Transformer FAQ

Adding  center taps or windings to transformers

This page shows how to add a center tap to a 5Spice transformer winding. The same techniques can also be used to add another winding.

  • using the linear transformer
  • using the saturating core or linear transformer

Using the linear transformer

A linear transformer with center tap is combined with the original  transformer. Connection is made by paralleling the windings so that magnetic flux from all transformer cores is coupled together.


Set one of the transformers (Xfrmer1) for the turns ratio and mutual inductance you want. Set its Ls1 for the transformer's leakage inductance, if desired.

Set the other transformer (Xfrmer2) to be nearly ideal. Set the turns ratio to 1:1. Set the mutual inductance to 1000 times the value of the first transformer. Set leakage inductance to 0.

Add the winding resistance as separate resistors (not shown) in each side of the input and output windings of the composite transformer. Note: If winding resistance is set as a transformer parameter, any DC current will not flow through all the winding resistance.

The resistor in the schematic is necessary to prevent Spice's "singular  matrix" error. Do not set R1 smaller than 1E-3 ohms unless you use 5Spice's Project Defaults Wizard to set Spice for lower resistance values.

Using the saturating core transformer

This method can also be used with the linear transformer. Just harder to understand.

To add a center tap or another winding to the saturating core transformer, we must use a "transformer" that couples DC. Otherwise the DC current in the core will not be correct and saturation will be wrong.


V1 and I1 form a "DC transformer" with a voltage ratio of 0.5, from right to left, and a current ratio of 0.5, from left to right. V2 and I2 form a second "DC transformer". We have connected them in series but they could be used as separate windings.

V1 and V2 are in series, each with half of Xfrmr3's winding voltage, to create the center tap. Looking at current flow on the left side of the schematic, half of the current flowing in connection 1 plus half of the current flowing in connection 2 is applied to Xfrmr3's winding.  The "DC transformer" works in both directions.

Set the parameters of the saturating core transformer as you normally would.

Note: If you  get "singular matrix" error, add a small resistor in series with Xfrmr3's winding 1. Do not set it smaller than 1E-3 ohms unless you use 5Spice's Project Defaults Wizard to set Spice for lower resistance values.

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