Transformers

[sailinon]

I have two Questions I wish someone could answer for me. One is can two

isolated transformers be connected in series to get both voltages.( two 12vac

outputs hooked in series to get 24vac).

Can two 12vdc pc power supplys be hooked together to get 24vdc. If this can

be done then tell me how. Thanks

[ViperTX]

Yes.....the pos on PS1 is left alone, the neg on PS1 is connected to the pos on PS2 and the neg of PS2 is left alone. Now if you measure between the pos of PS1 and the neg of PS2 you will have the sum of both supplies. It helps if the power supplies outputs are isolated....which in the case of most DC supplies is how they are.

[widgitmaster]

http://www.mpja.com/productview.asp?product=7846+TR

$17 for a 24v 10a transformer

[gar]

051022-0716 EST USA

sailinon:

On your first question --- the answer is yes. But you must phase them correctly. If you randomly connect them you will get either near 0 volts, or the sum of the two.

If you label the output wires as S11 and S12 of transformer 1, and S21 and S22 of transformer 2, then connect S12 to S21 and if the voltage from S11 to S22 is near zero, then connect S12 to S22 and the voltage from S11 to S21 will be the sum of the two outputs.

Your second question has been answered. At most only one power supply can be grounded. Note: many personal computer power supplies have the common lead tied to the power supply enclosure and in turn connected to the ac power cord safety ground.

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[sailinon]

Thank you people so much for this information, Im just a newbe but I have a lot of junk that might be put together as a cnc, I have some transformers, both ac and pc power supplys that i can gang together to get the votages i need. again thanks for the replys. I just hate to see good junk go to waste.

[gar]

051022-1155 EST USA

sailinon:

If you want to series the PC power supplies you may need to open the enclosure and cut the connection from common to the enclosure. Before you open the box you may be able to check whether common is tied to safety ground with an ohmmeter.

Also note for any one PC supply you have typically a +/-12 v and a +5 v supply all sharing the same common. Also the +/- 12 supplies are a lower current rating than the +5. Assume the common of one supply is conneted to chassis and ground, and a second supply is in series with the first +5 supply and its common is floating, then you will have -12, -7, +5, +10, +12, and +17 output voltages relative to the first supply common.

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[mxpro32]

keep in mind that the current decreases when you step-up the voltage with a transformer. If you step up the voltage by a factor of 2, the current is cut in half.

[gar]

051022-1515 EST USA

mxpro32:

sailinon has two transformers that he is connecting the primaries in parallel and the secondaries in series. Assuming identical transformers the maximum secondary current is unchanged by putting the two in series, but the voltage is doubled. The maximum VA (volt-ampere) rating of the two transformers is twice that of one, and this is also demonstrated by 2V * A.

If the two secondaries were connected in parallel you would also have twice the VA rating of one transformer. Here the maximum load current would be twice one transformer, but the same voltage. When paralleling secondaries it is extremely important that phasing is correct before applying primary power. Otherwise you have a very high circulating current thru the two secondaries. No big problem when you series the secondaries and the phase is wrong, you just get 0 volts.

Always keep in mind that the sum of the voltages in any closed circuit is 0. And the sum of the currents at any node is 0.

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[OCNC]

Originally Posted by gar 051022-0716 EST USA sailinon: On your first question --- the answer is yes. But you must phase them correctly. If you randomly connect them you will get either near 0 volts, or the sum of the two. If you label the output wires as S11 and S12 of transformer 1, and S21 and S22 of transformer 2, then connect S12 to S21 and if the voltage from S11 to S22 is near zero, then connect S12 to S22 and the voltage from S11 to S21 will be the sum of the two outputs.

Then if you want to connect the secondaries in phase in parallel you would connect S11 to S22 and S12 to S21?

[sailinon]

I take my hat off to you gentleman, you guys are so helpful.

I also had a good day, for a week been going nuts but found the problem was the

software, downloaded updated version and my motors began to go.

Again thanks there is so many places A person can go on this site that he dont want

to get out any more. Ive got to get a life

[gar]

051023-0809 EST USA

OCNC:

The easy experimental way to determine if phasing is correct for paralleling is:

1. Connect one end of one secondary to one end of the other secondary.

2. Measure the ac voltage between the two open ends. If this voltgae is near zero, then those two open ends can be connected together, thus paralleling the secondaries.

If the voltage was double the voltage of one secondary, then the phasing is wrong. Swap the primary wires of one transformer, or swap the wires of one secondary, but not both. Then check again for correct phasing. Note both secondaries can be on the same transformer.

You can series the secondaries of transformers with different secondary voltages. The current rating will be that of the secondary with the lowest current rating.

You can not parallel secondaries of different secondary voltages.

It is very common to see transformers with two primary and two secondary windings, all being of equal voltage rating. These transformers can be wired for 1 to 1, 2 to 1, or 1 to 2 ratio.

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