Need help with center tap Xformer

[Chris Smith]

Glad to Philba
From the first year in any Electronics class room, You will be taught that....<p>It Alternates, it doesn’t go in a circle, it doesn’t flow from top to bottom, the center tap DOES require a diode for full wave if the center tap is used, ect, ect, etc, <p>It ALTERNATES from one direction to the other, at 60 hz in the US. <p>Statements like this are false...<p>”If the primary side has positive on the top and negative on the bottom then primary current... “<p>You have positive on the top, then negative on the top, alternating at 60 times per second. <p>
“On the secondary side the top would be positive and the bottom would be negative. “<p>60 HZ back and forth<p>
“To get both +/- without a CT is more difficult. You need to reference them to ground somehow” <p>Negative is a term relative to a positive value, ground is considered a absolute. Floating grounds or negatives, are found on all batteries and never touch the actual earth. <p>“The current flow in the bottom half is also counter-clockwise. The center tap would be positive relative to bottom. The direction of center tap current would be opposite the current from the top.“<p>There is no counter clockwise, clock wise except at 60 hz.

[philba]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Chris Smith:
Glad to Philba
From the first year in any Electronics class room, You will be taught that....<p>It Alternates, it doesn’t go in a circle, it doesn’t flow from top to bottom, the center tap DOES require a diode for full wave if the center tap is used, ect, ect, etc,
...
<hr></blockquote><p>I dont have a problem with the rotating terminology. If you look at the circuit symbol, it kind of does rotate. I doubt many are confused about what "alternate" means.<p>So, the diode has to be connected to the center tap for full wave. hmmm, how does that work? do you connect the top and bottom to ground? In my first year electronics, they taught me the first circuit I posted. Funny, I sure remember it working. Glad you are able to straighten us out.<p>By the way folks, here's a nice page that describes the various transformer configurations.
http://www.powervolt.com/techan01.html<p>Phil

[Chris Smith]

There is no symbolic flow, as in DC From top to bottom. <p>The current changes back and forth negating any symbolic flow, from top to bottom.<p>And Enzo said it best as to your other question,...Two halves make a full wave, or one long coil makes a full wave by negating the center tap. <p>However using the center tap with diodes halves the voltage while doubling the current, and That’s all it does.

ENZO says....the CT arrangement allows me to get both supplies off one winding rather than using two separate windings and two bridges. At that point I do indeed have a full bridge across the CT winding, but it is operating as a pair of half bridges.

[Bernius1]

Phil, what if you stack two bridge rectifiers? Stacked, with the Pos of one tied to the NEG of the other, using the conjoined AC terminals as a center tap ? The only problem I see is that depending on current draw, the center-tap ground could float wildly. But that in itself may have advantages.....

[philba]

This is, I think (though I'm sure our local savant will have a variant opinion) the correct way to use a bridge and CT xformer to get a bipolar supply.

[Chris Smith]

That forms TWO half transformers producing two separate quantities of voltages available each with half the total current.<p>Tying the center tap to EARTH provides a Negative value to the equation. <p> Floating the center tap to the chassis, also provides a negative value. <p>Used as two separate voltage values, you can have half the current in each at half the available voltage. <p>Combined and minus the center tap produces twice the voltage at half the total current available to the lower voltage.<p>To get twice the current, you pair the two halves in a series mode for the same voltage as each winding, but the current doubles. <p>And still NO Flow in a circle, in a loop, from top to bottom or any of the other incorrect statements expressed before such as......<p>"If the primary side has positive on the top and negative on the bottom then primary current... “ <p>“On the secondary side the top would be positive and the bottom would be negative. ““ <p>“To get both +/- without a CT is more difficult. You need to reference them to ground somehow””

“The current flow in the bottom half is also counter-clockwise. The center tap would be positive relative to bottom. The direction of center tap current would be opposite the current from the top.““

[sofaspud]

All of which could very well be why your electrical outlets are labeled "hot" "neutral" and "ground" rather than any of that +/- stuff.
But if the original question actually referred to a half cycle of the AC waveform....

[philba]

sheesh chris, can you just lighten up??!? How many times are you going to flog people for using "incorrect" terminology? Why don't you take out an ad in N&V to proclaim their complete worthlessness to human kind or something. This is getting ludicrous.<p>If you are going to teach, teach. If you are going beat people up, please just go away.

[Chris Smith]

Philba, people die for less ignorance. <p>Especially with AC. <p>Learning the correct terms, values, and ways of electricity is what a Electrician or electronics person does. <p>That and frying them self or the circuit, when they don’t learn so well. <p>Im not being hard on any one, Im just correcting a massive mistake in terminology. Like the post above, yes the current swings in a clock wise fashion, for each HALF of one cycle. Then It doubles back to bite you in the ass if you think its cold? <p>If you go around telling people that the current flows from top to bottom, I cant think of anything worse than thinking the bottom is some how safe to touch, because its flowing AWAY from me? <p>Its up to us to teach it right, not half ass.

[sofaspud]

I for one appreciate that clarification, Chris, but it's hard for me to imagine what the person who posted the original question has gotten from all this. I'd like to give him the benefit of the doubt and that designating a single point as positive (or negative) throughout the full 360° AC wave is totally invalid. And I take no responsibility if he wants to stand in a tub and work on his toaster.

[Chris Smith]

You hit the nail on the head sofaspud.<p>I can think of the perfect example of how to kill your self by thinking a + reference point [as in the opposite point or lead] flows away from you. <p>The AC variac, has TWO hot leads coming out of it.<p>[ March 11, 2005: Message edited by: Chris Smith ]</p>

[rshayes]

The Physics course that is usually taken before that "first electronics course" usually teaches you that current flows around a continuous path. I don't see any problem with referring to this path as a loop or a circular path.<p>Under the circumstances, referring to the current flow as being either clockwise or counterclockwise is not unreasonable. Of course, a particle physicist might prefer "spin up" or "spin down".<p>The first chapters of books on circuit analysis usually introduce Kirkhoff's Laws. One states that the sum of the currents into and out of a node in a circuit are equal to zero. The second states that the sum of the voltages around a mesh in a circuit is also equal to zero. These conditions apply to both DC and AC. The current around a mesh is usually described as either clockwise or counterclockwise.<p>The direction of the current depends on the polarity of the applied voltage and the convention being used. "Conventional current" assumes that current is caried by positive charges. "Electron current" assumes that the charge carriers are negative and results in a current in the opposite direction for the same polarity of applied voltage.<p>As long as your assumptions are consistent, the results come out the same. Benjamen Franklin assumed that charge carriers were positive. This was shown to be incorrect about 100 years later when the "Edison effect" was observed. Either assumption produces usable results.<p>The polarity of a transformer winding is usually indicated by putting a dot next to the ends of the windings that have the same polarity. All of the dotted ends of the windings will go either positive or negative at the same time. This is shown in Philba's posting. Incidently, that symbol for a transformer is more common in Europe than in the United States.<p>A center tapped secondary winding can be viewed as two secondary windings that are connected in series-aiding. This means that the dotted end of one winding is connected to the undotted end of the other winding. The voltage across the series connected windings will be the sum of the voltages across each of the individual windings.<p>The transformer can then be viewed as two transformers with the primaries in parallel and the secondaries in series. Each of these transformers has only two windings, a primary and a secondary.<p>The critical characteristic of a transformer is the turns ratio between the primary and secondary. If the primary has N turns for each turn on the secondary, the secondary voltage will be equal to the primary voltage divided by N.<p>In an ideal transformer, no primary current flows until there is current in the secondary winding. When a circuit is connected to the secondary winding a current will flow. This current results in a primary current that is divided by the turns ratio, N.<p>Since the transformer does not store energy (ideally) the direction of the current is opposite on the two windings. If current flows out of the dotted end of the secondary, it flows into the dotted end of the primary. In other words, the power flowing out of the secondary is equal to the power flowing into the primary.<p>Since the two primary windings are connected in parallel, the total primary current is equal to the sum of the two individual primary currents. When a full wave rectifier is used, only one of the two transformers will supply current to the load at any given time depending on the polarity of the input voltage. The primary current in the other transformer will be zero since the secondary current is zero. The primary current in the active transformer will be equal to the load current divided by the turns ratio. On the other half-cycle, the other transformer is loaded and the first transformer is unloaded. If a bridge rectifier is used to get both positive and negative outputs, current will flow in both transformers at the same time, but the values may not be equal, depending on the loads on the positive and negative outputs. The primary current will be the sum of these currents divided by the turns ratio.

[Chris Smith]

Your confusing AC with DC currents. <p>AC Current does not flow in any type of circle for more than one half cycle before changing direction. <p>With AC, first its pushed at the speed of light in a clock wise manner, and then Its pulled in the opposite direction at the speed of light in a anti clock wise manner, give or take a few feet per second, in copper wire,... for example.<p>So stating that AC current actually continually flows in one direction is a complete misnomer and a Unreasonable hypothesis to teach to any student.<p>Kirchoffs law is a approximation for summing up total amperage usage, for math purposes, and not a model of actual flow of electrons, acording to physics.<p>The word "Alternating" speaks for it self.<p>[ March 12, 2005: Message edited by: Chris Smith ]</p>

[jimandy]

I'm confused here. Stephen said
<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> In an ideal transformer, no primary current flows until there is current in the secondary winding. <hr></blockquote>
So, assuming we had that "ideal" transformer and then removed the secondary (unwrapped it and threw away the wire) all that would be left is the coil formerly known as Prince - (er uh I mean Primary). Apply an alternating current to THAT coil and there would be current flow - and there would certainly not be current flow in the seconday that is now a ball of fluff on the floor.<p>Wait a minute - maybe there wouldn't be current flow! After all, upon the collapse of the magnetic field upon each reversal of the AC, there would be an opposing emf that would "null out" a measurable current flow...
Wouldn't there? - I'm way in over my head on this one.

[Chris Smith]

Yes its weird, and yes its true.<p>The IDEAL transformer is in such a balance, the push and pull with out a load, equal exactly ZERO. <p>And thus with out a load, no actual current has been consumed in the primary. <p>BUT, in the real world we has R, and thus it never happens. <p>If you take a perfect Gold winding transformer, matched and balanced for impedance, and drop the coil windings to absolute ZERO and run these coils, you will have as close to perfect as one can achieve. ZERO consumption.<p>You can also inject a gold ring with electrons at this temperature, and it will store the excess electrons and “Flow” in a continual circle,... for ever,.... so long as it remains cold.