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#1
12-09-2005, 03:48 PM
 terrakota Member Join Date: Feb 2005 Posts: 67

Hi,
can somebody explain to me what is negative voltage?, how a negative voltage is produced from a typical battery?, when and why you must use a negative voltage?.

can someboady drives me to a site with some usefull tutorial, guide, etc. explaining this?

thanks for an help
and please excuse my poor english
#2
12-10-2005, 06:11 AM
 Ron H E-book Developer Join Date: Apr 2005 Location: Idaho, USA (GMT-7) Posts: 7,050

Quote:
 Originally posted by terrakota@Dec 9 2005, 10:48 AM Hi, can somebody explain to me what is negative voltage?, how a negative voltage is produced from a typical battery?, when and why you must use a negative voltage?. can someboady drives me to a site with some usefull tutorial, guide, etc. explaining this? thanks for an help and please excuse my poor english Quoted post
When we measure voltages in a circuit, they are usually referenced to a common node in the circuit called "ground". Ground does not have to be connected to earth. It is just a term that indicates the common node in the circuit.
The schematic below should help explain this concept. As you can see, whether a voltage is positive or negative simply depends on which end of the battery (or other power supply) is connected to ground.
#3
12-10-2005, 08:51 PM
 terrakota Member Join Date: Feb 2005 Posts: 67

Quote:
 Originally posted by Ron H@Dec 10 2005, 02:11 AM When we measure voltages in a circuit, they are usually referenced to a common node in the circuit called "ground". Ground does not have to be connected to earth. It is just a term that indicates the common node in the circuit. The schematic below should help explain this concept. As you can see, whether a voltage is positive or negative simply depends on which end of the battery (or other power supply) is connected to ground. Quoted post
ok i understand but,

with a diode if you set the polarity of the battery to forward bias the diode you are applying negative voltage no? becasue the electros leave by the negative side.
if reverse polarity you are applying positive voltage to the reversed bias diode, so so, whats the difference?

thasnk for reply
#4
12-10-2005, 10:18 PM
 Brandon Senior Member Join Date: Dec 2004 Posts: 307

Forget about moving electrons because they teach you incorrectly to make it easier in the beginning. Electrons don't move, well, they do, but they move like 1 meter every 20 seconds in copper so its obviously not the electrons delivering the energy. You're looking to deep into this, its very basic.

BY CONVENTION, you assume current flows from + to - for all calculations. You also assume that current goes in the direction of the voltage drop. I.e. if point A is 3v and point B is 4v, since A is lower that B, the current would flow from B to A, the direction of the voltage drop. This also works with negative voltages. A is -4 B is -5. It would flow from A to B since B is lower than A.

Diodes (+) --[>|--- (-)

The polarity on diodes is used to tell you which way current will only typically flow in the diode in normal operation. It flows from the + to the - if you apply enough voltage to the + side such that the difference between the + and the - is atleast the diodes forward on voltage. Typically the ones u use in school n what not are 0.7 volts.

Remember, voltage is just a difference. It is that difference which forces current to flow and the current flows from the higher voltage to the lower voltage.
#5
12-10-2005, 10:28 PM
 JoeJester Senior Member Join Date: Apr 2005 Location: Grand Prairie, TX, USA Posts: 2,361

terrakota,

Look at the attached circuit. All the voltages are negative with respect to ground. Is the diode forward biased?
__________________
Joe
Navy Electricity and Electronics Training Series (NEETS) - pdf files - http://www.fcctests.com/neets/neets.htm
#6
12-10-2005, 11:13 PM
 n9352527 Senior Member Join Date: Oct 2005 Posts: 1,198

Quote:
 Originally posted by Brandon@Dec 11 2005, 12:18 AM Electrons don't move, well, they do, but they move like 1 meter every 20 seconds in copper so its obviously not the electrons delivering the energy. Quoted post
???
#7
12-11-2005, 12:40 PM
 n9xv Senior Member Join Date: Jan 2005 Posts: 329

Quote:
 Originally posted by Brandon@Dec 10 2005, 06:18 PM Forget about moving electrons because they teach you incorrectly to make it easier in the beginning.* Electrons don't move, well, they do, but they move like 1 meter every 20 seconds in copper so its obviously not the electrons delivering the energy.Quoted post
Ok - I'am gonna have some fun with this just to warn ya!

FOR ILLUSTRATON ONLY!!! - KIDS DONT TRY THIS AT HOME!!!

Brandon, if electrons dont move - or - they only move at the incredibly slow pace of 1-meter / 20-seconds, then you should be able to plug an AC line cord into your 120-VAC outlet with the ends stripped and hold one in each of your (wet) hands. If what your saying holds, then you have some 20-seconds before you get "bit" by that all mighty force behind the fuse pannel!

Now, I have a feeling that you wont actually try this but - - - what might be stopping you?

M-M-M-M-m-m-m-m-m-m-m!
#8
12-11-2005, 01:18 PM
 n9xv Senior Member Join Date: Jan 2005 Posts: 329

"can somebody explain to me what is negative voltage?"

Any voltage can be negative with respect to a more positive point.

Example:

Take two 12-volt batteries, connect in series - negative terminal of one battery to the positive terminal of the other battery. Now, for purposes of reference and this explaination, drive a ground rod into the ground and connect the "tied end" of the batteries (negative-to-positive connection you made earlier) to the ground rod. The ground rod is the reference or common connection for our 12-volt battery arrangement.

So, if you measure between the ground rod and negative terminal of one battery (black lead to ground rod & red lead to negative terminal) you will read "- 12 volts". The voltage at this point is more negative with respect to ground. Conversely, you could say that ground is more positive with respect to that batteries negative terminal.

Electrons always flow from negative to positive. The positive-most point in any circuit represents a depletion of electrons and the negative-most point represents a surplus of electrons. The surplus (-) condition is always trying to satisfy or equilize the depleted (+) condition. Thus, electron current flow.

"Conventional current flow" depicts current flowing from positive to negative as a matter of conveinience. All is good and well as long as you realize that it is actually the movement of electrons doing the work.

"when and why you must use a negative voltage?."

The reasons are as varied as the circuits themselves. One common application of a negative voltage is for biasing circuits. A negative voltage may be used to turn a device on whereas a positive voltage may be used to turn it off and visc-versa.

BTW, JoeJester provides a good example for thinking "outside the box" - if you will.
#9
12-11-2005, 01:31 PM
 terrakota Member Join Date: Feb 2005 Posts: 67

Quote:
 Originally posted by JoeJester@Dec 10 2005, 06:28 PM terrakota, Look at the attached circuit. All the voltages are negative with respect to ground. Is the diode forward biased? Quoted post
yes its forward bias, well i think :-)

thanks to all for your help now i have a clearer image
#10
12-11-2005, 01:33 PM
 JoeJester Senior Member Join Date: Apr 2005 Location: Grand Prairie, TX, USA Posts: 2,361

One prime example of negative voltages is RADAR.

If you have a radar aboard a ship that uses a waveguide connected to the anode of the magnatron ... that waveguide is typically metal. The waveguide is attached to the bulkheads and overheads working it's way up the mast to the rotating antenna. Now, last time I checked, large ships were METAL so anything attached to the bulkheads, overheads, and masts were at the same potential as the ship itself [ground].

In order to have the anode of the magnatron at 20,000 volts, you must make the cathode negative 20,000 volts.

Another example is in your kitchen. A microwave oven. Again, the magnatron anode is at ground potential. You need to apply a large negative voltage to the cathode to get that magnatron to be forward biased.

There are times you want and need negative voltages. The a\$\$ you save maybe your own.
__________________
Joe
Navy Electricity and Electronics Training Series (NEETS) - pdf files - http://www.fcctests.com/neets/neets.htm

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