I have an audio transformer (SP-66) with a 1:1 ratio and centre taps on the primary and secondary windings.

Sp-66 http://catalog.triadmagnetics.com/ImgMedium/a1232.jpg

Tests: When I input a 6.5 kHz 8 Vpp sinusoid on the primary (pins 1 & 3), I observe an 8 Vpp sinusoid on the secondary (pins 4 & 6). (1:1 ratio).

Input on pins 1 & 2, I observe a 16 Vpp sinusoid on pins 4 & 6. (1:2 ratio).

Input on pins 1 & 3, I observe a 4 Vpp sinusoid on pins 4 & 5. (2:1 ratio).

My pcb is already soldered with the sp-66, and I need to double the output voltage. I tried shorting pins 2 and 3 together on the primary to achieve a 1:2 winding ratio. Instead, I still observe 8 Vpp, a 1:1 ratio.

Question: Why does shorting the centre tap on the primary to pin 1 or 3 still result in a 1:1 ratio on the secondary?


Shorting the centre tap to either end of the winding is effectively adding shorted turns to the transformer.

You should connect to 1 & 2 or 2 & 3 and leave the 3 or 1 pin respectively OPEN.

If this was a power transformer connected to the mains, the results of your experiment might be a lot more dramatic.

  • \$\begingroup\$ The SP-66 is already soldered to the PCB. I can remove the transformer, cut off leg 3, and solder leg 2 to leg 3 trace on the bottom layer. However, this will take time to do as I have about 28 of them, and they are all currently wired up and stored in a housing. If I have no other choice that is my plan. For the first part of your answer, how is there any current flow between pins 1 and 2 if they are shorted? What is happening? \$\endgroup\$ – BCon May 14 '15 at 20:11
  • \$\begingroup\$ The "unused" part of the winding acts like another secondary so you're shorting that out. Current flows because they are shorted and because a voltage is induced. You have no other choice but to cut the connection if you need that ratio. Can't you cut the trace to pin 3 and jumper the input to pin 2? Or cut the trace to pin 1 and jumper the other input to pin 2? \$\endgroup\$ – Spehro Pefhany May 14 '15 at 20:27

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