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How to identify the "phase" between the two white wires at this primary
How to identify the "phase" between the two white wires at this primary
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Old 25th June 2019, 06:15 PM   #1
Karl vd Berg is offline Karl vd Berg
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Question How to identify the "phase" between the two white wires at this primary

Good afternoon/evening to all,

I bought an Amplimo toroid which shows two white wires for the primary. At diagram (below), it shows the dotted mark in one white wire for the "live/hot", but on the actual xmer there is no indication at all. The secondaries are easy to identify by the different colours.
An email was sent to Amplimo, but no answer/reply so far.

How could I identify "which" white wire should be wired to phase?

Thank you.

P.S.: Is it reasonable to put two ~10A fuses after the secondaries?

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Old 25th June 2019, 06:49 PM   #2
FauxFrench is offline FauxFrench  France
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Hi Karl,
Any of the white can be used as phase and the other white as neutral. It does not matter. Evidently, the phase of the secondary outputs change 180 degrees as well, but as AC voltage is "first one way and then equally much the other way" it has no effect in operation. This is why they made both lead-wires white.
The real information in that figure is how the secondaries are phased with respect to one another. Therefore, they have different colors.
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Old 25th June 2019, 06:52 PM   #3
Karl vd Berg is offline Karl vd Berg
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Hi FauxFrench,

Thanks a million for the info!

Do you think it's reasonable to put two ~10A fuses after the secondaries (for extra protection)?
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Old 25th June 2019, 07:13 PM   #4
cbdb is offline cbdb  Canada
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Yes to the fuses, but if the transformer output is only 3.75 amps You might want to use 5 amp fuses
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Old 25th June 2019, 07:26 PM   #5
Karl vd Berg is offline Karl vd Berg
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Quote:
Originally Posted by cbdb View Post
Yes to the fuses, but if the transformer output is only 3.75 amps You might want to use 5 amp fuses
Ok, thanks.
I remember a while ago someone here saying fuses should be around 3x times the secondary values, but 5A may be ok, indeed.
It's a 100W class AB amp, I'm replacing its old 220V transformer, as it gets quite warm after one hour running, has an annoying permanent buzz, also the rect. bridge gets bit warm. I suspect the amp is draining high(er) currents than it should (with old 220V trafo), so I want to put fuses after secondaries.
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Old 25th June 2019, 07:41 PM   #6
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by Karl vd Berg View Post
Hi FauxFrench,

Thanks a million for the info!

Do you think it's reasonable to put two ~10A fuses after the secondaries (for extra protection)?
As cbdb says, choose the fuses above the nominal current rating but not so much above nominal current rating that a short-circuit current is not certain to blow the fuses. I would also choose 5A-6A, slow-blow types.
NB: It is always better to realize that you have chosen fuse values such that they are activated a bit before you expected, than after the damage is done. It is not exact science to choose fuse values and wire-fuses are not very precise themselves.
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Old 25th June 2019, 09:47 PM   #7
Mark Tillotson is offline Mark Tillotson
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Secondary fuses protect the downstream circuitry and wiring, the primary fuse will blow if there's a secondary short due to the massive primary current draw that will trigger. Basically size the primary fuse to prevent the transformer catching fire. A hard short will either blow the rectifier or the primary fuse anyway, but secondary fuses are probably cheaper than bridge rectifiers(!) If you omit secondary fuse the downstream wiring should be generously rated (as least as thick as the secondary windings).


If there's an auxiliary secondary of lower power, that could short without pulling enough power to blow the primary fuse - that kind of winding does need a fuse I think, especially if at the surface of the winding stack.
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Old 26th June 2019, 07:23 AM   #8
Karl vd Berg is offline Karl vd Berg
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Thank you, Mark and FauxFrench. Much appreciated!

Last edited by Karl vd Berg; 26th June 2019 at 07:28 AM.
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Old 26th June 2019, 11:37 AM   #9
Kay Pirinha is offline Kay Pirinha  Germany
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+1 to Mark!


It is always better to put the fuse between the power line and the PT's primary, not only for safety reasons, but also in order to minimize losses.


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Old 26th June 2019, 12:56 PM   #10
Karl vd Berg is offline Karl vd Berg
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Thanks again, and just one more question: the 3.47A at secondaries refer to full power? I'm buying several slow fuses: 5A, 6.3A and 8A.
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