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Secondary's C.T. tied to Totem Pole Caps???

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  • #1

    Secondary's C.T. tied to Totem Pole Caps???

    Hi Folks!

    I was just curious as to why the center tap of the mains transformer secondaries are tied to the junction of the power supply's (totem pole config'd) first stage filter capacitors? Does this effectively create a voltage doubler arrangement? With the FWB rectification scheme, I would think that one could float the C.T. entirely. TIA!

    Oh, here's some schematics that illustrate what I am talking about, they are all pdf's. The Orange schematics are the more simple ones (2nd & 3rd):


    http://www.schematicheaven.com/marsh...100w_2203u.pdf

    http://www.schematicheaven.com/newam...e_mkii1973.pdf

    http://www.schematicheaven.com/newam...e_mkii1972.pdf

    http://www.schematicheaven.com/newam...ek_mig100h.pdf
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  • #2
    ...it's a voltage doubler circuit
    ...and the Devil said: "...yes, but it's a DRY heat!"

    Comment

    • #3
      The circuits you linked are not voltage doublers. The center tap connection is just used to insure equal voltage across the two filter caps that are in series. It's the same configuration that you see in solid state power amps with bipolar supplies except that the minus supply is grounded.
      WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
      REMEMBER: Everybody knows that smokin' ain't allowed in school !

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      • #4
        Originally posted by loudthud View Post
        The circuits you linked are not voltage doublers. The center tap connection is just used to insure equal voltage across the two filter caps that are in series.
        ...I stand corrected, so it's just a "hard-wired" voltage balance point. Obviously, I didn't look hard enough at the schematic.
        ...and the Devil said: "...yes, but it's a DRY heat!"

        Comment

        • #5
          Originally posted by loudthud View Post
          The circuits you linked are not voltage doublers. The center tap connection is just used to insure equal voltage across the two filter caps that are in series. It's the same configuration that you see in solid state power amps with bipolar supplies except that the minus supply is grounded.

          Thank you VERY much guys! If I may pry just a tad more about this connection:

          Is it the 'balance' of the secondary windings that does this? I'm trying to visualize how this functions... TIA!

          HAPPY NEW YEAR!
          Last edited by Tobacco Soup; 01-01-2008, 04:06 AM.

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          • #6
            ...I did a little "homework" and textbook "researching" and it's actually a 'voltage-stacking' re-configuration of the normal "Dual Complementry" rectifier circuit (Linear Magnetics App Note):



            ...the re-configuration from "dual ± voltages" to 'voltage-stacking' is accomplished by simply moving the "ground" from between the two capacitors (and xfmr center-tap) over to the 'negative-side' of the two stacked capacitors; however, the final 'stacked' voltage is same as from normal BRIDGE ckt, so it's not a voltage-doubler.

            ...and, the xfmr-winding acts as the voltage-balancer for the two caps (taking the place of the 220K voltage-balancing resisitors typically used on stacked capacitors).
            Last edited by Old Tele man; 01-01-2008, 08:09 PM.
            ...and the Devil said: "...yes, but it's a DRY heat!"

            Comment

            • #7
              Umm...that's still not the Marshall connection. What you've got is a pretty standard split load supply. The Marshall version still grounds the anode side of the rectifier allowing only a single positive supply. You get two advantages here. First, a "standard" center tap transformer can be used with a bridge rectifier without fouling things up and the second advantage is that, as loud thud already stated, you've got an equal voltage across both capacitors.

              Cornell Dublier used to have a rather large white paper on their site explaining all the pros and cons of several different capacitor connection arrangements. Don't know if it's still up or not as the site as undergone a massive overhaul since I was there last.

              -Carl

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              • #8
                ...the pix is of a "dual complementary" circuit, not the "voltage-stack" arrangement, it's just to illustrate where & how the "stack" arrangement is derived from.
                ...and the Devil said: "...yes, but it's a DRY heat!"

                Comment

                • #9
                  Ok, found the app note. Very informatie read!

                  http://www.cde.com/catalogs/AEappGUIDE.pdf

                  -Carl

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                  • #10
                    I PSpiced the 1st link that Tobacco posted, and it's a doubler.....

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                    • #11
                      Originally posted by Earl View Post
                      I PSpiced the 1st link that Tobacco posted, and it's a doubler.....
                      ...but did that assume PT-voltage was 'end-to-end' or PT-voltage was 'end-to-centertap'?
                      ...and the Devil said: "...yes, but it's a DRY heat!"

                      Comment

                      • #12
                        OTM - I just used 2. 1V peak SIN sources in series, and CT'd the junction of the two.

                        I guess, in 'normal' terms, that would be a 0.7 - 0 - 0.7 VRMS output..?

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