Could somebody please explain full wave rectifiers using ohms law?
- Thread starter peteb
- Start date
Jack S
Friend of Leo's
I see a basic misunderstanding of simple DC circuitry. Electrons are negatively charged and when presented with an imbalance of positive and negative charge will seek the balance. Balance stops electron flow. Electrons do not flow through a battery, they flow from negative to positive and as long as the positive is in excess the electrons will flow. That is current. Eventually enough electrons are stripped away from the negative side that the battery stops working. When Ohm's Law was devised they did not understand fully how current flowed and got it backwards, so, as a rule, it is stated that current flows opposite the actual direction of the electron flow.
Do yourself a favor. Go through this post and remove all referenceas to 'ground' and replace them with 'the other side of the circuit'. I think a lot of people impose a magical meaning to grounds when they there is no magic whatsoever. Ground is just another conductor.
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Maxwell ain't Ohm's law.
And, here-in lies the rub. Belief isn't fact or science. Belief resists all efforts at education.
If I'm following your premise, you ground one of the legs of the transformer and move the diode string to the center tap? You might see some half wave voltage on the output of that rig, but you certainly won't be able to draw any current. The diodes would effectively biased against each other.
Diode Bridge Rectifier Current Flow During Positive Half of AC Wave
All four diodes in a bridge rectifier act as one-way valves that allow current to flow in only one direction. The two diodes on the left side form the 'bridge' from the amp circuit back to the power transformer so a center tap is not required. As the outflow of current (shown with orange arrows) is 'pushed' by the transformer, the return path (shown with blue arrows) is simultaneously 'pulled' by the transformer's negative voltage so a bridge rectifier can extract twice the voltage of a conventional two diode rectifier which only 'pushes' because the transformer center tap is grounded at zero volts and does not pull.
Bridge Rectifier Current Flow During Negative Half of AC Wave
Note: All diagrams in this All About Rectifiers section show conventional current flow when in reality to create a positive voltage electrons must be removed from a conductor (+ voltage = scarcity of electrons, - voltage = excess of electrons).
All rectifiers must have a DC current return path back to the power transformer because the rectified DC flows in only one direction--away from the transformer. The transformer center tap or rectifier bridge provides the current return path back to the transformer.
Although a bridge rectifier will extract twice the voltage from a transformer compared to a conventional rectifier, a bridge rectifier will only supply 62% the rated current at that higher voltage. If you tape off the center tap of a power transformer and replace a conventional rectifier with a bridge rectifier it will generate twice the voltage but the winding wires would have to be of a heavier gauge to generate the same current.
andrewRneumann
Tele-Afflicted
I respectfully disagree with some of your statements. Unfortunately this thread has gone off the rails (was it ever on the rails?). I’m willing to learn a thing or two. Happy to start another gentlemen’s thread if you want to pursue it further.
SRHmusic
Tele-Afflicted
Since I had a similar simulation laying around, here's a bit more that might help illustrate the operation. This has a simple transformer model with a tapped secondary, just like the highlighted section of Leo's schematic in the OP. (The full wave bridge examples above are good, but not exactly corresponding to the schematic in the OP.)
In the plot at the bottom we can see both the voltage at the primary (vac, in red), and the currents through the diodes D1, D2.
Why do the carriers (edited- I'm so used to thinking of positive carriers that I mistakenly copied the OP wording) at high voltage move toward ground? Because they can, and are attracted to the lower potential, pushed by the E-field, whatever explanation you prefer. It's not unlike any other circuit.
[Edit 2, 3 - The OP question was asking how electrons could move to a lower voltage point ("ground"). The answer is related to the EMF induced in the secondary as @andrewRneumann touched on. Each cycle, the potential at the top leg is raised - which means the carriers at that node, e.g. electrons, have their potenial raised, as well. And their potenital is lowered on the other half cycle. The transformer acts sort of similarly to a battery in a sense, though a battery is DC and the EMF arises from electrochemical changes rather than electromagnetics. I agree with @Jack S that there isn't electron current flow in the battery, rather a restoration to a discharged state. But we might think of it as a virtual current. It's similar to a capacitor in that sense.]
The "trick" here is that with the AC on the transformer, on one half cycle leg "a" is at a high potential, forward biasing D1 to the load, and leg "b" is negative with respect to ground, and D2 is reverse biased and does not conduct. On the other half cycle, the situation is reversed with leg "b" high, D2 forward biased, and leg "a" low and D1 reverse biased. If you can see the colors well enough in the bottom plot, you can see D1 conducts when vac is postive (and leg "a" is high), and D2 conducts when vac is negative (and leg "b" is high).
This is in LTSpice. I can post the file if anyone is interested to play around with it. (LTSpice is a free simulator from Linear Technology, now part of Analog Devices.)
(Notes - The voltages, component values, etc. are not representative of anything real, but the circuit functions correctly. This model has equal turns in each leg, so when tapped only 60V at the output for a 120V AC input. Also, Leo used three diodes in series as the reverse breakdown of a single diode is not high enough for the voltages generated at the output.)
In the plot at the bottom we can see both the voltage at the primary (vac, in red), and the currents through the diodes D1, D2.
Why do the carriers (edited- I'm so used to thinking of positive carriers that I mistakenly copied the OP wording) at high voltage move toward ground? Because they can, and are attracted to the lower potential, pushed by the E-field, whatever explanation you prefer. It's not unlike any other circuit.
[Edit 2, 3 - The OP question was asking how electrons could move to a lower voltage point ("ground"). The answer is related to the EMF induced in the secondary as @andrewRneumann touched on. Each cycle, the potential at the top leg is raised - which means the carriers at that node, e.g. electrons, have their potenial raised, as well. And their potenital is lowered on the other half cycle. The transformer acts sort of similarly to a battery in a sense, though a battery is DC and the EMF arises from electrochemical changes rather than electromagnetics. I agree with @Jack S that there isn't electron current flow in the battery, rather a restoration to a discharged state. But we might think of it as a virtual current. It's similar to a capacitor in that sense.]
The "trick" here is that with the AC on the transformer, on one half cycle leg "a" is at a high potential, forward biasing D1 to the load, and leg "b" is negative with respect to ground, and D2 is reverse biased and does not conduct. On the other half cycle, the situation is reversed with leg "b" high, D2 forward biased, and leg "a" low and D1 reverse biased. If you can see the colors well enough in the bottom plot, you can see D1 conducts when vac is postive (and leg "a" is high), and D2 conducts when vac is negative (and leg "b" is high).
This is in LTSpice. I can post the file if anyone is interested to play around with it. (LTSpice is a free simulator from Linear Technology, now part of Analog Devices.)
(Notes - The voltages, component values, etc. are not representative of anything real, but the circuit functions correctly. This model has equal turns in each leg, so when tapped only 60V at the output for a 120V AC input. Also, Leo used three diodes in series as the reverse breakdown of a single diode is not high enough for the voltages generated at the output.)
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The diodes attached to the HV secondary of TR1 are not a full bridge.The signal passes through two 7025 tubes before feedin into the 12AT7 twin triode. Each side of the triode feed a control grid of a 6L6GC tube in a push-pull fashion. The incoming signal first turns on one side of the dual triode which in turn fires off it's companion 6L6. the 6L6's are tied to opposite ends of the primary of TR3 which converts the HV signal into low voltage current to drive the speakers.The little white triangles are a common signal ground. The HV secondary is connected through the chassis to the common between the 6L6 tubes completing the circuit. Remember, AC voltage/current travels in both directions before it goes through the rectifier. Electrons flow up through the ground into the 6L6 tubes toward the HV supply, first through one tube and then through the other.
Jack S
Friend of Leo's
I am not looking to start anything, but if you want to start another thread to discuss, I share what I know and you can tell me where you disagree. I would like to know where you disagree.
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It is not so much about "ground", it is more about syntax. Other than actual earth, "ground" is typically a syntactical reference to a common circuit point that may not be at earth potential. I seem to recall some instructors/engineers making a big deal out of incorrectly using the term "ground" instead of "common". But so much for that.
When a charge potential exists such as with a (good) battery, and there is a completed circuit, the electrons will flow from the more negative to the more positive potential as a current through some resistance I=V/R (even a short piece of wire has some resistance) and will be dissipated as power P=V*I (or sometimes I^2*R losses). Side note: A battery does contain some internal resistance.
Electrons will flow into or out of ground (common) depending on the signal polarity relative to this reference point. "Conventional" current flow can be explained as "hole flow" where the gaps between the electrons appear to move in reverse. Whether or not they move as if banging bowling balls is another issue.
When a charge potential exists such as with a (good) battery, and there is a completed circuit, the electrons will flow from the more negative to the more positive potential as a current through some resistance I=V/R (even a short piece of wire has some resistance) and will be dissipated as power P=V*I (or sometimes I^2*R losses). Side note: A battery does contain some internal resistance.
Electrons will flow into or out of ground (common) depending on the signal polarity relative to this reference point. "Conventional" current flow can be explained as "hole flow" where the gaps between the electrons appear to move in reverse. Whether or not they move as if banging bowling balls is another issue.
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another thing is people tend to think of ground as the end of the circuit. it is not, it's the half way point. in AC, using conventional flow, the power from the top of the sine wave flows from the windings, through the diodes, to the load... to ground. it cant stop there, it has to return to its source. a circuit is a circle right? (Jerkoffs Law). also.... you can't have a north pole without a south pole.... anyway, the electrons flow through the "ground" to the other set of diodes which are connected to the coils in the negative half of the sine. that puts the two coils in series, with the load in between. and, it matters not, series is series. visualize the 2 halves of the sine wave as separate power sources. when you are not at the peak of the sine, both polarities are working together. because they are in series, on opposite ends of the circuit the voltages of the two halves add. it is even more apparent on 3 phase
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SRHmusic
Tele-Afflicted
@popcat I think you put it succinctly in your post #3 (https://www.tdpri.com/threads/could...ctifiers-using-ohms-law.1099413/post-11345856 ), and @andrewRneumann I think you also have touched on the confusion with (and importance of) EMF. (OTOH, I think the ground concept is important but not quite at the center of the question here. The same results occur regardless of which node is chosen as ground.)
In the spirit of 'there are no dumb questions,' I gave this some more thought about the basic question of why things keep going in a circuit and how to explain it.
The overview in the Wikipedia page notes the definition of electromotive force: An electromotive force causes charge separation that then leads to the potential difference seen at the terminals of the source (e.g. a battery or a transformer secondary). For a battery, the charge separation is caused electrochemically when it's charged, and in coupled inductors (secondaries) it's caused electromagnetically by the changing magnetic flux from driving AC current into the primary. (@andrewRneumann mentions this in his post.)
So @peteb 's question in the original post kind of hit on this basic question/concept of why things keep going in the circuit. In this case, the AC current from the generator way back at the power station induces EMF in the secondary, causing charge separation and continually providing the energy to do the work of keeping the charges moving (work = energy = force on something moved through a distance in an opposing field). This this continuing, periodic charge separation that leads to the potential (voltage) difference at the secondary, and those separated charges are the ones the keep moving around to get back to a lower overall state (electrons toward positive potentials).
(This reminded me of what my high school electronics shop teacher used say. Since it's AC, the electrons really are just pushed back and forth. "You're not buying electrons from the power company. The same electrons that were in your TV when you bought it are still there."
)
en.wikipedia.org
Also, here's a decent online text on EMF, Faraday's Law, more history of the early experiments, etc.:
phys.libretexts.org
In the spirit of 'there are no dumb questions,' I gave this some more thought about the basic question of why things keep going in a circuit and how to explain it.
The overview in the Wikipedia page notes the definition of electromotive force: An electromotive force causes charge separation that then leads to the potential difference seen at the terminals of the source (e.g. a battery or a transformer secondary). For a battery, the charge separation is caused electrochemically when it's charged, and in coupled inductors (secondaries) it's caused electromagnetically by the changing magnetic flux from driving AC current into the primary. (@andrewRneumann mentions this in his post.)
So @peteb 's question in the original post kind of hit on this basic question/concept of why things keep going in the circuit. In this case, the AC current from the generator way back at the power station induces EMF in the secondary, causing charge separation and continually providing the energy to do the work of keeping the charges moving (work = energy = force on something moved through a distance in an opposing field). This this continuing, periodic charge separation that leads to the potential (voltage) difference at the secondary, and those separated charges are the ones the keep moving around to get back to a lower overall state (electrons toward positive potentials).
(This reminded me of what my high school electronics shop teacher used say. Since it's AC, the electrons really are just pushed back and forth. "You're not buying electrons from the power company. The same electrons that were in your TV when you bought it are still there."
)
Electromotive force - Wikipedia
Also, here's a decent online text on EMF, Faraday's Law, more history of the early experiments, etc.:
23.5: Faraday’s Law of Induction- Lenz’s Law
Faraday’s experiments showed that the emf induced by a change in magnetic flux depends on only a few factors. First, emf is directly proportional to the change in flux ΔΦ. Second, …
phys.libretexts.org
Yes, it is why AC transmission is known as standing waves. Trying to simplify explanations for such forums and not write a white paper always leads to more questions anyways -- probably beat this horse enough.@popcat I think you put it succinctly in your post #3 (https://www.tdpri.com/threads/could...ctifiers-using-ohms-law.1099413/post-11345856 ), and @andrewRneumann I think you also have touched on the confusion with (and importance of) EMF. (OTOH, I think the ground concept is important but not quite at the center of the question here. The same results occur regardless of which node is chosen as ground.)
In the spirit of 'there are no dumb questions,' I gave this some more thought about the basic question of why things keep going in a circuit and how to explain it.
The overview in the Wikipedia page notes the definition of electromotive force: An electromotive force causes charge separation that then leads to the potential difference seen at the terminals of the source (e.g. a battery or a transformer secondary). For a battery, the charge separation is caused electrochemically when it's charged, and in coupled inductors (secondaries) it's caused electromagnetically by the changing magnetic flux from driving AC current into the primary. (@andrewRneumann mentions this in his post.)
So @peteb 's question in the original post kind of hit on this basic question/concept of why things keep going in the circuit. In this case, the AC current from the generator way back at the power station induces EMF in the secondary, causing charge separation and continually providing the energy to do the work of keeping the charges moving (work = energy = force on something moved through a distance in an opposing field). This this continuing, periodic charge separation that leads to the potential (voltage) difference at the secondary, and those separated charges are the ones the keep moving around to get back to a lower overall state (electrons toward positive potentials).
(This reminded me of what my high school electronics shop teacher used say. Since it's AC, the electrons really are just pushed back and forth. "You're not buying electrons from the power company. The same electrons that were in your TV when you bought it are still there."
Electromotive force - Wikipedia
Also, here's a decent online text on EMF, Faraday's Law, more history of the early experiments, etc.:
23.5: Faraday’s Law of Induction- Lenz’s Law
Faraday’s experiments showed that the emf induced by a change in magnetic flux depends on only a few factors. First, emf is directly proportional to the change in flux ΔΦ. Second, …
andrewRneumann
Tele-Afflicted
Meh, I’m not feeling pugilistic. I’m going to read up on batteries and make sure I’m clear on where the electrons start, where they go, and where they end up. My understanding is that they not simply capacitors (IE “charge holders”), electrons do indeed flow through them, and they have only a small internal resistance to current.
Jack S
Friend of Leo's
Two unlike materials in a solution (electrolyte) creates ions. The ions create a buildup of electrons on the anode. This is not taking the electrons from the cathode and pushing them through to the anode. It is a chemical reaction between the unlike materials creating ions that completes the circuit. The electrons themselves do not move through the battery. At a certain point the unlike materials can no longer create ions and the battery dies. In this sense I referred to the stripping away of the electrons. In fact, what actually happens is a balance has been reached no longer allowing electron flow due to chemical changes within the unlike materials. This is how I understand it.
andrewRneumann
Tele-Afflicted
Ok I read this fascinating blog
https://www.comsol.com/blogs/does-the-current-flow-backwards-inside-a-battery/
And it turns out you are right. The electrons do not complete the circuit. The circuit is completed internally in the battery by positively charged cations migrating in the conventional current direction. So I think I just took the abstraction a little too far. The abstraction that a battery is nothing more than a DC voltage source with a small internal resistance in series, although useful for correctly analyzing a circuit, should not be taken literally as far as electron flow goes internally in the battery.
The question still remains—why do the positive cations migrate from negative to positive? The blog post, which I admittedly didn’t fully comprehend, explains the chemical process that produces this result. I learned that electrical current is not always produced by moving electrons (easy to forget). It can be produced by positively charged particles moving just the same. Very interesting and thanks for pointing out my error.
SRHmusic
Tele-Afflicted
Yes, current is often discussed as counting the electrons that move past a point in time, but really it's the amount of charge that's moving that defines the current, not the carrier (1 ampere = 1 coulomb of charge per second).
That Comsol page is good. Batteries are quite complex, involving materials science, chemistry, physics, etc. in a lot of detail.
Conventional Diode Rectification
Conventional two diode rectifier showing current flow during the positive half of the AC wave. The power transformer's center tap provides the current return path from the amp circuit back to the transformer. The center tap is grounded at zero volts.
Conventional rectifier during the negative half of the AC wave.
Conventional Tube Rectification
A standard dual plate rectifier tube like the 5Y3 works in exactly the same manner as the above two diode rectifier. That's why tube rectifiers are always paired with power transformers with center taps--the center taps are required to provide a current return path from the amp circuit back to the transformer.
Full Wave Rectification
Full Wave Conventional and Bridge rectifiers convert both the positive and negative AC wave (full wave) into DC voltage so they create 120Hz pulsing DC. Compare this graph with the "Half Wave Rectification" graphic above showing 60Hz pulsing DC. Knowing that power line and bias voltage are at 60Hz and high voltage rectified DC is at 120Hz helps with tracking down the source of amplifier hum
Thanks SRHmusic.
very thorough.
are you able to shift the center tap away from center on your model?
that would be interesting to see.
