Back to back Toroidal isolation transformer wiring

[Endckr111]

Hi
Iv two identical Toroidal Transformer 120va, and would like to build a back to back isolation transformer.
My question is, should both toroidal transformer secondaries be wired in parallel , then link both transformers together.
Basically what I’m asking is, what is the correct method in joining the two together
Your help much appreciated
Many thanks
Mark

[snowman_al]

Your question is not quite clear to me.
So,
If the transformers have one single output secondary each then yes just join the secondaries together.
If the transformers have two out put secondaries wire them in series at each transformer. Then join the 2 together just at the outer ends.

In fact either way, series or parallel secondaries will work, but 'standard practice' is to go for higher voltage (series) over higher current (parallel).
Alan

Sorry for the very quick sketch...

[GrimJosef]

Yes. The cost of higher voltage is more stress on the insulation. But it should withstand that, so there will be no undesirable consequences. The cost of higher current is greater ohmic losses in the copper. Those will be undesirable consequences.

Cheers,

GJ

[Endckr111]

Many thanks guys, all seems clear now, series is the way to go, there isn’t much current been drawn on output for the items I shall be using it for so I’m happy now
Cheers
Mark

[Refugee]

If you use two identical 240/120 volt transformers you will get better regulation wiring the primaries in parallel and the secondarys in series and get double the power handling as a bonus.

[broadgage]

I do not think that the O/P is considering use of two transformers interconnected so as to give twice the power handling.

What I believe they propose is two identical transformers, the first one to step down mains to say 12* volts, and the second one to step this 12* volts back up to mains voltage, but isolated from the mains.

This apparently rather clumsy approach is often much cheaper than a purpose made isolating transformer and works well in practice.

*or any other voltage.

[GrimJosef]

Actually, thinking about this again, I was overstating the case earlier. With the secondaries in parallel the total current will double, but each secondary will still be passing the same current as it would if they were in series. So the copper winding losses will be the same. The only place where the increased current will make losses worse is in whatever wires are used to connect the transformers to one another.

I suspect that in practice the better option will be determined by the details of the transformers' winding schemes.

Cheers,

GJ

Just in case one winding has a turn or two more or less I would go for series.

[wave solder]

I've always ended up with too high a voltage when using two identical transformers back to back so I ended up with a 9v transformer on the input and a 12v one on the output, I would rather have it a bit low than too high. that still gave over 210V loaded for the small loads I needed it for.

[Endckr111]

So if I go back to back in series with theses 120va transformers, my max Current would be somewhere around 6-7 amps, but generally 500 mah to 2 amp,is that a safe limit ?

What output voltage are they?

[Endckr111]

240 volts on input , secondaries are 15 volts, 30 volts in series

[Endckr111]

Sorry they are 160va toroidal transformers

[snowman_al]

You do not get something for nothing...

They are both 160VA transformers, so the maximum current they can handle is 0.66 Amps at 240 volts, 240v x 0.66A = 158W, simple ohms law.
Because of the losses in going 'back to back' you should not exceed 0.4 to 0.5 Amps (400 to 500mA) at the final '240 volt' out put. Less would be better.


Alan

PS, If you want / need 2A at 240 volts you need at least 500VA transformers (240v x 2A = 480W).

On the secondaries 30V 120W = 4A.