[Mark Korda]

Hi,in my Basic Electronics book to train the Navy,1955,VanValkenburgh,tubes,I have hit a snag understanding 1/2 wave vs.full wave rectification.I understand that full wave utilizes the negitive part of the wave,but for what reason,why,how,when and all that.If some one out there could explain this a little better than the book does I think I could move on to the filter circuit chapter.Remember,I'm not the sharpest tool in the shed,so you'll have to go slow.Thank you all.When I tend to hit a snag like this(not really understanding full wave rectifying)I've been known to put the texts down and move on to some thing else,but I don't want to just quit,I really want to learn at this stage.Thanks again for any tutoring....sincerely...Mark Korda

[Ericus Rex]

Tubes only conduct current when the signal is in the positive half of the cycle.  So with a half wave rectifier, the tube passes half the sine wave, then stops conducting altogether during the negative half of the wave, then conducts again when the signal returns to positive.  So you have complete gaps in conduction during half of the sine wave cycle; not very efficient.  A full wave rectifier is two half wave rectifiers in one tube.  But the two halves are wired on opposite sides of the  center-tapped transformer; when one half goes negative (and cuts off), the other goes positive (and conducts).  So when you sum up the two halves after rectification, you have a continuous, bouncy positive wave that never has any gaps and never goes negative.  Then the signal is filtered through caps and/or inductors to make it flatter and less bouncy.  Does this help?

[Ericus Rex]

I've come up with some analogies to help me understand electronics.  Maybe this will help you too.

Think of the center-tapped tranny as a see-saw; the pivot is the center tap (ground), which is what both see-sawers cycle around.  When one person goes up, the other goes down.  But one person is always going up (except for the split second when they change direction).  Now imagine there is a pump of some sort attached under each person that operates only when that person is going up.  The pump would be the rectifier.  If there is only 1 pump under one person you'd have a half-wave rectifier; the pump would only work when that one person was going up.  There would be no pumping at all when that person was headed back down.  If you have a pump under each person you'd have a full wave rectifier; the combination of the two pumps means that the pump system is working all the time, with no gaps, since one of the two persons is always heading up and pumping (rectifying).

More confused now?

[Mark Korda]

Ericus,that helps tremendously.With what you wrote,and me looking back at the diagrams with the arrows showing the path of current,I think I can finally grasp it.I don't think the book had stated very clearly what you just did.I'm still a little foggy on how the neg. becomes positive to fill in the gaps on the 1/2 wave signal,but like every thing else,I'll have to read it over a few times,just as many times it takes me to remember someones name before it's cemented in my noggin....thanks Ericus...you the king!....Mark K

[Mark Korda]

Ericus,I typed you a message while your second answer was coming in.I just read it,and that was the perfect and easy way to explain to some one who was in the lower percentile on their SAT's,yours truly.In fact I just got some new ink for my printer,and I'm going to post that (pump)message on the wall right in back of my computer screen so it's drilled into me.You should be a teacher!,if you already aren't....thanks again...Mark.

[Ericus Rex]

You're welcome, Mark!

I'm actually a lot like you.  I just started a few years earlier.  I don't think like an EE and reading most books is like reading greek, I can't seem to get all that stuff to stick in my head.  I consider myself a moron when it comes to this stuff.  So I've had to 'translate' the books into real world analogies to understand it.  Still working on it.

The full wave rectifier is still not the most efficient since the negative portion of each half is still wasted, though it certainly is better than a half wave.  The most efficient way to wire it would be the full wave bridge rectifier.  It actually recycles the negative halves back into the signal as positive, which then combines with the first stage of rectification so 100% of the signal is utilized.  Like having two more pumps above the see-sawers that work while the person is headed back downwards.  This requires 2 full wave tubes.  Usually people just use silicon bridge rectifiers in this case.

[Mark Korda]

Ericus,it's Saturday and I vowed to clean out my truck.I am an interior house painter at this stage in my life.It's so full of single slice pizza paper plates,paint thinner,drop cloths ,McDonald wrappers,and coffee cups that I can't see out of the right side.I got to go out an reorganize.Any way,what I'm saying is I might not have the time to thank you enough,your incite on these messages keeps getting better and better.I have seen the silicon diode bridge diagram,but have not got to the part or theory of how it works.I will though,and any info from you is absolutley great and appreciated.When I rebuilt a Dyna ST-35 it had only 2 silicone diodes as I recall in the diagram.I did learn that the current flows in the opposite direction of the arrow,which dated back to (conventual theory)of the thinking (positive to negative)instead of what is excepted today,negative to positive after the discovery of the atom,the old Navy book,and Radio Shacks(Getting started in Electronics by Forest Mims).I'm probably going to have more questions in the future,if you got the time....got to get to the truck...thanks Ericus...Mark K.

[Mark Korda]

Ericus,I've hit another snag I bet you or any one else too,might know to help me.In my Navy Basic Electronic book on tubes,the half wave or full wave rectification diagrams show the secondary transformer tap(ground),half wave to begin with,shows that the voltage to the cathode,directly or indirectly heated,goes from the secondary center tap to another ground further away on the chassis,then to a resistor(25k 10w), the load ,then to the cathode,which shown is directly heated.My question is then:They say the current which shown is going from ground to ground thru the chassis.Is this zero voltage?Is this the 180 degrees point where the positive sine wave and negative wave meet?Is this why I can touch the chassis without getting some kind of shock?Is the voltage so low(positive)that I can't feel it like a 9 volt battery,(unless you touch it to your tongue).I know any one can say you have to study basic electricity first,and I did buy the other set of books from the same Navy publisher,got thru the first,am currently fighting my way thru Ohms and Kirchhoffs laws,throw in magnetism to confuse me more,and just got to AC current,which I never had in high school,or my 1 year in college,but I think if any one could give me a heads up on those questions noted previously,I might have a little head start.To me the (Lab) part of the Audio Circle is the most helpful and useful,and better than going to school,cause all you guys,gals(Elizebeth),are giving me free tutoring!Thank you all.......Mark Korda

[mboxler]

Hi Mark...

Is this the book you are referencing?  If so, what page?  Might help us help you  .

http://jacquesricher.com/NEETS/14178.pdf

Power supplies...Chapter 3

Mike

[wakibaki]

The full wave rectifier is still not the most efficient since the negative portion of each half is still wasted, though it certainly is better than a half wave.  The most efficient way to wire it would be the full wave bridge rectifier.

Sorry, this is incorrect.

The full-wave rectifier and the bridge rectifier are roughly equally efficient, certainly in the case of solid-state diodes. Both use the negative-going and the positive-going parts of the cycle.

The difference is that the full-wave rectifier requires 2 diodes and a centre-tap. The bridge requires 4 diodes and needs no centre tap.

If the transformer has a centre tap then it can be used either in a full-wave arrangement or in a bridge arrangement. The difference is that if the transformer has a 100V secondary (50-0-50-0) then, ignoring diode drops, the full-wave arrangement will have 50V 70.7V output, and the bridge will have 100V 141V output (with half the current available).

Edit: corrected output voltages for capacitor input filter. This was a slip of the pen. Nobody noticed apart from me, however.

The transformer in the picture has a dual secondary, so it can be wired as a single secondary or single secondary with centre tap. The third arrangement shows how to get 50V 70.7V from the same transformer, but with a bridge.

Dont forget that this is an idealised circuit. In practise a voltage is lost across the diodes. Remember that the bridge has 2 diode drops, where the full-wave has only one, so the voltage will be slightly less in the case of transformer 3 as opposed to transformer 1. This is a small difference when solid state diodes are used, but can be quite substantial in the case of valves.




I'm looking at your last post, Mark. I need to figure out exactly what you mean.

w

[wakibaki]

The reason why you can touch the chassis without getting any shock is because the chassis is connected to ground, or it should be.

This is why mains equipment with an exposed chassis has a 3-wire mains lead, so that the chassis can be connected to ground.

Ground is also called earth.

This is because the planet is a big conductor. It may not look like a very useful conductor, you wouldn't use a tube full of soil instead of a wire, but in big enough quantities, especially when it's wet, soil is a conductor. Thus the planet is the reference point for zero volts. It could be at 1000 volts, but with reference to what, the moon? Since we can't make a connection to the moon, it's not relevant. It's like Einstein said, everything is relative.

All the ground connections in the sockets in your house are wired to a central grounding point. Often this is the point where the mains water supply enters the house (the rising main), sometimes it is a large (12 foot) copper spike driven into the ground (ground!). They test this point with special equipment to make sure that the resistance to ground (the earth) is below a minimum requirement when the wiring is first installed.

When you get a shock (simple shock) it is because you have touched a point which is at some potential (voltage) with reference to ground (earth). Current flows from the point of contact, through your body and out through your shoes, or your backside, if you're sitting on a metal bench, to ground. You can also get a shock (more dangerous) if you happen to be touching a grounded point (chassis) with one hand, and touch a point with high potential with the other hand. In this case the current flows in one hand, across your chest, and out the other hand. This can stop your heart. This is why it's often recommended to keep one hand in your pocket when you're working on tubes. The other kind of shock can stop your heart too, or burn you to a frazzle, but a shock across the heart requires the least current to kill you.

That's basically it. Think about what I have written for a while, and if this doesn't explain what you wanted to understand, ask another question.

w

[Ericus Rex]

Sorry, this is incorrect.

The full-wave rectifier and the bridge rectifier are roughly equally efficient, certainly in the case of solid-state diodes. Both use the negative-going and the positive-going parts of the cycle.

I'm looking at my book right now (1963 Radio Amateur's Handbook).  Full wave center tap config results in .45 times the RMS voltage of the transformer secondary and the full wave bridge results in .9 times the RMS voltage of the transformer secondary.  To quote "For the same total transformer-secondary voltage, the average output voltage when using the bridge rectifier will be twice that obtainable with the center-tap rectifier circuit."  Now, they're talking about tube rectifier circuits, as is the OP.  Maybe it's different with diode rectification.  Have I read something wrong here?

Mark, I'll have to look at the diagram.  I have those books at home.  I'm not sure I'll be able to help you though.  Tube rectifiers look backwards to me and I haven't fully gotten my head around them.

[wakibaki]

I'm looking at my book right now (1963 Radio Amateur's Handbook).  Full wave center tap config results in .45 times the RMS voltage of the transformer secondary and the full wave bridge results in .9 times the RMS voltage of the transformer secondary.

Yeah, that's totally off the wall. That's the output voltage in the case of a choke-input smoothing circuit. In the case of a capacitor input smoothing circuit the output voltage is 1.414 times the RMS voltage in the case of the bridge, and 0.707 times the RMS voltage in the case of the centre-tap. As long as the capacitor is suitably dimensioned. Neglecting diode drops.

"For the same total transformer-secondary voltage, the average output voltage when using the bridge rectifier will be twice that obtainable with the center-tap rectifier circuit."

Yeah, that's what I said, but the reason is that with a centre tap, only half the secondary is driving relative to ground (to which the centre tap is connected) not because only one half of the cycle is being used. The only case where one half of the cycle is unused is half-wave rectification.

The output voltage is irrelevant to the efficiency. In a half-wave rectifier one half of the cycle is unused, in the case of both the full-wave (centre tapped) and full-wave bridge both the positive- and negative-going halves of the AC cycle are used.

The question of efficiency is moot, since half of the cycle (in the case of the half-wave rectifier) is rejected onto the mains, it's not wasted strictly speaking as in wasted as heat, it's a question of how you think of efficiency.

[Mark Korda]

Hi Mike,thanks so much for the copy of that Navy book and the chapter to help me.I noticed the copyright was 1998.The Navy books I am referring to are the paperback red and white,and red and orange colored Basic Electronics and Basic Electricity by  Van Valkenburgh and Nooger and Neville Inc.They were copyrighted in 1953.They were made it said to train as many people quickly as possibly and with simple drawings.I know you have seen them some time in your life.They show protons throwing charges with electrons like baseball players.Looks like Mel Ott and Joe DiMaggio rather than A-Rod and Derek Jeter.I was hoping Ericus Rex would chime in as he can make it simple enough for me to have a shot at getting it.I went to high school in the mid 70's,struggled through Algebra1,did a little time in the summer,(summer school Geometry),and senior year was in a class we called (fun with numbers).I'm trying to re educate myself mathmatically as with grouping and a new thing called the calculater,sans abacus.Thanks Mike.Wakibaki,thank you too.Your a little ahead of me,I do understand your earth explanation,but the Dyna ST-35 I rebuilt has a 2 prong plug as well as my wall outlet,yet I have no sound problems at all.It is plugged into a power strip.I did not mean to start any conflict here,just wanted to know if ground was zero voltage on the chassis...thanks all of you....Mark Korda.

[wakibaki]

I do understand your earth explanation,but the Dyna ST-35 I rebuilt has a 2 prong plug as well as my wall outlet,yet I have no sound problems at all.It is plugged into a power strip.

OK. Nobody is suggesting that all devices have a connection to ground, or that a connection to ground is necessary for good sound.

The reason that you don't get a shock from the Dyna ST-35 is because no part of the chassis is connected to any point in the circuit having a high potential with respect to ground.

A direct connection to ground is not necessary for a high potential with respect to ground to develop. The very currents which can kill are leakage currents passing through your shoes.

The ST-35 design is 50 years old, the first production models appeared in 1964.

No modern devices having a 2-wire mains cable are permitted to be sold in most of the Western world unless they are double-insulated.

Take a look at the Wikipedia entry for Appliance classes:- http://en.wikipedia.org/wiki/Appliance_classes

Without poring over the manual for some time (which I have no desire to do) I cannot tell whether there are any explicit connections between the circuit and the chassis, but I suspect that there are not, but if there are, the device is potentially dangerous.

The device is potentially dangerous in any case.

At the very least you are depending on the integrity of the insulation in both the input and output transformers, which are old.

A single fault could cause an electric shock or other dangerous occurrence. Sales of such items have been banned in the UK since 1975.

Many of us still have legacy items of electrical equipment which we choose to operate, but care must be taken to ensure the safety of operators and onlookers.

As currently configured, you should not permit anyone to touch the device while connected to the mains, and you should warn anyone in the vicinity when operating the device not to touch it. You could be held legally liable in the event of an accident. Do not touch the chassis or any conductor connected to it yourself. This includes the screws typically used to secure plastic knobs.

You should not IMO in any case permit children in the vicinity of the device while connected to the mains.

You can fit a safety ground using a 3-wire cable and plug and operate the device from a 3-pin outlet, either on a circuit with a RCD at the consumer unit or via an adapter with an RCD. This will probably meet the requirements for due diligence in your jurisdiction.

w

[AVnerdguy]

The main advantage of a bridge full wave rectifier is reduced manufacturing cost - no need for a center tapped transformer. Diodes are much cheaper than copper windings. Other than that, they produce the same waveform and essentially the same output (the voltage drop of a solid state device is negligible).

[JohnR]

Full wave center tap config results in .45 times the RMS voltage of the transformer secondary and the full wave bridge results in .9 times the RMS voltage of the transformer secondary.

That would be for a choke-input filter.

[Ericus Rex]

W and JohnR, you are right.  The diagram I was referencing was for a choke-input filter.  Thanks for the clarification!

Mark, I'm still looking for my book.  I have the exact book(s) you're currently reading.  The simple explanation for why you don't get shocked when touching your chassis is that nearly all of the voltage is consumed by the circuit before being sent to ground.  The way you describe what you've read implies that the full B+ is sent through ground to get to the filter caps.  That doesn't seem right to me but I'll have to find the book first.  Sorry, we moved into a new house a year ago and most of my library is still boxed...somewhere.

[mboxler]

The main advantage of a bridge full wave rectifier is reduced manufacturing cost - no need for a center tapped transformer. Diodes are much cheaper than copper windings. Other than that, they produce the same waveform and essentially the same output (the voltage drop of a solid state device is negligible).

For us newbies...

I agree with this statement.  However, the original post is Vacuum tube full rectification, not full rectification for a vacuum tube amp.  As I see it, one would need 2 dual diode tubes if the transformer is not center tapped.  Would this still be cheaper than one tube and twice the windings?

Thanks, Mike

[JohnR]

As I see it, one would need 2 dual diode tubes if the transformer is not center tapped.

3, as tube rectifiers are internally connected at the cathode. The common ones are anyway.

It's possible to use a single rectifier tube and two diodes to implement a full-wave bridge.