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Old 1st March 2009, 04:44 AM   #1
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Default Isolation after transformer considered isolated?

Hi guys.

I'm building my next power supply and I need to clarify something.

If I parallel multiple rectified AC after a transformer's secondary , is it considered isolated?

I originally intended to use 1 transformer for each output but I find that it's a waste of money and space due to the low current I intend to supply for about half of the 10 outputs.

e.g.
Code:
AC->(1) Rectified DC->Smoothing+Filtering+Regulation
  ->(2) Rectified DC->Smoothing+Filtering+Regulation
  ->(3) Rectified DC->Smoothing+Filtering+Regulation
  ->(4) etc
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Old 1st March 2009, 04:48 AM   #2
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I for one have no idea what you are trying to say. How about a sketch?
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Old 1st March 2009, 05:39 AM   #3
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Haha... I myself thought I sounded cryptic..

Apologies for the bad drawing. I can't draw for nuts, on a computer AND in real life. haha.

Is the schematic below considered "isolated" or isolated? There is only 1 secondary.
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Old 1st March 2009, 05:47 AM   #4
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Use a flyback converter with a separate winding for each of the outputs. It can be designed to operate from low voltage if you are new to power supply design.
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Old 1st March 2009, 07:23 AM   #5
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isn't that usually part of an SMPS circuit?

means i have to read up on this.. man.. SMPS are way out of my league at the moment.

But I didn't know that flyback converters are isolated till now. Interesting.
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Old 1st March 2009, 08:31 AM   #6
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Quote:
Originally posted by firethorn
Haha... I myself thought I sounded cryptic..

Apologies for the bad drawing. I can't draw for nuts, on a computer AND in real life. haha.

Is the schematic below considered "isolated" or isolated? There is only 1 secondary.
Isolation is with respect to the mains service; so it is isolated.
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Old 1st March 2009, 10:46 AM   #7
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I mean with regards to ground loops. Not in terms of safety. Is it considered isolated in that respect?
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Old 1st March 2009, 11:59 AM   #8
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Quote:
Originally posted by firethorn
I mean with regards to ground loops. Not in terms of safety. Is it considered isolated in that respect?
My experience with personal equipment has involved ground loops being caused by interaction with the mains. Yet there is just about an infinite number of possible set-ups. Maybe just try minimalism and address problems as needed.
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Old 1st March 2009, 12:02 PM   #9
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You seem to miss something elementary.
3 times 9V supplies are not isolated in any way, because you achieve isolation by magnetic (or optical) coupling only. Keep in midn that normally you connect GND to one of secondary wires.
But you can achieve ground loop braking by some common mode filtering, I mean if you put R+L on both wires of rectified voltage and C between them after RL.


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Old 1st March 2009, 12:49 PM   #10
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You are allowed one isolation card with that scheme. When any one of the 9V commons gets tied to ground, the others automatically follow this reference, though you need to understand the others do not have their commons directly connected to ground, nor are you permitted to. Nor does the measured common-ground voltage on the other supplies equal zero, it is somewhat dynamic and variable.

So, as long as you have no ties to ground on the secondary supplies, they are isolated from ground and from the mains voltage. But they are never isolated from each other, they are bonded together through the p-n diode junctions.

You only get one opportunity to ground a secondary. So with regards to ground loops, it will fix the bonded secondary, but I doubt you will like what happens to the others.
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