Using multiple voltage transformer to get higher voltage

[hondatech739]

Hi, I have a toroidal transformer that has three output voltages. One set is listed as 15.9V-0-15.9V, the second set is 7.5v, the third is 12.0V-0-12.0V.
My question is, can I use the 15.9V and the 12V outputs together to get 27.9 volts? If so, how. I have tried a few different things , but nothing gives me what I need. Am I chasing my tail here?

Thanks in advance

[hondatech739]

What I mean is can I configure it in a way to get +/- 27.9 volts for a dual power supply?

[glennb]

Are the output voltages truely center tapped windings, or can they be separated into 0-15.9, 0-15.9 ? (ie. Does the center tap have 2 wires) Most toroidals I have seen have separated secondary windings.

[DF96]

If they are centre-tapped windings then the answer is no. If they are separate secondaries wired to provide a CT then yes, you can rewire them to get what you want.

[gmphadte]

If u can get to one of the centre tap and open it to make two separate windings of atleast one, u can do it.

Gajanan Phadte

[DUG]

Originally Posted by glennb

Are the output voltages truely center tapped windings, or can they be separated into 0-15.9, 0-15.9 ? (ie. Does the center tap have 2 wires) Most toroidals I have seen have separated secondary windings.

And your current limit will be as the lowest of the two windings.

[hondatech739]

The 15.9v set has orange-brown-orange wires, and the 12v set has yellow-black-yellow wires, so I am assuming that since the two CT leads are different colors, that they are separated. I have measured voltage from each CT lead to all of the others and they all produce voltage:
black to yellow gives me 13.25v on both yellow,
brown to orange gives me17.4v on both orange,
brown to yellow gives 9.1v on one and 16.51 on the other,
black to orange gives me 20.73v on one and 13.06v on the other.
Orange to orange is 34.77v,
yellow to yellow is 26.7v
orange 1 to yellow 1 is 33.44v,
o1 to y2 is 7.8v,
o2 to y2 is 26.07v,
black to brown is 3.6v.
Then there are 2 green ones that measure 8.3v between them. I won't go into all of those measurements unless someone asks or thinks it's needed.

Here is a few pics of what I'm working with.

[DF96]

orange 1 to yellow 1 is 33.44v,
o1 to y2 is 7.8v,
o2 to y2 is 26.07v,
black to brown is 3.6v.

That does not make much sense. The label implies separate centre-tapped secondaries, but your measurements seem to show they are connected - unless you are measuring capacitive pickup, which is always possible with a high impedance DMM.

Could you repeat the orange-yellow and black-brown measurements with a resistor connecting your DMM probes? Anything in the range 10K to 100K will do. The idea is to present a lower impedance than the bare meter, so stray capacitance will have much less effect. I would expect you to see near zero results this time.

[hondatech739]

Originally Posted by DF96

That does not make much sense. The label implies separate centre-tapped secondaries, but your measurements seem to show they are connected - unless you are measuring capacitive pickup, which is always possible with a high impedance DMM.

Could you repeat the orange-yellow and black-brown measurements with a resistor connecting your DMM probes? Anything in the range 10K to 100K will do. The idea is to present a lower impedance than the bare meter, so stray capacitance will have much less effect. I would expect you to see near zero results this time.

Originally Posted by DF96

If they are centre-tapped windings then the answer is no. If they are separate secondaries wired to provide a CT then yes, you can rewire them to get what you want.

Okay, with a 22k resistor I got .044v across the black to brown
orange 1 to yellow 1 is .558v
o2 to y2 is .429v
o2 to y1 is .401v
o1 to y2 is .132v
You were right. That makes a lot more sense. At least for the way it is marked. So I guess the answer is no, all I can get is what's marked on the label.

[AndrewT]

to be able to stack (add) the voltages you will need to get inside, as Gmp described

If u can get to one of the centre tap and open it to make two separate windings of at least one, u can do

There is an alternative.
Use the 15.9+15.9 as one winding. = >32Vac when open circuit.
Use the 12+12+7as the other winding. =>32Vac when open circuit.
But your output current is limited to 750mAac by the 7Vac winding rating.

A third alternative is to add a 2A rated winding of ~7Vac to make a 31.8Vac + 31.8Vac @ 2A output. That is a total VA of 63.
A lot of work for 63VA !