# How is there a voltage on the secondary coil when di/dt=0 on the primary coil?

I have made a simple pulse transformer circuit in LTSPICE using a pulse and two coupled inductors. L1 has a resistance of 1 Ω and L2 has a resistance of 0 Ω.

I am having trouble understanding why this circuit does what it does. I have graphed the primary coil current vs the secondary voltage output. What is very strange is that from 0.1 μs onwards, primary current is constant yet secondary voltage is nonzero.

I was under the impression that the secondary voltage was equal to mutual inductance times di/dt on the primary coil, yet di/dt=0. Can somebody help me understand why LTSPICE is is producing these results?

• You have a voltage (L2?) plotted there. Show a plot with all the interesting variables shown together. Commented Jun 28, 2022 at 0:34
• V2 / I1 ~ 9 Ohms and coupling factor is unspecified as well as turns ratio but is implied by sqrt(inductance) ratio Commented Jun 28, 2022 at 1:51
• Try simulating for a longer period of time, say 1m, or use smaller values for inductors. The magic will be gone. LTspice (note the spelling) shows the correct output for the given input (GIGO). Commented Jun 28, 2022 at 5:57
• You don't need dI/dt for a transformer to function in simulation. It will always copy over voltage from the primary to the secondary according to turns ratio. Commented Jun 28, 2022 at 7:03