Using 2 Transformers to build Power Supply

[millman52]

Using 2 of the following spec transformer: DUAL INPUT VOLTAGE OF 110 VAC, OR 220 VAC 3 AMPS. OUTPUT 27 VOLTS @ 12 AMPS. TOTAL VOLT-AMP RATING 335.

It's not that I am planning on using this particular transformer. I am posting the pic & specs to use for discussion purposes.

Imput voltage isn't an issue for me to wire, pretty much straight forward there.

It's the output(s) that it all becomes foggy.

If the outputs are wired parallel. Am I correct in assuming you still have 27V, but wattage & amperage doubles to 24A, 670W?

Now what happens if wired series? Do you get 54V, 12A, ?W. ???

[H.O]

Hi,
Secondary windings in parallel will give you 27V, 24A. 27 * 24 = 648
Secondary windings in series will give you 54V, 12A. 54 * 12 = 648

As you can see you get double 'power' either way, it's just a matter of deciding how to use it - voltage or amperage.

[Al_The_Man]

Series or parallel, phasing is important.

[millman52]

Series or parallel, phasing is important.

Thanks to both. I am working on calculating for the Capacitors now just to follow through with this type of design.

Al can you elaborate a bit on "phasing"?

[Al_The_Man]

There is several previous post here on this subject, if you have identically marked transformers, then often it is not a problem, but if you were say, to connect the primaries in parallel it is possible to connect in such a way that the secondaries are out of phase, (produce opposite polarity) this would appear as a short circuit load to the primaries if the secondaries were paralleled.
If in series, they would produce zero voltage across the outer ends.
If in doubt, and the desired result is to parallel the secondaries, the primaries can be paralleled and one end of a secondary connected to one end of the other secondary, the two open ends should have ~zero voltage on them. If they have double the voltage then swap one of the secondary connections and try again, when zero is read, they can be paralleled.
Al..

[millman52]

In using series arrangement of transformer(s) 54V, 12A. 54 * 12 = 648


& this formula from Campbell Designs tutorial. C=(80,000 * 12) or 960,000. 960,000 / 54 = 17778 uf.

Conclusion: Need capacitors in the 60+V range & uf at 18,000 - 20,000 ?

Is this correct?

[millman52]

There is several previous post here on this subject, if you have identically marked transformers, then often it is not a problem, but if you were say, to connect the primaries in parallel it is possible to connect in such a way that the secondaries are out of phase, (produce opposite polarity) this would appear as a short circuit load to the primaries if the secondaries were paralleled.
If in series, they would produce zero voltage across the outer ends.
If in doubt, and the desired result is to parallel the secondaries, the primaries can be paralleled and one end of a secondary connected to one end of the other secondary, the two open ends should have ~zero voltage on them. If they have double the voltage then swap one of the secondary connections and try again, when zero is read, they can be paralleled.
Al..

Ok I understand. THANKS.

For this example I was intending 2 identical transformers & series connecting the secondaries.

I fully appreciate the caution(s) you have outlined above. I was attempting to keep this example as straight forward as possible, just to understand the basic principal of a power supply.

[vtxstar]

Take a look at the wiring of these toroidal transformers (don't worry about the difference in style, just focus on the connections) - From another post in this section

Those transformers are dual primary types - a similar concept to what you are doing with two frame type transformers.

Notice the "dots" on the winding schematic. The dot is the phasing "key" (for the lack of a better phrase) When the AC voltage goes positive in relation to the dot, the secondary winding voltage goes positive in relation to its dot.

That is what Al is talking about. Get these screwed up and it won't work (the reason for your question afterall).