications involving low voltages, triacs are commonly utilized as control elements for AC voltages. However, a significant drawback arises in the form of power dissipation within the control element, especially noticeable for currents exceeding 1 ampère. To address this issue, a heatsink becomes necessary for the triac. For instance, when controlling the brightness of a halogen lamp using this setup, the voltage drop across the triac leads to a notable reduction in the lamp’s maximum brightness. This drawback can be circumvented by employing two MOSFETs in place of a triac as the control element. The key lies in connecting the two MOSFETs in series with opposite polarity, while their gates are parallelly linked to the control circuit.

AC Controller using MOSFETs Schematic Circuit Diagram

Virtual Ground and Modern MOSFETs

In this circuit, the junction of the two gate leads serves as the virtual ground, establishing the reference point for all other potentials within the system. Contemporary MOSFETs like the 20N03 from On Semiconductor (, boasting an RDS(ON) value of 0.035 Ω, can effectively control a 50-W halogen lamp without requiring an additional heatsink.

Negligible Brightness Loss and Versatile Controller Compatibility

The decrease in brightness is practically insignificant, given the minimal voltage drop of (0.035 Ω × 4.2 A) = 0.147 V. It’s worth noting that the specific 20N03 MOSFET isn’t mandatory; essentially, any n-channel MOSFET with a low gate voltage, preferably a ‘logic FET,’ can be employed. Furthermore, this circuit offers the advantage of ‘zero-power’ gate drive, a notable contrast to triacs that demand drive currents of 10 mA or higher. This feature allows the utilization of various controllers such as microcontrollers, TTL gates, or 555 timer ICs.

Understanding MOSFET Basics

MOSFET stands for metal-oxide-semiconductor field-effect transistor, representing a field-effect transistor featuring a MOS structure. Typically, the MOSFET comprises three terminals: gate (G), drain (D), and source (S), which are integral to its operation and control.


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