Two transformers in series, what happens to the ratings?

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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 ?
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.
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.
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.
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.
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.
Geoff said:
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.
I'm sorry, Paul, but I have to disagree with you. Resistors do vary within their tolerance range, even when selected from the same batch.

I have measured many 1% metal fim resistors taken from the same reel and two adjacent resistors on the reel can be up to 1% high and 1% low respectively. I have also measured a lot of 10W 5% power resistors (again from the same batch) to obtain specific values for crossover networks etc and again the values varied up to the tolerance limit.

I have not measured any low value (sub 1 ohm) resistors, due to the difficulty in taking accurate measurements, but I have no reason to doubt that they will also vary in a similar manner.

You are right that toroidal transformers with dual primary and/or dual secondary windings often have the windings connected in parallel. However, the two primaries or secondaries will normally be bifilar wound and will therefore be well matched, thus ensuring equal current sharing between the two.

Current sharing is not the only problem with paralleled windings. The voltage generated by each winding must also be the same othewise there will be a high current circulating through the two windings (even with no load connected). With a 1V difference between the windings and a winding resistance of say 0.18 ohm, a current of nearly 3A will be generated. This can cause overheating and/or reduce the maximum current that can be drawn from the transformer.

Transformer secondaries should only be connected in parallel when it can be assured that the voltages generated on each winding remain within a maximum of 0.5V of each other under all conditions of loading. This usually requires bifilar winding. Transformer manufacturers normally specify whether the tolerance on a particular model will allow for parallel connection of the secondary windings

In your case, with two separate transformers having the primaries in series and the secondaries in parallel, it will be difficult to ensure that the secondary winding voltages remain closely matched under all supply voltage and load conditions. I suggest you take some measurements of each transformer, before paralleling the secondary windings, at no load, part load and (if possible) full load to see if the secondary voltages are sufficiently well matched.

Geoff, your experience is quite opposite to mine but I'll take your word for it. Larger valued resistors is probably the answer.
You know, the best approach would be to keep the transformers separate. How about just building two power supplies, say one for each channel?
I totally agree with doing the measurements, a very good idea.
Remember, though that any unbalance is compensated for because the primaries are in series. Suppose you hooked the primaries in series and kept the secondaries seperate. Then you shorted one of the sencondaries out, almost no power would be drawn from either transformer. The unloaded transformer would impede the current into the shorted one. This effect is still present with the secondaries in parallel, except now, with both drawing the same current, the primary current also remains the same.
It appears that I started a bit of a debate as to whether or not it is ok to put the primaries in series and if so, how to do it. I guess I just shouldn't use my transformers and just buy new ones. i hate not being able to use good stuff especially since I have 4 of these transformers that I purchased from apex jr. These were for the amps that I had mentioned in a previous post that kept blowing the output transistors. I need to know what can be done. I guess I'll just have to find some new transformers for the amps. I could use a variac, but thats just another piece of equipment I have to buy and then worry about.

If I were to use just one transformer at 120v primary and 110CT secondary at 7A, what would the current be if I just use the center tap of the transformer, ala 55V. What amperage would the transformer be capable of supplying? Not that this work work for my situation, it is just a question I need answered because I don't know.

Is what subwo1 saying correct, it is ok. I have actually done this and everything seems to work ok. I have put the two primaries in series (each gets 60 volts) and the secondaries all in parrallel (55 Volts Center tapped now because of the primary) and connected them to a bridge rectifier wich feeds some large caps and then the amps are connected to these caps. I have been using this for a while (several months) and it seems to work ok and the transformers don't get very warm. I am asking now, because I have only been doing temporary setups until now. I now want to complete this project and put everything into rackmount cases, make it look pretty, and use it.
So, if I understand this well, the meaning of putting those primary windings in series is to get half the voltage on the secondary windings.

Maybe you want to reach two equal DC-voltages after rectifiing, in that case I would do the following:

1) put those secondary windings of the two transformers in parallel
2)rectifie the AC-voltage at the secondary windings.
3)put two equal capacitors in series, and connect the two ends on the "DC-output" of the rectifier.
4)you can simply tap the two equal but opposite voltages at the two ends; the connection between the two capacitors will act as the ground.

I'm not 100% sure about this method, but I think I've seen or read it somewhere. I would ask someone of you to tell this method is right or not.

Best regards,

Definately not a good thing when running near the maximum rating of the caps either. My experience has been the same. You can not hook up a voltage source to two caps in series and then use the conection between the caps as the ground. As soon as you start drawing current, the balance will be upset and one of them will get the full charge and the device you are supplying, such as an op amp, may not be very happy about this.
Jeremy_Wolf said:
If I were to use just one transformer at 120v primary and 110CT secondary at 7A, what would the current be if I just use the center tap of the transformer, ala 55V.
You'll still be able to get 7A at 55VAC.

I have actually done this and everything seems to work ok.
It would be useful if you can figure out whether the current is actually being shared evenly between the transformers (do they all get just a little bit warm? Measure their temperatures?). Otherwise, I would say you've proved that it works so build that box!

Don't worry about starting a debate here. You've surely noticed how everyone is generally very polite about it all.
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