DIY Isolation Transformer

After I bought my test gears such oscilloscopes, I noticed that I should have an isolation transformer in my bench. Then, I forgot again with the other projects. One day, after seeing Dave and Todd videos blog about isolation transformer, I started the construction of DIY isolation transformer. For those interests, here is Dave and Todd explained videos. Thanks both.

Some designed by cascading back to back secondary winding of two transformers (220V||12V<->12V||220V). It also be worked fine but I build my own. One of my buddy’s local transformer manufacturer wound this transformer and give me as present. Thanks buddy again.

It was designed as

  • Rating : 1KVA, 220V
  • The core  cross sectional area : 60mm x 60mm
  • Primary winding & Secondary winding : 300 N, 17SWG

The two winding should be wounded together to avoid turn ratio mismatch.

DIY-Isolation-Transformer (1)

I found old charger chassis and collected some some required staffs such as sockets, fuse etc.

DIY-Isolation-Transformer (2)

Seated transformer at chassis base. Prepared face plate, cover hole with plane PCB, installed input and output fuse houses, lamp. And wiring,


I agree with Todd suggestion. This is the schematic from commercial isolation transformer. The output winding NEUTRAL is CONNECTED to main GROUND!!!


This is the edited diagram as per suggestion. Disconnected secondary winding (NEUTRAL) from main GROUND. Mine has no circuit breaker and surge protection but both input and output fuses.


Final assembly is look like that. A new tool is at my bench.

DIY-Isolation-Transformer (7)

Note :

– DUT (device under test) should be isolated with transformer rather than oscilloscope.

– BUT, Dave scenario-3… if  DUT power supply is grounded to chassis (main ground), the problem is still existed. Disconnect DUT ground ?? or use with floating power supply?! or with battery? 😛

– Test device such as oscilloscope should not be floated (must be grounded) without special reason.

– Most forum and articles talked about isolating oscilloscope not DUT. If someone know the best practice and pros, cons, let me know.

– Differential probe, usb isolator are also good but cannot still effort.

– I need GFCI outlet at my bench.


I recently found a post at Keith’s Electronics Blog. There is also original schematic from manufacturer. It isolates even ground pins of each output. Yes, system ground (Chassis ground) is purposely isolated.

– My thought is that isolating system ground has some pro and cons. Sometime we need to isolate for full safety or purposely. In most case, test equipment should be grounded for safety.

Refrubrishing My Isolation Transformer

and schematic from Keith’s Blog


8 thoughts on “DIY Isolation Transformer

  1. Your output can be floating without connecting neutral to ground, just like you did, but you risk damaging your transformer’s winding insulation with static electricity. You need a “fail-safe” transzorb between neutral and ground, set to 90% of the insulation voltage rating of the transformer. A “fail-safe” transzorb is simply a couple of them connected in series. This is not something theoretical. It’s very easy to damage transformers that way, and is often a source of “unexplained” leakage currents through locations where the winding insulation broke down — usually to the core of the transformer.

    You must also note that the benefits you get from floating the output the way you do are mostly imaginary. In other words: people imagine the benefits that aren’t there in reality. Look at any switching power supply that you may connect to such an isolation transformer. There will be capacitors between L and PE (Earth), and between N and PE. Similar capacitors are in EMC filters in many pieces of equipment you might want to isolate with such a transformer. Those capacitors un-isolate your secondary, and make it dangerous to work on. YOU HAVE BEEN FOLLOWING SOME SERIOUSLY BAD ADVICE AND IT CAN KILL YOU. Don’t say I didn’t warn you.

    A true floating-output isolated supply has, in fact, isolated ground, but that isolated ground must be connected to the center tap of the secondary winding and nowhere else. If the secondary has no tap, like in your case, you can connect to the secodary’s “neutral”, but that’s sub-optimal. For protection from static electricity you still need transzorbs between the floating PE and “real” PE.

  2. Hi Kuba Ober,
    Thanks for your explanation. I will find suitable transzorb and install it.
    I also note that my approach is still un-isolated in most case.
    I am still not clear about connecting isolated ground to secondary center-tap. My transformer has no center tap as you mention. What means a center-tap? If my transformer has center-tap, it will be 110V tap. ?? I am confused. Pls kindly explain it. A schematic may clear than more words.

  3. A center tap is a tap in the middle of the winding. That way the voltage on both L and N is balanced with relation to ground (same voltage, 180 degrees out of phase), and usually results in lower current through the PE (ground) connection to the isolated device. Again, realize that most equipment has one capacitor between each of N and L and PE. I know that your transformer’s secondary doesn’t have a center tap. That’s a mistake.

  4. The other thing I would think is dangerous and would have problem with, is the suggestion that the “two winding should be wounded together to avoid turn ratio mismatch”.
    That way, the two wires (the primary and the secondary coil) literally TOUCH each other, with only two thin layers of enamel as isolation between them. You may think it’s OK, assuming the enamelled surface of the wire provides enough electrical insulation. You are happy, you have nice, safe piece of equipment. But:
    I am sure you are familiar with ‘transformer hum’, and can feel the tiny vibrations in the transformers body when it operates? Those are effects of the AC current – see eg. here: After hours?days?months?years? of these tiny vibrations, one of the turns will become loose enough to start scratching enamel insulation of the neighbouring turn-the other coil, and eventually, you get short connection between the two. At that moment, you don’t have an isolation transformer any more, you have a KILLING MACHINE instead, and you don’t know about it.
    If that happens in ‘normal’ transformer, you get short between two turns of the same coil – the shorted turn may get hot, eventually destroying the whole transformer. To prevent that, in the past we used to ‘boil’ home-made transformer coils in a wax-like substance, so that the liquid wax penetrated inside the coils, impregnated them and prevented such vibrations. Modern coils are completely encased in a plastic or resin-like substance, I believe, for the same reason.
    (The wire on the photo of your transformer is freely visible, so obviously such a treatment wasn’t applied to its coil. Further, the core itself looks like it may become little looser with time, which will ‘improve’ those damaging vibrations; a coat or two of a thinned lacquer may penetrate between individual layers of the core, and stop that.)
    To conclude, I would suggest to wind two identical coils, and mount them on the transformer core side-by-side, preferably with some isolation between them. Alternatively, wind the primary coil first, followed by a layer of insulating material, and then continue with winding the secondary coil. Also, I would try to find modern replacement of the wax, and apply it, too.

    1. Thanks Bob K,
      I will try to seal with transformer coated lacquer (I noticed this type of paint in local supplier). Actually, I know that it should be coated layer by layer but I have to resin from outside this time.
      I am also considering a new transformer. Yes, your idea is right, the two windings should be side by side or layer by layer.

  5. Have a look at The Post Apocalyptic Inventor vids on youtube

    TPAI Lab Report #005: Vacuum Tube Amplifier and Power Supply, Circulating Pumps Update

    Isolation Transformers, Variacs and Autotransformers

  6. An Isolation Transformer is one that gives protection from electric shock to the man and the material. It is widely used in the circuits which are too sensitive to handle the pressure of the voltage directly. These are manufactured with high insulating material and special shedding techniques to minimize transverse mode noise. It gives protection against strong lightening, impulse noise, accidental discharge of capacitors and many other hazardous situations.

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