Connecting two transformers to mains voltage - linear power supply

[maor]

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.

[maor]

Can I connect two transformers in parallel?

[G7PSK]

On the mains side, Yes, there is no difference between fitting two plugs and plugin them side by side into the mains  and fitting one plug for both transformers.

[maor]

Yes, you can connect two transformers in parallel on the main side. Think about it, if you connect two them to the wall on separate cable it will be the same to put them on one cable.

I was thinking about that going for the obvious answer, but what about the inductance from the two coils?

[IanB]

I was thinking about that going for the obvious answer, but what about the inductance from the two coils?

It depends on the electrical supply to the transformers. Connecting two items in parallel allows them to be considered completely independently and separately from each other as long as the supply is unaffected by either load. However, if the supply should be inadequate, for instance on the end of long thin wires, then one load can affect the supply, which can in turn affect the other load.

This is seen most commonly in American homes where things like ovens, air conditioners or electric irons will make the lights dim when they turn on. The lights dim because the wiring is inadequate for the task and experiences a voltage drop due to the current flowing.

Normally considerations like this don't apply when connecting small devices in a project box with a proper mains cable and proper internal wiring.

[ptricks]

A few things to consider.
Is the transformer from the other supply capable of supplying the current you need ? It will not work if you have a better voltage but not enough amperage.

The schematic you posted shows 2200uf capacitors on the outputs of the voltage regulators, that is way too big. With something like a 2200uf the regulator is not going to accurately control the voltage because it takes too long for it to respond with such high capacitance. Something like a 100uf would be better, I even use 10uf sometimes, depends on what it powers. You want the majority of the filtering on the input of the regulators.

You can connect transformers in parallel and even series if it is done properly.  You can also connect the output of the first transformer to the input of the second transformer to change voltages further. Something like a 1:5 ratio second transformer would be great in your case, stepping the first transformers ~25V down to 5V.
Or you can connect two identical transformers so that they share the same secondaries which creates an isolation transformer for mains power. 220VAC converts to 12VAC on the first transformer, 12VAC secondary converts back to 220VAC on the second transformer.