I have a dual primary transformer out of an old amp and would like to use it for another project. I can't find a datasheet or a way that explains how to wire it for 220 operation. All the diagrams that I have seen have only 4 input wires.

One post suggested connecting the black and white wires together but it then just overheats and shuts down.

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Any help or info will be greatly appreciated. Thank you


2 Answers 2


Looks like Blue and White wires are the same thing - You can confirm this with resistance measurements.

Join Yellow and Black wires together.

Apply 220V across White (or Blue) and Brown

  • \$\begingroup\$ Thank you Icy. As I said on @andyaka Blue - White measures 0.5ohm (on the 200ohm setting) and Blue - Yellow measures 4.1 ohm. But when I do as you say, connect Black and Yellow, then measure over Brown - Blue it jumps to 7.7 ohms \$\endgroup\$
    – Adrian
    Jan 14, 2016 at 11:43
  • \$\begingroup\$ @Adrian Which sounds just about right - Roughly double what a single 115V winding resistance was... \$\endgroup\$
    – Ecnerwal
    Jan 14, 2016 at 13:47
  • 1
    \$\begingroup\$ I suspect that the way you had it wired (black connected to white), the two primaries were wired in series, but with the windings being powered in opposite directions. So the magnetic field of one cancelled out the field of the other. This loses the back-EMF that normally protects the coil, and is effectively sticking a 7.7 ohm resistor across the mains. The way Icy suggests connects the two primaries in series, but with the current going in the same way for both, so the magnetic fields add. \$\endgroup\$
    – Simon B
    Jan 14, 2016 at 16:05
  • \$\begingroup\$ Thanks for all the help guys. I tried what @Icy said yesterday about the black and yellow together and transformer gave a stable 27.6 volts. Now I can try and build a little audio amp with it. [![AC Transforemer][1]][1] [1]: i.stack.imgur.com/gZlDQ.jpg \$\endgroup\$
    – Adrian
    Jan 15, 2016 at 5:17

I think blue (or white) might actually be a screen between the primaries and secondaries. Use a DVM and check connections between blue and white and report back what impedance you measure. If it measures open circuit, and you can measure capacitance, you might find it's somewhere like 200 pF to 800 pF between blue and white. This would confirm it as a screen and then you'd need to wire it directly to safety earth.

It's important you find this out.

For the rest of the installation neutral to white (or blue), join yellow and black and connect live to brown.

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    \$\begingroup\$ the line between the two 0V labels suggests they are connected together. \$\endgroup\$ Jan 14, 2016 at 9:48
  • \$\begingroup\$ @Jasen Oh, and I suppose the line between "0V" and "115V" suggests those two points are connected together - now that would be interesting. There's a certain amount of naivety in your comment! \$\endgroup\$
    – Andy aka
    Jan 14, 2016 at 10:17
  • \$\begingroup\$ they sure are, any ohm meter will tell you that. \$\endgroup\$ Jan 14, 2016 at 10:25
  • \$\begingroup\$ @Jasen and if you measured a capacitor as open circuit does that tell you AC won't pass - think about what I'm saying - a line on a label can mean anything or nothing. \$\endgroup\$
    – Andy aka
    Jan 14, 2016 at 10:44
  • \$\begingroup\$ Thanks for that @Andyaka Blue - White measures 0.5ohm (on the 200ohm setting) and Blue - Yellow measures 4.1 ohm. Blue -Black and Blue Brown are nothing. I don't think my DVM has capacitance. \$\endgroup\$
    – Adrian
    Jan 14, 2016 at 11:37

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