Dual voltage power supply question

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dual voltage 7812 7912

HERE's a schema for a +/- 12 volt power supply. What I don't unserstand is the input voltage to the regulators.
Does the input voltage to the regulators become 9 or 18 volt, and why?
To me it seems that the voltage will become 9. But then the regulator would not work...
 

dual voltage power supply 7912

There are different ways of marking a transformor. As faar as practical issues are concerned, each winding of the transformor should supply Minimum 15 VAC so as to get 7812 and 7912 working and to get 12 volt in dual track.
 

pranam77 said:
There are different ways of marking a transformor. As faar as practical issues are concerned, each winding of the transformor should supply Minimum 15 VAC so as to get 7812 and 7912 working and to get 12 volt in dual track.
Click to expand...


It does seem to be a 18V transformator used. If you look in the picture the secondary winding is marked 9 - 0 - 9. I would read that as a 18V output with a center wire, wouldent you?
I know a 7812 or 7912 alone would need minimum 15 volt too operate as they should.
When the bridge rectifier is wired to both ends of the secondary coil this would create ~18V DC, but when the center wire is used as ground to the regulators they will get 9V in?
Or do they get all 18V because of how it is wired?
 

As per the given schematic, as the center wire is used as ground to the regulators they will get 9V in. Good luck
 

Re: dual voltage 7812 7912

hi.. can anybody provide me the schematics, connecting 7812 & 7912, for dual tracking??
 

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