# Why can you add two dc lines together but not two ac lines together?

Discussion in 'General Electronics Chat' started by foolios, Nov 24, 2009.

1. ### foolios Thread Starter Active Member

Feb 4, 2009
160
1
I am looking at a voltage mod for case fans that will allow your fans to run at 7v instead of 12v. Still moving a good volume of air but also being a little quieter.

What I don't understand is why can you add a 12v d/c line to a 5v d/c line and get 7v? Why can this happen with d/c without having to include a ground while in a/c trying to add two lines together would cause them to buck.

Why is this so between the two types of power?

2. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
Adding AC can be done. In fact, it is done with almost any center tapped secondary in a transformer. Two windings are simply placed in series with each other and phased properly for addition. The junction point of where they tie together is brought out as the center tap. If, in fact the two secondaries are not the same voltage, with proper phasing, they can either add or subtract. It is all in the phasing. (Phasing means reversing the two wires for the secondary)

3. ### foolios Thread Starter Active Member

Feb 4, 2009
160
1
If you took one of those phases like you're saying. Lets say at the 2 lines coming in to your house and touched them together, wouldn't they buck?
If so, why?

But your scenario includes a ground right? But this dc scenario doesn't. A 5v + a 12v is making 7v.

Also, you brought up something very interesting. Where the two windings are center tapped. Isn't the center tap acting as a ground? How does the secondary side have this center tapped ground that doesn't just take all the electricity away, completing the circuit?
I will try to draw a picture of what I am asking.

Last edited: Nov 24, 2009
4. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
The +12v and +5v share a common ground.

If you connect a fan between +12v and ground, you get +12v across the fan motor.

If you connect a fan between +5v and ground, you get +5v across the fan motor.

If you connect a fan between +12v and +5v, then you get 12v-5v=7v across the fan motor. The 12v supply will source current, while the 5v supply will sink the current.

You should add your location information to your profile; we don't have a clue where you live. Just your Country is enough.

Here in the U.S.A., homes have 60Hz AC supplied as two "hot" lines, L1 and L2, and one Neutral line. L1 and L2 are the opposite ends of a transformer secondary, and Neutral is the center tap of that same secondary winding. Neutral is connected to earth ground for safety reasons both at the transformer, and at the electrical service panel in the home.

Between Neutral and either L1 or L2, 120VAC is measured. Between L1 and L2, 240V is measured. This is because L1 and L2 are equal in amplitude, but opposite in polarity.

Large appliances, such as electric clothes dryers, ovens/stoves, water heaters, etc. are operated from 240VAC, because otherwise the wiring would dissipate too much power. Small appliances operated from 120VAC.

5. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
In my illustration using transformer secondaries, I did not include ground. Depending upon the phasing and how you connect the windings and where you put the ground, if used, many things are possible. If you want to experiment with such things, get a tranformer with multiple low voltage secondary windings. That will help you stay safe. Not understanding the ins and outs of power distribution can possibly cause you (and possibly your home) great damage if you experiment using it.

6. ### foolios Thread Starter Active Member

Feb 4, 2009
160
1
The 12v supply will source current, while the 5v supply will sink the current.

It seems to me you're saying that 12v will have more power to take electrons away from the 5v side. So the 12v side will now become the positive end of the circuit and the 5v side will become the negative side of the circuit. (D/C)

Neutral is connected to earth ground for safety reasons both at the transformer, and at the electrical service panel in the home.

This is the center tap connected directly to the windings though right? What keeps the induced current that is now on those windings from traveling along this return? Why isn't this a short circuit? It seems like it's a complete circuit right at the winding.
EDIT: I dunno, it seems kind of fuzzy but maybe it's because unless the loads in the house are using power, then the induced current isn't being drawn. But how is it that current doesn't just jump from the primary right along that center tap? It's a lot of voltage, arcing happens at tha voltage easily doesn't it?

Between Neutral and either L1 or L2, 120VAC is measured. Between L1 and L2, 240V is measured. This is because L1 and L2 are equal in amplitude, but opposite in polarity.

Ok, so the reason why L2 is phase shifted is so that it's of negative polarity. This will cause it to add to the voltage of L1.
Could you explain this further? The way I am thinking of how it should work is that since ac is moving back and forth, it's using itself as a return, so if L2 is pushing towards L1 but they are moving in the same direction, things would seem to be right, but that's a nono. So I'm confused because since instead when shifted in polarity L2 is now pushing towards L1 which is now pushing back since it's opposite.
How does that add up? It would seem it should subtract, like in the DC example where 5v takes away from 12v...

I found a picture to draw on. Look at my confusion:
::"image removed due to hosts image bandwidth constraints"

Last edited: Jan 23, 2010
7. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Yep.
The +12v is more positive than the +5v supply.
To the fan, the +5v supply becomes its' return path, and the +12v it's supply.
The fan doesn't know anything about "ground"; it's isolated from it.

Have you looked at our E-books on AC and transformers?

They might help you understand that whole thing.
In dry air at sea level atmospheric pressure, it takes about 3,000,000 volts to jump a 1 meter gap of air. To scale that down, it takes about 3,000 volts to jump a 1mm gap, which is about 0.039" or roughly 1/26 of an inch.

However, once the air across a gap is ionized, that gap can be considerably widened. Ever seen a Jacob's Ladder in the old science fiction movies like Frankenstein? A spark gap travelling up between two divergent wires, like a "V"? The high voltage causes the initial arc at the bottom of the V, and the hot ionized gas rises upwards due to it's lighter weight than the surrounding air. Eventually, the arc becomes too long to sustain at the top of the V, the arc terminates, and then re-starts at the bottom of the V.

Take a look at the attached. It's a plot of what L1, L2, and Neutral look like. Notice that the vertical distance (voltage) from L1 to Neutral is always half what it is to L2, right up until they cross at 0v.

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8. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
OK, L1 and L2 are equal in amplitude, but are 180° out of phase with each other. When L1 is going more positive, L2 is going more negative, and vice versa. They are "anchored" to ground by the Neutral line.

If you put a load across L1 and L2, there will not be any current to or from the Neutral/Ground, because L1+L2=0.

Example: at a given point in time, if L1=55v and L2=-55v, then L1+L2= 55v +(-55v) = 0v.

Last edited: Nov 25, 2009
9. ### foolios Thread Starter Active Member

Feb 4, 2009
160
1
I will try to be patient and read through that section then reread these explanations.
I will post back with more questions. Thank you for sharing your knowledge and trying so hard to explain it.
I know it's not an easy subject, especially when trying to relate it to someone that doesn't have enough knowledge to relate it to something that they should have had an understanding of first.

10. ### mik3 Senior Member

Feb 4, 2008
4,846
63
When you work with AC voltages you have to consider their phase too.

For example, if you add two 100V AC voltages which are in phase then you will get 200V. If they are 180 out of phase then you will get 0V. The phase difference can be any value and each value will give different output (added) voltages. You can represent the AC voltages with vectors and add them by changing their angle and see the resulting vector.

If you read this you will understand more:

http://www.allaboutcircuits.com/vol_2/chpt_2/1.html

11. ### foolios Thread Starter Active Member

Feb 4, 2009
160
1
Thank you for this.