USB-Powered Appliance Circuit: 150W Triac Control

Circuit diagram:

Description

During computer startup and shutdown processes, numerous external devices – including printers, displays, and scanners – frequently necessitate activation and deactivation simultaneously. Utilizing the 5-V power supply generated from the PC’s USB port, these peripherals can be switched on and off concurrently with the computer itself. This methodology can also be applied to other devices with USB interfaces, like contemporary televisions and radios. This technique, known as a ‘USB-standby-killer’, can be achieved using only five components. The USB output voltage activates the triac opto-driver (MOC3043), which incorporates zero-crossing detection. Subsequently, this action drives the TRIAC, type BT126.

The circuit presented here is utilized by the creator for controlling loads consuming approximately 150 Watts and is safeguarded by a 1-A circuit breaker. However, the circuit possesses the capability to manage considerably larger loads. In such instances, or when working with highly inductive loads, a so-called snubber network must be implemented across the triac, and the fuse value should be adjusted accordingly. This circuit can be seamlessly integrated into a multi-way mains powerboard, provided that adequate electrical isolation exists between the USB and mains sections.

More about components

The core of this circuit relies on the MOC3043 integrated circuit, a versatile optocoupler. This IC functions as a solid-state relay, efficiently transferring electrical signals from the USB supply to control the TRIAC. Specifically, the MOC3043 contains a MOSFET and a phototransistor, which collectively provide switching capabilities. Its zero-crossing detection feature ensures that the TRIAC is only triggered at the exact moment of the AC waveform’s zero-voltage point, minimizing noise and maximizing efficiency. This integrated circuit’s compact size and robust design contribute significantly to the circuit’s reliability and ease of implementation. The MOC3043 is commonly used in power supply applications and small motor control circuits.