Electronics > Projects, Designs, and Technical Stuff
Testing back to back power transformers for use as an isolation transformer
duak:
Series/parallel windings are wound bifilar, ie. two wires wound at the same time, to ensure they have the same number of turns and identical coupling inductance. This means that you've only got two layers of magnet wire insulation for galvanic isolation. Bifilar windings also have more coupling capacitance between the two windings than if the windings are separated.
As an aside, I encountered a toroidal power transformer where the primary winding count differed by one turn. It worked fine when the windings were in series, but not in parallel because there was a circulating current flowing that would trip the line breaker. Going back to the build sheet, I found that the designer did not specify bifilar winding.
David Hess:
--- Quote from: duak on May 10, 2019, 07:13:47 pm ---Series/parallel windings are wound bifilar, ie. two wires wound at the same time, to ensure they have the same number of turns and identical coupling inductance. This means that you've only got two layers of magnet wire insulation for galvanic isolation. Bifilar windings also have more coupling capacitance between the two windings than if the windings are separated.
--- End quote ---
The dual windings on the transformers I used are on separate layers; they are not bifilar wound. But I still chose to wire them back to back for increased isolation.
soldar:
--- Quote from: intabits on May 10, 2019, 04:51:18 pm --- With the big ones, I could connect the 220V primaries in parallel to the mains, and their 120V secondaries in series.
--- End quote ---
That doubles the power they can handle. Back to back they can only handle the power of the smallest one but in parallel they add up.
Be careful about the secondaries in parallel having the same voltage. A volt or two will not really matter, especially under load, but if there is much difference you will get wasted heat.
I connect one end of the secondaries and measure voltage between the other two open ends. That way you also make sure they're in phase.
soldar:
--- Quote from: duak on May 10, 2019, 07:13:47 pm ---Series/parallel windings are wound bifilar, ie. two wires wound at the same time, to ensure they have the same number of turns and identical coupling inductance.
--- End quote ---
Definitely not the ones I have. I guess mine were wound by someone who could count :)
--- Quote from: duak on May 10, 2019, 07:13:47 pm --- As an aside, I encountered a toroidal power transformer where the primary winding count differed by one turn. It worked fine when the windings were in series, but not in parallel because there was a circulating current flowing that would trip the line breaker. Going back to the build sheet, I found that the designer did not specify bifilar winding.
--- End quote ---
I doubt that. One turn is not going to blow anything. If the transformer tripped the breaker when the secondaries were connected in parallel it was because they were connected incorrectly, not because there was a one turn difference.
soldar:
Some years ago, for many months, my building was undergoing renovations and the electricity would we cut off randomly and without warning which was a pain when I lost my current work on the computer. Even worse, The work was still ongoing when I was leaving for some weeks travel in China and I wanted to remotely connect to my computer at home.
So, I talked to my neighbor in the next building and he had no problem letting me hook up to his electricity and we threw a cable from my window to his window. I could have just plugged my computer into his supply but that risked the hanging cable failing so I rigged something very simple.
For isolation I used those two transformers in series as described in my earlier post. Then I put a diode bridge and connected it in parallel with the computer PSU bridge.
With my mains working most, almost all, of the power came from my side because of the resistance of the long cable to the neighbor. When my power was cut off the computer wouldn't even notice and would continue to draw from the neighbor.
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