The Shocking Truth About Transformerless Power Supplies

Transformerless power supplies are showing up a lot here on Hackaday, especially in inexpensive products where the cost of a transformer would add significantly to the BOM. But transformerless power supplies are a double-edged sword. That title? Not clickbait. Poking around in a transformerless-powered device can turn your oscilloscope into a smoking pile or get you electrocuted if you don’t understand them and take proper safety precautions.

But this isn’t a scare piece. Transformerless designs are great in their proper place, and you’re probably going to encounter one someday because they’re in everything from LED lightbulbs to IoT WiFi switches. We’re going to look at how they work, and how to design and work on them safely, because you never know when you might want to hack on one.

Here’s the punchline: transformerless power supplies are safely useable only in situations where the entire device can be enclosed and nobody can accidentally come in contact with any part of it. That means no physical electrical connections in or out — RF and IR are fair game. And when you work with one, you have to know that any part of the circuit can be at mains voltage. Now read on to see why!

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Upcycle An Isolation Transformer

There are several reasons you should have an isolation transformer. They can prevent ground loops and also prevent a device under test from having a DC path to ground (or isolate an oscilloscope from DC ground, which can be dangerous in its own right, but that’s another discussion). [Tanner_tech] noticed that finding ballast transformers for sodium vapor street lights is getting easier as more street lights move to LED technology. What to do with these transformers? Build an isolation transformer, of course.

Of course, your dumpster transformer might be a little different than the one shown in the post (and the video, below). [Tanner] shows how to work out the leads you need. A little wood work and a PC power supply case finished the project.

Judging from the comments, some people take [Tanner’s] talk about safety as an implication that a transformer makes working on mains safe. It doesn’t. It makes it safer if you know what you are doing. Working with high voltage isn’t a place to learn by doing.

If you want some practical advice, [Jenny List] has a good read for you. You probably also ought to invest an hour in watching this video that has a lot of practical advice.

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Isolation Transformer 101

[Todd Harrison] has released a ~50 minute video covering everything you want to know about Isolation Transformers for protecting yourself and your costly equipment. Admittedly I have not given the subject much thought, but if you need to measure high voltages you should probably watch this video.

[Todd] Explains the differences between different types of transformers, including cost, why you would want one, safety and plenty of theory. Whiteboard in hand, all the details are clearly explained. If that’s not enough to convince you, there are some fun “don’t try this at home” experiments that hit the point home.

He has a pretty beefy 8 amp 40 lb monster that cost a fairly large amount, though its worth it to be safe. Unless you think you’re going to need that type of amperage, you wold probably be fine with a smaller model. The product shown in the video is a hospital grade, and requires a mod to make it safe for your bench. [Todd] explains that mod as well. Even if your not planning to do this, its still cool to see a transformer bigger than your hand. Plus it makes a pretty cool sound.

Join us after the break for the video.

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