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what is negative voltage/power supply ?

khankll

Feb 6, 2011
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most op amps require two voltages positive and negative ..
my question is what is negative voltage ?
aint the postive and negative just related to polarity ? :confused: just reverse the two leads of positive supply and u get negative ?
 

duke37

Jan 9, 2011
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Yes but the two supplies need to be independant with the positive output of one connected to the negative of the other.
 

davenn

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Sep 5, 2009
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most op amps require two voltages positive and negative ..
my question is what is negative voltage ?
aint the postive and negative just related to polarity ? :confused: just reverse the two leads of positive supply and u get negative ?

its more commonly called a dual rail supply or a split rail supply
here is an example of a regulated + and - rail 12VDCsupply

attachment.php


ideal for Op-Amps and other circuitry that needs 12V you could always replace the 12V regulators for 5V ones for a + and - 5V supply

NOTE how one regulator is a positive one (7812) and the other is a negative one (7912)

aint the postive and negative just related to polarity ? :confused: just reverse the two leads of positive supply and u get negative ?

NO not in this case its showing you that the 2 rails are positive or negative in relation to the 0V rail (in the diag shown as GND)
your confusion comes from the way we label batteries etc as with positive and negative terminals .... really you have a positive V and a 0V terminal

Dave
 

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khankll

Feb 6, 2011
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Yes but the two supplies need to be independant with the positive output of one connected to the negative of the other.


actually i ewant to know what beast iss this negative ?
if i have a battery and if i connect the other way around so i have negative .. i m mentioning a case where we have a single battery/source ..

@davenn
i m trying to understand ur reply ..i m re reading it .. will try to digest as much of it as possible.. will report back ...
 

donkey

Feb 26, 2011
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in most cases a schematic will show a ground, any voltage away from that is either positive or negative. in MOST circuits the ground is negative, in some there are both positive and negative as shown above. and in other cases the positive is ground while negative is shown.
there are many reasons to do this the one that comes to mind is where a single circuit requires 2 vvoltages but has a common positive. thus it will be easier to make said positive the ground.
 

JMW

Jan 30, 2012
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Voltage is nothing more than potential. The points where you measure it make it + or -.
This reference point typically ground aka earth. There is no such thing as negative voltage, at least in this universe. It is all in how and where you are measuring.
 

gorgon

Jun 6, 2011
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Voltage is nothing more than potential. The points where you measure it make it + or -.
This reference point typically ground aka earth. There is no such thing as negative voltage, at least in this universe. It is all in how and where you are measuring.

In my world the reference point is called 0V, and has little to do with earth or ground. Sometimes this reference point is connected to ground or earth.

Being a 0V point, it also indicates that there is both voltages more positive and 'more' negative. This is then the +V and -V we're talking about. The value(s) of these voltages is of course an expression of the potential difference from 0V.

Any isolated(or floating) voltage supply can be both a negative supply, with (+) connected to 0V, or a positive supply, with (-) connected to 0V.

All these voltages are referenced to the local 0V, and not to a universal whatever.

For the opamps you'll always have a +V and a -V supply. A bit depending on the opamps actual parameters you can supply it with a single voltage, nomally a positive references to a 0V. The -V terminal will then be connected to 0V.
For a dual voltage supply the -V will be connected to the -Vsupply.

However, any opamp need a virtual GND to operate. For a single supply system this will be at +Vsupply/2, and for a split supply it is normally at 0V.

No opamp will work properly if the virtual GND is set to one of the supply voltages, regardless of, if it is a single or split supply system.

Please note that some opamps really need larger supply voltage(s) than the ones used in logic circuits.
Both the inputs to and the output from an opamp can be restricted from getting too close to the supply voltages. Datasheets is your friend here.

TOK ;)
 

Enigma

Jul 15, 2012
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sort of like, you can apply 0V at any part of the spectrum.. like an anchor?
 

Mongrel Shark

Jun 6, 2012
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if i have a battery and if i connect the other way around so i have negative .. i m mentioning a case where we have a single battery/source ..

You can solve this really easily with 2 batterys...

http://www.societyofrobots.com/images/electronics_negative_voltage_battery.jpg

If you want to read the article I got the pic from it is here: http://www.societyofrobots.com/electronics_negative_voltages.shtml

I found it all a but tricky until I did the two batterys in series test.

It's all about reference points as others here have pointed out.


Ground is often the reference point, and sometimes marked 0v . If you have more volts than the reference point you have +v and less than the reference point and you get -v

Put two equal batterys in series and do some measurements...


Hope that helps.
 

khankll

Feb 6, 2011
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You can solve this really easily with 2 batterys...

http://www.societyofrobots.com/images/electronics_negative_voltage_battery.jpg

If you want to read the article I got the pic from it is here: http://www.societyofrobots.com/electronics_negative_voltages.shtml

I found it all a but tricky until I did the two batterys in series test.

It's all about reference points as others here have pointed out.


Ground is often the reference point, and sometimes marked 0v . If you have more volts than the reference point you have +v and less than the reference point and you get -v

Put two equal batterys in series and do some measurements...


Hope that helps.
thaks i will do the two cell experiment.. but how do i create the ground physically ?
just take a wire and insert one end into grond/mud ? :confused:
 

mattsains

Jun 8, 2012
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I think anything can be ground. You just pick a wire and say "this is 0V. When I say voltage I mean the difference between this point and another" because voltage is always a difference between two places, unlike, say, current. It's like a comparison of two places in a circuit.
Think about a building. The tenth floor really means ten floors above the ground floor. If there were two basement levels, you would still call the tenth floor that, even if there were eleven actual floors below it. So you've just decided to call the ground floor the zeroth floor. then the basement floor is the minus oneth floor, relative to the ground floor!
So really, I can decide that what was the tenth floor is the zeroth (or "ground") floor, which means I'll walk in the front door into the -10th floor, again relative to the "ground" (zeroth) floor. Do you see how we are comparing floors to a certain floor (the ground floor)
 

CiaranM

May 19, 2012
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aha, electronics is certainly misleading! 'Ground' doesn't usually mean ground, as in.. well.. dirt and stuff. It means a reference point. If you had a 9V battery and used the +V terminal to represent your source, you would use the -V terminal to represent ground. When you compare the terminals together with a multimeter, it should read '9V'. This is the potential difference and, as you can guess by the name, represents the difference in voltage between two points.
Note: Another word for voltage is electromotive force. Read this term as 'electron motion force' and you'll understand what voltage actually is; it is the amount that something is able to 'push' or 'pull' electrons towards itself. Thus, when you compare two points on a circuit, you are finding out the difference in their ability to push or pull electrons..
NOTE: AC sockets in your home are earthed. Not to be confused with 'ground', this earthing configuration quite literally means a connection to the Earth. A metal pole or something is connected to the earth terminal, and put into the actual Earth. You know how lightning heads to Earth? When AC in your sockets is short-circuited, hopefully it will do the same.

anyway, I'm rambling on, I think, so I'll help you with op amps instead. See next post!
 

CiaranM

May 19, 2012
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I recently was helped by the community, so I should pass along some info.
With an op amp circuit and an AC power supply: See picture 1.
The AC power supply's + and - outputs are connected to the power input pins on the op amp. The ground is connected to the circuit ground, which is for connecting ground terminals from speakers, input signals, inverting or non-inverting op amp signal inputs, etc. Take note that an op amp circuit therefore uses three rails, these are +V, -V, and ground/0V.

With two DC batteries: See picture 2. (the capacitors are to ensure good operation of the op amp)
Firstly, you take the middle rail to represent ground. This means that whenever you measure a voltage, you compare it to this. So, to measure the top rail's voltage, you compare it to the middle rail. This shows a difference of 9V, so the top rail represents 9V. Next you measure the middle rail's voltage. Since you're comparing it to itself, you'll see a difference of 0V. Lastly, you compare the third rail to the second rail. This is like what you did for the first rail, however instead of comparing a positive terminal's output to ground, you are comparing a negative voltage to ground. The result: you'll see a difference of -9V.
Use this circuit to power op amps if you have no AC sources available!

NOTE: I forgot to re-draw the voltmeter display screen. Sorry!
 

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CiaranM

May 19, 2012
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Another circuit suitable for use in DC powered op amp circuits is a potential divider. See pic A. The equation to work out the voltage at the middle rail is Vin x (R2/R1+R2). In order to get the middle rail voltage to be equal to half of the input voltage, you use resistors that are the same value.
E.g. 10V supply with 5K resistors.
Vin x (5000/5000+5000) = 10 x 5000/10000 = 10 x 0.5 = 5V. Neat huh?! So rail 1 = 10V, rail 2 = 5V, and rail 3 = 0v.

Why is this useful though? Well, in pic 1, I used the last rail to represent ground. You'll notice that there is 5V difference between rail 1 and rail 2, and rail 2 and rail 3.
Now I'll say that the middle rail represents ground, i.e. '0V'. when I compare the first rail to this, I will get +5V. When I compare the third rail to this, I will get -5V. It all depends on what rail you use as your reference!

By the way, this potential divider needs capacitors connected to it when used for an op amp circuit.

See pic B for an inverting op amp circuit with a gain of 100 powered by 9V.
 

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john monks

Mar 9, 2012
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The ground is a definition. Please do the two cell experiment. You will hook up both cells in series in the same direction, one cell positive lead tied to the negative lead of the other. Now you will want to define that junction as "ground" or as your reference. You will define that voltage as zero because this will be the point that all other voltages will be compared with and measured from. This may or may not be connected to earth, a water pipe, a car chassis or anything else. This is defined as zero volts and is your reference ground. So in an op amp circuit your output, input, and every other voltage will be measured with a voltmeter lead connected to that point and the other lead on ground, the connection between the cells. So I believe you already realize that a negative voltage tends to push electrons away and a positive lead tends to draw electrons relative to your ground. Now inside a cell this is the opposite but we are looking outside the cell. Please try connecting two cells together and report back what you find with a voltmeter.
 
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