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Old 20th November 2012, 01:05 AM   #1
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Default Power transformer question

I have an unknown transformer question. As you can see, the secondary is made up of two yellow, two orange, two red and one black. There is continuity between orange, red and black, not yellow. Voltage readings are:
Yellow - 15v
Orange - 56v
Red - 96v
Assuming the black wire is a center tap, I get ~26v on each orange and ~46v on each red. So, what is the black wire?

Tom
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Old 20th November 2012, 01:40 AM   #2
glennb is offline glennb  Australia
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Assuming the black wire is a center tap, I get ~26v on each orange and ~46v on each red. So, what is the black wire?
If you are using a meter with high input resistance you are probably measuring leakage between the windings. I suggest you put a 10K Ohm resistor across the meter probes to shunt away the miniscule leakage currents, and then measure again from black to each of the other secondary wires. If the reading is zero the black is probably an electrostatic shield that should be grounded.
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Old 20th November 2012, 02:58 AM   #3
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Glenn, I tried the 10k resistor and it's still the same thing. The black seems to be common to both the red and orange wires.

Tom
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Old 20th November 2012, 03:02 AM   #4
AJT is offline AJT  Philippines
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it is the center tap....except that the readings would have been 1/2 of what you measured between red and red and orange and orange....
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Old 20th November 2012, 09:52 AM   #5
glennb is offline glennb  Australia
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Glenn, I tried the 10k resistor and it's still the same thing. The black seems to be common to both the red and orange wires.
OK, it seems one of the windings is red - orange - black - orange - red. ie. 46 Volts aside of the CT, with taps at 26 Volts. From either red, do you read ~20 Volts to one orange and ~72 Volts to the other orange?
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Old 20th November 2012, 10:23 AM   #6
DF96 is offline DF96  England
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The orange is probably for powering preamp stages, and the red for power amp of an integrated amp. The yellow does something else - maybe (after regulation) +12V for an FM tuner?
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Old 20th November 2012, 12:17 PM   #7
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Yes, that's exactly what I get when I measure from any red to the orange wires. I think it may have come out of one of those all-in-one integrated amp things. So, no matter which color I end up using, (red or orange), I should treat the black as a center tap?

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Old 20th November 2012, 11:41 PM   #8
glennb is offline glennb  Australia
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Yes, that's exactly what I get when I measure from any red to the orange wires. I think it may have come out of one of those all-in-one integrated amp things. So, no matter which color I end up using, (red or orange), I should treat the black as a center tap?
Good. Yes, black is CT. Here is an example of generating two different positive DC supplies. Overlay the same design with reversed diodes to add negative supplies, relative to the CT.
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Old 21st November 2012, 01:44 PM   #9
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The missing 4 volts is kind of curious. I wonder if the centre tap has been back-wound.
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Old 21st November 2012, 05:27 PM   #10
DF96 is offline DF96  England
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Good point. Maybe the OP can recheck his voltage readings and confirm that the black really is a CT and not offset a little.
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