HELP with 220v step up transformer wiring

[Kevinkay]

transformer came to me "broken".  Plugged it in and tested output voltage sockets-  One is marked 220V and other 110v (for step-down).  On 220v socket only 120v measured across terminals no voltage measured across 110 socket.  Opened the case and saw one of the three lines coming off the center coil transformer was broken off its soldered connection but I can not figure out where it belongs/broke free from.
Very much an amateur and I am SURE I will use incorrect phrasing in my description but doing my best.

MSKJ ST-3000VA step up & down transformer - Chinese made unit and I have tried and failed trying to contact them for a wiring diagram.
It has an input switch (120 or 220)  I have it set as 120v input plunged into a 120v power outlet
There are 3 wires coming off the center  a toroidal autotransformer ( I think), ONE of those wires broke free
Each of those 3 wires to ground  measure 120V
The sockets have a "N" and "L" marketed on each terminal - I assume load and neutral
There is a breaker, BUT it is in line with the neutral white wire of the 120v polarized plug.
I get 120v on the output 220v across the terminals, nothing from "L" to ground and 120v from "N" to ground
On the 110v outlet- nothing across the terminal, 120v from "N" to ground and 120v from "L" to ground
The input switch baffles me - the continuity does not change from switching from 120 to 220 input. The switch is not malfunction as when I removed connection wires it was working as should.  The input Positive (black) wire goes to the input switch and then BOTH of the wires coming off that switch go to the 110 output one to N and other to L.  The L of the 110v in jumped onto the N of the 220v outlet.  The negative (white wire) of the 110 input goes to the breaker and then to the L of the 220v outlet.   3 wires from the coil - 1 to the N of the 110v, one to the N of 220v outlet and the third is the loose connection.   I tried connecting the loose wire to the 220v N (along with another coil wire) thinking 2 wires with output of 120v volts off the coil would = 220v when joined, this  did not work and still only measured 120v across terminals. The attached wiring  diagram i sketched and attached I left off the ground wires, power light and USB output terminals for simplicity.   SORRY for being so wordy, thanks for reading and appreciate any help. THANKS

[Kevinkay]

trouble attaching photos

[SparWeb]

Can you measure current, or resistance?

Before plugging in anything questionable like that, I'd do some continuity checks first, to make sure the wires I think I should connect to Mains are the right ones, and that they aren't shorted out.

[Kevinkay]

I thought I was measuring "current" when I was checking for voltage??
Where should i test for resistance, I am assuming thats OHM's?

I did plug it in outdoors and was wearing safety glasses 

[Mary B]

A picture from the back side of the bracket/power socket would be more useful... and of the wires coming out of the transformer.

It may have a common ground between all windings, something an ohmmeter can tell you. It may be an isolation transformer wit the secondary not connected t the primary side..

If I can see the color coding of the wires off the transformer it might help... there is a standard color code...  not that the Chinese always follow standards!

[Kevinkay]

I am away from home but will provide more photos when I return, BUT the 3 wires coming off the center transformer are the same - green with a sleeve with red stripe, partly over them.  They come off the transformer at the same point of the circumference, but possible at different heights but hard to tell.
I should measure OHMS between the wires off the transformer to ground and find continuity?

THANKS!

[joestue]

Sounds like an auto transformer.

Connect the wire and see if it blows the fuse, produces 60 volts, or nothing happens and report back which one.

[Kevinkay]

connect the loose wire to WHERE?

tried connecting it to the 220v "N" terminal along with another transformer wire already attached there.  It did not blow the fuse but it also did not change the output between terminals, stayed at 120v.

[mab]

 Based on your diagram, i would try connecting the 'green' on your diagram to '220v L'

If that works i would then consuder reversing the input connection to put the breaker in the live side.

[MagnetJuice]

It is kind of difficult to trace the wire in the drawing.

Maybe this will help.



Ed

[Kevinkay]

YES!
Thank you mab and magnetjuic!
I have to say I doubted this and took extra precautions connecting a wire giving me 120v line to the side that has a wire going back to the neutral of the incoming voltage, but it worked!

I now get 245v across the 220 outlet terminals.  "N" to ground gives 245v, "L" to ground results in 0v
the 120 outlet give me 120V across terminals. "N" to ground gives 120v, "L" to ground results in 245v

when I switch the input switch to 220v (even though its only getting 120v) it "halves" the output at the outlets, 60v at the 120 outlet and 120 across the 220 outlet.

There is a "buzz/hum" now while the unit is plugged in and running, switching the breaker off, kills the output voltage and the buzz.
I am thinking this buzz is normal and getting 245v (on a 220v-240v outlet) is with in normal range?
And I am mistaken on L meaning load and N meaning neutral???

Thanks for all the responses, I posting the working wiring diagram

"If you wanna go fast - go alone, if you wanna go far, go together"

[MagnetJuice]

Hi Kevin,

Good to hear that you got it working.

245V is a bit high, but it will come down when a heavy load is connected.

As for the hum, all transformers do that, except for the very old ones. For some reason the old transformers were built tighter and a lot of them had the coil potted in some epoxy or polyester, then enclosed in a metal case. I have some of those and they run very quiet.

Noise drives me crazy. On some of my big power supplies, I remove the transformer and mount it back using rubber grommets to isolate it from the metal case. That usually kills the humming. Most of the time, the hum can be reduced by adding soft rubber feet to the case.

Ed