What is the best way to implement a power supply?

Question

Electronics

Hello!
There is a power supply (see figure) for 5V. It is also necessary to implement an output for +15 and -15V.
How best to do it.
Thank you.

Answer 1

It is necessary to replace the existing transformer with a transformer with 3 windings, two for 15V, one for 5. Diodes are connected to the 15V windings with an anode and a cathode to form +15 and -15 a common wire to the winding.
But especially stubborn ones can connect 7805 to +15 to get 5v and use the transformer that is - connect the diodes like this:
although 15 is unlikely to be there if it used to be 5, but raise the voltage on the primary and probably last.

Answer 2

Once upon a time I saw this scheme in the Radio magazine. There, emnip, this PSU was positioned as a very low-power, adapted to the small dimensions of powered devices. Therefore, I personally would not add functionality to it.
But if you really need it, then you can, and it’s elementary: you need to wind another winding on the trans with a midpoint, with the number of turns three times greater than on the 5-volt winding, and connect two rectifiers to it - one exactly the same as on 5 -volt, the other with diodes turned on the other way around (well, smoothing capacitors for each, in the appropriate polarity). These midpoint rectifiers will give + and - 15 volts. But the stability of these voltages will be low. However, there and on the main exit it is small.