15-12-2016, 08:10 PM
This sort of thing is very common. It makes the outboard PSU very simple (and cheap!). The same PSU "brick" will be used for many models in the range, which saves cost (design and approval). It means a simple 2-pin connector (and single switch) can be used for power. Perhaps not the most obvious way to do it, but good production engineering.
Would I do it this way myself? Perhaps. Indeed, I have.
In this case, it means I can spec the transformer to deal with the 5V load, and not have to have extra windings for the +/-12V rails.
Alternatively, if I did use a circa 12-0-12 transformer in the usual way to make +/-12V DC (post-regulators), I'd have a problem with heat dissipation in the 5V regulator. A switched-mode DC-DC converter might not be welcome, and a custom transformer with tappings to do the 5V rail might not be desirable either.
Vive la difference
Would I do it this way myself? Perhaps. Indeed, I have.
In this case, it means I can spec the transformer to deal with the 5V load, and not have to have extra windings for the +/-12V rails.
Alternatively, if I did use a circa 12-0-12 transformer in the usual way to make +/-12V DC (post-regulators), I'd have a problem with heat dissipation in the 5V regulator. A switched-mode DC-DC converter might not be welcome, and a custom transformer with tappings to do the 5V rail might not be desirable either.
Vive la difference
