what's the difference between -12V and GND?


first, i thought that if some electronics said -X V, it was just GND, but then i saw both in the same circuit,  so what's the difference?
and how can you use -12V

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Re-design7 years ago
In some circuits negative is the same as ground, but in some circuits ground = 0 volts and in other parts of the circuit there is a negative voltage that is below ground. If you connect a voltmeter between ground (0 volts) and -12 volts it will read a difference of 12 volts, with the ground being most positive. Electrical parts work because of a difference in potiential. Many don't care which way the difference is (light bulb) and some work in reverse (dc motor) and some won't work at all or are destroyed. IT's a hard concept to explain and goes much more into detail than what I have just explained but google "negative voltage" if you wish to learn more.
godofal (author)  Re-design7 years ago
so, if i take -12V and GND (0V) and hook it up to a LED for example, i should consider the -12V as minus (cathode) and GND as + (anode) or wont this work? and is that also where the negative voltage regulators are for? (those L78XX (if i remember correctly) series, but then in negative, i've seen them somewhere) that will change -12V to -5V? i presumed that they would just go after the circuit, like with the LED, instead of going to the anode, to the cathode. but after writing and re-reading it, it seems a bit strange i presumed this, becouse it will still change the positive voltage, like a resistor, and if you connect LED's in series, and all that... prove my own point wrong, yay for me XD
Voltages are always measured and named relative to other voltages. Any one of them can arbitrarily be called 0V. Simple example that may help clarify: Think of two 1.5V batteries in series. If you measure across the two of them, that's a total voltage difference of 3V. If you pull a wire out from the point between them, that's at 1.5V on the same scale. But you can also call that middle wire 0V, in which case one battery is supplying +1.5V and the other is supplying -1.5V. So: you can look at the three voltages you named earlier as either (0, +12V, +24V) or (-12V, 0, +12V), or (-24V, -12V, 0) or even (-19V, -7V, +5V). Which set of labels you choose depends entirely on what makes sense for the circuitry you're connecting them to.
godofal (author)  orksecurity7 years ago
ah, i get it, thanks!
framistan7 years ago
This is easy to understand if you think of your car battery. You know the metal body of the car (the chassis) is negative. (on most cars not some foreign ones). You really only need to run ONE wire to anything you want to power with 12 volts. You must run a POSITIVE wire from the battery and that is called PLUS 12 volts. Now PRETEND that we are going to put ANOTHER battery in the trunk of your car. This battery will be connected with the POSITIVE terminal to the cars chassis. Now you can connect anything to this battery by running ONE wire to any item you want to power with 12 volts... but the wire will come from the NEGATIVE side of this new battery. This will be your MINUS 12 volts wire. So... you see the chassis can be NEGATIVE ground with respect to(connected to).. the 1st battery.... or it can be POSITIVE ground with respect (connected to) the 2nd battery. It is exactly the same inside of computers or televisions or other circuits. the CHASSIS of the equipment is GROUND and you can measure NEGATIVE or POSITIVE voltages at various points in the circuit.
rmikel framistan2 months ago

if I connect a multimeter to the positive terminal of and chasis of the car will I get 12v?

frollard7 years ago
Short answer: + and - show you which way the electrons flow. Long answer: Ground is where all electrons want to go to - a throwback from the old nomenclature where it literally is the endpoint of a circuit. These days its used as a safety circuit in household wiring -- all the water pipes in the house and every recepticle should be 'grounded' to zero volts. Electrons (negatively charged) flow from negative to positive. Ground is arbitrary, but considered to be in the middle at 0 volts. Ground does not carry a charge, but it can be relative to any other voltage, positive or negative. Volts are a measure of potential energy, just like gravitational potential is generally measured by distance (height). The more something wants to go from A to B the more energy it has. By this analogy, higher voltage is higher height. Furthermore, let's say that ground is the same as ground level. Negative charges (electrons) try to get to positively charged areas. Consider negative volts the same as mountains, and positive volts as canyons. An object will fall off a mountain and land on the ground. An object can also fall from the ground into a canyon. Same thing here: Power can flow from -Negative voltage to ground, and the ground can also give up electrons to a + charged object. Example: North American cars (and most asian cars afaik) are Negative ground. That means that of the 2 connections in the circuit, everything metal in the car is connected to the negative terminal on the battery and is considered zero 0 volts. Any wires in the car are then considered to have +12 volts to show a total difference of 12 volts. Electrical circuits are just that - a circuit. They require a full connection from a voltage source through conductors to a load then back to the other end of the source. Hypothetically by hooking another battery's positive terminal to the car's frame, you could run a positive ground with negative wires at the same time, so long as the wires never crossed. Between the +12 and -12 volt wires you would have a voltage of 24V (the difference between the mountain and the canyon).
godofal (author)  frollard7 years ago
HUGE WALL OF TEXT HURTS MY EYES! but seriously, i dont understand half of what your saying, and the other half, i already knew XD
Well we don't know what half you do know so he's got to say it all.

Take the stuff you don't know and learn what it means.

Sometimes it takes a wall of text to learn a concept.  And if you think this is a wall of text then you have a long road ahead of you.  I have a book over 400 pages just on ac electricity.  No circuits or diagrams or components just ac electricity.
godofal (author)  Re-design7 years ago
yeah, i know :D, it was just late yesterday, and i wasnt in the mood to go figure it all out. btw, it wasnt ment negatively, now i read my post again i understand that it might look that way, but it wasnt ment like that.
That's good. I didn't really think you meant it negatively.

I was just trying to point out the way this works. And, even though you were the one asking the question, it's best to answer the question giving an overview of it so that when someone searches the same subject it is answered in a way that they will understand even if they only know 1/4 of it.
Huge wall of text hurts all of our eyes. It's a bug in the new "plain text" editor that was rolled out earlier this week. Hopefully will be fixed soon....