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Old 29th November 2005, 04:26 AM   #1
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Default center tap to dual secondary conversion

Ok i did this and it seems to have worked fine.

I got some Avel lindberg 100VA 50vCT from ApexJr for some small amps

I decided to go with dual rectifiers, so i needed dual seperate windings so i did a little surgery.
I sliced the tape over the middle wire of the CT and pulled it out till it met with where it joins to the actual winding wire. it joins to the middle of the 50v so there are 2 wires coming off it. i cut one of them away from the CT wire, scraped it and soldered a new wire to it. it actually went very easilly with this particular transformer.

instead of 25 0 25 now i have 2x 25v. now the only thing is, if i measure the voltage between the newly seperated center tap, now i can get up to 25v with just the meter. if i put even a 10k resistor there it drops to 0.2v or so (varies)
same thing on the scope.

If i measure the resistance between the 2 newley seperated coild (with the transformer not plugged in of course) it is not even measurable with a fairly decent digital meter (fluke 73) it just reads as a completely open circuit.

i measured current as well and after trying it with resistors to be safe, i tried shorting them through the ammeter, and the current is 0.03mA

is this going to cause any problems? I tend to think not, but maybe someone knows better.
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Old 29th November 2005, 07:44 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
a 100VA transformer should have measurable resistance on each secondary winding. Are you measuring the two windings but out of phase?
Connect the old centre tap to the new extra wire. This creates the original 25-0-25. Measure the voltage - 50Vac?
Separate the two central wires. Measure centre to end on both windings - 25Vac + 25Vac?
Now try your resistors across each winding & measure.
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Old 29th November 2005, 09:46 PM   #3
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no, i was measuring the resistance between the adjacent windings to make sure there was no short or anything. i was trying to find out why i can measure a voltage between the 2 windings. the windings themselves are fine. both at 25v.
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Old 30th November 2005, 03:18 AM   #4
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i tried it with the PSU today, it worked fine. the 2 halves of the PSU are completely floating from each other, either when they are in phase or out of phase.

there was a tiny mystyreous bit of noise i can pick up, which i can only see on the scope at its lowest voltage and fastest timebase. its not from the PSU though, because its still there after i turned the psu off!
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Old 30th November 2005, 09:08 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
after reading post 4, I now understand what you were measuring in post 1.

The two floating windings can sit anywhere in voltage terms. What fixes them is a reference. The only reference they have is either leakage to the primary or leakage to each other.
these leakage currents should be very small, probably less than a few pA.
When you connect a voltmeter across them the voltmeter becomes the reference and changes the floating voltage.
Similarly when you connect a resistor between the secondaries this becomes the reference for the voltage difference between the secondary windings. The currents and voltages bewteen two floating secondaries tied with a reference reisitor should be almost zero but you may be able to measure something with very sensitive instrumentation. All completely normal.
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