# Negative Power supply

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#### gaboor

##### Member level 1
Hi,

This is laways confusing me, how can you design a negative power suplly?! i mean what is the difference between - 5V power supply and +5v power supply with revrse polarity?!

I am sorry if it is a stupid question!!

#### Siniša

##### Full Member level 6
There is not much difference if that is only power supply used. If you have ±5V, try building it with only positive regulators and you will see difference. You can't just stack positive regulators on top of each other as regulator is only sourcing current and does not sink, so your 0 won't be 0 any more.

#### senthilnathan.rajesh

##### Junior Member level 3
Actually a positive supply can be treated as a negative supply if the supply terminals are interchanged. i.e "+ Terminal" of the positve supply can be treated as "GND Terminal" of Negative Supply. And the "GND Terminal of the positive supply can be treated as "-ve terminal" of the Negative power supply.

This how you can convert a positive power supply in to a negative (direct method).

But in the case of Dual Power suplies the above explanation won't hold good.

Example of constructing a Dual Power Supply from 2 individual positive power supplies as below (Basic - its very simple and self explanatory)

Image format see the link below

https://images.elektroda.net/82_1168361510.JPG

I think you could have understood.

#### sivakumar_tumma

##### Full Member level 4
both postive and negative power supplies are same. Genarally same current will flow in both the terminals of the supply. we can use one as refernce. when positive terminal is used as refernce it is nagative power supply or else it is positve power supply..........

#### wisepie

##### Member level 4
This is depends on what is the reference voltage.

#### ryusgnal

+5V power supply is when you connect the +ve terminal to the VCC node and -ve terminal to ground. And -5V is when you connect +ve terminal to ground and -ve terminal to VCC. As shown in the picture

#### kender

gaboor said:
i mean what is the difference between - 5V power supply and +5v power supply with revrse polarity?!
There's no difference, if both supplies are floating with respecto to each-other. In other words, they are not referenced to the same ground. Or in other words (yet again) the impedance between the 2 grund of 2 power supplies is high.

#### babinton

##### Member level 4
HI, why not simply use an Ic voltage regulator. What you need to do is just ad an L7900 IC to the ouput of your bridge rectifier circuit. The particular IC ypu will use will depends on your desired output negative voltage. For example, if you need a
-5dc supply, use L7905. You can check the manufaturer datasheet on how to build the regulator.
regards.
babs

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