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Center-tap to ground?
Center-tap to ground?
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Old 17th January 2007, 08:42 PM   #1
cbutterworth is offline cbutterworth  United States
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Default Center-tap to ground?

I finally got my Aikido hum down to tolerable levels, after much advice from folk on the Tubes forum.

Now, my question relates to the center-tap of my Hammond 269AX 125-0-125 transformer. I am using full-wave recitification and get around 360VDC after the cap input filter system. My heaters are powered by a separate 6.3VAC transformer whose center-tap is referenced to 1/4 B+.

So, back to the HT transformer. Do I need to ground the center-tap? If I do, where's best? I could connect it to the ground of the recitified voltage, in other words, I'd connect it to the -ve end of the first cap (a 1.5uF motor-run). Or, I could connect it directly to the chassis.

Right now, my PSU/signal grounds are run into the Broskie Aikido board utilizing the star-ground on the PCB. The PCB is direct coupled to the chassis using the designed-in PCB mounting bolt. I will experiment with this a little to see if diode/cap/resistor combination can lift this ground a little from the chassis/IEC earth.

I would appreciate any help on this.

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Old 17th January 2007, 09:02 PM   #2
zigzagflux is offline zigzagflux  United States
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Default Re: Center-tap to ground?

Originally posted by cbutterworth
So, back to the HT transformer. Do I need to ground the center-tap?
A schematic would help, but you must be using one of two rectification methods. First would be graetz bridge, using 4 diodes and the two end leads of the transformer secondary. Center tap is unused. If this is what you are doing, DO NOTHING with the center tap lead. If you tried to ground it, you shorted the secondary of your transformer.

Second method would be using two diodes, which requires the center tap be the negative terminal on your motor run cap. In this instance, the center tap is already used, and tied to circuit common, eventually finding its way to ground.
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Old 17th January 2007, 09:06 PM   #3
refference is offline refference  Brazil
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Hi Charlie ,

If could understand right , you are using the extremes taps
of your power tranformer , to get the H.T. , correct ??

And , you are using a bridge rectifier , correct ??

Then : 125 + 125 VAC = 250 VAC x 1.414 = ~ 353 VDC

In being so , you must to ignore the H.T. winding’s center tap ,
because , if you connect it to the ground ( or - HT ) , you
will short circuit the secondary winding and ..... as
zigzagflux said , the transformer will .... blow up !!!

You must to insulate the H.T. center tap , and forget it .

I hope it helps you ,

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Old 17th January 2007, 09:18 PM   #4
cbutterworth is offline cbutterworth  United States
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Great, I am doing the right thing by not connecting the center-tap. Yes, I am using a Graetz bridge. I will replace the temporary electrician's tape with a peice of stout heat-shrink.


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