Author Topic: What is the ground in battery powered devices?  (Read 12811 times)

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Offline Mint.

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What is the ground in battery powered devices?
« on: December 18, 2011, 01:10:01 am »
What is the ground in battery powered devices? Since they are not always connected to ground. Is it just the negative terminal of the battery?
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Offline Psi

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Re: What is the ground in battery powered devices?
« Reply #1 on: December 18, 2011, 01:16:04 am »
If you're making a battery powered device you get to decide which is ground, + or -

For example, 99% of cars are ground negative, but there have been some cars where the car metal is connected to positive.

But yeah, in 99.9% of cases you would make the ground connection negative.
You would only ground positive if you wanted a negative voltage supply.

I seem to remember cars with a positive ground rusting faster due to electrolytic corrosion

« Last Edit: December 18, 2011, 01:18:12 am by Psi »
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Offline 8086

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Re: What is the ground in battery powered devices?
« Reply #2 on: December 18, 2011, 01:17:07 am »
Yes, generally it is the negative terminal of the battery. Though it isn't a "ground", it's 0V. With a battery you have the option of doing whatever you like really. You choose where "ground" is. You can connect "ground" as the positive terminal and get a negative potential on the other side of the battery.
 

Offline amspire

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Re: What is the ground in battery powered devices?
« Reply #3 on: December 18, 2011, 01:22:25 am »
It is often the 0V, but ground is usually assigned to the most convenient common connection in the circuit.  If a battery circuit has an analog 0V that is half the battery voltage, then that may be the best connection to called ground.

Or you can have separate digital ground and analog ground. It is not like the "ground" has any magical properties - if is just convenient to have a set reference point for the rest of the circuit.  So if you say that Q1 base has a DC bias voltage of 2.5V and the collector is 4.1V, you know that these voltages will be relative to the assigned ground.

Also for many circuits, but not all, the signal that propagates through the circuit uses the ground as its 0V (for analog) or False/Low for digital. So having an assigned ground can just make is easier to understand a circuit.

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Offline ivan747

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Re: What is the ground in battery powered devices?
« Reply #4 on: December 18, 2011, 01:22:42 am »
Yes, on a battery pack, the negative side is the circuit ground. If you are using a split power supply like this, you normally call the center ground:




If you connect the circuit to earth's ground, then the point where you connected the earth should be ground in most cases. Let's say you connected earth ground between the battery cells in the circuit above. That would be ground. When earth is disconnected, the circuit is floating and you can call anything ground, but the convention is to call ground the common point in the circuit where most stuff goes to.

The circuit ground is just a reference. It is used to simplify circuit equations and sometimes it is also used to indirectly indicate where earth ground should be if it is ever connected (for example, the Arduino's negative part of the DC jack is ground because it is also connected to the USB cable gorund, which is connected to the earth).
« Last Edit: December 18, 2011, 01:24:20 am by ivan747 »
 

Offline Mint.

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Re: What is the ground in battery powered devices?
« Reply #5 on: December 18, 2011, 01:41:08 am »
Can't you just use an insulator instead of the battery terminal, maybe something like a piece of plastic or wood?
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Offline 8086

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Re: What is the ground in battery powered devices?
« Reply #6 on: December 18, 2011, 01:52:55 am »
Can't you just use an insulator instead of the battery terminal, maybe something like a piece of plastic or wood?

What do you mean? Having an insulator as a ground wouldn't do anything.
 

Offline bfritz

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Re: What is the ground in battery powered devices?
« Reply #7 on: December 18, 2011, 02:04:12 am »
Can't you just use an insulator instead of the battery terminal, maybe something like a piece of plastic or wood?

Think about this as a circuit.  Ground is just a concept.  What is ground?  Is it the same in the space shuttle while orbiting the earth, as it is in your computer?  Probably not, there is likely a voltage potential between those two grounds, so they are not the same.

Sometimes it is easier to think of abstract concepts when we choose something more visible.  I am sitting in a chair at my desk.  How fast am I moving?  My chair is moving at 0mph with respect to the earth's surface, but the earth is moving around the sun at about 66,600mph!  So, how fast am I moving?  Until we set a referance, you really can't answer.

So, the circuit is much the same.  For any power source to work, a circuit must be formed.  You can draw the circuit showing all the nodes normally connected to ground as a wire, and the circuit will work just fine.  The ground symbol is simply a convenience for us to label those nodes, in a way that shows what our reference is.

As far as connecting the bottom of the battery terminal to plastic or wood, those are both good insulators.  What happens if on the positive side of the battery you wind a wire around one end of a wood stick, and the other end of the wood stick attached to a wire that goes to a light bulb, and the other end of the bulb to the negative terminal of the battery?  The bulb doesn't light, as the wood doesn't conduct electricity, so no electrons flow.  The wood acts like an open switch.  If you put the open switch on the bottom side of the battery, do you expect a different result?  Think, how will the electrons flow to complete the circuit?
 

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Re: What is the ground in battery powered devices?
« Reply #8 on: December 18, 2011, 02:06:03 am »
Current flows through loops, so you need a closed loop (eg. a combination of batteries, resistors and LEDs, not an insulator like wood) for current to flow. Voltage is measured between two nodes (eg. battery terminals). It's somewhat inconvenient, however, to state that the voltage between the negative terminal of the battery and the output pin of an IC is 5V, so it's common to label one node (eg. the negative battery terminal) as ground, and measure all voltages relative to this node. This allows you to simply state that the voltage on the output pin is 5V.
 

Offline Mint.

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Re: What is the ground in battery powered devices?
« Reply #9 on: December 18, 2011, 02:07:20 am »
Yeah but isn't the earth an insulator too, so it has 0v. So wood and anything non conductive will have 0v too?
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Offline 8086

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Re: What is the ground in battery powered devices?
« Reply #10 on: December 18, 2011, 02:13:53 am »
Yeah but isn't the earth an insulator too, so it has 0v. So wood and anything non conductive will have 0v too?

You can't think of things as "having voltage"!

Voltage is the DIFFERENCE in potential between two points. Insulators do nothing to a circuit, so connecting any part of your circuit to an insulator will do nothing at all!

You think of earth as being an insulator, but in fact it is pretty conductive. It's condutive enough that if you touch the live mains, and you don't touch anything else, the current will flow through you, and through the ground, to complete the circuit, and electrocute you. (DONT TRY THIS)

Side note: The reason that electrical items connected to the mains have an earth connection is so that if the live connection inside the device touches any exposed metal (that a person may be touching), the current is diverted straight to ground, as both you and the grounded metal have the same potential.
 

Offline IanB

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Re: What is the ground in battery powered devices?
« Reply #11 on: December 18, 2011, 03:08:20 am »
Can't you just use an insulator instead of the battery terminal, maybe something like a piece of plastic or wood?

Start at the beginning.

Get a battery. Get a torch bulb. Get some wires. Learn how to make the bulb light up by forming a circuit. Now try to put other things in the circuit instead of a wire, such as wood, plastic, glass, coins, wax, other things. Find out which things allow the bulb to light up and which ones don't.

Proceed from there.
I'm a ChemE--I know all about the flow of fluids.
 

Offline Mint.

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Re: What is the ground in battery powered devices?
« Reply #12 on: December 18, 2011, 07:11:14 am »
Yes I know about that, and to complete the circuit we need to form a complete path I know that bit also. However ground is used as a reference point isn't it? So why can we use a piece of wood because we will probably have guaranteed 0v? Since it does not conduct? ::)
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Offline IanB

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Re: What is the ground in battery powered devices?
« Reply #13 on: December 18, 2011, 07:18:50 am »
Yes I know about that, and to complete the circuit we need to form a complete path I know that bit also. However ground is used as a reference point isn't it? So why can we use a piece of wood because we will probably have guaranteed 0v? Since it does not conduct? ::)

The reference point has to be somewhere on the complete path of the circuit, not external to it. There has to be potential for electricity to flow from the measurement point to the reference point. There is no potential for electricity to flow from the circuit to some external piece of wood, so it cannot be used as a reference.

If you imagine connecting your bulb between two points instead of the voltmeter, your bulb must have the potential to light up. If the bulb is not likely to light up, there is no voltage.
« Last Edit: December 18, 2011, 05:48:35 pm by IanB »
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Offline bfritz

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Re: What is the ground in battery powered devices?
« Reply #14 on: December 18, 2011, 07:19:31 am »
Yes I know about that, and to complete the circuit we need to form a complete path I know that bit also. However ground is used as a reference point isn't it? So why can we use a piece of wood because we will probably have guaranteed 0v? Since it does not conduct? ::)

Do exactly what you have said.  What you will find, is that where you touch the wire to the wood, the voltage will be at the battery voltage, because wood doesn't conduct electricity... no current flows.

Think of ohms law.   V = I*R   If no current flows, then the voltage drop through the wire and bulb is zero, so the battery voltage will be present where the wire is touching the wood.  Wood is not a very good conductor.  We don't use copper in wires because it is inexpensive, we use it because it is a fairly decent conductor.  If we could use insulators like wood, then plastic cord would be less expensive, and we'd be using that.
 

Online Zero999

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Re: What is the ground in battery powered devices?
« Reply #15 on: December 18, 2011, 10:38:28 am »
Yeah but isn't the earth an insulator too, so it has 0v. So wood and anything non conductive will have 0v too?
The earth isn't an insulator, it's a poor conductor and if there's a large surface area between two conductors buried in the earth, a significance current can flow when a voltage is applied. Try burying a metal object (e.g. a tin can) in the earth outside with a wire attached to it and measuring the resistance between it and a known grounded object such as a water pipe or the earth pin on a mains socket with a multimeter. You'll find it doesn't read open circuit, the resistance will probably be between 1k and 1M. The resistance between two large metal objects buried in moist earth can be even lower, in fact the electricity companies sometimes use the earth as a return conductor to save money on cable as only one wire is needed to distribute single phase power rather than two.

Yes I know about that, and to complete the circuit we need to form a complete path I know that bit also. However ground is used as a reference point isn't it? So why can we use a piece of wood because we will probably have guaranteed 0v? Since it does not conduct? ::)
Firstly an insulator doesn't conduct electricity so it's potential difference relative to earth could theoretically be anything from -millions of volts to + millions of volts.

Secondly, you're missing the point about a reference point.

The earth in an electrical circuit is just the part of the circuit we decide to call 0V because it's the most convenient point to measure all voltages with respect to i.e. where you put the negative lead of the meter.

In a portable battery operated the voltage between the earth ground and circuit ground is irrelevant as far as the operation of the circuit is concerned.

Attached is a virtual earth circuit which devised the power supply from a single 12V battery into a bipolar (+V and -V) 6V supply. If the earth symbol were connected to the battery's negative terminal, the voltage at Tr1's and Tr2's emitter would become 6V relative to 0V. What would the voltage at Tr1's and Tr2's emitter be if the earth symbol were connected to the battery's positive terminal?

Oh and I suggest you reread all the replies to the other threads you've created about earth before asking any more questions.
https://www.eevblog.com/forum/index.php?topic=5580.0
https://www.eevblog.com/forum/index.php?topic=5557.0
« Last Edit: December 18, 2011, 10:45:21 am by Hero999 »
 


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