How To: DC +/- Power Supply?

Opamps usualy run off +15 and -15. You need 2 power supplies... both +15..... to hook one up as -15, hook the + to the gnd of the other one. Or just buy a

+-15V supply.... check jameco.....

Hi, Jeremy. Good newbie question.

You're stuck with 15 volts potential between the two wires coming out of your wall wart, and a +/-15V supply has 30V potential between the + and -.

One possible solution is a "rail splitter", which sets an artificial common halfway between the two. As long as you didn't require very much current, you might try this with two low ohm resistors splitting the difference like this (view in fixed font or M$ Notepad):

+o---o-------o +7.5V | .-. | |22 ohms 3 Watt | | '-' 15V | o-------o"COM" | .-. | |22 ohms 3 Watt | | '-' | -o---o-------o -7.5V created by Andy´s ASCII-Circuit v1.24.140803 Beta

Of course this wastes a lot of power just to get a low output impedance for the artificial GND. A better solution would be something like this:

+o---o--------------o----------o+7.5V | | .-. .----|----. | |10K | | | | | | || | '-' '--|- | 15V | | >---o-----oCOM o------------|+/ | |/| .-. | | |10K | | | | '-' | | | -o---o--------------o----------o-7.5V created by Andy´s ASCII-Circuit v1.24.140803 Beta

The op amp creates a low impedance for the virtual GND at the expense of 1 IC and a couple of resistors.

These are the two basic ways to do a "rail splitter". But neither will get you where you want to go, which is a +/- 15V supply. In order to do that, you can do a bit of a trick with an oscillator and a few caps and diodes like this:

+15V +15V +15V | + + .-. | | | |24K .---o----o---. C = 100uF 25V | | | 8 4 | D = 1N4002 '-' | | C | | | o-----o7 | +|| D -14V | | 3o---||--o----|

If you open the wall-wart you will probably find a transformer, a rectifier, and a capacitor, which produces the 15 Volt DC output.

You can use the transformer to get plus minus 15 Volt with a suitable rectifier stage and two capacitors.

If you add regulators you can get plus minus 12 Volt, which is good for most op-amps and other projects.

Scroll down to 70 percent of the web page. Look at "Doubler - Version 2". That is a way to get plus minus outputs from a single winding.

Building your own power supply is a good first project in electronics, and you will have use for it for many years.

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Roger J.

look in any commercial power supply catalogue and you will see they are classified as single, dual or even triple voltage output. what you want is a dual voltage supply.

A really cheap and simple way to get a dual supply: Get two 15 Volt wall-warts, connect the minus output of one to the plus output of the other and call that connection Ground.

Now you have a dual power supply, which has three connections, Ground, plus 15 and minus 15 Volt.

This dual supply is now unregulated.

Add regulators to get a regulated dual power supply. The regulators need a few Volt to work, so you can now have +-12 Volt, if you used 12 Volt regulators.

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Roger J.

If the two wallwarts are differently sized, if one is a 1 Amp and the other is a 300 mAmp, for example, put the heavier one as the positive supply, because you will need 0-15 Volt for some parts of your circuits, and +- 15 (12) Volt for other parts of your circuits.

You can use the unregulated 15 Volt for some purposes, where power is more important than smothness, and the regulated +-12Volt for other parts of your circuits, like powering op-amps.

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Roger J.