Positive and negative voltage with just two 7805 regulator

[RGA]

Hi everybody,

I know that we can make positive voltage with 7805 and negative voltage with 7905.
I want to make positive and negative voltage at the same time with just two 7805 regulators.
can you suggest a circuit for it?

I'm looking forward your answer!
Thank you very much!

 

[BradtheRad]

If you wish for two positive supplies to create a bipolar supply, then you need to start with two isolated supplies. Connect the positive end of one load to the negative of the other load.

The 7805 is represented by a zener & NPN below.

By using a PNP you can make a negative polarity regulator, similar to the positive regulators shown above.

 

[d123]

Hi,

Have you got a copy of "L78 Positive voltage regulator ICs" (DocID2143 Rev 32) by ST Micro? On page 30 there is a circuit you describe, maybe you can adapt it to your needs.

 

[andre_luis]

The main problem on using two positive voltage regulators supplied with a single power source, is that you could not use the same ground reference for both.

 

[RGA]

Dear BradtheRad,d123

It is a challenge that i use just two 7805 regulators. I must not use other components.
so circuits that have not two 7805 regulators are useless for me.

 

[RGA]

Dear andre_teprom

yes, that is true. how can we design this circuit? it is a challenge.

 

[andre_luis]

Unfortunatelly this is not possible to achieve without using a symetrical topology.

 

[c_mitra]

It is a challenge that i use just two 7805 regulators. I must not use other components. ...

Use two isolated supplies and get 0,5 and 0,5 volts using two 7805 regulators.

Connect them in series (make sure that the supplies do not share the common ground) and get 5; (0+5); 5 volts. You will get 5-0-5 V

What exactly is the problem?

 

[betwixt]

Not efficiently but you can....

Use two conventional 7805 regulator circuits. Sit one 'on top' of the others +5V output so the bottom one has 0V on it's ground pin and the other has the 5v output from the bottom regulator as it's ground. You now have a stacked regulator 0V, 5V and 10V, it needs more than about 13V at their joined inputs to work properly. Then take the +5V output as your 0V point and you have one 5V above it from the top regulator and the ground from the bottom regulator as your -5V output.

It isn't efficient for several reasons but for low current it may be acceptable. The main problem is the bottom regulator needs to dissipate more heat because of it's higher voltage drop (>8W per Amp) and load variations on the -5V (bottom) regulator can influence the top one. There can also be issues with thermal shutdown as either regulator overheating will influence the heat dissipated in the other.

Use with caution.... but it is possible.

Brian.

 

[RGA]

Dear betwixt,


I connected regulators next to each other ,it was useful . I got good result.

Thank you very much.