Toroidal transformers wiring configuration.

[davelectronic]

I have two toroidal transformers, both are 2 x 115 Volts primary windings. And 2 x 12 Volt secondary windings. I wanted to find out if I could put both primary windings on each transformers, and both secondary windings series to get 6 Volts output from both transformers under voltage input on the primary windings. Hope that makes sense. There 300 VA each transformers. I think the primary windings under voltage might sag under secondary load, dropping the output secondarys below 6 Volts. Thoughts on this appreciated.

[Benta]

If the transformers are identical, there's no problem playing around with the windings.
But it's unclear what you want.
1: a 6-0-6 V output?
2: a single 6 V output?

There's zero problem in running the primaries at half voltage, in fact it's advantageous. Much lower core losses and less inrush current problems.
But  you'll have increased resistive losses due to thinner wires. All in all, derate the power to 270 VA per transformer, and you'll be OK.

But whatever you do, be damned certain about the polarity/phase of all windings.

I've attached the options:

[davelectronic]

Thank you for the help, and the diagrams. Yes I wanted a single high current 6 Volts output from two 300 VA 12 Volt toroidal transformers. Yes I will play about with the windings. I know I can get 12 Volts or 24 Volts from the secondary windings. I just thought I could derate the twin primary windings so the output drops to half the rated voltage. But I know lowering the primary voltage might sag the secondary voltage to much. I think I have to try it and see.

[Benta]

But I know lowering the primary voltage might sag the secondary voltage to much. I think I have to try it and see.

Why would it "sag" the outputs? Not running the core at full magnetization actually makes it a better transformer, but larger and heavier than it needs to be.

[davelectronic]

I thought the secondary voltage might sag under load at the primary. As i would only be putting half the rated voltage into each primary side of the transformers. They would be wired in parallel, for the two primary windings, them put the single inputs primary in series with each other. So a parallel series input, and a series secondary output. As each transformer would be receiving 120 Volts AC.

[Benta]

Sorry, but your location says UK, and you have 230 V service, right?
If you're somewhere else with eg, 120 V mains, please let us know.

[IanB]

Sorry, but your location says UK, and you have 230 V service, right?
If you're somewhere else with eg, 120 V mains, please let us know.

Let's simplify it by ignoring the multiple windings complexity.

Suppose we have two identical transformers, each rated 100 VA, with 230 V primary, 12 V secondary.

We place the two primaries in series so that each primary individually sees 115 V (and each secondary produces 6 V).

We place the two 6 V secondaries in parallel (observing polarity), to give a higher current 6 V output.

What do we now have?

Do we have a 200 VA arrangement with 230 V in, 6 V out?

Or do we have a 100 VA arrangement with 230 V in, 6 V out?

Or do we have something else?

[davelectronic]

Yes I'm in the UK, so 240 Volts AC, in the case of the 100 VA transformers, I would be inclined to see it as 100 VA 6 Volt output. But because the primary windings are running at half there rated voltage, I'm thinking the power factor has dropped.

[Phil1977]

We place the two 6 V secondaries in parallel (observing polarity), to give a higher current 6 V output.

What do we now have?

Do we have a 200 VA arrangement with 230 V in, 6 V out?

Or do we have a 100 VA arrangement with 230 V in, 6 V out?

Or do we have something else?

A big part of the transformers losses is created by resistance losses. And these losses do not care about the voltage but only about the current flowing through the wires. That means, if you use a 200VA-transformer and derate it to half of its nominal voltage, the current stays the same and you have half of the nominal power.

Because resistive losses are only a part of the game you could carefully watch the transformers temperature while you exceed the nominal current. As long as the windings have a healthy temperature, I see nothing that could go wrong.

[Benta]

For two ideal transformers, it would be a 200 VA arrangement. But due to thinner wires, it'd have to be derated a bit, prolly to around 180 VA.
But the only way to find out is to run it and measure temperature rise at different load currents.

[davelectronic]

Yes I understand resistance losses, but I'm no expert. I can measure temperature rises, after I get a running idle temperature first. Taking ambient temperature in to consideration. These transformers have a thermal fuse fitted to each transformer, it's 130°C if that pops through thermal overload, there a pain to replace. I had popped the same fuse in a smaller 160 VA transformer of 18 Volts. Then replaced the fuse, bit tough to get to it. I've got a thermal coupler for my multimeter, but not got a thermal imaging gun.

[Benta]

Well, you can get a good estimate by just measuring the resistances of primary and secondary.

Careful!!! Be sure to short the secondary (or the other primary) before measuring, otherwise you and your multimeter will get a whack.

[davelectronic]

Well I wired the primary windings singley as for 240 volts on each transformer. I then put each transformer in series with the other one. I was getting a reading of 7 Volts approximately, each transformers primary read 124 Volts each on the input. And the secondary windings where in parallel. I was hopeful, but when I loaded the 7 Volts output, the voltage collapsed to 0.9 Volts. There are a purple and grey wire, they are linked as if each transformer was a single 240 Volt unit. I don't know if I need to do something with the purple and grey wire. The resistance of each transformer was 10 ohms.

[Benta]

Look, telling us about the wire colours is like telling us which M&M you liked best yesterday.

But you don't answer questions or give relevant information.
Are the transformers really identical?
Do you have a data sheet?
Why don't you post a picture of them, especially the labels?

My crystal ball is in the workshop for service this week.

[davelectronic]

Yes there both identical. I will get photos of them tomorrow.

[davelectronic]

This is the wiring of each transformer. I can't see what I did wrong, to allow two transformers in series on the primary input. I tied grey and violet together, leave brown and blue input for each transformer. Then put both transformers in series on the AC input. All voltages where present when read with a multimeter, both primary windings where receiving 124 Volts AC, both secondary windings read 7 Volts AC . But on connecting a load of 50 watts the secondary voltage collapsed to 0.9 Volts AC on both secondary windings. So what can I do to get these transformers to run on half the supply voltage. I don't have any ideas.

[davelectronic]

The only other wiring configuration I can see, is grey and violet on both transformers in series together. Then brown and blue both transformers in parallel. I'm unsure if that would work, I could measure the winding resistance of the primary in that configuration and see what that looks like.

[Phil1977]

How did you exactly connect your load to the secondaries?

I assume you need to connect the secondary outputs in parallel so that the load is equally distributed over both transformers. If you connect the load to only one of both transformers, then it output voltage quickly collapses while the output voltage of the other transformer probably doubles.

[davelectronic]

Yes I connected the secondary windings in parallel all together. The voltage was reading fine at 7 Volts, but collapsed under a 50 watt load.  I will try again in a while, see if I can figure this out. I really need approximately 6 Volts AC from these two 12 Volt toroidal transformers.

[Phil1977]

Just check carefully for some loose connections. If one of the secondaries is not connected, then both transformers are out of balance and the voltage collapses.

You could also check the primary voltages *while* the output is collapsed. If it´s an out-of-balance-situation then it should also be visible on the primary side.

If all that is not helping it would be good to know the input power in idle and loaded state. Do you have a simple powermeter you can switch into the mains supply?

[davelectronic]

Yes I will try this, and the measurements of the primary under collapsed secondary voltage. I don't have a watt meter, but I do have AC DC clamp meters up to 20 Amps measurement. That should see the idle state, and what's going on when the secondary voltage collapses.

[davelectronic]

Thank you for all your help, I missed a bad solder connection on one of the secondary winding output (not enough heat when soldered) wrapped the output wires with 1mm solid copper wire. Now it's all good, secondary output is 7.00 Volts AC, each transformers primary is receiving 125 Volts AC approximately. I don't know the VA rating of this configuration. The project is a single unit charger that can charge multiple 5 Volt devices at the same time. I've yet to build a circuit. Hopefully a total transformers output of 10 Amps is not over optimistic.

[IanB]

I'm confused.

The picture you showed has the transformer labelled like this:

30 VA
Primary: 0-115 V (Blue, Grey), 0-115 V (Violet, Brown)
Secondary: 0-6 V (Orange, Yellow), 0-6 V (Black, Red)

Can we confirm that the secondary windings on your transformers are marked 0-12 V, 0-12 V, rather than 0-6 V? And the transformers are still 30 VA?

If so, for each transformer individually, the max current in each winding would be 15 VA / 12 V = 1.25 A. Combining transformers and putting all secondary windings in parallel, that would give a total of 1.25 A x 4 = 5 A.

You might get a bit more current by halving the primary voltage on each transformer, but resistive losses go with the square of the current, so trying to operate at 30 VA with halved voltages will give you twice the current and four times the resistive heat generation. So 10 A at 6 V AC might be pushing it.

Now you say you want to charge batteries, which presumably means rectifying to DC? If so, you have to considerably de-rate the transformer. I would think 10 A DC is highly unlikely, certainly not at a reasonable voltage. You are probably better being conservative and assuming 5 A rectified DC, and likely not at 6 V. Maybe at 5 V DC perhaps, but that would need some measurements and design calculations.

For DC applications, transformers need to be uprated, with both higher VA rating and higher secondary voltage.

[Benta]

I think the photo is from a general data sheet on a series of transformers and not the actual one.
But the OP is extremely secretive and selective about providing just the tiniest tidbit of information, so I don't know.
I'm out, it makes no sense.

[davelectronic]

I'm not trying to confuse anyone. This is the series of transformers I have two of. 300 VA 115 - 0 -115 - 0 and 0 - 12 0 - 12 both primary and secondary windings are dual windings. The wiring in the picture I posted is the configuration of the windings before I wire them up. First primarys in series and secondary windings in parallel. I'm getting 7 Volts AC from these two 12 AC transformers. I haven't mislead anyone. I figured you would know what dual windings where, maybe you don't, and that's made you confused. Or you didn't understand I was trying under volt the primary windings in series, to get 6 Volts AC high current from the secondary output. I know have that, and above thanked you all for the help. I genuinely haven't tried to mislead anyone. Just because it's not an exact photo of the transformers, doesn't mean the drawing in the photo isn't accurate. Below is the actual toroidal transformers that are now working perfectly. Thanks again.