Need help with center tap Xformer

[rshayes]

An ideal transformer is a device that has these characteristics:
1) It has a turns ratio, N:1
2) The primary voltage is N times the secondary voltage.
3) The primary current is 1/N times the secondary current.
4) The secondary current is in the opposite direction from the primary current.
The fourth condition indicates that the transformer does not store energy, since the power in equals the power out.<p>There are many ways that a transformer can be non-ideal. The best that you can hope for is a "good" transformer over a restricted frequency range. One factor is the inductance of the primary winding. This can be represented as an inductance across the primary terminals. Ideally, this inductance is infinite. In practice, this causes an additional primary current in addition to that caused by the secondary current. In a good audio transformer, this inductance may be several hundred henries and the additional current may be only 5 or 10% at the lowest frequency that the transformer is designed for. It becomes less at higher frequencies. In a power transformer, this current may be a substantial fraction of the reflected secondary current. This is tolerable since it is nearly 90 degrees out of phase with the line voltage and does not result in very high losses. This allows a smaller core to be used.<p>The primary and secondary windings have winding resistances. These are usually represented as resistances in series with the primary and secondary terminals.<p>Some of the magnetic field generated by the primary or secondary is not enclosed by the other winding. This is represented as a leakage inductance, usually in series with the primary winding.<p>Both the primary and secondary have stray capacitances. These are usually placed in parallel with the primary and secondary terminals.<p>Loss in the transformer core due to eddy currents and hysterisys can be represented as a resistance in parallel with the primary.<p>I used an ideal transformer in my previous post since this describes the essence of a transformer's action without being distracted by the non-ideal characteristics of the transformer. In practice, the best you can hope for is that none of the non-ideal behaviour causes a significant problem.<p>[ March 13, 2005: Message edited by: stephen ]</p>

[rshayes]

Incidently Chris, you are doing a great deal of barking up a tree that has nothing in it.<p>There is no difference between alternating current and direct current. Direct current is simply alternating current of zero frequency. All of the physics and mathematics that apply to alternating current also apply to direct current. Some of the math is simpler because the frequency dependent terms become zero or infinity.<p>Fourier analysis can give a DC term as well as AC terms. Multiplying two signals together can give either AC or DC terms. Laplace transforms can represent DC as well as AC, and so forth. DC can be sent down transmission lines as well as AC. There is no basic physical difference between the two.

[Chris Smith]

Steven
You confuse the Theory and Math, with the actual facts.<p> The voltage /current still moves electrons in one direction and then the other. The Mathematical SUM, is irrelevant to this fact.<p>Electrons move from orbit to orbit, and only in a mathematical summary at the end of the day, does the “Theories” and “laws for counting beans” and basic Math come into play for the purpose of bean counting in a mathematical manner of “just how many electrons did move? The fact is, they moved, and Alternated directions in doing so. <p> Algebra does not negate a motion of electrons, It merely makes it easier to count them by placing all actual numbers to one side of the equation as if it were a DC formulae. <p>However, because the current moves actual electrons, and because there are a finite number of electrons in each Amp, You are incorrect. <p>The physics backs up this statement. <p>Whether or not they SUM at the end of a day into a mathematical number for OUR understanding has little to do with what the actual electron is doing in its actual days worth of movements. <p>And still at the end of the day, one word wins out over all presuppositions, IT STILL “Alternates”. <p>Never confuse the mathematical theories with the realities. Their purpose is for Math Purposes, and not the Physics of what actually happening in the electron world.

[Bernius1]

I don't want to simply throw another terd in the pool, but; In most of my textbooks, rectified DC is shown as consecutive parabolas ( this may be 2-diode center-tap rectification ). However, going through a regular bridge rectifier, it seems that the AC pole reversal occurs at the peaks, not at zero. So for an input sine wave, the output should be sine, where the 'lower' peaks rest at ground, and the entire wave is 'above' ground. You could say that as long as the slope of the sine wave is positive, the two diodes which are on will remain on, until the slope becomes negative, and the pairs of diodes switch states. Correct ?

[rshayes]

Hello no_vice:<p>The actual waveform would ideally be a series of half sine waves if the input is a sine wave. A parabola is actually a pretty close approximation to a half sine wave, and the difference is hard to see with your naked eye.<p>In a full wave rectifier, the most positive end of the winding is connected by its diode to the load. The other end of the winding is negative and its diode does not conduct current. The center tap is connected to ground.<p>When the waveform goes through zero, the half of the winding which used to be negative is now positive and connected to the load by its diode. The other winding is disconnected.<p>In effect, the load is alternately connected to the most positive voltage at any given time. Only half of the winding supplies current at any given time.<p>In the bridge rectifier, two of the diodes act in a similar way to connect the most positive end of the winding to the load. The other two diodes connect the most negative end of the winding to ground.<p>In effect, both end of the winding are reversed as the waveform goes through zero. The full winding is supplying current all of the time. With a given winding, the bridge connection will supply twice the output voltage.

[rshayes]

Hello Chris:<p>First: I see that you are still barking.<p>Second: Your Physics and Chemistry are about 75 years out of date.<p>Current is not carried by electrons sequentially hopping from atom to atom. This would require that they move at the speed of light. This cannot be obtained with any reasonable accellerating field. I believe the the physicists are up in the Tera electron-volt range and they haven't gotten there yet.<p>Current can be carried in either direction without changeing the direction of electron motion. Consider two electrons, one moving east and one moving west, both moving at the same speed. There is no net movement of charge. If you slow down the east-bound electron and speed up the west-bound electron, there will be a transfer of charge to the west. I can transfer charge to the east by slowing down the west-bound electron and speeding up the east-bound electron. This can create an alternating current without changing the direction of motion of either electron.<p>Now expand this to an infinite number of electrons,half moving east and half moving west. I can still transfer charge in either direction without changing the direction of any electron. This is still almost true if the electrons start out with random velocities. In this case, only a few electrons with the lowest velocities might change direction.<p>Now expand this to three dimensions. I can transfer charge in any direction without significantly changing the direction of motion of most of the electrons.<p>An individual electron will probably change direction for other reasons long before the applied field changes. On the average, the charge will move in response to the applied field in the same way, whether the field is steady or alternating.<p>Electronic circuits do not normally deal with individual electrons. The average is what matters, and the average is what the equations normally used describe.

[MrAl]

Hi there,<p>I dont know if this helps or not, but how you look at ac current depends on
what you are trying to analyze or how you want to look at it.<p>In the time domain, ac current is described by a sine wave which goes both
positive and negative and so it changes direction every half cycle.
In the frequency domain, ac current flows in one direction and so the equations
can be written just as if it were dc current flowing (and of course using
complex variables when appropriate).<p>Of course time domain analysis seems closer to reality, but that doesnt mean
frequency domain analysis is totally useless, or is not close to reality.<p>Examples:<p>Time domain voltage: v(t)=sin(wt)
Freq domain voltage: V(s)=Va(s)<p>Time domain resistor: R
Freq domain resistor: R<p>Time domain cap current: i=C*dv/dt
Freq domain cap current: I(s)=V(s)*sC<p>
Note in the above examples working in the freq domain simplifies everything to
algebra, while working in the time domain means having to use trig and calculus
for some things. Working in the freq domain also means you can assume current
flow is in a single direction, similar to dc current flow.<p>Hope this helps <p>
Take care,
Al

[Bernius1]

Stephen, I alluded to your post in a different thread. If you slosh water in a roasting pan, you feel net forces on the outside, without actually losing any water. Like a capacitor. As the charge sloshes back & forth, the variances are output as an AC voltage, albeit small, which we call 'signal'. But the voltage only, NOT actual electrons, or current, is commuted. So, in the thread about AC vs. DC for line power, I said that AC can power a home better because the capacitive effect of the earth ground ( theoretically infinite ) will allow transmission even without a 'near-zero-resistance' ground path. To me, that explains why poor grounding in a house results in various voltages which are not divided by resistance.
Dick Feynman would add that low frequency photons actually do the voltaic work.

[Chris Smith]

Steven, You shoot your self in the foot eloquently. First you state that they don’t need to move in a direction, then you make claim that they move in all directions at the same time? <p>As to the speed of light and the electron, that’s exactly what it does and as stated, is slightly slower in copper because of its path ways and resistance. Electricity travels in a copper winding at approximately the speed of light. <p>In order to measure one Amp, it has to have a finite charge of electrons which is counted in theory and given a name and amount and this figure and name, called the coulomb, has apx.6.241 506 x 1018 e.<p>These numbers of electrons actually alternate and can be counted as doing so to form the AC Amp, thus forming the Statement “Alternating. Current. <p>The mere fact that most if not all of the electrons move in this orderly fashion is depicted by the need of the diode which acts like a ratchet, allowing these electrons moving in one direction to pass through the gate and not be allowed to return through the gate, thus further reinforcing the words Alternating Current as they move, and they are restricted from migrating back, thus once again, the words “alternating current”,....which blows all the hypothetical theories out of the water that state AC moves in a clock wise or anti clock wise fashion, except for one half of each cycle before changing directions. <p>AC even pulls the electrons back through the opposite diodes, that are opposite the positive ones equaling the balance of electrons in the total figure. That why some face one direction, while the others oppose this direction. <p>You can argue your point all day, but until you solve your semantic problem with the word “Alternating” you wont be able to win. Perhaps you can rename Ac to be called “Fuzzy Current” and then depict the electron as being random, traveling in a unorganized fashion with some going up and down and sideways, but until that day arrives, The electrons [for the most part, the majority because not all electrons have the same force on them equally] move in a orderly fashion in one direction through the gate, [@ 6.241 506 x 1018 e per amp] and then change directions to move in the opposite direction and thus they are named AC because of this simple fact.<p>If the right hand rule of EMF is a fact and is accurate for a DC current, then logic dictates that both directions must apply to AC, which is merely DC acting in Two Axially different directions, for the two halves that make up one cycle. <p>As the force is applied in one direction, it is also removed in the other direction with each rotation of the armature thus first pushing, and then pulling, as the current also follows this motion.
**********************************
The right-hand rule<p>Remember the magnitude of the force felt by a charge q moving with velocity v through a magnetic field B is:
F=QVsin0
As stated previously, the direction of the force is perpendicular to both v and B. Of course, if both v and B are pointed in the plane of the screen, F could be either into or out of the screen to be perpendicular to both vectors. One uses the right-hand rule to determine the direction of F.
To find the direction of F use your right hand and:
Point your thumb into the direction of v.
Point your fingers into the direction of B.
The direction of the force will be out of your palm. Your fingers will curl into the direction of the force.
If the charge is negative, one must remember the direction of the force will be opposite. To calculate the force on a segment of wire, use the direction of the current I instead of v.
If you had used your left hand the force would have been directed oppositely. It seems as if this violates fundamental American principles of equality. However, we will learn that the direction of B is also determined through a right-hand rule, and the application of two right-hand rules to get to something meaningful (the force) means that two left-hand rules would have given the same result. Thus the laws of electromagnetism do not favor right-handed vs. left-handed people, they only favor consistency. If a physical result depended on the right-handed rule that would constitute violation of parity. This indeed occurs in weak decays which will be studied at the end of this course. In weak decays, if one considers a nucleus where the charge rotates as your fingers wrap around your right hand, the emitted electrons all go along the direction of your thumb. Curiously enough, if one did the same experiment with antimatter, one would have to use one's left hand to find the direction of the emitted antielectrons.
http://www.pa.msu.edu/courses/1997sprin ... drule.html<p>[ March 15, 2005: Message edited by: Chris Smith ]</p>

[rshayes]

Hello Mr. Al:<p>My point exactly. AC currrent is physically no different from DC current. Both are simply convenient to generate and analyze.<p>DC can be represented in Laplace form as a unit step. Then do the analysis and take the limit as time goes to infinity. Only a masochist would do this but it could be done. There are also transforms for the sine wave and cosine wave (ie alternating current). The analysis procedure is the same.<p>Then you get to fourier analysis. Here DC and AC are merely two different frequencies.<p>My point, which Chris seems determined to ignore, is that there is no physical difference between AC and DC. Both are simply waveforms that are exceptionally easy to generate and to analyze.<p>DC makes analysis easy. All of the reactive elements disappear.<p>Analysis with sinusoidal waveforms is a little more complex, but still simple. Due to the characteristics of the sine and cosine, the solution to the analysis of a linear circuit excited with a sine wave will be a sine wave, possibly of a different phase. I suspect that this is unique to the sine and cosine waves.<p>That is the main benefit of these special cases. It is actually possible to analyze networks with five or six parts by hand (pencil and paper) and get a usable result before an algebraic error makes the whole analysis useless.<p>DC can be generated with a battery. AC can be generated with an alternator. These are the easy cases. Other waveforms are a bit harder.

[Chris Smith]

Steven<p>The only point[s] of contention here was “Does AC like DC go in a clock wise fashion”?<p>NO It alternates each cycle. <p>The right hand rule for Electro Motive Force backs this up.<p>Can math and algebra be used to formulate it into a DC like component to account for the same work done, YES. But this was never part of the contention. <p>From the original post...
Statements like this are false... <p>””If the primary side has positive on the top and negative on the bottom then primary current... ““
“On the secondary side the top would be positive and the bottom would be negative. “
“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.“<p>[ March 16, 2005: Message edited by: Chris Smith ]</p>

[rshayes]

Actually, all four of these statements are true. The terminology is also correct, and has been in use for at least 50 years and possibly 100 years. I don't have a copy of Steinmetz' text, but he may have used these conventions and terminology as early as 1900. The books that I have from the 1930's and 1940's are mainly oriented toward graduate students, and they assume knowledge of these conventions.<p>"If the primary side has positive on the top and negative on the bottom then primary current would flow counterclockwise for this cycle."<p>This statement establishes a reference polarity and an assumed direction of current flow. Counterclockwise is an acceptable way of indicating the direction of current flow around a loop. From his definition, it appears that he is using the "electron current" convention, and this is perfectly acceptable.<p>"On the secondary side the top would be positive and the bottom would be negative."<p>This defines the polarity of the secondary winding with respect to the primary. Another way of doing this would be to use dots to identify the polarity of the secondary, but this way is adequate.<p>"On the top half the center tap would be negative and current would flow counterclockwise."<p>This reiterates the previous definition of polarity for the secondary and defines an assumed current direction for the loop containing the top half of the secondary. Again, he appears to be using the "electron current" convention.<p>"To get both +/- without a CT is more difficult. You need to reference them to ground somehow."<p>Until this winding is referenced to ground by an external circuit, the voltages with respect to ground at the plus and minus terminals are undefined. Using a single winding and rectifier to provide two supply voltages is not common, but it has been done on occaision over at least the past fifty years.<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>This defines the polarity of the bottom winding, even though that could be inferred from the description of the winding as center tapped. It continues the use of the "electron current " convention, and indicates (with possible ambiguity) that the center tap current is the difference of two loop currents.<p>In AC circuit analysis, the directions of the currents in each loop are usually unknown until the analysis is complete. It is usual to assume that they are all clockwise (or counterclockwise), set up the equations, and do the analysis. The results of the analysis will identify the magnitude and phase of the current in each loop relative to the assumed phase and this will allow the determination of its magnitude and direction at any given time.

[philba]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by stephen:
...
"To get both +/- without a CT is more difficult. You need to reference them to ground somehow."<p>Until this winding is referenced to ground by an external circuit, the voltages with respect to ground at the plus and minus terminals are undefined. Using a single winding and rectifier to provide two supply voltages is not common, but it has been done on occaision over at least the past fifty years.
...<hr></blockquote><p>This is usually done so as to use an AC wall wart. Not pretty but it does allow one to avoid having to get UL approval.

[rshayes]

It used to be done even with vacuum tubes. Getting a positive supply was easy, that was simply a full wave rectifier. This was one tube with two plates sharing a common cathode. A negative supply would require two tubes, since the cathode couldn't be shared and no one made a tube with two cathodes and a common plate. Heating the two cathodes might require two more transformer windings also. This could get expensive.<p>Some DC coupled oscilloscopes needed a negative supply to allow the input grids to be at ground potential. A negative supply might also be needed to shift the level of the signals between stages. A cheap way of getting this supply was to increase the voltage of the positive supply and disconnect it from ground. A voltage divider across the supply would have its tap connected to ground. This would put the negative terminal of the floating supply below ground. The positive terminal would be above ground. Most of the load would be connected between the two power supply terminals. This reduced the current necessary in the voltage divider.

[philba]

I have to say those tube eletronics designers were incredibly good at efficient use of components. I have an old AK Receiver that I'm restoring. They used the speaker bias coil as a power supply filter!! I bet its rated in henrys. I'm not sure why there was no 60hz hum, though...