I've built this 220v DC motor controller. I use a 2x110v - 2x15v transformer to power up the stator.

enter image description here

The transformer is a Myrra 45064 with the primary windings connected as follows: pins 1 and 9 connected to the mains, pins 4 and 6 in short.

On the secondary windings, i've connected pin 12 and 19 to a rectifier, and 14 and 17 in short.

When I plug it in at 220v AC, the transformer heats up very fast and outputs ~12V out of the expected 30V. Didn't keep it powered more then 6 seconds.

I thought that this pin configuration should connect the 2 halves of the transformer in series and output 15+15V.

Am I connecting them wrong, or there could be another cause for the high current through the transformer?

P.S.: The schematic on github shows a 3 port primary winding, however, the board is built with the connections done as explained above.

  • \$\begingroup\$ Try open secondary first and see that you have ~30V, then connect a few hundred ohms to it and see if it behaves as it should. If yes, then check the bridge, the capacitor, the wires, more than that I cannot help, this is a "blind answer" for my part. \$\endgroup\$
    – Vlad
    Sep 10, 2012 at 13:43
  • \$\begingroup\$ Have you tried it with the load disconnected? It sounds like you're simply overloading the transformer. \$\endgroup\$
    – Dave Tweed
    Sep 10, 2012 at 13:44
  • \$\begingroup\$ I've totally isolated the transformer, ran 220 through the extreme pins in the primary, short the middle primary pins. I've also separated the 2 secondaries. Voltage/secondary ~6V instead of 15. The trafo is new. No idea what's going on. \$\endgroup\$
    – Valer
    Sep 10, 2012 at 14:52

1 Answer 1


The Farnell page has a little better description of the wiring needed:

Primary Pin Connections        |    Secondary Pin Connections           
Style   0V  115V    0V   115V  |    0V  Vsec    0V  Vsec
UI39    1   4       9    6     |    17  19      14  12

It looks like connecting per this table parallels the primary winding to set the turns ratio for 115V input.

Based on that idea, it looks to me like pins 1 and 6 should see the AC and pins 4 and 9 should be connected together, to maintain the correct phasing of the primary.

  • \$\begingroup\$ Thanks! That was indeed the issue. I totally missed that wiring info. Fixed now :) \$\endgroup\$
    – Valer
    Sep 10, 2012 at 19:20

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