Negativ voltage from Positive

[Kryten]

Hi
I havent found any thing on the forums about this
Anyways

I need to create +- 5Volt and have 12 volts as supply.
I know there is a way to do this (not that i can recall it ) by using some caps and res and diodes.
Does anyone have a link or a schematic (whit formulas if any, on how to calculate the values)

 

[Nigel Goodwin]

You need to post exact details of what you're trying to do, a generic solution may not be the easiest solution for your requirements.

 

[Leftyretro]

Try https://tangentsoft.net/elec/vgrounds.html

Lefty

 

[Kryten]

You need to post exact details of what you're trying to do, a generic solution may not be the easiest solution for your requirements.

Well i need +-5v to an opamp (yes its analog) it will bee used in a car so ~12volt as supply. I dont need much current

 

[Nigel Goodwin]

That's hardly 'exact'? - but the normal method is two resistors and a capacitor to form a 'virtual earth'. However, that depends on the exact configuration.

 

[picbits]

Use a MAX202 and tap off the negative supply from a cap if you don't need much current.

 

[Kryten]

Im going to mak a litteli line driver for my reversing camera as the picture is bad on ca 5m cable and not on 2m cable (witch isent enought).
Exact enought?

 

[Nigel Goodwin]

I take it you are aware of the frequency and power output requirements for a video amplifier?, you can't just use any old opamp.

It makes it more difficult because the 'virtual earth' method requires capacitor coupling, and to drive video into 75 ohms requires a large value coupling capacitor.

Are you sure the cable isn't faulty?, or the wrong type?, 5m shouldn't be a problem?.

Funnily enough we've just fitted a reversing camera on a van at work, but it's wireless - really cool thing, and only cost about £50 for camera and monitor.

 

[Kryten]

Yeh i know its cool...
Well im using a TI opa2690 for it (it should do the trick)
No the cable isent faulty. I have tried several and it seems that the output from my (crappy) camera is at the (bottom)edge of what it should be
I was thinking about using it as a single suppy but i dont know wheter the cvbs signal svings around 0v or if i can lift it to 2,5 volts and have a 1volt sving.

But the OPamp is for a "power switch" (i have bustet a couple of fuses using everything on revers light), make it as a impedance sencer of sort

 

[Nigel Goodwin]

Yeh i know its cool...
Well im using a TI opa2690 for it (it should do the trick)
No the cable isent faulty. I have tried several and it seems that the output from my (crappy) camera is at the (bottom)edge of what it should be

For a 'quick and dirty' solution, remove the 75 ohm load resistor in the monitor! - that will probably cure your problem!.

 

[Kryten]

Souds really dirty. But i will try anything

 

[Nigel Goodwin]

Souds really dirty. But i will try anything

The load resistor cause a 50% drop in the signal, so removing it effectively gives you a gain of two!. Generally doing so has no ill effects at all, and I have an old B&W monitor where it's switchable.

 

[Gordz]

How about a one transistor 'booster' at the camera end if taking the 75 ohm resistor out of the monitors input does not work ?

 

[Nigel Goodwin]

How about a one transistor 'booster' at the camera end if taking the 75 ohm resistor out of the monitors input does not work ?

One transistor will invert the video!

You need two transistors.

 

[Gordz]

Doh !!

*note* Engage brain BEFORE keyboard....

How about a TWO transistor 'booster' at the camera end if taking the 75 ohm resistor out of the monitors input does not work ?

 

[Krumlink]

If all you want is a + and -5V, just grab yourself a 7805 and a 7905. Real easy.

 

[audioguru]

A 7905 needs a negative input voltage. There isn't a negative input voltage.
That is the problem.

 

[Gordz]

Look up the ICL7660, it is easy to use and should meet your needs.