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Old 24th July 2014, 02:32 AM   #1
BigE is offline BigE  Canada
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Default Universal transformer

I have a transformer whose primaries can be wired to run anywhere in the world. It has 5 sets of secondaries that each can put out 4A with 120 V C.T. at no load. It will be wired to produce between 64 and 70 V C.T. for an F5T.

What should I do with the unused primaries? Can I do something to them to reduce noise?

Also, it seem that each polarity of each set of secondaries must be independently fused -- necessitating 10 fuses. Is that correct?

THANK YOU!

BTW: It is a 2.4 KVA transformer, that I wired as 1.2 KVA and successfully started it with the diyaudio store soft start today, using the parts from the regular BOM. When wired for the F5T it will be about 1.4 KVA.
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Old 24th July 2014, 07:43 PM   #2
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I don't think there is anything you can do with the unused primaries to reduce noise, just make sure they are insulated and cannot be touched, as they will have full voltage on them. Same goes for any unused secondaries. (No need for fuses in any unused secondaries of course.)
The pair of fuses in each secondary protects the transformer against a short circuit from either side of that secondary to the centre tap.
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Old 25th July 2014, 01:18 AM   #3
BigE is offline BigE  Canada
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Thanks Malcolm! It's a bit of a chore to put in 10 fuses, but it's probably worth it in the end.
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Old 25th July 2014, 02:29 AM   #4
sreten is offline sreten  United Kingdom
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Hi,

What transformer has unused primaries ? And how ?
(Other than primaries having some extra taps.)

There is no point independently fusing parallel secondaries,
what are you protecting against by doing so ? Not the load.

rgds, sreten.

At such power levels fuses could be part of of a CRC
power supply, replacing the R, if I'm following.
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Old 25th July 2014, 03:25 AM   #5
BigE is offline BigE  Canada
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The transformer has two primary windings.

The first winding has taps for 100V and 115V.
The second has taps for 100V, 115V, 125V.

I was asking how to treat the unused wires. Apparently, it is enough to make sure they don't short out or touch.

If I don't have to fuse the secondaries, I won't! That's a significant expense and takes up quite a bit of real-estate.
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Old 25th July 2014, 03:38 AM   #6
gootee is offline gootee  United States
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Aren't those "universal" primaries usually used either in parallel or in series, and there are no "extra wires"? The secondaries are often done similarly.

As long as the secondary voltages are right, why would you ever want to wire for a lower-than-maximum VA rating?
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Last edited by gootee; 25th July 2014 at 03:43 AM.
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Old 25th July 2014, 06:01 AM   #7
udok is online now udok  Austria
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Quote:
Originally Posted by BigE View Post
I have a transformer whose primaries can be wired to run anywhere in the world. It has 5 sets of secondaries that each can put out 4A with 120 V C.T. at no load. It will be wired to produce between 64 and 70 V C.T. for an F5T.

What should I do with the unused primaries? Can I do something to them to reduce noise?

Also, it seem that each polarity of each set of secondaries must be independently fused -- necessitating 10 fuses. Is that correct?

THANK YOU!

BTW: It is a 2.4 KVA transformer, that I wired as 1.2 KVA and successfully started it with the diyaudio store soft start today, using the parts from the regular BOM. When wired for the F5T it will be about 1.4 KVA.
You can reduce noise (due to AC current between primaries and secondaries) by connecting one end (and only one!) of each unused primary or secondary go ground or earth.
If the winding is between another primary and secondary it will act as a shield.

Often such big transformers have build in thermofuses. A fuse at the AC inlet may be enough.
You may need fuses at the secondaries too to protect against fire hazards...
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Old 25th July 2014, 07:14 AM   #8
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The primaries must be wired in parallel for 100 or 115 volt inputs.
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Old 25th July 2014, 08:23 AM   #9
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Can you give us some more details of how you are planning to wire this transformer? A diagram would be great, if possible.
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Old 25th July 2014, 08:47 AM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
unused primaries
??

A dual primary transformer can have two sets of 115Vac windings.
These two winding can have extra tappings to make the transformer even more universal.
eg 100Vac for Japan
Extra tappings at 5Vac or 10Vac increments for close setting to suit supply voltage at location.

For a 230Vac supply both primary windings are used in series.
For a 115Vac supply both primary windings are used in parallel.

The transformer does not have any "unused primary" windings.
The 100Vac tappings are for Japan
The 125Vac tapping is for 240Vac countries (not Europe). 115+125 = 240

It will have unconnected tappings if extra have been fitted. These extra tappings are on the used windings and are Live.

DO NOT parallel the mixed voltage tappings eg. 115 & 125 windings. The primaries set with the wrong tappings will draw differing currents and the higher current (lower voltage tapping) will overheat.
You can series connect different voltage tappings. Because the same current passes through all windings.

Your transformer can be set for 100Vac, 115Vac, 200Vac, 215Vac, 225Vac, 230Vac, 240Vac. That's very universal for the cost of three extra tappings.
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Last edited by AndrewT; 25th July 2014 at 08:59 AM.
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