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Old 12th October 2011, 08:48 AM   #1
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Default Dual secondaries with one bridge

Hi to all I have a transformer with dual secondaries and dual primaries. First question is Can I use the dual secondaries as center tap with one bridge without having any 50 Hz hum? Do I have to be carefull in the phases?
Also Is there a posibility to have a hum if I connect the primaries in series (240Volt) but in not correct phase? What if I have two of those trafos? Do I have to pay attention in the phase between the two trafos? The problem or the key here is the 50Hz hum. I have the situation described above and I experiance some audible hum.
I would apreciate any comments
Thanks
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Old 12th October 2011, 10:24 AM   #2
DF96 is offline DF96  England
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You can use two separate secondaries as one CT secondary. Just connect the end of one winding to the start of the other. The join is the centre tap. The phasing will probably be indicated by leadout colours, but you can check with an AC voltmeter. You can use one bridge with a CT sec to get + and - DC.

If you connect primaries in the wrong phase you will hopefully blow a fuse or trip a breaker. Failing that the transformer primary windings will get very hot and emit smoke.

Put up a diagram if you want further comments on what you have done.
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Old 12th October 2011, 10:38 AM   #3
AndrewT is offline AndrewT  Scotland
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If you use a Dual secondary with a single bridge rectifier, you MUST convert the Dual secondary to Centre Tapped. Do as DF suggest.

To avoid potentially damaging mishaps with incorrect wiring, always power up mains powered projects via a bulb tester.
It is that good that even with the Dual Primaries mis-wired as an effective short circuit that the mains fuse does not blow.
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Old 12th October 2011, 10:52 AM   #4
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Thanks for the replies. Also if we have two seperate trafos, can I connect its primaries either way or I have to be carefull for the phases. Is any situation that can cause 50hz noise? If I connect the primaries of the two trafos in antiphase do I cancel any noise?
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Old 12th October 2011, 10:56 AM   #5
AndrewT is offline AndrewT  Scotland
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The Dual Primaries are usually 115Vac.
Connect them in parallel for 110/120Vac operation.
Connect them in series for 220/240Vac operation.
Connecting one primary with the incorrect phase will turn the transfomer into an effective short circuit across the mains.
The transformer Primaries will try to absorb many kW of power until something blows.

Use a bulb tester.
Quote:
To avoid potentially damaging mishaps with incorrect wiring, always power up mains powered projects via a bulb tester.
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Old 12th October 2011, 11:01 AM   #6
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I am not talking about the primaries of one trafo but for the primaries of TWO DIFFERENT trafos?
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Old 12th October 2011, 11:05 AM   #7
DF96 is offline DF96  England
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Primaries of two different transformers can be connected either way, however you will need to get the secondary phasing right if you are joining these together too.

What do you mean by 50Hz noise? A little hum from the loudspeaker? Mechanical noise from the transformer?
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Old 12th October 2011, 11:17 AM   #8
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Mechanical and noise at output
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Old 12th October 2011, 11:24 AM   #9
DF96 is offline DF96  England
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A little mechanical noise may be unavoidable. Try mounting using rubber grommets, but be sure to maintain a ground connection for the frame.

Hum on the output, unless severe, is unlikely to be a transformer issue but due to poor grounding or inadequate smoothing. Many people route the reservoir charging currents through their star ground, then wonder why they get hum.
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Old 12th October 2011, 11:35 AM   #10
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Quote:
Originally Posted by DF96 View Post
A little mechanical noise may be unavoidable. Try mounting using rubber grommets, but be sure to maintain a ground connection for the frame.

Hum on the output, unless severe, is unlikely to be a transformer issue but due to poor grounding or inadequate smoothing. Many people route the reservoir charging currents through their star ground, then wonder why they get hum.
What do you mean by "Many people route the reservoir charging currents through their star ground" exactly? It is not clear to me.
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