# Nature of batteries and wall-warts, negative voltage?

Discussion in 'General Electronics Chat' started by magnet18, Feb 9, 2011.

1. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
124
I was doing some thinking today in math class about my project of creating a plasma arc using an ignition coil, and I figured if I had two, one operating on the negative side of ground and one on the positive side, I could double the voltage difference.

My question is, what are the voltages of the terminals pf a battery with respect to ground (like the screw on an outlet plate)?

For example, a car battery, is there a -6 volt charge on the negative terminal, and a +6 volt charge on the positive terminal, or is there a +12 volt charge on the positive terminal and no voltage difference between the negative terminal and ground?

I was wondering the same thing about wall-warts.

And ultimately, if a circuit had +12V and ground, would it work the same if the +12V was replaced with ground and the ground with -12V?

(I'll post a schematic sometime soon, until then its a 555, a PNP, an NPN, and an N-channel MOSFET)

This is probably a fairly simple question, Ive just been seeing - and ground used interchangeably and was slightly confused.

Thanks

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
Without a definite connection to ground the battery terminals are floating. They can't be measured with respect to ground. All voltages are relative, but some common connection must exist before you can measure them.

BTW, that arc is not very dangerous, but how does the school feel about it? You will, of course, enclose the arc so a second-grader can't poke it with a finger or a pencil?

magnet18 likes this.
3. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
124
Heres the sketch I made in math, (girl next to me actually taking notes gave me a funny look )

So you're saying that if i was to connect the positive terminal of a car battery directly to ground (ground prong or screw on outlet) it wouldn't just start discharging but it would give me a steady -12V power source?

And I thought voltage could be determined by the electron density, giving it an absolute value relative to ground.

Its actually a fallback plan for a science fair project, actual project being to build a plasma speaker. Its for a science class full of seniors... and me

When I got up and presented my idea the teacher didn't shoot it down, so i guess its OK (i love it when i impress the captain of the football team), Ill just be bringing it in and displaying it during a presentation, so the only safety risk is me

File size:
84.9 KB
Views:
32
4. ### spinnaker AAC Fanatic!

Oct 29, 2009
4,874
997
You should be paying attention in math class and not day dreaming young man!

5. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
124
It's not daydreaming if its practical, daydreaming would be planning on building a Tesla coil the size of a skyscraper and fending off aliens with it.

Besides, its not like I'm ever going to need to be able to find the solutions of a trig function of half an angle anyway
(seriously though, will I?)

Last edited: Feb 10, 2011
6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
No, that is not the case. Without there being a connection in place between the negative terminal of that battery and the ground terminal, the positive terminal has no reference. No voltage can be measured, and no circuit can be made to work. The path for current has to exist.

7. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
In schematics and circuits, ground is basically the name of a node that is used as the reference point for all other voltages in the circuit. It is sometimes connected to earth, but usually is not. Think of portable devices such as cell phones, laptop computers, Ipods, Ipads, etc. If you looked at the schematic of each one, you would find a node designated as ground. It is not connected to Earth ground, or to the ground of any other device.

8. ### Audioguru New Member

Dec 20, 2007
9,411
896
Magnet18, the "ground" of a circuit is simply a 0V reference voltage.
You talk about ground like it is connected to earth like mains electricity.

The electical sytem in a car or in a portable radio is not connected to earth but their 0V is called "ground".

Usually the negative terminal of a battery or of a power supply is 0V and is called ground then it has a positive power voltage but some circuits use a negative power voltage.

I think a plasma speaker is just a low loudness very distorted whistle.

9. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
124
I know, thats because I'm wondering what happens when mains ground is used, because I'm considering using mains ground.

10. ### Audioguru New Member

Dec 20, 2007
9,411
896
I don't think I have ever used earth ground as a ground in any of my thousands of projects. None of my battery powered circuits have anything to do with earth ground and the power transformer in my mains powered projects isolate them from earth ground.
I have never made an old fashioned AM crystal radio that needs an earth ground.

11. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
124
Alright, i wont worry about it then, but for the sake of discussion and so i can learn something here, would this circuit work the way in thinking, one side creating a high positive voltage and the other a high negative voltage?
(i replaced my actual circuitry with a switch)

File size:
9.6 KB
Views:
48
12. ### Audioguru New Member

Dec 20, 2007
9,411
896
No.
Because there is nothing to turn off the Mosfets. A high voltage is created at the output of the transformer when the mosfets turn off. To turn off, the Mosfets need an active circuit with transistors discharging the high capacitance of the gate, or a low value resistor.

13. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
124
I know, i built the top half side, video is avaliable here http://www.youtube.com/watch?v=HBMvNkakibw

I apologize if i wasn't clear, what i meant was that for the simplicity of the schematic i replaced the 555 and transistors with the switch symbol

What im asking is if this would work with regards to the positive and negative power aspects

14. ### Audioguru New Member

Dec 20, 2007
9,411
896
Cars make arcs without the negative half of your circuit.

15. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
And you don't need earth ground.

16. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
124
Thanks for the replies, but I know I don't NEED earth ground, and I know I don't NEED the bottom half of the circuit, I'm asking what would happen IF I did this, for some obscure reason.
would I get a -12V charge??

17. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
What do you mean by "a -12V charge"?

18. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
124
Earth ground is zero, right? so would the negative terminal of the second car battery be -12 volts relative to ground?

19. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
You mention in your videos' comments that it doesn't work reliably unless you're running something across the timer's Vcc wire (or something similar) - and now you want to make it more complicated (more than TWICE as complicated), even though you don't understand the original circuit?

Why?

Last edited: Feb 10, 2011
20. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Yes, it would.

magnet18 likes this.