The USB Isolator That Saved My Laptop From a Burning Motor Inverter

It was just another normal day programming a motor inverter. The system was running a 1.5 HP motor at around 30 Hz when something started to look wrong. The rotation became unstable, the motor began to wobble, and then it happened.

The board suddenly started throwing sparks, fire, and small explosions. The lights went out, flashes and bangs echoed around the room, and for a moment you don’t really know whether to run or just stand there staring at the disaster unfolding. The circuit breakers tripped, but there were still occasional explosions coming from the two 1000 µF 350 V capacitors discharging into who knows what.

The board burning in the dark because the lighting breaker also tripped.

Once the initial shock passed and the power came back, I managed to put out a small flame that had started in the insulation of some wires. What remained was not pretty: a charred PCB, a microcontroller that had literally exploded, programming pins that simply no longer existed, and the unmistakable smell of burned rubber in the air. Some wires had melted together and the room had that strange silence that follows electrical chaos.

Oh no… the laptop!

I quickly looked to the side, already imagining the worst. In that instant I thought about all my data, my projects, and the cost of replacing the computer — especially these days when laptops are anything but cheap.

But then I noticed something unexpected.

It was fine

The screen was still on.
VSCode was open.
No signs of damage.

The only reaction from the laptop seemed to be the cooling fan spinning a bit faster because of the heat in the room.

That’s when I looked to the left and saw the small device that had probably prevented a much bigger disaster: a simple USB isolator sitting between the computer and the programmer.

At that moment it became clear that it had done exactly what it was designed to do.

The danger of debugging motor inverters

When I first started working with inverters, I already knew that something like this could eventually happen. In many power boards the microcontroller shares the same ground as the power module, with no real isolation.

This means that the computer connected via USB is effectively tied electrically to the power circuit.

In inverters powered from rectified mains, the DC bus can easily reach around 310 V. Any fault, transient, or imbalance between ground references can put dangerous voltages on the path between the board and the computer.

In practice, the USB cable can suddenly become an unintended path for fault currents.

It was even somewhat common for me to feel small electric shocks when touching the metal shell of the USB connector during testing. At the time I ignored it, but that was already a clear sign that something in the system was not exactly healthy.

The cheap solution I found

Some time ago, while casually browsing AliExpress, I came across small USB isolators normally used in audio applications.

They were cheap and promised something interesting: galvanic isolation between the computer and the USB device.

So I decided to try one.

And it worked.

These isolators typically use the ADuM3160 chip from Analog Devices, which is specifically designed for USB signal isolation. Internally it uses magnetic coupling to transfer digital signals across an isolation barrier, breaking the direct electrical path between the two sides of the system.

This means the computer and the connected circuit no longer share the same current path.

In other words, even if something terrible happens on the power side, the chances of that energy reaching the computer drop dramatically.

Limitations

These devices are not perfect.

Although they support USB 2.0 connections, in practice the speed is limited to roughly USB 1.1 levels. For most lab or industrial applications this is usually not a big problem.

In my case, using USB-to-serial converters, I managed to reach stable communication at 576000 baud, which is more than enough for firmware debugging and telemetry.

For programming microcontrollers and standard serial communication, the speed limitation is barely noticeable.

The moral of the story

That incident left a simple lesson:

Debugging motor inverters by connecting a computer directly to a live power circuit is not a good idea.

A USB isolator costs only a few dollars.

A laptop costs a few thousand.

On that day, looking at a burnt circuit board and an intact computer, it became very clear which one is worth protecting.