Transformer with DC

A step produces a secondary pulse, but a constant DC input eventually produces no secondary voltage in coupled windings.

V1 applies a 0–5 V step through R1 = 100 Ω to two coupled 100 mH windings. Their coupling coefficient is 0.99 and the secondary load is 1 kΩ. Unlike a pure turns-ratio element, coupled inductors retain magnetic energy and show the difference between changing and constant excitation.

$$v_L=L\frac{di}{dt}$$ $$v_s=M\frac{di_p}{dt}+L_s\frac{di_s}{dt}$$ $$I_{p,\mathrm{DC}}=\frac{5}{100}=50\,\mathrm{mA}$$

During turn-on, changing primary current induces a secondary pulse. As primary current settles, both winding voltages approach zero and the secondary stops delivering power. The primary still draws DC current through its series resistance.

Read the chart. Vpri is measured across the primary winding, after R1. Vsec is the loaded secondary voltage. Ipri rises toward 50 mA. The 12 ms view shows the pulse and the settling current; zoom into the first millisecond to inspect the fast edge.

Try it. Change R1’s Resistance to 200 Ω. The final primary current falls to 25 mA and its transient changes. Restore 100 Ω. The winding fields are Primary coil inductance, Secondary coil inductance and Coupling coefficient.

Model limits. The windings are linear and omit saturation and core heating. A real transformer driven with DC can saturate and overheat; this model demonstrates induction, not a safe DC operating condition.