Why do voltages cancel in D.C. but not in A.C.?

Thread Starter

foolios

Joined Feb 4, 2009
163
How does the two voltages add up? Why can we tap into one 120v line and into another and get voltage across this wire? Especially all the voltage added up together.
 
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russ_hensel

Joined Jan 11, 2009
825
AC and DC both add. With DC you must take into account the relative polarity of the voltages, in AC the analog ot polarity is phase. Most easily seen treating AC as a complex expotential.
 

zgozvrm

Joined Oct 24, 2009
115
Here is an example of two 120 lines coming together to make 240v. It would appear to me that this shouldn't work at all.

[picture omitted]

How does the two voltages add up? Why can we tap into one 120v line and into another and get voltage across this wire? Especially all the voltage added up together.

The two 120 Volt lines are actually in phase with each other (they came from the same secondary coil on the transformer) and are, therefore, additive. If they were 180 degrees out of phase, they would cancel each other out. If they were at some phase offset between 0 and 180 degrees, the result would be greater than 0 volts and less then 120 volts.
 

Thread Starter

foolios

Joined Feb 4, 2009
163
AC and DC both add. With DC you must take into account the relative polarity of the voltages, in AC the analog ot polarity is phase. Most easily seen treating AC as a complex expotential.

Ooh... Ok, so in DC both are traveling at each other as ouput only. In AC the two lines are moving back and forth at the same time. Soo... hmm... when one line is ... uggh.. thinking...

The two 120 Volt lines are actually in phase with each other (they came from the same secondary coil on the transformer) and are, therefore, additive.

Ok, I think I need to see the way that the power is flowing through them in a diagram with arrows. I will see if I can redraw what is happening.
 
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Thread Starter

foolios

Joined Feb 4, 2009
163
If I draw out how I think the voltage is moving along, it appears to me that they will buck when they meet at the top.
Since they are in phase they are pushing at the same time and pulling at the same time. This seems just like the DC version.
Now if they were out of phase then maybe I could see that one side will draw while the other pushes.

Can you please show me what's going on here? This isn't right but this is how I understand it.

::"image removed due to hosts image bandwidth constraints"
 
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russ_hensel

Joined Jan 11, 2009
825
Try not to mix up concepts. Power flowing is dubious, current flowing is fine. We normally do not think of voltage as flowing either, unless we are involved in relativistic situations. For example in the ideal wire the voltage is the same at either end, the voltage does not flow down the wire. In an RC circuit ( series ) current flows thru the wire to change up the voltage ( and charge ) in the cap.


There are some who do not even like the idea of current flowing, because current is the flow of charge they do not like pizza pie, very unique, or rio grande river either, but that is how most of us talk.
 

Thread Starter

foolios

Joined Feb 4, 2009
163
I will try to use this in another way. Sorry for the confusion. I am confused but I will try looking at it another way.

Lets look at it as the difference of potential. The difference in potential of the DC 12v versus the 5v is the 7v because one side has more electrons(is pushing more electrons) than the other. I guess making the lesser voltage side the positive side. The difference in potential in this case would be that the 12v side is the more negative side and forces its will that it needs to get rid of electrons more so than the 5v side. The difference is in that the 5v side becomes 0 and that the 12v side is different by 7v.

Where is this difference in potential in the AC circuit pictured? Being the same phase, I would think then that the difference in potential from 120v to itself would be zero.

Maybe this description will show the flaws in my understanding. Hopefully you can use this to correct my thinking. Of course, that's if I am even in the ballpark of knowing whether that's an accurate analysis of what's happening.

Thanks so much in advance!
 

t_n_k

Joined Mar 6, 2009
5,448
I will try to use this in another way. Sorry for the confusion. I am confused but I will try looking at it another way.

Lets look at it as the difference of potential. The difference in potential of the DC 12v versus the 5v is the 7v because one side has more electrons(is pushing more electrons) than the other. I guess making the lesser voltage side the positive side. The difference in potential in this case would be that the 12v side is the more negative side and forces its will that it needs to get rid of electrons more so than the 5v side. The difference is in that the 5v side becomes 0 and that the 12v side is different by 7v.

Where is this difference in potential in the AC circuit pictured? Being the same phase, I would think then that the difference in potential from 120v to itself would be zero.

Maybe this description will show the flaws in my understanding. Hopefully you can use this to correct my thinking. Of course, that's if I am even in the ballpark of knowing whether that's an accurate analysis of what's happening.

Thanks so much in advance!
The power transducer you are looking at is for a 3-wire split-phase system.

The two 120V lines are 180° out of phase and as you see them portrayed in the wiring diagram, will add algebraically to give a total of 240V [not 0V] across the voltage sensing coil in the transducer.

You might refer to the following link which explains 3-wire split-phase systems in more detail.

http://en.wikipedia.org/wiki/Split-phase_electric_power
 

Thread Starter

foolios

Joined Feb 4, 2009
163
Incidentally, the fan trick is neat, but has its potential safety issues as well. They are just less significant than wiring two 120V phases together.


Is this going to blow up my power supply unit?
 

BillB3857

Joined Feb 28, 2009
2,493
It probably won't hurt your power supply but since the fan runs at a lower speed, you may allow the temperature of whatever the fan was cooling to become too high.
 

Thread Starter

foolios

Joined Feb 4, 2009
163
I decided to mod only the coolest, less stressed system I have. I guess I have the cheapest fans in that one since it's the cheapest and less powerful system.

I wouldn't try this on one of the more stressed systems that I have where the temps are much higher for cpu and chassis. I've monitored the temps previously. So far, this has worked out quite well. I am happy with the difference in sound. It's more in moderation with the other systems and their noise output.

The noise stressed me out. I could have replaced the fans but this seemed more interesting and free.
 

BillB3857

Joined Feb 28, 2009
2,493
AC is simply DC that varies with time, even to the point of changing polarity. If you pick a single point in time and evaluate the instantaneous voltages, it behaves exactly like DC. Of course with the time element in place, other things like reactance come into play, even with DC and reactive components. Think about RC or LC time constants with switched DC.
 
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