Ground or Negative?

BillyGalbreath · 04-30-2008, 10:41 PM

I'm a bit confused.

I've always learned that electricity moves from a positive source to a negative (like positive/negative terminals on a car battery). However, with all these diagrams I keep seeing and studying, barely anything is ever wired to a negative post, or even back to the negative terminal on the battery. Everything goes to Ground. I am a bit at a loss of words for this. What confuses me even more is that I've always learned that Ground is a 3rd wire (like the 3 prong plugs in your home [+/-/G]) and is not treated as a negative terminal.

Now, with all that in mind, when I build my diagrams for my own project I keep wanting to draw negative wires going to negative terminals, all which lead back to the negative terminal on the battery. I have yet to see a single diagram from anywhere else that does this, and I'm starting to wonder why...

Sure, the negative terminal on the battery is bolted to the car's frame. And in a round about way, everything else that's grounded to the frame is technically wired back to the battery... But wouldn't this create a charge throughout the entire car's frame, because ultimately anything on the negative flow had to come from the positive flow, right? Seems very odd to me!

Could someone shine some light on this dark subject for me! Thanks!

h3rk · 04-30-2008, 11:02 PM

You're thinking right in a lot of ways. That round about way that things are connected, through the body of the vehicle is precisely the way that the electrical circuits were designed. There is very little resistance in the return path to the battery through the car. Crank your tunes and somewhere in the sheets of metal that make up your car, is an invisible snake of electrons flowing from your battery's negative post on it's way through the amp, speakers, cables, fuses, and into the positive teminal of the battery (we talk about current in the other direction because it's easier to imagine...ok...sorry about the tangent). AC uses 3 wires beacuse all current flows back and forth between neutral and 120 (or whatever). Ground there, is earth, and it is a bad thing if one of the wires is touching ground (as is your car, but with less obvious results much of the time), it's a saftey issue, ground is not used as a common there. EDIT: So say, if you had a drill and it's motor blew up inside, and somehow an AC wire was touching the case, when you picked up the drill, your body would be shorting that voltage to ground, and you would feel it. The ground wire, along with gound-fault detecting and protective devices are designed to both; ensure you aren't the most attractive path to ground, and hopefully they de-energize the circuit.

In that sense your car is not as safe as a residence, but in another since 12V is 'low voltage' and normally considered non-lethal. The free electrons in your cars body want to get to positive, so there is no worries when stepping into or out of your car, since earth is not connected to the positive terminal, and you don't complete the circuit. Current needs a complete path to flow.

warnockm · 04-30-2008, 11:18 PM

technically, electrons move from negative to positive

numbers · 05-03-2008, 10:17 AM

And don't blame the folks who taught you the positive to negative flow. Sometimes you do have to think of it that way. In fact, at any given time, you may need to be using one or more of three different ways of visualizing electricity depending on what it is you're thinking about. Negative to positive flow for power consumption and most other situations; positive to negative because of the funky way transistors and diodes are annotated; and mere voltage levels when working with digital logic and waveforms. Don't let it give you a headache. Just step back and realize - 99% of the time, it doesn't matter how it works as long as it just works.

parksgm · 05-03-2008, 01:22 PM

Quote: Originally Posted by BillyGalbreath

Now, with all that in mind, when I build my diagrams for my own project I keep wanting to draw negative wires going to negative terminals, all which lead back to the negative terminal on the battery. I have yet to see a single diagram from anywhere else that does this, and I'm starting to wonder why...

Cars ARE wired in this manner because it halves the amount of wire needed to wire the electrical accessories of the car. In a car with numerous electrical accessories, doubling the amount of wire used would have a significant impact on the price of the car, as well as create difficulty routing all of those wires in constrained areas (behind the dash, through the firewall, etc.)

Cars CAN BE wired in this manner because they use a low voltage electrical system. 12 volts is not enough electrical potential to shock you because 12 volts cannot overcome the resistance of your skin, and so electrons can't flow (in most cases...don't ever lean on the terminals of a car battery with a sweaty arm duing the summer...the salty sweat significantly lowers the resistance of your skin, and in that case, 12 volts is definitely enough to shock you). For this reason, it's ok to use the chassis as a ground/negative, even though it's essentially a large, exposed conductor, because your skin provides enough resistance to prevent your body from shorting the circuit when you touch it.

However, 12 volts is more than enough to cause current to flow through metal wire, which has much lower resistance.

BillyGalbreath · 05-04-2008, 01:01 AM

I see.

So, technically, it doesn't really matter if I wire to ground or back to the negative terminal on the battery then... I'll use the ground, as that seems best and easiest.

Thanks guys! Ya'll been a real help!

warnockm · 05-04-2008, 06:55 AM

Quote: Originally Posted by BillyGalbreath

I see.

So, technically, it doesn't really matter if I wire to ground or back to the negative terminal on the battery then... I'll use the ground, as that seems best and easiest.

Thanks guys! Ya'll been a real help!

correct! good luck. Remember to make sure the ground wire from your device to the chassis is as thick as your power wire from the battery.