sam9
Having bought a transformer or two, I can appreciate the problem. Some like Avel-Lindberg include a state called " regulation", expressed in percent. For instance, if you buy a 13.5V transformer with 6% regulation from them and measure the secondary voltage without a load it will be 13.5V x 1.06 =14.3V. This accounts for what they expect the drop under load to be. On top of that, manufactures want those specs to always be met so they tend to de-rate them a little, maybe another 5%. Transforners I've bought from Plitron and Avel-Lindberg both were this way.
This sounds nice unless you are in a situation where you have an upper limit on the rail voltage due to possible damage to components. I ran into this once and had to do a quickie fix by adding a pair of + & - voltage regulators to what should have been an unregulated supply. Eventually, I replaced the transformer.
So anyway here is the modified calculation I apply to the spec sheets:
(Stated Voltage) x 1.1414 x (1+stated "regulation" %) x (1.05 for under-rating).
Example (13.5V x 1.1414 x 1.06 x 1.05)-1 = 16.1V expected unloaded reading at the rsulting rails . The "-1" is what I expect a 25 amp bridge rectifier to drop, other rectification methods my differ.
