Could somebody please explain full wave rectifiers using ohms law?

peteb

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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.
Sorry AR if you don’t feel like this is a gentlemen‘s thread.

if you don’t feel like interacting positively, please feel free to not participate at all.

to quote Skynyrd,

“A southern man don't need him around anyhow”
 

BelairPlayer

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and additionally, there is no reason why rectification cannot be described in terms of ohms law. The maxwell equations fully define all facets of electricity including electro magnetic forces and potential, but even they are based on ohms law.
Yes, there is a reason:

Ohm's law states that the R in this relation is constant, independent of the current.[3] If the resistance is not constant, the previous equation cannot be called Ohm's law, but it can still be used as a definition of static/DC resistance.[4]

Source:https://en.m.wikipedia.org/wiki/Ohm's_law

In ac circuits, there is no resistance. There is reactance. Reactance in a rectifier is continually changing. You now have to account for watts reactive, watts real, and watts apparent. Power factors come into play. The equations become much more complicated and they don’t really explain why rectification is occurring, just what the instantaneous circuit state is at any given moment.
 

BelairPlayer

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No. Reactance is the opposition to current flow caused by inductance or capacitance. Resistance can exist in an a.c. circuit. Resistance combined with reactance is impedance.
I’m tired. Agree 100%. I maintain my original point with the word impedance inserted everywhere I used reactance. 👍🏼
 

SRHmusic

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Just a note on this discussion of Ohms Law. The Wikipedia article lays it out pretty well. Ohm's "law" isn't really a law. It was based on empirical (experimental) observations. Maxwell's Equations still pretty much hold everywhere, and handle varying quantities well (calculus, basically, but even in multiple dimensions). Ohm's Law still pretty much holds in many practical situations with adjustments for varying quantities, but it's not exactly the original limited form. And for constant resistance (assumption) the basic V=IR form is still useful for circuit calculations, and initial teaching. Also |V|=|I||Z| holds for complex impedances, too. It's a useful relationship, with caveats.

@peteb Can you expand on what you mean by 'away from the center'?
And, by the way, @andrewRneumann did come back after reading up more and the discussion continued.

(edited for clarity)
 
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peteb

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The power transformer excites the electrons up to a voltage. At each step of the way through the bias resistor, the tube and the load, there is a voltage drop that will follow Ohm's law. When the electrons get back through the diodes they are all tired out but the transformer excites them again, pumping them back up to a negative voltage.
Thanks Philosofriend.

you see it the closest to how I see it.
 

peteb

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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.
Thanks d Sutton.

that sounds reasonable. I am still interested in seeing this done on a modeling app.
 

peteb

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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).
Thanks SRHmusic,

this sounds reasonable.



thanks everyone for a lot of other reasonable replies, that I didn’t comment on.
 

peteb

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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.)
Thanks SRHmusic, I may need to try LT spice.
 

dsutton24

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isn’t that Ohms’ law?

No. Ohm's law describes the relationship between current, voltage, and resistance. It doesn't explain why current flows.

At some point the argument gets silly. There's an old saying around the trades: Too many people want to learn the tricks of the trade without learning the trade. You're assembling a bunch of disjointed ideas and turning them into a self-styled Grand Unified Theory. At some point you just need to pay your dues and learn the basics. As I said about forty pages ago, if you get yourself a good grounding in theory a lot of this stuff will become self-evident.

They are sympathetic with each other which means they do not conflict and are copacetic.

That's ludicrous. There are a lot of things that don't conflict, but are not equivalent to one another. If you're going to use the word 'copacetic' as a technical derivation I can't take you seriously any more.
 

SRHmusic

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They are sympathetic with each other which means they do not conflict and are copacetic.
Ohm's "Law" is a narrow result that was first found experimentally, and that does fall out of Maxwell's equations under the same strict assumptions. But Maxwell's equations cover a huge range of situations, and can be used to derive everything in the rectifier circuit from the transformer action to the diode behavior, and more with varying temperature effects, nonlinear device behavior with voltage, etc. Ohm's Law is a handy rule of thumb, that is at best a subset of Maxwell's Laws. It's similar (slightly) to how Newton's laws of motion can be derived from relativity.
 

rdjones

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No. Ohm's law describes the relationship between current, voltage, and resistance. It doesn't explain why current flows.

At some point the argument gets silly. There's an old saying around the trades: Too many people want to learn the tricks of the trade without learning the trade. You're assembling a bunch of disjointed ideas and turning them into a self-styled Grand Unified Theory. At some point you just need to pay your dues and learn the basics. As I said about forty pages ago, if you get yourself a good grounding in theory a lot of this stuff will become self-evident.



That's ludicrous. There are a lot of things that don't conflict, but are not equivalent to one another. If you're going to use the word 'copacetic' as a technical derivation I can't take you seriously any more.
"Copacetic" is a fictional word, created to legitimize a constructed concept in a novel.
By using it in a technical sense reduces the discussion to creative over-simplification.
 

peteb

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Ohm's "Law" is a narrow result that was first found experimentally, and that does fall out of Maxwell's equations under the same strict assumptions. But Maxwell's equations cover a huge range of situations, and can be used to derive everything in the rectifier circuit from the transformer action to the diode behavior, and more with varying temperature effects, nonlinear device behavior with voltage, etc. Ohm's Law is a handy rule of thumb, that is at best a subset of Maxwell's Laws. It's similar (slightly) to how Newton's laws of motion can be derived from relativity.
Sounds reasonable, thank you, SRH music.
 

Paul-T

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I teach physics to 16 and 18 year olds and these explanations are confusing me. I think robrob's explanation is the best.

Ohm's law applies to metallic conductors at a constant temperature: a diode is a semiconductor, and operates entirely differently.

I wouldn't say electrons are 'attracted' by anything. An EMF supplies them with energy: just like I might lift a ball to the top of a hill and give it potential energy. The ball will then roll down that hill and dissipate that energy.

Electrons are being 'pushed' by the battery or EMF in the sense that the battery's electromotive force moves them to a higher energy state, with more potential energy, like the top of a hill. If given a path to earth, ie a lower energy state, they will travel down the energy gradient just like a ball rolling down a hill.

(except for when we are looking at quantum quantities of electrons, in which case one or two might just pop to the top of a hill on their own without being pushed...)
 

andrewRneumann

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I teach physics to 16 and 18 year olds and these explanations are confusing me. I think robrob's explanation is the best.

Ohm's law applies to metallic conductors at a constant temperature: a diode is a semiconductor, and operates entirely differently.

I wouldn't say electrons are 'attracted' by anything. An EMF supplies them with energy: just like I might lift a ball to the top of a hill and give it potential energy. The ball will then roll down that hill and dissipate that energy.

Electrons are being 'pushed' by the battery or EMF in the sense that the battery's electromotive force moves them to a higher energy state, with more potential energy, like the top of a hill. If given a path to earth, ie a lower energy state, they will travel down the energy gradient just like a ball rolling down a hill.

(except for when we are looking at quantum quantities of electrons, in which case one or two might just pop to the top of a hill on their own without being pushed...)

I learned more in high school physics then probably any other class. It was where the physical world finally started to make sense. Physics teachers rock, and I salute you.

Agree with everything you said, but one statement in your battery paragraph seems misleading.

If given a path to earth, ie a lower energy state, they will travel down the energy gradient just like a ball rolling down a hill.

I used to think that if I touched the terminal of a battery, and I was grounded, I could get shocked. After all, there is a voltage printed on the battery and I can be a “path to earth.” No one ever explained to me that’s not how it works. With a battery, it’s not a path to earth that matters, it’s a path to the other terminal.

For whatever reason, I believed that there was an accumulation of electrons on the negative terminal, and if I touched it, electrons would flow through me to earth—just like static electricity flows when you touch a metal door knob after walking across carpet in the winter. I was wrong. There is an EMF in the battery, but it is measured from one terminal to another. Earth doesn’t come into it. Current flows only if there is a relatively low-resistance circuit between both terminals of the battery.

I only write this because somewhere out there is a young me, trying to make sense of the lowly battery. Hopefully I helped him understand. Maybe he’ll make better life choices because he’s not afraid of getting shocked by touching the terminal of a car battery. :D
 




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