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Old 2nd October 2006, 07:28 PM   #21
JesseG is offline JesseG  Canada
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From AndrewT
Quote:
+-34Vdc from 2*38.5Vac is not possible. Ignore that poster until he can prove his assertion.
C'mon guys - lighten up a little

We are not ADDING the two 38.5 together, this is a BIPOLAR supply. Each side is 38.5v.

Taking one side as a simple example:

38.5v AC secondary is peak voltage. into a bridge (full wave) rectifier produces RMS DC of 33.87V, when the 3% ripple is smoothed by the caps, the result is about 34.2V DC.

Respectfully, I think you fellows are confusing peak AC and RMS.

John65b - would you like a schematic or photos?
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Old 2nd October 2006, 07:38 PM   #22
poobah is offline poobah  United States
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John is probably stating an RMS voltage as is the norm...

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Old 2nd October 2006, 08:44 PM   #23
john65b is offline john65b  United States
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JesseG,

Please post pics and schematic anyway...it would be good to see you system regardless of what the nomenclature diffences are...

I will most likely unwind the trannie as Poobah instructed (Thanks Poobah).

But just for S&G, why couldn't wire up 40V & 0V to a rectifier to get same results as 20V 20V? Understand that I won't do it, but my curiosity won't let this go...
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Old 2nd October 2006, 09:41 PM   #24
poobah is offline poobah  United States
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John,

It can be done... but you have to create a virtual ground. You place 2 large caps in series across your rectified output, then you connect the center point between the caps to ground. It has problems... low valued bleeders across the caps for one.
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Old 3rd October 2006, 02:21 AM   #25
john65b is offline john65b  United States
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So this can work? Wow... remember, I have two centertaps (black wires), and two 40V leads (blue wires)... and all four wires, (blue, black, black, blue) are the same winding...voltage across any blue to any black is 40V....blue to blue is 80V, etc.

And the virtual ground (0V) between the pair of smoothing caps is made by a single connection to chassis ground and nothing else...

Don't understand the bleeder resistor issue...attached is a schematic...
So if anyone has a high voltage centertap trannie (CT needs to be split), they can do this to get something that is identical to two separate trannies at 1/2 the voltage for a dual mono power supply??
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Old 3rd October 2006, 04:08 AM   #26
poobah is offline poobah  United States
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Yep...

Place bleeder resistors in parallel with each cap such that they burn 3 to 5 Watts. The resistors should be 1 or 2% as they will ultimately determine the balance between rail voltages.

Me... I would unwind the the tranny. But I am unclear as to your tranny wiring. Can you make a picture?

What you have drawn presumes 2 isolated windings, but you describe all 4 wires as being "the same winding".

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Old 3rd October 2006, 12:30 PM   #27
john65b is offline john65b  United States
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I get 0 ohms between both black wires - there is only one winding. The two centertaps are the same.

In other words, if I had a centertap tranny (three wires) and I took the centertap wire and split it, so there are now four wires, this is what I have

I do not know why the manufacturer of this torroid split teh centertap, but it is the same wire.

Knowing this, would this still work? I have tried it a while back I remember it working....
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Old 3rd October 2006, 01:06 PM   #28
AndrewT is offline AndrewT  Scotland
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Hi,
although I said rewind the toroid, you do not need to unwind all of the secondaries.

Find a point about midway along each winding. Cut the two windings at about the mid-points.
Measure the voltage in each of the four new shorter windings.
Add turns to each to bring them up to the desired voltage.
Now you can run four windings into two recifiers to give two dual polarity supplies, one for each channel or you can parallel two windings to double the current capacity and end up with one dual polarity supply.

Buy some enamelled copper wire of the same diameter as you have on the secondaries.

By adding turns the total VA rating is not reduced. Some would argue that the VA rating is slightly increased.

If you reduce the turns on the secondary then the VA rating is reduced in proportion to the reduced voltage you end up with. i.e. the current stays the same.


Do you follow all that?
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Old 3rd October 2006, 03:30 PM   #29
poobah is offline poobah  United States
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VA rating is perhaps not a good way to look at this. VA is really is power rating, based on an optimal design, that proiduces a given temperature rise. That is 50% iron loss, and 50% copper loss.

Reducing turns could be viewed as reducing VA, because of the obvious loss in voltage. But also winding resistance decreases, in turn heat. The flip side, increasing turns, does not increase VA. You have the same core and iron losses, but more winding resistance. Bottom line is that winding currents should be considered priority over VA. Regardless, removing turns, can be done safely without concern, as long as the winding current is not proportionately increased by presuming the VA remains equal.

John,

The decision to unwind should really be made based on whether the secondaries are wound side by side (good thing)... or one on top of the other (bad thing). To implement the drawing you have would require that the center taps be split anyway. How about pulling away some tape and checking the lay of the windings?


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Old 3rd October 2006, 03:55 PM   #30
john65b is offline john65b  United States
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Andrew, I think I understand...I will look at the winding tonight if possible.

I suspect both black centertaps are connected at the same point on the winding. If I cut the winding at that point, I create two separate windings, and thus a dual secondary. No unwinding, or rewinding would be nessessary...see my attachment...

From there it becomes as simple as connecting the power supply with dual secondaries.
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