I have a transformer with dual input 115v and dual output 9v and rating power 1.15va.

My goal is to connect it in parallel to produce single input 115v, single output 9v and rating power 2.3va.

The primary doesn't have a Center Tap, but the secondary has it and it confused the hell out of me.

The secondary has 5 connections.

How do I achieve the 9v and 2.3va on the secondary ?

enter image description here

enter image description here Thank you

  • \$\begingroup\$ Do you have a schematic of the transformer? I don't understand how a dual 9v secondary could have 5 pins. \$\endgroup\$
    – Mark
    May 31, 2016 at 21:51
  • \$\begingroup\$ @Mark The schematic that this transformer suppose to be has no center tap. Tow winding input two output. With a multi meter set to ohms I got no reading from Center Tap to any other winding, Does that mean that it's not even connected? \$\endgroup\$ May 31, 2016 at 22:03
  • \$\begingroup\$ DO you have a scope? Does the transformer schematic show winding phase relationship, like a dot on one end of each coil? \$\endgroup\$ May 31, 2016 at 22:15
  • \$\begingroup\$ OK, then. Bob's and Transistor's answers below would both be correct then. Is it possible that the center pin is a Faraday shield? \$\endgroup\$
    – Mark
    May 31, 2016 at 23:13
  • \$\begingroup\$ NOTE te rated power will still be 1.15VA. \$\endgroup\$
    – user16324
    May 31, 2016 at 23:31

2 Answers 2


I have a transformer with dual input 115 V and dual output 9 V and rating power 1.15 VA.

  • The most you can get out of this is 1.15 VA.
  • You can wire the primaries in parallel for 115 V or in series for 230 V.
  • You can wire the secondaries in parallel for higher current at 9 V.
  • You can wire the secondaries in series for lower current at 18 V.

Output current will be \$ I_{MAX} = \frac {P_{MAX}}{V} \$.


simulate this circuit – Schematic created using CircuitLab

Figure 1. 230 V and 115 V primary connection.

I got no reading from center tap to any other winding.

It's probably a polarising pin to prevent it being inserted the wrong way into a PCB. Other similar transformers would have different pin spacing on primary and secondary for this reason.

  • \$\begingroup\$ @Bob Underwood, you can see in the second picture how I wired it. I fed it 110v and the output was 13v multi-meter set to 500 ACV. Tried 20 DCV and it was 0. I thought I was suppose to get 9v \$\endgroup\$ Jun 1, 2016 at 0:56
  • \$\begingroup\$ You've commented under my post instead of Bob's (but that's OK). Your 13 V was open-circuit. Put a 1k resistor across the output and you should see the voltage collapse. You will find the regulation of the small transformer is not very good. Since there is no direct current from a transformer your 0 V DC reading is correct. \$\endgroup\$
    – Transistor
    Jun 1, 2016 at 6:12
  • \$\begingroup\$ I commented on your post, and mentioned Bob so that both of you will be notified. The system won't allow me to mention both names. Thank you \$\endgroup\$ Jun 1, 2016 at 12:40

I suspect the primary is the 4 pins on the left side, two independent 115V windings. They could be connected in series for 230V input, or paralleled for 115V input. The "normal" convention would be to connect pin 1 to pin 3 for the hot side of 115V and pin 2 to 4 for the neutral side.

It sounds like the center pin on the right is indeed floating, probably a shield.

Suggestion: energize the primary using just the upper two pins on the left. I THINK you will see 115 appear on the lower right pins, and I further THINK you will find two independent secondaries on the right side.

There are a million other possibilities, but the above is my best guess; if you can confirm, we can talk about phasing later.


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