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Author Topic: Silly question about voltage and current  (Read 49287 times)

Dave45

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Re: Silly question about voltage and current
« Reply #75 on: March 01, 2014, 02:49:25 AM »
Yes please. If we use the term "current" can we agree to stick to the established convention of the direction of "travel"? And if we are talking about charges, then let's call them charges and accept that they go the other way around the circuit. But if we are analyzing circuit behaviour from the standpoint of electromagnetism, power dissipation, and all that, we should stick with the established convention, even though we know it is "wrong".  Drive on the left side of the road if you are in Britain or Japan, please, and if you are on Earth.... current (consider "conventional" as always implied) goes from positive  to negative.

MarkE

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Re: Silly question about voltage and current
« Reply #76 on: March 01, 2014, 03:11:10 AM »
Dave, which convention is used does not end up mattering mathematically.  Negative current convention dictates that current flows from the negative pole of a power source back to the positive pole.   Positive current convention follows the opposite path.  Negative going CCW or positive going CW yields the same results.

TinselKoala

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Re: Silly question about voltage and current
« Reply #77 on: March 01, 2014, 03:32:29 AM »
Very funny Dave. If I were to use a cartoon to describe your viewpoint, the head would be stuck up a different hole.

SeaMonkey

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Re: Silly question about voltage and current
« Reply #78 on: March 01, 2014, 04:07:52 AM »
Circuit Diagram posted by Dave45 here:

(http://www.overunity.com/14343/silly-question-about-voltage-and-current/dlattach/attach/134173/image//)

Open the above link in  a New Tab in order to see the
image.

There really isn't anything wrong with the depiction
of an N-Channel MOSFET functioning as a Low Side
Switch.  The arrow depicting the direction of switched
inductor current flow, when considering the polarities
of the semiconductor components, must refer to
"electron flow."  The associated power supply polarity
is therefore self evident.

The red-letter label starboard of the Spike Suppression
Diode which parallels the inductor is incorrect, however.

The Inductor Discharge Kickback will forward bias the
suppressor diode with a resultant current flow consistent
with the Time Constant of that portion of the circuit.

The direction of current flow through the inductor in
both cases (charging and discharging) will be the same.


Farmhand

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Re: Silly question about voltage and current
« Reply #79 on: March 01, 2014, 04:17:12 AM »
OK so I was mistaken in my first test and admit it, I'll explain where I went wrong later.

Anyway With the circuit posted below, I see that there is indeed a current through the coil snubbed by the diode to the supply, and it seems to cause a continuous current in the coil I used, which heated it right up.  ;D However when the coil was discharged to a higher voltage the current went to zero. Which is desirable to me.

I don't like coil snubbing, I was never sure why, then I thought I knew why, but now I know I know why I don't like it.  ;D

First picture is the circuit the dashed portion is the alternative/second arrangement.

Second shot is with the diode snubbing the coil to the supply.

Third shot is coil discharging through a higher voltage.

..

P.S. I better recheck things.  :)

..

Seems correct Yellow trace is inverted on the scope to make it correct I think.

With the diode snubbed to the supply rail it uses 500 mA (maybe more) and heats up the coil but with the coil discharged to the battery it uses only 200 mA (maybe 300 mA) and doesn't heat up the coil and the current goes to zero.

I did the original test to try to show why I think snubbing is bad and this test shows even more reason fo me so I am very glad I did it.



..

Farmhand

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Re: Silly question about voltage and current
« Reply #80 on: March 01, 2014, 04:41:35 AM »
I know if I were switching a coil to turn it on and off which waveform I would prefer to see.

Look at all that current in the snubbed to supply shot, maybe I should do a Rosemary and claim OU on that. Circulating continuous current. hehehehe.

It's a heater !  ;)

..

P.S. The previous test I tried this on had a charging circuit which allowed me to make a bad probe placement which simply did not read the current through the diode back to the supply rail. Operator error. Thanks to Milehigh I know see what I think is a real result and it still back up my dislike of the practice of coil snubbing to the supply. So I am happy in two ways. I realized a mistaken result and also keep my dislike of coil snubbing for a good reason.

My apologies MilHigh, and Thanks.  :)

..

forest

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Re: Silly question about voltage and current
« Reply #81 on: March 01, 2014, 10:46:33 AM »
Now, please FOCUS, this is very simple question : if electrons movement is the reason for electric current then why it require CLOSED CIRCUIT ? Take a charged electrolytic capacitor with HUGE energy stored (like those big ones used for power factor correction) , connect negative terminal to some place having less electrons like maybe other capacitor positive terminal), then measure how much electrons moved to that empty place by simple voltmeter. I think you would find nothing changed, capacitor still charged ,energy still there, electrons won't move easily...


Waiting for your comments

MarkE

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Re: Silly question about voltage and current
« Reply #82 on: March 01, 2014, 10:50:08 AM »
Now, please FOCUS, this is very simple question : if electrons movement is the reason for electric current then why it require CLOSED CIRCUIT ? Take a charged electrolytic capacitor with HUGE energy stored (like those big ones used for power factor correction) , connect negative terminal to some place having less electrons like maybe other capacitor positive terminal), then measure how much electrons moved to that empty place by simple voltmeter. I think you would find nothing changed, capacitor still charged ,energy still there, electrons won't move easily...


Waiting for your comments
If you do not apply a potential across something, why do you expect charge to move?

Dave45

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Re: Silly question about voltage and current
« Reply #83 on: March 01, 2014, 12:14:10 PM »
Very funny Dave. If I were to use a cartoon to describe your viewpoint, the head would be stuck up a different hole.
Im sorry the sarcasm was rude, just take a little time study electron flow, the direction it moves through a diode, the bemf and the direction it moves through a diode.
You will realize the bemf has an opposite polarity, this imply's that a neg pulsed coil will have a pos return and a pos pulsed coil will have a neg return.
Study ionization, the plarity of the electrodes, the ion clouds formed around the electrodes, how that relates to a pos and neg pulsed coil.

Alternating current is more that it imply's, a coil pulsed with alternating current receives a pos and neg pulse.
Check out static neutralizers and you will see how your neutralizing any extra energy you may be drawing from the ambient by powering a transformer with both legs.
Split the pos and neg so they cant neutralize each other.

MarkE

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Re: Silly question about voltage and current
« Reply #84 on: March 01, 2014, 02:52:49 PM »
Im sorry the sarcasm was rude, just take a little time study electron flow, the direction it moves through a diode, the bemf and the direction it moves through a diode.
You will realize the bemf has an opposite polarity, this imply's that a neg pulsed coil will have a pos return and a pos pulsed coil will have a neg return.
Study ionization, the plarity of the electrodes, the ion clouds formed around the electrodes, how that relates to a pos and neg pulsed coil.

Alternating current is more that it imply's, a coil pulsed with alternating current receives a pos and neg pulse.
Check out static neutralizers and you will see how your neutralizing any extra energy you may be drawing from the ambient by powering a transformer with both legs.
Split the pos and neg so they cant neutralize each other.
Dave why don't you study up on things like solenoid drivers?  Low-side only, high-side only, and "X" drivers have been in the market for decades.  Variants of single sided drives using bi-voltage, single sided diode clamps, zener clamps, and simultaneous two switch switching are old hat.  If you are listening to music on any kind of computer, or portable device, the Class D amplifier that is driving your speakers or headset is switching the output up to 1 million times per second.  Class D amplfiers do not pull energy out of the ambient environment.  Engineers carefully design the PCB layout and EMI filtering so that they do not emit excessive RF into the environment.

TinselKoala

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Re: Silly question about voltage and current
« Reply #85 on: March 01, 2014, 04:41:45 PM »
Now, please FOCUS, this is very simple question : if electrons movement is the reason for electric current then why it require CLOSED CIRCUIT ? Take a charged electrolytic capacitor with HUGE energy stored (like those big ones used for power factor correction) , connect negative terminal to some place having less electrons like maybe other capacitor positive terminal), then measure how much electrons moved to that empty place by simple voltmeter. I think you would find nothing changed, capacitor still charged ,energy still there, electrons won't move easily...


Waiting for your comments

I think you would find that the _other_ terminal of the capacitor has the opposite charge, and opposites attract, across the cap's dielectric. There is no current pathway in your hypothetical question for the charges to equalize _across the capacitor's dielectric_. Why should the electrons, or rather the charge they carry, travel away from someplace that they are strongly attracted to by the presence of all that opposite charge on the other side of the dielectric? You've offered them an extremely high-resistance non-pathway in the other direction along a very strong electric field gradient .... an open circuit as you have set up. There is no pathway for the equal and opposite charges in the capacitor to neutralize.

Farmhand

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Re: Silly question about voltage and current
« Reply #86 on: March 02, 2014, 03:06:45 AM »
Im sorry the sarcasm was rude, just take a little time study electron flow, the direction it moves through a diode, the bemf and the direction it moves through a diode.
You will realize the bemf has an opposite polarity, this imply's that a neg pulsed coil will have a pos return and a pos pulsed coil will have a neg return.

Study ionization, the plarity of the electrodes, the ion clouds formed around the electrodes, how that relates to a pos and neg pulsed coil.

Alternating current is more that it imply's, a coil pulsed with alternating current receives a pos and neg pulse.
Check out static neutralizers and you will see how your neutralizing any extra energy you may be drawing from the ambient by powering a transformer with both legs.
Split the pos and neg so they cant neutralize each other.

Dave take a good look at the scope shot and the circuit that produced it ( a few posts back), now notice that all the voltage is positive and all the current is positive,

If you look at the grounding point you will see that when the switch turns on the scope probe for voltage drops to zero, however the current rises positive and then declines positive. The  applied voltage is DC and on switch on the scope probe drops to zero volts then on switch off the voltage rises positive as the current declines positive.

There is no negative to it. Unless a wanted or unwanted oscillation causes it.

People say the voltage of the coil reverses but going by my scope shot I do not see how, the voltage is scoped across the coil and the voltage is always nil or positive.

If someone else would like to point out where the voltage across the coil reverses I would be glad to try to understand it.

Cheers

Farmhand

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Re: Silly question about voltage and current
« Reply #87 on: March 02, 2014, 03:27:00 AM »
What happens is the coil changes from being a "load" to a "supply" and so the actual voltage "polarity" does not change nor does the direction of the current the only thing that changes is the end of the coil we think of as positive with relation to "load" "supply" situations, this is due to voltage level not positive negative as such. In other words when the coil is taking current the dot to designate the positive end of the coil is at the top, then when the coil is supplying current the dot to designate the coil polarity is moved to the bottom. It's a change in the mind only, to rationalize the thing we are seeing and so like minded people can communicate about it. There is much disinfo about DC pulsing of coils. And much is not explained by some of the other trained guys or too many people ignore them.

I don't mind to argue with them, because I feel anyone can be mistaken, I don't mind being wrong if I learn something, but most of the time they are correct, it's just that most of us do not understand how they say it. Or misunderstand what they say. And the misunderstanding goes both ways of course.

The discharge of a coil is forward emf (or just plain emf) as opposed to counter or back emf. This is seen by the current remaining to flow the same way, the voltage reversal across the coil happens when the coil discharge causes a higher voltage than the supply, but it does not go negative.

Cheers

TinselKoala

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Re: Silly question about voltage and current
« Reply #88 on: March 02, 2014, 04:00:10 AM »
Farmhand, your coil circuit appears to be "overdamped" or even critically damped,  in that there is no trace of an oscillatory ringdown when the supply is cut off.  Other circuit parameters could be adjusted so that a ringdown at the resonant frequency of the circuit could appear, maybe. This may or may not have a good or bad effect on the performance depending on what the goal is.

MarkE

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Re: Silly question about voltage and current
« Reply #89 on: March 02, 2014, 05:00:40 AM »
When an L/R or L/C circuit is switched must faster than the reactive network time constant, trapezoidal waveforms such as seen in farmhand's oscilloscope captures result.