Discrepancy Between LTSpice Simulation and Real Measurements

The current range I need is from 0 to 3 A, so I varied R6 from 100 Ω to 3000 Ω and noted both the current through R6 and the voltage across R1.

It appears that you are trying to simulate a current transformer (CT) that uses a circuit on the secondary that is somewhat problematic when the primary current falls below a certain value. It's problematic because of the secondary-side bridge rectifier.

So, at some low value of primary current, the induced secondary voltage becomes insufficient to properly forward-bias the bridge rectifier diodes and, this results in what you observe; a falling-away of rectified and smoothed voltage on the output.

Just consider what happens...

The bridge rectifier will start struggling to produce an output voltage when the secondary winding voltage falls to a level of two forward diode drops (call it 1.2 volts p-p). So, with 1.2 volts p-p on the secondary winding, we can calculate the primary voltage as 1.2 volts p-p divided by 125 (the turns ratio).

This equals 9.6 mV p-p.

_{I've calculated turns-ratio based on what you say (although I have no idea what 1.5 to 1.7 turns is because turns need to be whole numbers and not fractional). Nevertheless it approximately matches the turns ratio implied from the inductance values in your spice circuit.}

So, with only the primary inductance in play (secondary rectification almost neutralized), the inductive reactance of 30 μH at 50 Hz is 9.42 mΩ (that's milli ohms). With 9.6 mV p-p across it that's a primary current of about 1.02 amps p-p.

And, that's approximately/ball-park where you appear to be seeing the discrepancy in your numbers. As current falls below this value the discrepancy gets worse!!

Hence, my conclusion is that it is doing what it's expected to do. Of course if you modelled different diodes (in the bridge) you might get a smaller forward volt drop and the onset of the discrepancy will be at lower primary currents. A larger forward volt-drop diode and, the discrepancy will be at higher primary currents.

Look like issues related to numerical precisions. The fact that you observe a clear jump once a parameter crosses some threshold without any physical reason behind makes me think about the solver precision.

Several things to check:

• Make sure you don't have nets with no DC path to GND. Here the primary of your transformer is floating. Adds a 1GOhm resistor to GND somewhere to help the solver. Sometimes theses are added automatically by some component or directives. Check the documentation. But adding it manually is safe anyway. Numerically speaking, you don't need that primary to be floating at all to study your circuit. You could just tie the negative of V1 to GND. Because there is no loop, no current will ever flow through that GND tie. But the voltage of the primary will be fixed and not wandering around. Easier for the solver.
• Solver parameters. The numerical solver can be tuned for high accuracy (but slower speed). There are a lot of ways to do this. Search for "ltspice increase accuracy" and try some of them.