HOW TO WIRE THE POWER TRAFO
In the assembly guides, I have made a decision to do pictorials with a minimum of text, but for those who want a supplement and prefer to understand what they are doing, I will do a few posts on some key subjects. What does the power transformer do? For powering our units, the power transformer is used to take one voltage (110VAC or 220VAC, depending on your mains), and step it down to our needed voltage (typically 15VAC). And how does the power transformer do this? The most basic power transformer consist of two coils of wire.
To step the power up or down, the coils are wired in a way, so that they are not connected directly, but the coil used as input (Primary) generates a magnetic field, which induces a voltage into the coil used as output (Secondary). The coils have a different amount of turns, which gives the ratio between them (the stepping up or stepping down part).
You may find a transformer rated "Primary 220VAC:Secondary 15VAC" . In reality, that is just a way of expressing the ratio and common use. It would work equally well as a 110V -> 7.5V step down transformer, or a 15VAC -> 220VAC step up transformer. The ratio is about 1:14.7, and you can use either side as the primary. In principle, it would also work as a 220VAC -> 3226VAC step up transformer, but voltage is not the same as wattage/power (well touch very lightly on that later), so realistically, you would fry your small, DIY transformer, and quite likely yourself, if you tried it. Center tapped secondary
For audio circuits, we typically need +/-15, and a center tap for reference. To achieve that, we use a transformer with dual, 15V secondaries.
If we assume the primary is 220VAC, the two secondary windings will each have an identical ratio of approximately 1:14.7, which will step them both down to 15VAC. If we combine the secondaries in series, we effectively combine the two coils, and change the ratio, so the end-to-end voltage induced (now into twice as many turns) is 30VAC - a ratio of about 1:7.3.
That is if we measure from end to end. If we use the point where we combined the two secondary windings as our point of reference, we have 2 x 15VAC, relative to our center tap.
For most projects, you will need the center tap on the secondaries as a common ground reference, so you cant just use a 1x30VAC secondary and save yourself the trouble of combining the coils.
Dual primaries The Primaries never need the center tap, but sometimes, you will be facing a transformer with both dual secondaries, and dual primaries - The voltage on each primary will typically be 110/115V. The great thing about that is, you can configure your transformer for either 220VAC or 110VAC mains power. Combining the coils in series will give you 220VAC on the primaries, and combining them in parallel will give you 110VAC.
Why not just use one coil for 110V? You could, but if you do that, you don't just leave one side hanging, you also halve the effective power handling capability of the transformer. I might explain why in a later post, but for now, wire in parallel as a rule of thumb! :) Hope that helps!