Understanding the negative voltage supply

[catfish]

Hi,

I have a brief question for those who not only built these detectors but also understand how they works.

I've been trying to understand how the negative voltage is created. Maybe I don't see it but from my point of view it just makes no sense.

I can't post attachments so I'll upload them instead. Here are the links:

http://postimg.org/image/nrnopkwvv/
http://postimg.org/image/dvmlpxr3v/

start with the first image.

From the battery, the 9..12V goes to the regulator
The regulated 8V goes then to the 358. Which has its negative input connected to the output and (Ground ?!?).
The battery's minus (ground or P1 in the schematic) is in fact the ground (0V) of the system. And the output of the 358 should be the negative voltage. Ok.. wrong used symbols. But that's ok... It still makes no sense.

Connected like this the 358 outputs (+!!) 4V which makes perfect sense. The voltage divider on the positive input brings down the 8V to 4V.

So .. if anyone understands the point in this, and could tell me where are those -4.. -6V coming from, would be nice.

the second image is the entire schematic of the IDX pro. On the TGSL schematic it's the same thing



Oh and by the way. This is my first post here.
I find it a very nice forum.


catfish

[Silver Dollar]

Opamps generally need a plus and a minus supply. You can bias one like U6 in your example
by using equal resistors to the plus and minus of the supply. It's output is then at 1/2 the main
supply. If you then label this as ground then the minus of the main supply becomes negative.

So the main supply is effectively split in half using a virtual ground.

[kt315]

Ok.. wrong used symbols.

nothing wrong. GND is only a symbol. if a reference point of the schematic is GND, output of the opamp, you get +4 and -4 comparatively (sic) to the level.

[catfish]

oh man,

how could I miss that ? of course ... now it makes perfect sense. It's a basic circuit

And that ground symbol (the 358's output) is in fact the ground of the system and not a false used symbol.

I guess I was misguided by all the negative supplies in the schematic, and I thought whoever drew the schematic screwed the symbols up.

Thanks for opening my eyes on this.
catfish