Stacked Power Supplies and Other Craziness

[Fresh_Start]

Instead of continuing to clutter up the very interesting thread on plate vs. screen grid supply voltages, this is a continuation of the discussion about power supplies with multiple voltage outputs.

Here is a schematic of a "full wave bridge with center tap" supply as discussed

A follow up question:
Does the "center tap" need to be in the center of the PT secondary(ies)?

IOW could you use the full-wave bridge with center tap topology to create two power rails, one starting at 350 (a total of 250 volts for the secondaries) and another off the center tap starting at 245.  One secondary would yield 175 volts AC and the second 125 volts AC.

Most VVR circuits only adjust the power amp voltages while leaving preamp (and sometimes PI) voltages alone.  Use the higher voltage rail for the power amp and the lower voltage rail for the preamp.  Would there be any advantage over having a fork in the power rail?

Just as an example, here's a torroidal transformer with a 240 volt secondary and a 220 volt tap on the same secondary.  Not exactly the voltages I was thinking of but at least it's a PT diagram. Could you use the 220 volt tap as the "center tap"?

Thinking about that Antek PT again - if the unbalanced "center tap" concept would work, you could have a 20 volt tap and a 240 volt tap.  Might be useful if your amp has solid state circuitry, switching, etc.

Antek Torroidal

HBP's response:

In a way, this is how tube o'scope supplies are wired. I don't want to bust bandwidth and try to post a Tektronix o'scope manual here... But they have plenty hosted on BAMA (Boat Anchor Manual Archive).

These supplies are regulated, and the regulated outputs are stacked one on top of another to provide various voltages (typically from -150v to 500v). The high voltage CRT supply is generally separate, but still has some of its supply components run from the stacked regulated output voltages. Page 4-14 of the Tektronix 545a manual shows such a supply. The output voltages are -150vdc, 0v, 100vdc, 225vdc, 325vdc (unregulated), 350vdc and 500vdc.

You could do what you described is you use bridge or full-wave rectifiers for each tap, then stack the resulting d.c. voltage components on each other. Let's say that's a 250vdc and 100vdc set of supplies. You might have the "ground side" of the 250vdc connected to the actual circuit ground, while the "ground side" of the 100vdc supply is connected to the positive side of the 250vdc supply, giving a total output voltage of 350vdc.

The key is you can never reference the "ground side" of the 100vdc supply to anything but 250v. Your options must always be either the 250v output or the 350v output. A single tube could be connected such that it feels as though it has a 100vdc supply; you simply have to keep every part that would be "at ground" for that tube and its associated circuits riding on top of the 250v supply.

I've attached the schematic HBP refers to above.

Chip

[Fresh_Start]

(worried about the first post being too long...)

When I first looked at the Tektronix 545A power supply schematic I thought that the second full wave bridge rectifier down showed that my idea could work.  However, IF I understand the circuit correctly, both of the PT secondaries for that bridge are 125 volts each.  The whole bridge stands on top of (horrible language, I know) of the 100 volt rail just below it.  

100 + 125 = 225 from one rail.  

225 + 125 = 350 from the other rail.

However, the 325 volts coming off the "center tap" of those two secondaries does NOT look like it's in the middle of the two voltages.  It's identified as being "unregulated".  Note that the capacitors connecting each output from the full wave bridge to the center tap shown in the first post's schematic are missing.  Does this mean that the two secondaries for the second full wave bridge (down) are different?

Actually I think HBP has already answered my question about how to generate multiple supply voltages.  It looks like you need to "stack" full wave bridge rectifiers for multiple PT secondary windings.  The Antek with a 240 volt winding using the 220 volt tap on that winding as the "center tap" would not work AFAICT.  But there has to be a way to utilize that 220 volt tap.

Enough wandering thoughts for this morning...

Chip

[jjasilli]

OK. I think I got it.  Thank-you to Dummyload & Freshstart.  It appears that Voltage Doublers yield half-wave rectification at the half voltage tap.  The CT Bridge is not a voltage doubler -- it doesn't double anything, but rather provides a half voltage point, with full-wave rectification at both points.  I guess one disadvantage is that, in an amp, both taps require some duplication of B+ filtering, for the screens, etc., to get a full compliment of ripple reduction.

[sluckey]

You can stack power supplies in series just like stacking batteries in a 2 cell flashlight. Heck, stack as many as you like. Look at the Silvertone 1984 circuit. As long as you connect series aiding, the voltage across the stack will be the sum of the individual power supplies (or batteries). And the voltages don't have to be equal either. You can connect a 12V car battery in series aiding with a tiny 9V battery and get 21 volts across the pair, but the current capacity at 21v will be limited to the current capacity of the small 9V battery.

There seems to have been some confusion about this full wave bridge with center tap circuit, though it's probably much clearer now. I'm gonna stir it up some more. I've redrawn the original full wave bridge with center tap circuit in an attempt to explain from a different point of view. And I removed the ground symbol to minimize confusion. Don't think of this circuit as a full wave bridge at all! Instead, think of it as two separate conventional (2 diode) full wave rectifiers that just happen to be stacked up series aiding (think 2 cell flashlight again).

Fig 1 is the orig circuit.

Fig 2 has the lower 2 diodes blanked out just to make the conventional full wave rectifier stand out. The top diodes should look very familiar. It's the simple full wave ss rectifier we see all over the place. It's output is 1.414 x 212 = 300V.

Fig 3 shows the lower rectifier with the upper diodes blanked out. This circuit is not so common in tube amps because the junction of the diodes is negative in respect to the center tap. However, it's all relative and depends on how you connect your meter leads. Anyhow, since this rectifier is connected to the same transformer winding as the upper rectifier, the voltage output is the same.

Now put the ground symbol in. If you decide to put it on the PT center tap, then you will have a dual polarity supply, one positive 300V and one negative 300V. If you put it at the bottom of the circuit like the original drawing, you will have +300V amd a +600V outputs. All that changed was the ground symbol, your point of reference.

Hope I didn't create more confusion. My goal was to simplify the circuit operation using another POV.

[Fresh_Start]

sluckey - your explanation and especially the drawings are GREAT!

Thanks,

Chip

[PRR]

> Does the "center tap" need to be in the center of the PT secondary(ies)?

Yes.

> Use the higher voltage rail for the power amp and the lower voltage rail for the preamp.  Would there be any advantage over having a fork in the power rail?

K.I.S.S. Preamp power is small. Drop it however convenient robust and simple.

[DummyLoad]

playing with a stacked delon doubler.

yes it's they are true doubler circuits, but they are stacked series aiding.

there 4 possible taps 1/4, 1/2, 3/4 and the uppermost or, as i call it, the top tap.

henceforth we'll call the the top tap point D; the 3/4 tap point C; the 1/2 tap point B; the 1/4 tap point A. see the schematic for clarification.

i used two loads- resistor stacked load and capacitor stacked with a resistor load to increase the ripple. increasing the ripple will allow us to see the charge peaks on the oscilloscope. the resistor load lets us view the entire conduction angle with our oscilloscope.

interesting results. the since the lower doubler's mid-tap is what i call the 1/4 tap - we know from past discussions that it is 1/2 wave (60Hz charge rate) and that the top of the lower doubler is full wave - int this circuit it is now the 1/2 tap - we see a full wave, 120Hz charge peaks, business as usual. on the second (upper doubler) is where the fun starts.

look at the oscilloscope shots and see if you see some symmetry as to what's happening in the lower doubler's half tap (1/4 tap in this circuit) and with what's going on at the 3/4 tap - point C. also, notice the P-P voltage measured on the oscilloscope in the lower doubler is the same at the center tap and at the upper - the only difference is the charge frequency. now look at the schematic compare the DC readings made with a DMM. do you see a relationship of the measured DC voltage and charge frequency? keep in mind now, that you're measuring pulsed DC.

you ask why would anyone want to do this? why not? how else can you take 2 x 6.3V windings and turn them into 37VDC and get full wave at the 1/2 tap? now imagine you have 2 x 115V windings. yes, it's not quite full wave at the top tap, but it's close enough. the glass is half full.  

peace.

--DL

the schematic:



resistor load - vert: 5V/div. Point A - 1/4 tap.



resistor load - vert: 5V/div. Point B - 1/2 tap.



resistor load - vert: 5V/div. Point C - 3/4 tap.



resistor load - vert: 5V/div. Point D - top tap.



capacitor load - vert: 10V/div. Point A - 1/4 tap.



capacitor load - vert: 10V/div. Point B - 1/2 tap.



capacitor load - vert: 10V/div. Point C - 3/4 tap



capacitor load - vert: 10V/div. Point D - top tap



zero volt reference shot...  

[Fresh_Start]

DL - I don't understand where the voltage figures on the capacitor loaded model come from, but the wave form for Tap C certainly is interesting!  Are you saying that Tap C is close to full wave rectification?

Would two 115 volt secondaries yield about 365 VDC at tap D and 282 VDC at tap C?  If so, that seems like a nice starting point for an amp's power supply.

Thank you so much for digging into this some more.

Cheers,

Chip