I'm working on a basic linear power supply using some random parts I have here and an old kit I found.

The kit contains no taps 10:1 transformer going to a bridge rectifier to an lm317 with some filtering.

my first plan was to go simple and connect a 7805 to the bridge rectifier and get a constant 5v output, but I struck quite an obvious road bump, there will be about 25 volts dropped on the 7805.

So I opened up a onehunglow variable plugpack (which looks horrible, ugly unregulated linear design) where inside I found a small multi tapped transformer.

I wicked out the transformer and started measuring the ratios using my iphone as a signal generator and measuring the voltage on my oscilloscope.

I came out with these results:

the voltage is the simple attenuation ratio calculation for 220 line voltage in israel.

Full ratio - 15:1 -> 20.8vpk

First tap - 20.45:1 -> 15.26vpk

second tap - 27.7:1 -> 11.26vpk

third tap - 34.62:1 -> 9.012vpk

fourth tap - 44:1 -> 7.09vpk

final tap - 75:1 -> 4.16vpk

So I think I can use this transformer on another board where I'll make another bridge rectifier and the 7805 and have a constant 5v regulated.

now the question I have is can I do it?, can I simply connect the two primaries in parallel and call these two systems isolated?, is my assumption correct?

I added some images of the lm317 circuit and the little multi tapped transformer.