Two transformers in series, what happens to the ratings?

[Jeremy_Wolf]

Here is another quick question for you guys to answer. I have two 120primary 110V CT secondary transformers that are rated at 7 amps. If I were to put two of primaries in series, would the current half just like the voltage. So I would have 60V on the primaries and 55V CT on the secondaries. What would the current be? 3.5A ?

[subwo1]

Jeremy, you can still get close to 7 amps out from each transformer. But if you do that you have to have identical transformers, you have to put the secondaries in parallel and in phase. You should then get about 55Vac on the secondaries at about 14A. You have to put the secondaries in parallel or else one primary will impede the other because the loads on the secondaries will not be identical. Since you have twice the core material, if the wire in the windings were not the limiting factor, you would still be able to get about as much from each one as before.

[AudioFreak]

The above is correct but i cant stress this enough .... the transformers must be IDENTICAL.

[paulb]

I would strongly suggest current-balancing resistors if you are tying the two secondaries together (which is the only way this will work, I think). Even if they are identical models (which they must be), there will probably be a slight difference so they will not share the load evenly.
Maybe a couple of 0.1 ohm units? At 7A, they would drop 0.7 volt each and dissipate 4.9 watts.

[AudioFreak]

Yeah Paul thats sounds about right.

[Geoff]

The addition of 0R1 resistors could make the current imbalance worse not better.

Ignoring reflected resistance from the primary, a 110V CT secondary on an 800VA transformer will have a total resistance of around 0.36ohm, or 0.18ohm per half-winding. If the two transformer secondaries differ in resistance by say 10%, the addition of an ideal 0.1 ohm resistor in each winding (four in total) would reduce the resistance variation to 6.5%, which is an improvement but not exactly significant.

However, ideal resistors do not exist and low value power types tend to have a tolerence of +/-5% at best. If by chance resistors at +5% nominal were added to the transformer secondary with the higher resistance and ones with a value of -5% nominal were added to the other secondary, the resistance (and therefore current) balance would be worse than that without any additional resistors.

[Jeremy_Wolf]

I have seen many times where the primaries are put in parallel and the secondaries in parallel or series(toroidal transformers). Is current balancing a problem because I am talking about putting my transformers primaries in series and my secondaries in parallel? Is current balancing going to be an issue at all? What would happen, would one transformer try to supply all of the current like transistors like to do when you put them in parallel if you dont use resistors?

Would the current value be the same of different if I used the center tap(assume one transformer 120V primary 110CT secondary)?

Yes, they are exactly the same transformers.

[subwo1]

The result is that one may put out, say, 6 amps while the other does 5. this little unbalance causes no strain since the primaries "give" a little. In reality the difference should barely be noticable. I have hooked transformers in this way in the past to cut output voltage in half. I see no problems since your transformers are identical. To answer your last question, you would also connect the center taps to each other.

You know, I bet if you only partially overlap the secondaries by paralleling only half of each so that the other half of each "hangs off opposite ends" then you can get a +/- 60 volt power supply by then running the whole thing through a voltage doubler. It may be worth trying and seeing what happens. Na, it just doesn't quite work out right.

[paulb]

Originally posted by Geoff
The addition of 0R1 resistors could make the current imbalance worse not better.

A few comments:
My suggestion of 0.1 ohm was pretty arbitrary; your calculations indicate that you'd need slightly higher values to make this worthwhile (waste a little more power).
And even though the resistors say +/- 5%, the reality is that they will almost certainly be within 1 or 2% of each other, unless they are from different manufacturers or manufacturing batches. It's totally improbable that two resistors from the same batch would be at opposite ends of the tolerance scale.
But after reading subwo1's post, I agree: the transformers will tend to correct this by sagging a bit. I think I'd put them in resistors anyway; if nothing else, they'll blow up if the phasing is wrong and act like fuses.
Actually, separate fuses for each transformer may be a really good idea.

[joensd]

there exist special manganin resistors in TO-247 packages
they´re rated at 10 watts and 1% variation.
if dissipation gets more than 3W you´d need a heatsink.
maybe you could take those