isolation transformer from two 240/12, wiring?

[harp]

When is talk about making isolation transformer on poor-man way, always I see schematic 240/12-12/240.
So, I wonder, is this wiring from my attachment it work, bringing some good feature or it is bad idea and why?

[eccerr0r]

Re: isolation transformer from two 240/12, wiring?

should work as long as you get it hooked up correctly (polarity is important...) but be worried about dielectric strength and thus shock hazard from the transformer itself since the LV wiring is usually on the outside...

The traditional way also double insulates so it may be better in that respect, but this way adds more resistance in the coil paths so more loss this way, so six one way, half dozen the other...

[stj]

Re: isolation transformer from two 240/12, wiring?

keep them a few inches apart or the fields will interact and mess up the voltage.

[harp]

Re: isolation transformer from two 240/12, wiring?

Thank you, I guess that only secondary side must have correct polarity, like to adding the voltage on output, or the primary also must match - how to do simplest check of this?

[redwire]

Re: isolation transformer from two 240/12, wiring?

Where does that autotransformer connection come from? What's the advantage? Usually it's two back-back transformers, sec-sec. like Fig. 3 taken from Elliott Sound Products

[sam_sam_sam]

Re: isolation transformer from two 240/12, wiring?

Where does that autotransformer connection come from? What's the advantage? Usually it's two back-back transformers, sec-sec. like Fig. 3 taken from Elliott Sound Products

I second this this what I seen before not the other diagram that was posted earlier by Harp

[eccerr0r]

Re: isolation transformer from two 240/12, wiring?

I suspect the intent was to minimize stage loss since if you connect two transformers in sequence, you lose a bit in both transformers.

However connecting it the way suggested does not eliminate one stage of loss because you lose in the additional resistance due to series connection of the windings (versus transformer sequence). So ultimately you're probably not gaining much but losing insulation strength, hence my initial comment... and now have to say the traditional sequenced transformers is ultimately better at being an isolation transformer.

[redwire]

Re: isolation transformer from two 240/12, wiring?

I have a couple identical 120V-25.2VAC 2A Radio Shack 273-81512 transformers and just built both circuits, using a 40W load.

The usual back-back connection works fine but the sag under load is 21%. So 120.5VAC in, 24.66VAC middle, and 94.7VAC out into 432mA load. No load was 118.7VAC out, 27.3V middle.

OP's mysterious autotransformer sort of connection, does not work.
Why? What happens is almost all of the incoming mains voltage is dropped across the 120-25V winding. Because the current in the 25-120V transformer winding is almost 5x greater, it's never going to get the current it needs to output say 100VAC. I got 25-35VAC total output phased right, load or not.
Said another way, OP's circuit wrongly assumes things work with equal mains current flowing through the 120V and a 25V winding.
I think it doesn't work at no load because the magnetizing currents are also 5X higher in the 25V winding.

[eccerr0r]

Re: isolation transformer from two 240/12, wiring?

Ah thanks for trying it, yeah more current needs to flow through the primary side low voltage winding but is limited by the high voltage winding. Perhaps with ideal transformers this theoretically should work but breaks down rapidly when there's losses and real current that needs to flow through real wire. (or perhaps they need to be magnetically coupled!)

Good thought provoking question.

[stj]

Re: isolation transformer from two 240/12, wiring?

check arcade suppliers like WICO and HAPP.
older CRT based games use an iso transformer to drive the monitor for public liability reasons.

in europe it wont work because the transformers are 220 input and 120 output.