[ §6 · calculators · current transformer ]

Tesla coil current transformer

Ratio, currents, burden voltage and the lead the magnetising current gives, plus what wiring inductance in the sense loop costs you, which matters a lot once the burden is small.

second stage corefirst stage turnssecond stage turnsprimary current, peak · Aburden · Ωfrequency · kHzsecond stage core A_L · nH/turn²leads from the CT to the burden, one way · cmthe leads aretarget inductor L1 · µH

ratio 160:1CT secondary current 1.00 A at 160 A primaryburden voltage 3.90 Vburden power 3.90 W peakL of second stage 920 µHlead from magnetising 0.08° (subtracted, see CT timing) About 1 A: the loneoceans and WaskaLabs rule.Magnetising error is negligible. Your leads, 48 cm of twisted pair, come to 0.30 µH, andthat adds straight to L1: on its own it gives 13.36° = 24 % of the target angle forget it and the lead is 3.93° too much (7 %) fit a physical 1.54 µH instead of 1.84 Over 15 % of the angle comes from the stray part. Keep the burden and CT leads as short as possible, and set L1 ONLY on the scope.

formula

N = N₁·N₂     I₂ = I_pri/N     U = I₂·R     L₂ = A_L·N₂²
φ_mag ≈ arctan( R / (ω·L₂) )     (falls as 1/N₂²)
anything in series in the measured loop ADDS to L1
leads: twisted pair ≈ 0.63 µH/m, loose wires ≈ 1.29, coax ≈ 0.25

limits

The CT's own leakage is negligible. Two models disagreed on it, one putting the leakage at 13° and the other at almost nothing. What decides it is where the signal is sensed: across L1 + R, as it must be, or there would be no lead at all. That puts the leakage inside the magnetising term, where 920 µH of L_m swamps it. Moving the leakage from 0 to 1 µH shifts the total lead by 0.000085°. The 13° figure belonged to a topology that is not on the board.

Wiring inductance does matter. Whatever sits between the sense point and the burden adds straight to L1. At 51 Ω that was a couple of per cent; at 3.9 Ω, forgetting 0.3 µH of it turns a 55.5° lead into 59.4° (at 490 kHz), 3.9° or 7 % too much. (On its own that 0.3 µH would give 13.4°, 24 % of the target, the figure quoted here before. The inductance adds; the angle does not.) Set L1 on the scope, never by its nominal value, and keep the burden and CT leads very short.

There is no "optimum turns count" here. It assumed the stray phase goes as N², and the model that settled the leakage question above shows it does not.

The lead from magnetising printed above is the same quantity the CT timing page reports, and it enters the phase-lead budget with a minus sign.