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Center tap trans.
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Old 12th August 2018, 07:11 PM   #1
bluerooster is offline bluerooster
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Default Center tap trans.

Just for the sake of supposing. Suppose I had a 100-0-100 transformer.
We all know that I can do a full wave rectifier, and have 100v. Or a bridge, deleting CT, and have 200v. Now, can I put a bridge between one 100 and 0, and another bridge between the other 100 and 0, and have 2 100v legs out of phase?
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Old 12th August 2018, 07:24 PM   #2
scottjoplin is offline scottjoplin  Wales
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What do you mean "out of phase"?
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Old 12th August 2018, 10:28 PM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by bluerooster View Post
Now, can I put a bridge between one 100 and 0, and another bridge between the other 100 and 0, and have 2 100v legs out of phase?
Strictly speaking, getting "out of of phase" supplies from a single source/transformer is an impossibility.

If you mean two independent, floating supplies you can connect any way you want, it isn't possible either: you can connect two bridges and use the resulting supplies separately, but any attempt at connecting them galvanically downstream will result in a disaster: even trying to duplicate the normal -100/0/+100 configuration will cause shorts.

There is, in principle, a way to duplicate separate windings (not tied to a single CT): insert two capacitors in series with one of the bridges.
It is a theoretical option, but not actually practical in reality: the capacitors need to be large (ideally significantly larger than the filter caps), and because of the large value, they must be polarized, meaning you have to know the polarity they will be subjected to.
In addition, any cross-supply current will result in charging pulses through the the galvanic path connecting the supplies.

All this means that the scheme is not completely useless, just extremely limited at the very least...
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Old 13th August 2018, 03:12 AM   #4
PRR is offline PRR  United States
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Quote:
Originally Posted by bluerooster View Post
...Suppose I had a 100-0-100 transformer.
We all know that I can do a full wave rectifier, and have 100v. Or a bridge, deleting CT, and have 200v....
Before you go there...

With the usual capacitor-input filter, 100V AC gives 141V of DC.

Going on with where I think you are going: if you want two 141VDC outputs opposite polarity around a common ground, you use -one- full wave bridge, two caps, commoned with the center-tap. That's how "all" big audio power amps do it.
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Old 13th August 2018, 10:02 AM   #5
DF96 is online now DF96  England
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The general answer to this type of question is that two things (in this case PSUs) cannot be both connected and isolated, yet the question is usually of the form "can I run two separate PSUs from connected secondaries?". As Elvee says, you usually end up connecting them in some way which shorts something else so it doesn't work and may cause something to get rather hot before a fuse hopefully blows.
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