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Old 22nd February 2007, 09:33 PM   #1
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Default Joining two NCT transformers

I have two identical non center tapped 1:2 power transfomers left over from a couple of cheap monoblocks I once owned.

I am wondering if I can make a 1:2 center tapped transformer as follows:

1) Wire the primaries in series and attach 110 AC to both ends. 2) Wire the secondaries in series and use the middle point as the center tap.

Also if I need twice the voltage, can I just use each as a half wave rectifier, one for the positive and the other for the negative? In other words, wire the secondaries in series but connect each primary to line voltage separately.

Thanks
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Old 22nd February 2007, 10:02 PM   #2
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Your first method will result in a 120VCT transformer, with 60V on each secondary winding. You also are limited to using the transformers at 50% of their rating, as you are wiring 120V primaries in series.

Your second method is better, wiring the primaries in parallel and feeding them with 120V. You would then end up with 240VCT output at full transformer capacity.

Be careful to check your instantaneous polarities on the secondary: it's either right or wrong. If you get 0V output from outside leg to outside leg, swap the polarity of one of the secondaries.
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Old 23rd February 2007, 12:14 AM   #3
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All true, but one more thing to consider - if you are using half-wave rectifiers on each secondary, the current will be limited to a small fraction of the AC rating, since the DC flux will saturate the core. You COULD use separate full-wave bridge rectifiers (sand is cheap!) but DON'T connect the secondaries together anywhere - connect the DC outputs, - to +.

You also would not want to use the two transformers together center-tapped) with a 2-diode full-wave rectifier (half-bridge). Each transformer would have net DC current, the DC fluxes can't cancel when they are in different cores.

In essence, avoid half-wave rectifiers.
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Old 23rd February 2007, 01:53 AM   #4
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Then another question. I do have a proper full wave rectifier out of an old console. Trouble is I don't remember what was in the amp I pulled it out of. How can I determine its wattage capacity? Is there a general rule of thumb regarding weight versus power or some easy way to test when it overtaxes?

With the other two, it sounds like a better strategy could be to use one as a bridge rectifier using two tube half wave rectifiers (I want to use tubes) for the HT and the other could be the filament transformer. Unfortunately, this would mean extra filament current for an extra rectifier tube.
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