# Producing negative supply rails - Urgent

Thread Starter

#### strathy

Joined Mar 26, 2008
10
Hi,

This circuit was recently suggested in another thread:

"Try this, If you have room for 3 more components you could use 2 5 volt zener diodes and a resistor to limit the current. The resistor and the diodes are connected in series so that both diode will drop 5 volts, the GND is connected to the center point of the 2 diodes then you will get +- 5 volts at the other ends of the diodes. The 12v volts used is very marginal for running the 2 zeners but it may work, 15v would be much better also the resistor will determine how much current is available to the circuit and how much power it will draw from the power supply. You probably will not need more than 20ma for the circuit. If you are using 12v try a 68 ohm resistor to start."

What I'm trying to do is create a bi-polar supply from a + 12 V dc supply. This circuit was suggested, but I'm not sure if it will work or not. (I can't get it to work on a board anyway)

I think the original poster thought I was using a 12 V AC supply, but I'm not it is 12 Vdc.

Does anyone know what circuit he's referring to?

#### SgtWookie

Joined Jul 17, 2007
22,210
Well, if you really want to run a dual-supply type op amp circuit on a single supply, you'll need to use a capacitor in series with the input signal (thus blocking any DC or very low frequency component) and then use a pair of resistors from Vcc to ground to bias the input halfway between Vcc and ground. You'll need a similar divider for the other input to the opamp. One of the dividers will need to be adjustable.

The output signal may or may not need a DC blocking cap, depending on what you're doing.

#### techroomt

Joined May 19, 2004
198
the zener suggestion sounds reasonable. it essentially will take the original 12vdc supply, and create a new dc supply with a +5vdc, -5vdc, and a new dc ground (that is at a different potentail then the original 12 vdc ground) for your bipolar amp power requirements. therefore, the 2 grounds should not be connected. and the input and output signals will need the caps to block the (6v) dc offset created by the new supply (as sgt wookie described).

#### SgtWookie

Joined Jul 17, 2007
22,210
OK, visited your other thread.

I'm not certain exactly what you want to do with your output. However, I threw together the circuit you linked to with a single-supply version. Note that the output of the single supply version has an output that is offset from ground by 6v, due to the inputs' offset.

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#### Pich

Joined Mar 11, 2008
119
Wookie as you can see I'm new on this forum, for the life of me I cannot find a way to post a circuit do demostrate show trathy. Can you help?

#### SgtWookie

Joined Jul 17, 2007
22,210
Wookie as you can see I'm new on this forum, for the life of me I cannot find a way to post a circuit do demostrate show trathy. Can you help?
Here's what I do (I'm running Windows XP pro)
1) Make your circuit and run your emulation that shows output.
2) Press Ctrl + PrtScrn to copy an image of the screen to the Clipboard
3) Start Windows Paint
4) Press Ctrl+V (or select Edit/Paste)
5) Save the image file in .PNG or .JPG format
6) On the forum, start a reply in the Advanced mode.
7) Near the bottom, you'll see an option to attach files, click the button (you must have popups enabled for this site)
8) Navigate to where you've saved your file, and click on it.
9) Click the "Upload" button.
10) Close the manage attached files dialog.
11) Post the reply.

#### Pich

Joined Mar 11, 2008
119
Thanks will try it. hope I didn't hijack the tread.

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