Bi-Phase & Half-Wave Rectification

[Chumly]

Hi-yah all,

Here’s my views on DC power bi-phase rectification (that’s a two rectifier configuration with the secondary of the power transformer having a center tap). Comments?

During the tube era, DC power bi-phase rectification’s popularity over DC power four rectifier bridge was with tube power-supplies, given that only one tube would be needed.

Wiki tends to confirm: A very common vacuum tube rectifier configuration contained one cathode and twin anodes inside a single envelope; in this way, the two diodes required only one vacuum tube. The 5U4 and 5Y3 were popular examples of this configuration. https://en.wikipedia.org/wiki/Rectifier#Half-wave_rectification

In today's world, it seems to me that DC power solid-state bi-phase rectification has very little (if any) real-world practical advantage over DC power solid-state bridge rectification. Thus the chances of seeing a DC power bi-phase solid-state rectification circuit in the flesh as compared to a DC power bridge rectifier circuit in a modern, popular, solid state DC power supply would be quite unlikely.

My view is that DC power Bi-phase rectification’s main disadvantage over a DC power bridge rectifier circuit is that only 50% of the secondary of the power transformer is used at a given instant, thus if you have a 1000 VA power transformer you can only have a 500 VA (notwithstanding losses) DC power supply. This makes DC power Bi-phase rectification impractical now given the low costs of solid state diodes.

I'm not addressing guitar amps, their tube power supples and sag etc, nor so-called “audiophile” tube amps, nor vintage gear.

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My views on half wave DC power solid-state rectification is also that it has very little (if any) real-world practical advantage over full wave DC power solid-state bridge rectification. Thus the chances of seeing half wave DC power solid-state rectification in a modern, popular, solid state DC power supply would be like unlikely. Comments?

However Wiki has this to say about half wave for voltage-doubling (I'm not sure if it’s still popularly in use though) “The simple half wave rectifier can be built in two versions with the diode pointing in opposite directions, one version connects the negative terminal of the output direct to the AC supply and the other connects the positive terminal of the output direct to the AC supply. By combining both of these with separate output smoothing it is possible to get an output voltage of nearly double the peak AC input voltage. This also provides a tap in the middle which allows use of such a circuit as a split rail supply.

A variant of this is to use two capacitors in series for the output smoothing on a bridge rectifier then place a switch between the midpoint of those capacitors and one of the AC input terminals. With the switch open this circuit will act like a normal bridge rectifier with it closed it will act like a voltage doubling rectifier. In other words this makes it easy to derive a voltage of roughly 320V (+/- around 15%) DC from any mains supply in the world, this can then be fed into a relatively simple switched mode power supply.

Cascaded stages of diodes and capacitors can be added to make a voltage multiplier. These circuits can provide a potential several times that of the peak value of the input AC, although limited in current output and regulation. Voltage multipliers are used to provide the high voltage for a CRT in a television receiver, or for powering high-voltage tubes such as image intensifiers or photomultipliers.”

However with the cheapest modern car battery chargers and other battery chargers, maybe half-wave is still to be popularly found; but given the low cost of bridge rectifier modules and the fact that they are twice as efficient, maybe not?

 

[Ubergeek63]

In inexpensive low voltage circuits it is you only have one diode drop instead of the two you have using a bridge. It is, however, somewhat less effective use of power transformers (you can get a little less current out of a given transformer)

Dan

 

[Chumly]

Let me clarify a few things for you as per half-wave:

- You get 50% less VA not "a little less current out of a given transformer".

- Half-wave diode voltage drop is a non-issue because in full-wave you get double the VA, a more than fair trade-off.

To further edify:

The main point of the thread however is bi-phase, it's in that context I posted, I just threw the other stuff in there as it was no more work to post, it has some commonalty and I must teach it as well.

You see I am a teacher, and bi-phase comprises a substantive amount of the mandatory course material as it relates to power electronics.......but it would appear bi-phase (as it relates to power electronics at the least) only has a place in tube rectifier circuits.

I am not teaching vintage audio, guitar amps, nor tubes at all nor are they referred to at all in the power electronics section of my course.

I am teaching apprentice electricians actually, so power electronics is applicable to the learning but I don't see how an emphasis on bi-phase is of any net benefit when time is short and there is much material to cover.

I suspect this ongoing emphasis on bi-phase in the power electronics course material (and on the exam) is an antiquated holdover from the halcyon days of big industrial tube DC power!

 

[audioguru]

When only half of a winding is used at any given time then the average power dissipated by the transformer is half and therefore the current can be doubled.
A power transformer is designed not to saturate during the extremely high current that briefly charges the main filter capacitor.

 

[Chumly]

With all due respect!

What you say is not necessarily so, the current cannot necessarily be doubled.

More to the point you have to factor for the current & voltage rating of each of the transformer's coils, which should not be exceeded.

IOW you have to account for the fact that VA in equals VA out (not withstanding transformer losses) only within the current & voltage rating of each of the transformer's coils.

These rating are expressed as per the transformer's nameplate data.

In any case (as discussed above) the main thrust of my post is neither transformer calculations or half wave calculations it's bi-phase, which is neither a bridge nor a half-wave.

Thanks much!

 

[k7elp60]

Let me clarify a few things for you as per half-wave:

- You get 50% less VA not "a little less current out of a given transformer".

- Half-wave diode voltage drop is a non-issue because in full-wave you get double the VA, a more than fair trade-off.

To further edify:

The main point of the thread however is bi-phase, it's in that context I posted, I just threw the other stuff in there as it was no more work to post, it has some commonalty and I must teach it as well.

You see I am a teacher, and bi-phase comprises a substantive amount of the mandatory course material as it relates to power electronics.......but it would appear bi-phase (as it relates to power electronics at the least) only has a place in tube rectifier circuits.

I am not teaching vintage audio, guitar amps, nor tubes at all nor are they referred to at all in the power electronics section of my course.

I am teaching apprentice electricians actually, so power electronics is applicable to the learning but I don't see how an emphasis on bi-phase is of any net benefit when time is short and there is much material to cover.

I suspect this ongoing emphasis on bi-phase in the power electronics course material (and on the exam) is an antiquated holdover from the halcyon days of big industrial tube DC power!

Let me see if I understand what you are saying about a transformer and a bi-phase rectifier. If I have a transformer that is 28VCT and I use two diodes for rectifiers with the cathodes of diodes connected together and connected to the + terminal of a filter capacitor, and the CT connected to the - lead of the capacitor. The anodes of the diodes are connected to the windings either side of the CT. Are you saying this has no use?
If so, I highly disagree with you. In analog powersupplies it is used all the time.

 

[Chumly]

Let me see if I understand what you are saying about a transformer and a bi-phase rectifier. If I have a transformer that is 28VCT and I use two diodes for rectifiers with the cathodes of diodes connected together and connected to the + terminal of a filter capacitor, and the CT connected to the - lead of the capacitor. The anodes of the diodes are connected to the windings either side of the CT. Are you saying this has no use?
If so, I highly disagree with you. In analog powersupplies it is used all the time.

Nope, I’m sorry to say. No filtration of any kind has been referenced and you have not described a bi-phase configuration. It's all there if you read my posts carefully, and do the needed research.

Would you please clarify the following acronyms with full word definitions:

CT you mean?
28VCT you mean?

Cheers!

 

[saturn1bguy]

Nope, I’m sorry to say. No filtration of any kind has been referenced and you have not described a bi-phase configuration. Would you please clarify the following acronyms with full word definitions: CT, 28VCT

The circuit the fellow above described matches precisely the Wiki tube-based rectification scheme that you reference in your article, save for the filter.

CT - center tapped
28VCT - 28VAC center tapped

 

[JimB]

Consider this situation:

A centre tapped transformer winding, a bridge rectifier connected to the ends of the winding.
Now use the centre tap as the common connection and what have you got?
Two supplies, a positive supply and a negative supply, both using bi-phase rectification.

If you wanted two supplies, the other way to do it would be to use two separate windings on the transformer.

I think the original premise that bi-phase rectification belongs in a museum is somewhat simplistic, it is still a valid technique today.

JimB

 

[Chumly]

Consider this situation:

A centre tapped transformer winding, a bridge rectifier connected to the ends of the winding.
Now use the centre tap as the common connection and what have you got?
Two supplies, a positive supply and a negative supply, both using bi-phase rectification.

If you wanted two supplies, the other way to do it would be to use two separate windings on the transformer.

I think the original premise that bi-phase rectification belongs in a museum is somewhat simplistic, it is still a valid technique today.

JimB

Nope, that's a four diode, dual-DC-supply using a center tap transformer, that's not a bi-phase circuit as described (a two rectifier configuration with the secondary of the power transformer having a center tap).

Maybe dual bi-phase, but that's not what I'm referencing, if you read my text carefully I spec two rectifiers and a center tap. I suggest you check out the history of tubes in this context for background on the original rationale of reducing the number of rectifier tubes. But in modern solid state such a configuration is iffy for the reasons given. You're welcome to show otherwise with actual modern popular solid state DC power supply examples.

 

[Chumly]

The circuit the fellow above described matches precisely the Wiki tube-based rectification scheme that you reference in your article, save for the filter.

CT - center tapped
28VCT - 28VAC center tapped

That's why I asked for the clarification of the acronyms, and now I can say yep minus the filters sure!

However k7elp60 has yet to support his claim that "In analog power supplies it is used all the time" in the context given as per my opening post: "a two rectifier configuration with the secondary of the power transformer having a center tap".

Recall I do not mean a four diode, dual-DC-supply using a center tap transformer, that's not a bi-phase circuit as described. Maybe dual bi-phase, but that's not what I'm referencing.

If k7elp60 is correct he should have no trouble showing that bi-phase as I have described it (a two rectifier configuration with the secondary of the power transformer having a center tap) "is used all the time".

 

[Mikebits]

My views on half wave DC power solid-state rectification is also that it has very little (if any) real-world practical advantage over full wave DC power solid-state bridge rectification. Thus the chances of seeing half wave DC power solid-state rectification in a modern, popular, solid state DC power supply would be like unlikely. Comments?

Well, from a purely business point of view, the half-wave makes a lot of sense, dollars and sense that is. If Company XYZ is producing their widget at 100,000 units per year, and they can save 10 pennies/unit by reducing amount of diodes, then why not. I as company XYZ CEO could care less if your power bill goes up. Now in a mobile application, I would have a different perspective since the customer would not be happy with a short lived battery driven device.

 

[Chumly]

Well, from a purely business point of view, the half-wave makes a lot of sense, dollars and sense that is. If Company XYZ is producing their widget at 100,000 units per year, and they can save 10 pennies/unit by reducing amount of diodes, then why not. I as company XYZ CEO could care less if your power bill goes up. Now in a mobile application, I would have a different perspective since the customer would not be happy with a short lived battery driven device.

First of all half-wave is not a less efficient configuration in terms of actual power used (if one neglects transformer losses), so you are wrong when you suggest the "power bill goes up".

The "power bill" does not go up!

One's power bill would not go up with half-wave versus full-wave because VA in = VA out as I've discussed many times (notwithstanding transformer losses).

As to low end battery chargers or their equivalent, in order to make your case you would need to show the likelihood of hundreds of thousands of half wave rectifiers used in modern, popular, solid state DC power supplies.

What you have entirely overlooking however is that the VA rating of the transformer for full-wave would be 50% less than that of a half wave of equivalent VA output!

Thus your claim of "10 pennies/unit by reducing amount of diodes" savings could easily be destroyed by the need for a 100% larger transformer (yep that's right, you would need to double the size of the transformer to equal the VA output of a full-wave bridge).

You thus have yet to make your case, let alone prove the existence of hundreds of thou sounds of modern, popular, solid-state, half-wave DC power power supplies

In any case (as discussed above a number of times) the main thrust of my post is neither transformer calculations or half wave calculations it's bi-phase, which is neither a bridge nor a half-wave.

 

[saturn1bguy]

In today's world, it seems to me that DC power solid-state bi-phase rectification has very little (if any) real-world practical advantage over DC power solid-state bridge rectification. Thus the chances of seeing a DC power bi-phase solid-state rectification circuit in the flesh as compared to a DC power bridge rectifier circuit in a modern, popular, solid state DC power supply would be quite unlikely.

With respect to a full-wave bridge, the "bi-phase" rectification scheme is inferior in several ways:

1) Requires a center-tapped transformer with double the output voltage,
2) Requires a slightly beefier transformer build (Pt/Pd of 1.5 vs. 1.23), and
3) Requires higher voltage diodes (Vrrm/Vrms of 3.12 vs. 1.56)

But, it is superior in one way: Irms/Iout of 0.78 vs. 1.11 for a bridge (or 1.11 vs.1.57 with filter capacitor). None of this takes manufacturing or procurement cost into account, just the circuit specifics. But, alternative approaches are always useful. I've built this circuit before because it suited the parts I had on hand. As to your thesis, you may be right, the "bi-phase" circuit is likely not used as much as it used to be, for whatever reason.

(All values above from my Friedrich's Table of Circuit Data, 1975)

 

[Chumly]

saturn1bguy,

could you expand a bit more on the acronyms and other terms you are using (and the math too) so I can be sure of what you are talking about please?

 

[Mikebits]

First of all half-wave is not a less efficient configuration in terms of actual power used (if one neglects transformer losses), so you are wrong when you say "company XYZ CEO could care less if your power bill goes up" it would not versus full-wave because VA in = VA out as discussed many times (notwithstanding transformer losses).

You are mistaken. A half-wave only uses half the power put in and this is a loss of half the power that is provided. The fact is two cycles AC in and only use one cycle is inefficient.

In any case (as discussed above a number of times) the main thrust of my post is neither transformer calculations or half wave calculations it's bi-phase, which is neither a bridge nor a half-wave.

What for Pete's sake is a bi-phase rectifier? Never heard of it. Please explain. Do you have a diagram to enlighten me with?

Only thing I can think of is you are talking about a full-wave.

 

[saturn1bguy]

What for Pete's sake is a bi-phase rectifier? Never heard of it. Please explain. Do you have a diagram to enlighten me with?

Look up rectifier on Wikipedia, and look for the second of the full-wave rectifier circuits (third of the nice green AC half cycle drawings). I believe this is what the OP is targeting...

 

[Mikebits]

Ok, Don't we call that a Full-wave?

 

[saturn1bguy]

Ok, Don't we call that a Full-wave?

Yes, it's full wave, but there are two full-wave schemes, the bridge and the ?? (that's where we stumble into "bi-phase").

 

[Chumly]

You are mistaken. A half-wave only uses half the power put in and this is a loss of half the power that is provided. The fact is two cycles AC in and only use one cycle is inefficient.

Nope you are wrong again!

The 50% of the time the input sine-wave is not providing output power, said power is not (as you call it) "put in" at all.....it is in fact simply not used!

Again, there is no (as you call it) "loss of half the power", notwithstanding transformer losses!

VA in must equal VA out as discussed many times already, notwithstanding transformer losses. You do not understand how a transformer works vis-a-vis power transfer through mutual inductance.