# -ve voltage from +ve souce

Discussion in 'General Electronics Chat' started by vinodquilon, Jan 11, 2012.

1. ### vinodquilon Thread Starter Member

Dec 24, 2009
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Can I get -15V by reversing the polarity of +15V single power supply, as by connecting in reverse manner, MM reads -15V ?

2. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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You can get -15V by swapping leads, but NOT IN THE SAME CIRCUIT. The way to get -15 and +15 is to connect two 15V batteries in series, use the negative of the lower battery as -15V, the center as ground and the + of the upper 15V battery as +15V

If you need both +15V and -15V from AC, usually a center tap transformer is used, with center tap as ground.

If you need -15VDC from +15VDC (±15VDC at the same time), you can get a DC-DC converter, which is a switching power supply. There are many pre-built supplies out there.

How much current do you need at +15V and -15V?

vinodquilon likes this.

Dec 24, 2009
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2A current

4. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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Do you currently have a +15VDC 5A supply?

Or, more generally, what supplies do you have, along with rated currents?

5. ### vinodquilon Thread Starter Member

Dec 24, 2009
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I have two +15VDC 5A supplies.

6. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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See if they are isolated, e.g. if the negative sides are common to each other when plugged in and on (use a voltmeter first).

If they ARE isolated outputs, you could run them in series, but I'm thinking they not be.

Also check for continuity between metal case and negative terminal.

Otherwise, one would need to go through an inverting SMPS to get -15V out, which could get spendy.

How adept are you with electronics? Soldering, measuring, etc.?

7. ### mcgyvr AAC Fanatic!

Oct 15, 2009
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before thatoneguy goes into something you don't need...
If all you need is to power a single isolated device that requires -15VDC and you only have a +15VDC supply you can simply switch the leads..