Symmetrical supply rails with a 2 wire transformer, is it possible? - diyAudio
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Old 3rd March 2003, 06:56 PM   #1
Bricolo is offline Bricolo  France
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Default Symmetrical supply rails with a 2 wire transformer, is it possible?

I've got a transformer with only 2 wires on it's secondary (and only one secondary)

is there any method to obtain -,0,+ ?
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Old 3rd March 2003, 07:00 PM   #2
grataku is offline grataku  United States
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A DC-DC converter? I would guess that's only for low power stuff...
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Old 3rd March 2003, 07:40 PM   #3
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try this:
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Old 3rd March 2003, 07:49 PM   #4
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I suppose that'll work...but you can do better with a true full-wave voltage doubler. You can even use a bridge rect. here, though half the diodes act in parallel (IIRC), so it's less effective. (This strategy is used in dual-voltage switching supplies, where a switch or jumper changes it from FWB to FWD mode. Yep, the primary circuits have 340VDC on them!)
Anywho - it's basically two half-wave rectifiers pointing in opposite directions. One end of the transformer feeds a 'CT' between the two main filter caps, the other end feeds diodes which pump the caps alternately. This is known as a full-wave doubler because the ripple output is 120Hz, due to the stacked nature of the circuit.

I'll grab a circuit in a second...

Tim
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Old 3rd March 2003, 07:52 PM   #5
Rudy is offline Rudy  Belgium
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This is also a pssebillity, little bit easy'r

some minor points
- buffer elco's must be 2X bigger then normally
- diodes must be able to handle a lot of current

note, total output voltage is double then normally

Greetz Rudy ( dahm dots )
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Old 3rd March 2003, 08:05 PM   #6
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Yep, that's the full-wave I was referring to.

Here are some circuits to mull over. The CT between caps (on the FWD) can be used as a half-voltage (wrt. negative) or ground point.

Tim
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Old 3rd March 2003, 10:59 PM   #7
e96mlo is offline e96mlo  Sweden
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mermoz, if the earth symbols are supporsed to be the same node in your schematic, then there is no longer galvanic isolation.

/Marcus
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Old 3rd March 2003, 11:03 PM   #8
Bricolo is offline Bricolo  France
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wich one to choose?
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Old 3rd March 2003, 11:26 PM   #9
grataku is offline grataku  United States
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Bricolo,
no one has asked what do you intend to do with this...
Clearly voltage doublers are not the optimal solution if you care about ripple, otherwise why bother with the two secondaries, right?
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Old 3rd March 2003, 11:33 PM   #10
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Default voltage doublers and ripple

if you increase the capacitance the ripple can be dealt with-- or at least you can calculate the % ripple at a given load -- the problem is that capacitors quickly become more expensive than transformers.
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