power supply prblem..

[beginner Wan]

gud day sir!!

im working on a power supply with 3 different regulators to power my project(op-amp circuits).. +5v, +12 and -12v... the input for the rectifier is 12v 750mA AC.. (the schematic is attached) the problem is that im only getting 5v on the outputs... i've checked the circuit board, changed the regulators... and still, the same problem .. any suggestions or corrections on my circuits...

tnx in advance!!!

 

[ronsimpson]

First remove the 7912. It needs a need a negative voltage on it’s input!

With the 7912 removed try to get the positive side working. The input to the 78XXs should be about 16 volts. You should work on one supply at a time until the problems are found.

 

[beginner Wan]

oh... now i remember.. 7912 cant change the polarity of the input...one thing sir... is n't possible to input only 12v to 7812 for me to have a regulated 12v? what should be the value(range of the input to the output) of the input for me to have a better regulated output?
again... tnx sir...

 

[audioguru]

The minimum input to a 7812 or 7912 is 14.5VDC. The transformer winding must supply at least 11VAC to the rectifier bridge for each polarity. So use a transformer that is 24V center-tapped for 12VAC to the positive rectifiers and another 12VAC to the negative rectifiers.

I think your rectifiers are connected wrong for positive and negative unregulated supplies.

 

[Krumlink]

Heres the rectifier in correct orientation

 

[k7elp60]

In addition to what audioguru and others have stated you need to have the bridge connected properly. To get plus and minus voltages to the regulators you need two filter capacitors and a transformer that has a CT. The CT goes to ground and the other leads feed the rectifiers. You can use only one rectifier for each polatity, but it is not recommended as 1/2 rectifer cks are very poor.
See the attached

 

[audioguru]

The 7912 was connected with the wrong polarity input so it is probably destroyed. The rectifier bridge was connected wrong so the other two regulators are also probably destroyed.
I think all the regulators should be replaced when the wiring is corrected.

 

[beginner Wan]

thank you for all the help... now i'll change my connections and i'll tell you what happen... again, thank you guys!! one more thing, what is the best value of the regulator to use for 12v AC... coz if i cant use 12v regulators, im planning to change them.. again, thank you sir...

 

[audioguru]

If your transformer is 12VAC center-tapped, then each half is only 6V which makes a rectified and filtered 7.8VDC. If the filter capacitor is big then a 7805 and 7905 regulators can make regulated positive and negative 5VDC.

If your transformer is 24VAC center-tapped, then each half is 12VAC which makes a rectified and filtered 16.3VDC. Then a 7812 and 7912 regulators can make regulated positive and negative 12VDC.