Converting an Autotransformer into an Isolation Transformer

[MrAl]

Give some thought to how you are going to bring out the primary and screen terminations while maintaining safety clearances from the secondary. Bringing them up the edges of the secondary isn't an option without double or reinforced insulation (as well as being a pain for your secondary winding process)*.

Also consider how you are going to terminate the Aluminium wire, it won't solder and is soft, so tends to cold flow if crimped or put in screw connectors (screws loosen).

Better to think about this stuff now rather than when you're half way through.


Edit: *Drilling holes out through the cheeks of the bobbin is probably the safest option.

I’m not going to bring the primary lacquered wire out directly, instead I’ll solder it to a stranded insulated wire inside a fiber glass sleeve, which should provide proper insulation.

Yeah, aluminum tends to form a non-conductive oxide layer. I’ll use brass screw terminal and then fill it with Vaseline to keep the moisture out?

Hi,

The thing to use is called dielectric grease.  It's a special grease made for this.  It does not harm plastics like petroleum based greases do.

[Haenk]

Some UPSs transformers have twin 220V or twin 110V windings and can be used as-is for an isolating transformer.

I have a nice combo from a 3kW USV, autotransformer plus twin 220V isolation transformer. That combo is a beast, I guess about 25-30kg. (Unfortunately, that makes it impractical for home use...)
Really, I would not dare to rewind such a safety device, certainly not in the kW range.

[johansen]

Some UPSs transformers have twin 220V or twin 110V windings and can be used as-is for an isolating transformer.

I have a nice combo from a 3kW USV, autotransformer plus twin 220V isolation transformer. That combo is a beast, I guess about 25-30kg. (Unfortunately, that makes it impractical for home use...)
Really, I would not dare to rewind such a safety device, certainly not in the kW range.

The larger the transformer is, the easier it is to get it right.

Clearance and creepage can be handled by just 3 layers of a fairly tough plastic tape, whatever you choose, and it would be good to 10 thousand volts or so for personal use. Its not like its going to be resold as a medical grade isolation transformer.

Fiberglass reenforced packaging tape is great because even if you did have enough force on the windings to puncture the plastic tape, the glass fibers will keep the wires apart.

[ElectronSurf]

I've got the flu even though I was vaccinated. This season is crazy! So the project is on hold until I get better. In the meantime, I made some improvements to the PowerShell script. Here's what it does now:

• Calculates turns per volt, primary turns, and secondary turns.
• Lets you directly control flux density, so you can trade size versus losses instead of relying on fixed rules.
• Accounts for stacking factor and realistic core efficiency, not ideal cores.
• Automatically compensates secondary turns (+5%) for regulation drop.
• Calculates primary and secondary currents from real VA load and efficiency.
• Suggests wire diameters for both copper and aluminum based on conservative current density.
• Lets you override the wire sizes and immediately see the impact.
• Calculates window fill factor using actual conductor area and packing factor.
• Warns if the winding won’t physically fit or exceeds safe fill limits.
• Estimates mean turn length instead of using a fixed perimeter.
• Calculates total wire length for primary and secondary.
• Estimates wire weight for copper and aluminum separately.
• Includes a rough no-load primary current estimate typical for EI cores.
• Warns if wire size is too small for the current.
• Warns if copper fill exceeds realistic limits.
• Warns if the requested VA is beyond what the core can reasonably handle.

[David Hess]

I've got the flu even though I was vaccinated. This season is crazy!

The flu strains for the vaccination have to be selected months ahead of time and they do not always predict them right.

[golden_labels]

Vaccination is not a magical elixir that shields us from bacterias and viruses. It “only” allows the immune system to recognize the pathogen quicker and react more effectively. But we still get the immune response, necessarily. If we’re unlucky, with many of its downsides. The point is: the infection is much more likely to be nuked early (maybe without symptoms), less likely to cause serious harm, and not as likely to transmit.

Some people may still develop the illness, though it may be less severe it would be otherwise. Some may not develop immunity in time. Some may get in contact with strains they were not vaccinated against (evolution serves flu well). Some may have flu-like symptoms from a completely different source. It’s always throwing dice. We just “cheat” with vaccines by making the dice very favourable to us.

Since this is an electric safety thread (at least one related to), it’s really a similar concept. We have procedures, precautions, and best practices to keep us safe. But it’s not 100% risk free. We only shape probability to serve us, but once in a while an accident is deemed to happen regardless.

[ElectronSurf]

I've wound 260 turns of 1.8mm wire for the primary (I had to buy an extra 30 meter of wire). It used 30mm of the 56mm window space. It should have used less space, but because I reused the bendy wire, the winding wasn't very neat. I'm not sure if I can fit ~240 turns of 2mm aluminum wire in there, unless I wind it very tight and neat?

I also bought 0.2mm presspahn paper and a 0.2mm copper sheet.

I assembled the core with only the primary and measured 0.93VAC per turn. The current draw is 120mA at 220 VAC with a 0.65PF. It should run cool. There is no buzzing or humming.

My next steps are:

Fill the primary with wax, lacquer spray, or something similar.

Place an NTC thermistor on the primary.

Add the copper sheet.

Take it to someone who can tell me whether 2mm wire will actually fit. If not, I'll see how much I can fit and possibly compensate using a Variac at the output. IDK what to do if that didn't work...

[ME]

Looking at the pic, I dont think you are going to have room for a secondary winding that will have enough turns to output 230 volts!.

[ElectronSurf]

Looking at the pic, I dont think you are going to have room for a secondary winding that will have enough turns to output 230 volts!.

90mm (height) / 2mm (wire) = 45 turns per layer.
Let's say 40 turns per layer; then 6 × 40 = 240 turns (240 × 0.93 = 223VAC).
6 layers × 2mm (wire) = 12mm on each window, 24mm total.

A single layer of copper sheet is 0.2mm, and about two layers of 0.2mm presspahn paper are needed; that's not even 1mm total.
I have 26mm of window space left, so in theory it should fit.

But yeah, I'm still doubtful…

[ME]

I was going by the pic , you have to wind the same amount of wire as you already have wound on it plus the copper strip ,I think you are going to finish with an isolated step down transformer?, good luck with it.

[ElectronSurf]

I've added a layer of paper, then a layer of copper sheet (not a shorted turn, but a full turn), followed by another layer of paper.
Then placed the bobbin in the core and measured 15mm of window space. I only need 12mm for six layers.
I might be able to pull this off. God, I hope so…


There was a bug in the EI calculator where it didn’t account for the secondary being wound on top of the primary, so it was miscalculating the wire length. It should be more accurate now.
There are also some minor improvements here and there.

[Gyro]

Sorry to say that I'm still (as before) worried about the safe isolation of your secondary winding. The construction you show is fine for a ground referenced secondary, but for a safety isolation transformer, you need around 8mm of creepage distance from both primary and grounded screen. Please add 2 full turns of yellow adhesive Polyester (Mylar) transformer tape at the margins so that the overlap up the insides of the cheeks of the bobbin and over to the edges of paper by 8mm each (20mm is a common tape width). I suppose Kapton could be used instead but is probably harder to wrap with minimal crinkling.


P.S. Given the space you have remaining, you will need to take similar precautions on the outside of the secondary winding to isolate it from the core. In addition of outer paper, you will need to cover with Polyester transformer tape, overlapping the outside of the cheeks, again by 8mm, and bobbin slots. Transformer tape is so thin that it will not significantly reduce your winding space. I don't know the characteristics of your paper, but it would be wise to add a layer of the same tape (inner and outer paper).

[ME]

The term a fools errand springs to mind.

[BradC]

I re-wound a valve amp output transformer ~5000 turns by hand. I turned a wooden former on the lathe (dad's wood lathe), then just wound it on by hand turning the lathe spindle. The wire was very fine. I didn't replace it, just joined where it broke. Waxed paper between the layers. I was a lot younger and had more time back then.

Since then I progressed to the battery operated drill and a hall effect counter, and now I have a metalwork lathe with a VSD, so I use that and a hall effect counter. In all cases, I used a leather glove to hand tension and feed the wire on. I'm sure there's a better way to do it, but slow and careful can do much with little.

[ElectronSurf]

I intended to post this earlier, but a massacre occurred in our country and we experienced an internet blackout.

I switched to a split-bobbin design and replaced the plexiglass with wood fibre (same width, but more flexibility). The primary has 200 turns and the secondary 210 turns, resulting in about 10V (~5%) higher voltage on the secondary.

I tested it with a heating element at ~5A. With an input voltage of 210VAC, the output voltage dropped to ~195VAC. Drawing approximately 1kW from a 1.3kW core makes perfect sense.

The no-load current is 160mA. I also used 2mm aluminium wire for both the primary and secondary.

✅ Despite all the negativity in this topic, The winding is complete, and I have also developed FREE software for winding EI transformers, available for everyone to use.