30A AC Soft Starter (Inrush Current Limiter)

sam.moshiri · February 4, 2023

Are you tired of dealing with the damaging effects of inrush currents on your industrial devices? Look no further than an AC inrush current limiter (soft starter). Inrush current, also known as surge current, is the large amount of current that flows into a load at start-up. This can cause damage to equipment, reduce its lifespan, and lead to costly downtime. But with an AC inrush current limiter, you can eliminate these problems. Simply, a soft starter works by limiting the initial current flow, ensuring a smooth and efficient start-up, while protecting your equipment from damage.

So I decided to design this AC soft starter that is equipped with a fail-safe mechanism. During start-up, the inrush current passes through a power resistor, and after a delay (adjustable between 1ms to 1s), a 30A power Relay shorts the resistor and applies the full power to the load. If this Relay fails for whatever reason, the power resistor won’t melt everything; the logic circuit activates the fail-safe Relay that turns OFF the load to prevent any damage. 3 LEDs indicate the Supply, Normal, and Fault conditions. I selected the cheap ATTiny13 MCU as a controller.

To design the schematic and PCB, I used Altium Designer 23. The fast component search engine (Octopart) allowed me to quickly consider components’ information and also generate the BOM. To get high-quality fabricated boards, I sent the Gerber files to PCBWay. I used the Arduino IDE to write the MCU code, so it is pretty easy to follow and understand.

Let’s get started 🙂