rectifiers and voltage doublers etc...

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Doug H
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rectifiers and voltage doublers etc...

Post by Doug H » Fri Nov 04, 2011 9:12 pm

I see all kinds of different rectifiers looking thorugh the schematics. Do some of these use different transformer specs? or is it just a different way of skinning the same cat.

Some of the diode arrangements look like thaye would produce twice the power from the same system, so do these amps use transformers with half the secondary voltage? IF so how do you identify these when you are guying them?

I checked thorugh the wiki and it's still not sinking in.

Doug

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Re: rectifiers and voltage doublers etc...

Post by 51N15T3R » Sat Nov 19, 2011 7:25 pm

Transformers are set up for a certain rectifier type. A transformer set up for a full wave center tap rectifier will have double the required AC voltage across the full secondary whereas a transformer set up for a full wave bridge rectifier will have exactly the required AC voltage across its full secondary.

Also, the transformer set up for the full wave center tap rectifier will be rated at 1/2 the total required current. This is because when you reference to the center tap, each 1/2 of the secondary is on a 50% duty cycle, which allows you to pull DOUBLE the total winding current for 1/2 the total on time. Tranformers set up for full wave bridge rectifiers are on a 100% duty cycle, so they must be rated for exactly the total required current.

The circuit we call the "full wave bridge voltage doubler" isn't really a "doubler" per se. Matter of fact, that circuit is nothing more than a full wave dual polarity supply with a relocated zero reference. All they did was take a transformer set up for a full wave bridge rectifier and add a center tap to it.

If you were to take the FWB doubler circuit and relocate the zero reference to the center connection point of the two filter caps, you'd end up with a supply that kicks out a + and a - voltage at 1/2 the total voltage.

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Re: rectifiers and voltage doublers etc...

Post by Doug H » Sun Nov 20, 2011 1:18 pm

thanks, I've been trying to soak this up lately. I even read about "waterly watkin somebody" ?? rectifiers, where you can turn 10 volts into 5000, it's what they use in camera flashes and stuff like that.

That's a great way to put it. I think the page in the metroi wiki is wrong, because the author states that the only reason for the CT between the caps is to do away with the bleed resistors. Im still a little unsure cause of all the different ways of describing these things.

If we had a full wave bride with no center tap at all, it would only make .7 * CTV,

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Re: rectifiers and voltage doublers etc...

Post by 51N15T3R » Sun Nov 20, 2011 1:35 pm

Doug H wrote:
That's a great way to put it. I think the page in the metroi wiki is wrong, because the author states that the only reason for the CT between the caps is to do away with the bleed resistors. Im still a little unsure cause of all the different ways of describing these things.

If we had a full wave bridge with no center tap at all, it would only make .7 * CTV,
That is EXACTLY the reason why the CT is connected to the center point of the filter caps. For voltage balancing across them without having to use bleeder resistors. Basically what happens is that each cap is being charged up by 1/2 of the secondary. But...with the CT connected to that point, it just happens to be exactly the same circuit you'd use if you were doing a dual polarity power supply @ 1/2 the total voltage, but with the negative supply output being the zero reference instead of the center connection point of the caps like you would have on a dual polarity supply.

And you are incorrect on the FWB voltage.

When you rectify and filter DC, you get a DC voltage that is equal to the PEAK AC voltage that you are feeding it, which is 1.414x the RMS AC voltage that your meter sees. This is because the caps charge up to the peak AC voltage on the positive going cycle of the AC.

Basically you would have the exact same total B+ voltage whether the CT is connected to the center cap connection or not. The only reason it's called a "doubler" is because the schematic somewhat resembles a full wave doubler but with 2 extra diodes.

Image

Take a look at the image. Identical circuits...it just depends on which supply node you decide to treat as your "zero". ;)

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Re: rectifiers and voltage doublers etc...

Post by Doug H » Sun Nov 20, 2011 6:44 pm

the wiki hints that you would get the same power with the center tap pulled, ~500v from 350 CTV. I'm just unclear if the secondary CT between the caps is required to "elevate" the circuit to include both sides positive.

My full wave rectifier on my 50 gets around 475 from a 700ctv. If I simply added two diodes from ground it would put out twice as much? Wouldn't the ground just eat the other half without the CT sitting between the two somewherem like between those caps?

This is the sticky point to me.

I always though my meter read peak AC, so on half of a tranny say it's 300, going though a half wave rectifier you get 210 DC, which I figured was the actual rms of the AC 300 translated to DC. Now I'm all confused again :scratch:

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Re: rectifiers and voltage doublers etc...

Post by 51N15T3R » Sun Nov 20, 2011 8:03 pm

Doug H wrote:the wiki hints that you would get the same power with the center tap pulled, ~500v from 350 CTV. I'm just unclear if the secondary CT between the caps is required to "elevate" the circuit to include both sides positive.
Doug, not sure what you mean by "both sides positive". On a power supply you have only 2 sides: + and -. You pick one side to treat as "zero", and zero is neither positive nor negative. The supply voltage value and polarity is relative to this zero point. Voltage and polarity are not an absolute. They are relative to the point of the supply that you decide gets to be "zero".

On this supply, you're setting the negative side of the supply up to be "zero" by taking it to the zero reference we like to call "ground" (ground is ALWAYS zero). All positive voltage measurements are relative to this zero point.

The voltage on this supply is 495V UNLOADED. But since the supply is unregulated, the voltage will load down by about 20-25 volts once you put the valves in and load the supply.
Doug H wrote: My full wave rectifier on my 50 gets around 475 from a 700ctv. If I simply added two diodes from ground it would put out twice as much? Wouldn't the ground just eat the other half without the CT sitting between the two somewherem like between those caps?
No. Ground is just a "zero reference". If you try to bridge rectify a 700V transformer you will end up with twice the voltage at 1/2 the current. This is because you're feeding the full winding voltage to the rectifier, then treating the (-) side of the rectifier as "zero" rather than treating the center tap as "zero".
Doug H wrote:
I always though my meter read peak AC, so on half of a tranny say it's 300, going though a half wave rectifier you get 210 DC, which I figured was the actual rms of the AC 300 translated to DC. Now I'm all confused again :scratch:
No, the meter reads RMS AC. You end up with the DC value being the PEAK value of the AC because the filter caps charge up to the PEAK value of the incoming AC on the positive going AC cycle, then temporarily supply power to the circuit while the AC is on the negative going cycle of the AC.

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Re: rectifiers and voltage doublers etc...

Post by Doug H » Sun Nov 20, 2011 10:13 pm

51N15T3R wrote:
Doug, not sure what you mean by "both sides positive". On a power supply you have only 2 sides: + and -. You pick one side to treat as "zero", and zero is neither positive nor negative. The supply voltage value and polarity is relative to this zero point. Voltage and polarity are not an absolute. They are relative to the point of the supply that you decide gets to be "zero".

No. Ground is just a "zero reference". If you try to bridge rectify a 700V transformer you will end up with twice the voltage at 1/2 the current. This is because you're feeding the full winding voltage to the rectifier, then treating the (-) side of the rectifier as "zero" rather than treating the center tap as "zero".
by bioth sides positive I mean using the neg side as the zero reference so the voltage get's doubled, and ya I'd get twice the voltage if I bridged my 700ctv, what I don't understand is if that center tap going between the two caps is required for the "doubling" effect. The bottom of the rectifier wiki suggests that it isn't, and the voltage would be the same with the CT going to ground and the bleed resistors there. I'm thinking the CT going between the stacked caps is required in order for the neg side of the bridge to get used as the ground reference, without it you just get the same DC as a full wave. That's what confuses me, I have seen what seems to me to be differing explanations all over ther web.

thanks for your help.

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Re: rectifiers and voltage doublers etc...

Post by 51N15T3R » Sun Nov 20, 2011 10:21 pm

Doug H wrote:
51N15T3R wrote:
Doug, not sure what you mean by "both sides positive". On a power supply you have only 2 sides: + and -. You pick one side to treat as "zero", and zero is neither positive nor negative. The supply voltage value and polarity is relative to this zero point. Voltage and polarity are not an absolute. They are relative to the point of the supply that you decide gets to be "zero".

No. Ground is just a "zero reference". If you try to bridge rectify a 700V transformer you will end up with twice the voltage at 1/2 the current. This is because you're feeding the full winding voltage to the rectifier, then treating the (-) side of the rectifier as "zero" rather than treating the center tap as "zero".
by bioth sides positive I mean using the neg side as the zero reference so the voltage get's doubled, and ya I'd get twice the voltage if I bridged my 700ctv, what I don't understand is if that center tap going between the two caps is required for the "doubling" effect. The bottom of the rectifier wiki suggests that it isn't, and the voltage would be the same with the CT going to ground and the bleed resistors there. I'm thinking the CT going between the stacked caps is required in order for the neg side of the bridge to get used as the ground reference, without it you just get the same DC as a full wave. That's what confuses me, I have seen what seems to me to be differing explanations all over ther web.

thanks for your help.
That's what you're not understanding. There is no "doubling" effect really happening. You're getting the full peak AC voltage because you're connecting a bridge rectifier across the entire winding...NOT because you're referencing the center tap to the center connection point of the caps.

When the CT is connected to the center connection of the two filter caps, all it's doing is balancing the voltage on the two caps without having to use bleeder resistors to do the same thing. It does this by allowing 1/2 the secondary to charge up the top cap while allowing the opposite 1/2 of the secondary to charge up the bottom cap. Since both halves of the secondary are of equal voltage, this charges both caps up to an equal voltage.

The ONLY reason they call it a "doubler" is because it closely resembles a regular 1/2 wave doubler but with two extra diodes to form a bridge.

If the CT does not get connected to the center connection point of the caps, it just doesn't get connected at all (connecting it to ground in a bridge circuit will short the secondary). But then you're forced to have to use bleeder resistors to balance the cap voltages.

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Re: rectifiers and voltage doublers etc...

Post by Doug H » Sun Nov 20, 2011 11:48 pm

right, I was just about to post, if you lift the CT you're good to go. But if you put the CT to ground you are only going to see the same as a FW, cause the grounded CT is going to defeat the other half, shorting out half the tranny, which would be the more relevent issue I would suppose.

I call it a doubler, cause it puts out double what a FW does, which is what I I am used to seeing. It's pretty easy to see it as a doubler when it get's 500v dc from a 175 - 0 - 175 tranny, basically double what the FW on my 50 watter would pull. What I'm getting from you is it pulls a logical 500 from 350, which is the peak AC translated to DC, and the simple FW is only pulling half the available power.

Here's an interesting scenario. I completely rebuilt my tag board on my home plexi. After checking, rechecking etc...I fire it up, no sound. I had forgot ground the bias supply. At some point I took a reading, and unless I misread something, I had -380 volts on that circuit. WTF? It's a 220k resistor off the secondary goning into a half wave rectifier. So like, I'm thining 210 or something MAX, then after 220k, it's usually 60 volts. So how did the voltage get that high?

this voltage sounds suspiciously close to
a) the two bias caps together, raited at 200v with just a 15k resistor between them
b) the AC voltage off the secondary

WHat was scary is I have a Post PI MV, I just changed out my double cap version to the one which replaces the bias splitters, so basically I had the pot's shielding protecting me from -380v DC. I'm guessing most pots are rated at 250v max :(. Something to consider about this MV. If "b" is the case then there's no concern running the bias off of a 100v winding. OF course, you can always just make sure things are properly grounded. I got a shock too off this one, got fooled by the upside down cap thing...

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Re: rectifiers and voltage doublers etc...

Post by 51N15T3R » Sun Nov 20, 2011 11:59 pm

Doug H wrote: I call it a doubler, cause it puts out double what a FW does, which is what I I am used to seeing. It's pretty easy to see it as a doubler when it get's 500v dc from a 175 - 0 - 175 tranny, basically double what the FW on my 50 watter would pull. What I'm getting from you is it pulls a logical 500 from 350, which is the peak AC translated to DC, and the simple FW is only pulling half the available power.
No it's not "putting out half the available power".

With a Full Wave Center Tap Rectifier, you get 1/2 the full winding voltage, but DOUBLE the full winding current. Therefore the "available power" remains the same. This happens for two reasons -

1) The center tap is being used as the "zero", and relative to the center tap, each phase puts out 1/2 the full winding voltage.

2) You get double the current because each 1/2 of the secondary is only passing current for 50% of the time (50% duty cycle) as the diodes alternate on/off with the phases. Therefore, you can pull DOUBLE the instantaneous current because the AVERAGE current will average out to the rated winding current because it's passing double for 50% of the cycle, while passing nothing for the other 50%. Therefore, average current over 1 full AC cycle = rated winding current.

So the power (wattage) remains the same as if you were to bridge rectify the full winding -

990VAC Peak x 212mA Peak = 210 Peak VA

495VAC Peak x 424mA Peak = 210 Peak VA

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Re: rectifiers and voltage doublers etc...

Post by Doug H » Mon Nov 21, 2011 12:12 am

ya, half the voltage I meant, I didn't mean to get into ampage, wattage etc... thanks for all the explanations.

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Re: rectifiers and voltage doublers etc...

Post by 51N15T3R » Mon Nov 21, 2011 11:43 am

Just trying to get you to realize that the FWB doubler isn't really doubling anything. The FW center tap rectifier puts out 1/2 the peak AC voltage as DC because the center tap is used as "zero" and only 1/2 the winding is supplying current at any given time for 50% of the incoming AC cycle, whereas with the FWB rectifier, the entire winding is passing current 100% of the time.

A transformer that is set up for FWB rectification will have a secondary that is set up for the exact required AC voltage whereas a transformer that is set up for a FW center tap rectifier will have a secondary that is set up for DOUBLE the required AC voltage. It is the utilization of the center tap as "zero" that cuts the incoming AC voltage in 1/2 on the FW center tap circuit. On the FWB circuit, the entire winding is used 100% of the time while the negative going diodes are continuously switching which end of the secondary coil is being used as the "zero" leg. On FW center tap, the diodes are continuously switching which 1/2 of the secondary the circuit is drawing from while the center tap is always zero.

On FWB doubler, the center tap is just being used as a 1/2 voltage reference that ensures that both caps charge up to 1/2 the incoming peak AC voltage. But the voltage isn't really being "doubled" per se as having a bridge rectifier across the entire winding gives you the total peak AC voltage as DC anyway. I mean sure you could look at it as a doubler if you look at it from the perspective that each cap is charging up to 1/2 the peak AC secondary voltage via the center tap, then series wiring them doubles that voltage, but you'd end up with the exact same DC voltage if you were to omit the center tap altogether and use bleeder resistors instead.

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Re: rectifiers and voltage doublers etc...

Post by Doug H » Mon Nov 21, 2011 8:00 pm

I've been reviewing this for the last couple weeks, it's all starting to sink in. this shone some light on a few things.

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Re: rectifiers and voltage doublers etc...

Post by joey » Sun Dec 04, 2011 12:38 am

We'll actuallyyyyyyyy flemingmras, or 5ini573r or maybe JW, hooking the ct to the junction of the series connected caps will result in better balance as apposed to resistive balance. So there is merit to connecting it In that fashion, that way but won't have the beneficial discharge path. I do both really no drawbacks, and it's really still a matter
Of semantics!

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Re: rectifiers and voltage doublers etc...

Post by crumb » Sun Sep 22, 2019 3:40 pm

The important thing, I think..is not to think about how the 'alleged' voltage doubler would behave without bleeder diodes and a disconnected centre tap, but rather, that is does actually double the voltage as long as that centre tap is attached.
It may not appear in the text books as a voltage doubler, but only its because its an abstract variation thereof.
The problem with some ' engineers'...as opposed to backyard bandits, ( like jim marshall himself and the kid who stole the bassman design for him) is that they may RELY on the precious text books that they studied, sometimes a bit too much.
It simply offends some engineers as it doesn't appear in their textbooks...can text books really include all the abstract variations of something?
An example of this phenomenon is currently being played out in another forum called the ' marshall amp forum'
There is a post there, and the engineer.. refuses to extrapolate..refuses to talk shop ( I was the hack that confronted him in the post there called ' 4X12 8ohms and 4X12 16ohms question ?')
This guy is being treated like the pope over there, really, it sickening to watch the brown-nosing.
The thing is...he wont talk half waves, he wont talk diodes, he wont talk bleeder resistors...nothing...zilch...because he knows full well the second he admits there is voltage doubling going on, he loses his status as the Rat god.
His minions over there say he designed marshall amps like the jvm and some other ones too, and I've got him over a barrell.

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