I asked the manufacturer to give me 2 secondaries at 28V/120VA each as I'm planning to eventually end up with a power supply with + and - rails. Is your impression that this is not the case? Then I need to check with the manufacturer
The markings on the transformer indicate this is not the case. The total power is 120 VA shared by the two secondary windings. At 28 VAC for each winding, that works out to about 120 / 28 = 4.286 A shared across two windings, or about 2.15 A per winding. That's a pretty far cry from the 3 A per winding you were shooting for. Or are you still hoping to push this design to 5 A per output?
Why not just buy another transformer and (essentially) build two power supplies? If you need 5A at 30 V DC, that's 150 watts, so the transformer you purchased
is a bit on the wimpy side for that load. OTOH, 3 A at 30 V DC is only 90 watts, so your 120 VA transformer should be adequate for that... unless you want both positive and negative power supply outputs with respect to a common, in which case you need
two of those transformers.
I went back through this thread looking for an actual power supply design but failed to find one. I did find lots of stuff on the Internet with regard to the so-called Hiland power supply design, including numerous "hacks" of the Banggood product offering. One of the more interesting is described in a series of blog posts by a Norwegian hobbyist who goes by the name paulv. You can follow him starting
here:
http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html
But what I am wondering is: are you really learning anything from this thread? And are you any closer to constructing a "laboratory bench power supply" for use in your hobby? I do look forward to you posting pictures of your progress, but truth be known: power supply design is not exactly rocket science until you begin pushing the limits of adjustability, maximum voltage, and maximum current simultaneously. Adding "bells and whistles" in the form of metering, transient protection of the output when power fails (or surges!) and automatic fan control of the heatsink temperature are all interesting and, perhaps, valuable touches to incorporate into the final design.
However, I think most hobbyists would be satisfied with a pair of three-terminal voltage regulators (for positive and negative outputs with respect to a common terminal), a step-down center-tapped secondary winding providing 18-0-18 VAC (plus or minus a few volts) at a few amperes, a generic bridge rectifier capable of a few amperes with a few dozen (or a few hundred) peak inverse voltage rating, and a largeish electrolytic "smoothing" capacitor of ten thousand microfarads or so. This is all you need to play around with op-amp and other analog circuits. If you want to add some digital capability and resurrect some of those TTL parts gathering dust in your junque box, then add another low-voltage secondary winding, bridge rectifier, smoothing capacitor, and a 5V three-terminal regulator just for that. Or purchase some wall-wart power supplies.
If this were my project, I would be thinking of other ways I might accomplish my goal. Linear regulation may not be "dead" yet, but it is definitely on the way out. The old complaints about switching regulators introducing too much noise on the power supply rails, are pretty much addressed by proper circuit design and higher, more easily filtered and suppressed, switching frequencies. And there is a humongous reservoir of tried-and-tested circuit designs available on the Internet, along with application-specific integrated circuits to do the actual heavy lifting. Just add a MOSFET switcher and an inductor to finish the design.
But it's NOT my project... so onward to bigger and (hopefully) more suitable transformers.