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I have the following transformer that takes in 117V and steps down to 12V and 9V. However, I don't understand the label. "PUR" and "RED" are listed as 117V, does that mean PUR is power and RED is ground? Or is PUR power and BLK ground (since its 0)? Basically, which two wires do I connect to the power outlet wires? And what about the secondary wires? I'm new to transformers so I don't want to take any chances by wiring it up wrong. Thank you.

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It does not do 12V from 117V. or 234V. It does do twice 9V from either 117V or 234V.

In labels like these, you normally read them left to right. One side is primary, the other is secondary. So if it's a label like this, or very similar, the wires noted on one side together supply all the power that goes in, while all the power that can come out gets taken out by the wires noted on the other side.

In your case, presumably you want to put circa 115VAC in, since you are not referring to 230VAC at all. But to be sure, I'll handle both cases.

If a label on one side says "115 - 0 - 115" it means that side has two directly connected windings, like in this image on the top-side:

schematic

simulate this circuit – Schematic created using CircuitLab

In this case, you can connect 115V across one winding, or 230V across both. These transformers are a bit silly, since if you use them as 115V transformer, you are not using a bunch of copper that the manufacturer put on: inefficient.

So, what your manufacturer did was say: "Hey, why would we connect them at one point? If people have choices to make when wiring it up anyway, why not let them do all the connecting?"

Why?

Because now, you can connect the two windings in parallel on 115V and in series on 230V. The 0 and 117 just indicate which wires to connect together in which case.

The same happens on the secondary side, they give you two completely independent windings, so you can put them in parallel for 9V and in series for 18V.

If you can imagine the two transformers in the next picture to be linked together magnetically/inductively, you can see it like this:

schematic

simulate this circuit

Since no winding is connected to any other, you can connect them up any way you want. But of course, it would be silly to connect a 117V winding to a 9V one. And in many transformers they are no longer safety approved if you do put a mains AC voltage on a secondary winding, even if it makes sense to do so from your engineering point of view. That's because they have extra plastic between primary and secondary, but not between each secondary, so the guaranteed separation will no longer be guaranteed.

So, you can put 115V on it, by connecting the two "117" windings together in parallel, connect both "0" wires on that side (BLK & YEL) together, and connect both "117" wires (PUR & RED) together. This way you know that with 115VAC you can also put all the power into the transformer that you need to be able to take out without something getting too hot.

If you want to connect it to 230V, simply connect the "0" of one winding to the "117" of the other and put the mains 230VAC on the two wires that are left. (for example: Connect PUR and YEL together, and put 230VAC on BLK and RED).

The same tricks can be done on the 0-9-0-9 side to make one strong 9V or use them separately for two half as strong 9V's or in series for one 18V.

The VA rating of a transformer means Volt-Ampere, and is often given as the amount you can take out on the secondary. It is just the numbers multiplied. A 40VA transformer with one 10V output will be able to handle 4A on that output. A 28VA transformer with two equal 14V windings will be able to output 1A on each. Etc.

If a transformer like this is 18VA, for example, it will probably be made (unless a datasheet or label clearly notes otherwise) such that each 9V winding can handle a maximum of about 1A. So two parallel will be about 9V, 2A. Two in series will be 18V, 1A. So, unless something notes otherwise, this transformer is 12VA total, is 6VA per output. (= 6VA / 9V =~0.67A).

To be sure about the 6VA per output you could Google the number on the label, but if all the parallel wires have the same copper (or other metal) thickness, it's reasonably likely the windings share the rating equally.

And so also: Always make sure you have both primaries powered in either of the set-ups mentioned above if you want to be able to take all power out, because just like the secondaries, either winding will only be capable of about half the maximum power.

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    \$\begingroup\$ Really nice and thorough answer +1 \$\endgroup\$
    – Rev
    Jun 9 '15 at 17:36

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