Introduction

What Is a Variac?

A Variac, also known as a variable autotransformer, is an adjustable AC power supply that allows users to manually increase or decrease output voltage. Unlike fixed transformers, a Variac provides smooth voltage adjustment, making it a valuable tool for:

• Electronics development and testing
• Equipment calibration
• Motor speed control
• Burn-in testing
• Laboratory experiments
• Repair and troubleshooting

Because the output voltage is adjusted by rotating the Variac's shaft, precise motor control is often required when automating the process.

Project Background

This project is Part 5 of the author's element14 Start a Movement design challenge series.

The objective is to automate a laboratory Variac by replacing manual knob adjustment with a stepper motor. The controller precisely rotates the Variac shaft, enabling repeatable voltage adjustments without manual intervention.

The article documents:

• Mechanical mounting
• Stepper motor installation
• Coupling design
• Variac integration
• Motion testing

At this stage, the system operates as a standalone controller and does not include Ethernet or remote networking capabilities.

(Insert original mechanical assembly photos from the thread.)

Original System Architecture

AVR MCU
    │
Stepper Driver
    │
Stepper Motor
    │
 Variac
    │
 Adjustable AC Output

Community Discussion

One of the most interesting developments occurred in the discussion following the article.

Community member maxpowerr suggested evaluating W55RP20 as a future controller for the project.

While the original author did not implement this recommendation, it introduced a practical direction for expanding the design beyond a standalone laboratory controller into a network-connected automation platform.

The suggestion is technically compelling because W55RP20 integrates:

• RP2040 dual-core MCU
• W5500 Hardwired TCP/IP Ethernet
• Hardware TCP/IP offload
• Compact System-in-Package architecture

This reduces hardware complexity while enabling reliable wired Ethernet connectivity.

Proposed W55RP20 Integration

The following section is a conceptual engineering proposal inspired by the community discussion. It was not implemented in the original project.

Replacing the AVR controller with W55RP20 could enable:

• Ethernet remote control
• Browser-based voltage adjustment
• Modbus TCP
• MQTT
• Remote diagnostics
• SCADA integration
• PLC communication
• Industrial HMI connectivity

Updated architecture:

Remote PC / PLC
       │
 Ethernet
       │
    W55RP20
       │
Stepper Control
       │
Stepper Driver
       │
 Variac

The recommendation received positive recognition from the community because W55RP20 combines the RP2040 MCU and W5500 hardwired TCP/IP Ethernet controller into a single package. Although the integration was not implemented by the original author, it presents a practical path for adding industrial networking, remote monitoring, and Ethernet-based control without significantly increasing hardware complexity.

Why W55RP20 Is a Good Fit

The community recommendation is technically well aligned with the project's objectives because W55RP20 provides:

• RP2040 dual-core MCU
• Integrated W5500 Ethernet controller
• Hardwired TCP/IP offload
• Simplified PCB design
• Fewer external components
• Reliable wired Ethernet connectivity
• Lower firmware complexity than software TCP/IP implementations

These characteristics make it a compelling option for evolving the original laboratory prototype into a network-connected industrial controller.