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Old 10th March 2013, 05:26 PM   #1
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Default Power Transformer Wiring Question ?

I have two identical toroid power transformers, 120V primary and 350V-0-350V secondary.

Can I wire the secondary of each transformer together in series to get 700V-0-700V ?
If I can, what should I be aware of ?

The voltage will be rectified with 2 damper diodes and the CT will be grounded.
Is there a better way to wire this up ?
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Old 10th March 2013, 05:38 PM   #2
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Yes, you can.
But, Do be aware that you know the polarity of the primary windings winding's and that they are the same before you apply 120V from the line to them!!!!!

If they are Antek's they are known for not keeping note of the polarity of their winding's!!!!

FWIW

jer
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Old 10th March 2013, 05:38 PM   #3
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There was a discussion about this in an older thread, I'll try to look it up. Result was that using two separate transformers in a full-wave CT rectifier configuration might lead to transformer saturation and hum, as each transformer core will see secondary current flow in one direction only.

Bridge rectification would work, but means two more damper diodes and another heater circuit due to the elevated cathode potential.

Greetings,
Andreas

EDIT: Of course you could build two separate supplies and add them at the output, but the upper one would be floated by +350V. Doable but there might be safety issues if not implemented properly.
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Old 10th March 2013, 06:14 PM   #4
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Please find that thread.

Could you guys double check the wiring.
Attached Images
File Type: jpg Double Transformers.jpg (74.3 KB, 211 views)
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Old 10th March 2013, 06:18 PM   #5
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Is there any difference between the DOUBLE transformer configuration and ONE transformer with dual primary and dual secondary with the secondary wired in series and use the connected wires as CT ?
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Old 10th March 2013, 06:26 PM   #6
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Quote:
Is there any difference between the DOUBLE transformer configuration and ONE transformer with dual primary and dual secondary with the secondary wired in series and use the connected wires as CT ?
The physical difference is that the windings are on the same core in the second case, on two different cores in the first.

It seems this makes a difference, manp111. I didn't believe it either.

With two different transformers as shown in the schematic above, each secondary only conducts current during one half-cycle of the mains. This seems to lead to core saturation effects, as most people claim - also happens with half-wave rectification.

If you have one transformer, there is secondary current in both half-cycles, balancing the transformer load over the whole mains cycle.

Greetings,
Andreas

Last edited by Rundmaus; 10th March 2013 at 06:28 PM.
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Old 10th March 2013, 06:31 PM   #7
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Thank you Andreas.

I will not use the double transformer then.
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Old 10th March 2013, 06:37 PM   #8
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Don't thank me.

It's only a few weeks past that I argued there shouldn't be a difference, but some googling gives a lot of results and explanations. Of course the double transformer configuration works, but it will need serious derating to avoid saturation effects.

One solution would be paralleling the two transformers and then use bridge rectification. Simple with silicon, somewhat more complicated with vacuum diodes.

Andreas
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Old 10th March 2013, 06:41 PM   #9
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Thank you for the suggestion.
It is the voltage that I need, not current. I can just use one transformer with bridge.
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Old 10th March 2013, 06:52 PM   #10
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If you need much less current than the transformer is specified for, you may build it according to your schematic above. If I remember correctly, the derating factor is somewhat around 0.6 for half-wave rectification (and also for your setup).

If you are familiar with high voltages and safety measures, just make a test rig and increase the secondary current until saturation occurs. Then decide if your desired current is safe.

Andreas
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