DIY: Convert Old Transformer to Isolation Use

If you're into DIY projects and want to try building an isolation transformer, this article might be just what you're looking for.

Let's say you've got an old 750VA Mitsubishi transformer from Japan—something that was originally used in an elevator around 20 years ago. Good news: it's perfect for this kind of project.

This Mitsubishi transformer looks solid on the outside, and its build quality seems top-notch. On the primary side, it has multiple voltage taps: 380V, 400V, 420V, and 440V. On the secondary side, it has three outputs: 105V, 37V, and 100V. The current ratings for each output vary a lot, so you'll notice the wire thickness differs accordingly.

The full primary winding is rated for 440V. What you want to do is find the center point of that winding and split it, turning it into two equal and independent coils. That way, you can convert it into a 1:1 isolation transformer for 220V mains. The voltage ratio between the two coils is roughly 1:1.08—close enough to the midpoint for this to work well.

Here's what the transformer looks like:

And here's the data label on the side:

One thing to keep in mind — when modifying the primary winding for use as an isolation transformer, there's a small chance of insulation breakdown or leakage over long-term use, which could compromise isolation. That said, the risk is quite low since the operating frequency is low and there's not much spike voltage involved.

Also, if you're converting the 440V coil for 220V input, keep in mind that the rated power output will be cut in half. However, if the original primary winding had extra current capacity, you might not lose that much power. Still, it's important to avoid running it at overcapacity for extended periods.

That's it — transformation complete!