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# Negative D/C voltage?

#1
10-30-09, 08:14 PM
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Negative D/C voltage?

An electronics book I am reading states:

An op amp requires both negative and positive supply voltages. A positive
supply voltage in the range of 8 to 12 volts and a negative supply voltage in
the range of -8 to -12 works.

This is referring to a circuit I am wanting to build on an electronic breadboard powered by a battery. How does a DC circuit supply both a positive and a negative voltage? I've heard of A/C current moving back and forth but D/C only goes one way. How is it that voltage can be negative in this regard?

#2
10-30-09, 09:48 PM
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It means that negative is that in realtion to the ground rail.

For small experimentation, you would use two 9v batteries. On one the - terminal connects to your ground rail, + to the positive rail. The other + goes to ground, - becomes the negative rail.

To make it from AC, you would use a center tapped tansformer, with the outside windings, connedting to AC terminals on a bridge rectifier. The center tap on the transformer is ground, an the + and - on the rectifiers become the respecive positive and negative rails. You willl use filter capacitors though, filtering each leg to ground. Of course,, on the negative rail, capacitor - goes on the negative rail, + to the ground rail.

#3
10-31-09, 08:40 AM
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The other + goes to ground, - becomes the negative rail.

Positive to ground? Doesn't current come from the negative side of a battery? I mean, don't electrons want to travel from the negative post of the battery to the positive post of the battery? I guess the positive side has an abundance of protons, but protons don't move, so why would the + go to ground?
Ohh, unless the ground is going to supply electrons...
So, if you take a positively charged side of a battery and connect it to anything metal, whether the metal has a surplus of electrons or not, the positive terminal of the battery wanting some electrons will draw current from the metal. Is that it?

#4
10-31-09, 11:03 AM
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When playing tennis, they say you should stand with the tennis racket pointing forward with it "in the middle"...

A ball may come to your left or a ball may come to your right.

If the racket is in the center, you will be able to respond to a ball coming at you with equal quickness!

So let's say there is a servo motor controlling that tennis racket and connected to that op amp.

A forward voltage makes the motor go to the left and a reverse voltage makes the motor go to the right.

And...

Left for our power supply is +9 volts.
Ground (or common or 0) is the middle.
And right is -9 volts.

The op amp needs to be able to swing the racket to the left or to the right.

And there are only two wires connected to the motor.

Wire A.
And wire B.

We connect the ground or common to wire A.
We connect the op amp output to wire B.

For a left swing we get an output of...

A: ground/common/0 volts
B: +9 volts

For a right swing we get an output of...

A: ground/common/0 volts
B: -9 volts

And of course the question to the above would be how do you get a motor to go forwards or backwards where in one case you need - and + and the opposite + and -? The answer is a power supply and electronics which provides both!

#5
10-31-09, 11:28 AM
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I hate the use of "ground" anything in DC voltage.

More often, there is no "ground" but a "common" or a "+" or "-" designation.

ground means there is something that is grounded and this is simply not always the case, even when dealing with AC voltage and current.

so, you have +12 vdc and you want -12 vcd?

pos and neg are simply a manner of reference of current flow to or from a specific terminal.

If you are reversing a DC motor, often swapping the polarity is all that is required.

Now, depending what you are doing, that, sometimes is all you need to do: swap polarity. If you are building a circuit that uses a base point voltage and there is both - and + in relation to that point, you are in the situation Bill190 was describing. To create a midpoint so you have a base point to allow both neg and pos voltage, you need a power source with the ability to derive a lead from the center of the supply. The batteries hooked in series are the perfect example of that ability with the connection of a pos of one battery and the neg of the other battery, which are connected together when series connecting batteries would be the point which you would derive this set point from. From each other battery terminal, you would read either + or - voltage equally. You can also series such a supply with unequal voltage batteries and have unequal + and - voltage readings from the set point.

just as an addition to bill's post of the +9, 0, and -9 volt readings. each of those voltages would be from the common point. If you read from the terminals not including the 0 or common point, you would read 18 volts and depending on which way you placed your leads, you could read either + or -.

#6
10-31-09, 12:30 PM
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Thank you both. Those were very excellent explanations. I really appreciate them.

#7
10-31-09, 01:13 PM
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Originally Posted by nap
I hate the use of "ground" anything in DC voltage.
Agreed. Technically it is "zero volts".

#8
10-31-09, 01:24 PM
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Originally Posted by Rick Johnston
Agreed. Technically it is "zero volts".
I am trying to figure out this "difference in potential"

Are you saying that ground is anything that isn't trying to give or take electrons? That anything with voltage will try to find a way to neutralize itself via a ground. There is a difference in potential from one point to another because one point will give or take electrons?
And then there's those occasions where you will not see a difference in potential because either there is no voltage(nothing wanting to give or take electrons) or you are accidentally checking the same line(the same wire at two points - of which can't give or take from/to itself).

Q2)In house wiring, is the red considered positive and the black negative like we do for the car battery and small circuits?

#9
10-31-09, 02:22 PM
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Birds and power lines. And specifically a bird landing on a high voltage power line and not being shocked.

That is because the bird is at 69,000 volts or whatever, just as soon as it lands on the line.

If we were to place a volt meter from the bird to the power line on which the bird is resting, it would read 0 volts!

There is no difference in "potential" between the bird and the power line.

The earth or ground is also at a certain potential. If we were to measure between the bird (or power line) and ground, our voltmeter would read 69,000 volts. [WARNING! Don't ever try to measure high voltage as it will jump right through the insulation of the multimeter wire and electrocute you!]

But note that metal cases on home appliances (which are grounded) are also at ground potential or "grounded". If we were to place a voltmeter from the metal case of a kitchen range to a kitchen faucet, it would read 0 volts.

Then house electricity is 120 volts [AC] when measured to ground. If we measure from the black "hot" wire to ground, the meter will show 120 volts.

So what happens if a black "hot" wire touches the case of the kitchen range and the metal cabinet is NOT grounded?

Suddenly the metal case will reach 120 volts potential to ground (just like that bird)!

If we were then to measure from the electrified metal case to the kitchen faucet, the meter would read 120 volts AC.

Or if someone were to touch with one hand the metal case of the range at the same time they touched the faucet with their other hand, electricity would flow through their body, they would be electrocuted.

This has happened in the past and people have died. So what is done is the metal cases of electrical things are "grounded".

Then when grounded, what happens is if a "hot" black wire touches the metal case, this causes a short circuit which overloads the circuit breaker, and the breaker shuts off the electricity. And the metal case is always at "ground potential", thus someone touching it would never have electricity flowing through their body if they were also touching a faucet!

So grounding is for safety many times. (Other reasons with electronics.)

As to black and red wires and house wiring...

House wiring is AC or alternating current. Batteries are DC or direct current.

Typically house wires which are not white or green (black, red, pink, blue, whatever, are "hot" or at least 120 volts (or more in commercial settings) when measured to ground.

The white wire is called a neutral and is grounded or at ground potential. Note that a black and white wire can be used to wire a switch and in that case the white wire is a "hot" wire!

The green wire is grounded or at ground potential.

(There are exceptions to everything, so don't assume a white or green wire is at ground potential! Never touch any electrical wire unless the electricity to the entire house is turned off.)

This explains it pretty much...

Alternating current (AC)...
Alternating current - Wikipedia, the free encyclopedia

Direct current (DC)...
Direct current - Wikipedia, the free encyclopedia

Mains electricity...
Mains electricity - Wikipedia, the free encyclopedia

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