negative supply

[rehanessa]

i have two positive supplies made using lm317.

if i short the positive terminal of one supply with the negative terminal of the other supply and treat this terminal as common ground.

then will i get complementay outputs (ie positive and negative") from remaining two terminals???????

ie have i converted two positive supplies into a dual supply???????

[audioguru]

Hi Rehanessa,
Yes, you will have a dual-polarity supply, but only if the supplies have completely separate transformers or windings. You might be able to do it with a center-tapped transformer.

[rehanessa]

Hi Rehanessa,
Yes, you will have a dual-polarity supply, but only if the supplies have completely separate transformers or windings. You might be able to do it with a center-tapped transformer.

hi
audioguru


i agree with your first sentence that the two supples shall have separate winding (someone else also told me this).

could you tell why it is necessary to have two supplies utililzing two different windings and what would be the consequences if i use a common winding???

but, i dont agree with the last sentence of your reply that a center tapped trans could be used for this purpose, otherwise ic manufacturers would not have been manufacturing negative regulators like lm337.

[audioguru]

Hi Rehanessa,
You can't use a single transformer winding when you put two regulated power supply circuits in series because the winding will never produce a negative voltage with full wave rectifiers. You could do it with half wave rectifiers:
1) use one wire of the transformer as the common where the power supply outputs are joined.
2) Connect a positive-conducting rectifier diode from the active transformer wire to the power input of the positive regulated supply.
3) Connect a negative-conducting rectifier diode from the active transformer wire to the power input of the negative regulated supply.

The regulated power supply circuits are completely isolated from anything. You could ground the positive output of a "regulated positive supply circuit" and use its common as a negative regulated output.

A center-tapped transformer winding will have the center-tap as the common where the power supply outputs are joined.
OOOOps, I see your point, that's why I said "might be able". You cannot connect to the center-tap, the positive output of a regulated positive supply circuit that is being used for the negative regulated supply. You are correct, you need a negative regulator like an LM337. :-[

[rehanessa]

Hi
Audioguru

In the second paragraph, you have said something about

[audioguru]

Hi Rehanessa,
"Completely isolated" means having its own transformer winding and rectifier and nowhere in its circuit is grounded.