Is it possible to combine transformers?

[Morocotopo]

Is it?
I mean, two equal, separate transformers, can they be connected together to get double secondary voltage than a single one (for example, two 24 V ones to get 48 V)
Or not?
Am I getting too much cofee?

[BlueToad]

Yes... as long as you don't do anything too crazy with numbers or voltages, you can connect the primaries in parallel and the secondaries in series to get a higher voltage.
Carl

[Morocotopo]

So it´s not THAT crazy an idea... great.
Any concerns about the primaries (or secondaries for that matter) phase, or it´s not important?

[petemoore]

  Any concerns about the primaries (or secondaries for that matter) phase, or it´s not important?
  I'm assuming you are intending to rectify then filter to get a supply voltage ?
  At that point both outputs will have become DC, which has no phase, only polarity issues, which are easy to figure out then test.
  AC can be in or out of phase, ie it could be reversed, I doubt you'd want it reversed.

[R.G.]

Any concerns about the primaries (or secondaries for that matter) phase, or it´s not important?

Yes. If you make DC out of them first, you defer the problem to making sure the DC polarities are right, but you can connect transformer secondaries in series directly.

Measure the unloaded AC voltage of each secondary with your DMM. Now hook only one pair of the secondary wires together, leaving one end of each free. Measure the AC voltage at the free ends. This will be either (a) the sum of the two AC voltages or (b) the difference of the two.

You can then make DC out of either result. If the two windings are almost identical, then the resulting series AC voltage will be either twice the voltage of one of them, or nearly zero.

Phase, in the sense of "which side of this winding goes positive when the other winding goes positive" does matter on transformers.

[Morocotopo]

Great, thanks for the answers. Just to make it clear, I would use this connections scheme:

and just test wich secondary wire from each transformer I have to connect together to get V x 2 from the other two wires, right?
Now, for example, if I use two 12V transformers, I should get, with the right connections, 24V or thereabouts, right?
What about the amperage ratings? Let´s say that each transformer is rated for 1A, what would be the total Ampere rating? Still 1A? 2A? 500mA? Not too sure about this...
Anyone ever used this type of power transformers setup? I´m gonna use it to power an LM1875 amp (around 20W audio amp chip) after rectification and filtering of course.

Thanks again for educating me!

[Franky]

I think the overal consumption in series is limited by the weakest transformer...

If you want to make more current, maybe you should try them in // (only available if both secondary voltages are equal).

[Morocotopo]

So, Franky, in the example I gave the Ampere rating would be 1A (the same as the weakest transformer, in this case both are the same...)?
Yes, // would give more current but I need more V.

[Franky]

Yeah, if there is the same current (1A), there will be only 1 amp out..

When you want to double the current, you need the same voltage.
When you want to sum the voltages, you can draw the current given by the weakest xformer. Because the current will flow thru both secondaries, and thus is limited to the weakest.

[Mark Hammer]

Now, just to prevent any possible unfortunate mishaps involving either pedals, humans or small animals, let me ask a connected question.

I have a number of transformers I have purchased at surplus places that were obviously designed from some specific product/application.  They have two primary leads, as per usual, but have several sets of secondary leads, each at different AC voltages and each with their own centre-tap (i.e., 12Vct, 24Vct).

Should I assume that the sort of connection described in the first few posts here is NOT to be attempted when the secondaries are coming from the same transformer, no matter how many leads they have?

[R.G.]

No, you don't need to be that conservative, Mark. A transformer with multiple secondaries can have the secondaries connected IN SERIES in any combination whatsoever and there will be no ill effect. In series, only the current through all of them can flow, and that will be limited to the lowest-current-rated secondary. But the secondary voltages will add, including the "phasing" I mentioned.

In making transformers, there is what is called the "dot" convention. A secondary can only have two voltages, either following the primary voltage or inverted from it, depending on how you hook up the wires. If you put a dot on one primary wire, then the secondary wire which goes positive and negative in synch with the dotted primary wire should also be dotted. This should apply to all secondaries. The dotted (or paint splotched, or magenta nail-polished) wires all go the same direction, positive or negative, at the same time. So you can hook dots to non-dot and get the sum of the two winding voltages. You can hook dot to dot and get the difference.

Hooking secondaries up in parallel is very dicey. Unless the secondaries are very nearly identical, large currents flow because there is a difference voltage which is across only the wire resistance of the secondaries, and this can burn things out.

Did that make sense? I'm having trouble with my Techno to English translator recently.