# Wiring a transformer with two primary windings

I'm building a circuit that involves the use of multiple transformers. I've mainly used transformers that had one primary and one or two secondary windings. However, this time, the transformer I'm working with has two primary windings and two secondary windings, like in the diagram below:

simulate this circuit – Schematic created using CircuitLab

What is the proper way to wire such a transformer? Do both primary sides have to be wired to something in order to work? In my simulation, I'm only using one primary winding and one secondary as a bridge-rectifier, so the other one is not used at all. If I decide to make S1 and S2 center-tapped, will I have to use P2 as well?

EDIT: More info, I'm working with 115 Vrms. This is a signal transformer. If I have to give it a model, it's the 14A-56-28

• Can you provide a specification for this transformer? It is not uncommon to wire the primaries in parallel in 110V systems and series in 220-240V systems. But this is a guess without knowing the specification and intended application. Apr 3, 2020 at 8:34
• is this a power supply transfoirmwer or a signal transformer? Apr 3, 2020 at 10:47

First of all your diagram is showing 2 separate transformers. If it is one transformer there should be only one magnetic core like in picture below. There is no 2 primary winding transformer model in CircuitLab. So if you really need it you should create it.

A)"What is the proper way to wire such a transformer?"
You can wire in any order. Good practice is to wire all winding in same direction to avoid phase reversal.
B) "If I decide to make S1 and S2 center-tapped, will I have to use P2 as well?"
No. Just leave P2 ends not connected.

• Assuming P1 & P2 are the same voltage, you would want to connect them in parallel or the transformer would only handle 1/2 rated power. Apr 3, 2020 at 4:59
• You can't wire the primaries "in any order" - you must observe polarity. For 120 V operation, the dotted ends of the primaries must be connected together - doing otherwise will result in a short circuit. Apr 3, 2020 at 5:04
• @PeterBennett Polarity does not have to be observed during winding. Only during connection. Apr 3, 2020 at 7:17

You should connect the two primaries in parallel for 120 V input, and in series for 240 V (assuming the transformer is designed for 120/240V input).

You must observe the polarity of the primary windings when connecting them - the datasheet should show the correct connections.

A transformer with 2 identical primaries and 2 identical secondaries would be wired with the primaries in parallel for rated voltage and in series for double that. Likewise, with the secondaries.

Connected thus,the secondary current at double the secondary voltage would be half of that at the rated secondary voltage.

In parallel connection, identical ends would go together ('dot' to 'dot' and 'no dot' to 'no dot').

In series connection, opposite ends would go together ('dot' to 'no dot').

In practice, numbered terminals would be wired as per the wiring chart provided.

All windings would need to be used to utilise the transformer to its maximum VA rating.