Inductance needed for a target frequency
What choke do I need to make this cell resonate where I want it?
- L
- Required inductance, H
- f
- Target frequency, kHz
- C
- Cell capacitance, nF
LaTeX
L = \frac{1}{4\pi^{2} \cdot \left(f\right)^{2} \cdot C}
Method
- Convert the frequency to hertz and the capacitance to farads. Everything below is SI.
- Square the frequency. This is the step that makes the answer sensitive: halving the target frequency quadruples the inductance you have to wind.
- Multiply by 4π² — about 39.48 — and by the capacitance.
- Take the reciprocal. The result is the total series inductance in henries.
- If the design uses two chokes in series, each needs half of this, because inductances in series add.
- Check the characteristic impedance √(L ÷ C) before winding anything: it tells you the reactance the drive has to push against, and a number in the hundreds of kilohms means very little current will flow.
Assumptions
- The chokes are ideal inductors. A real coil with the henries this calculation asks for has substantial resistance and its own self-capacitance, and both pull the actual resonance below this figure.
- The capacitance is what you think it is. Cell capacitance moves with temperature, with gas in the gap, and with how much of the electrode is actually submerged.
- Series connection, with the chokes adding. Coils wound on a shared core couple to each other, and mutual inductance can add or subtract depending on winding sense — the total is then not simply the sum.
- The result must be below the chokes’ own self-resonant frequency to mean anything. A coil driven above its SRF behaves as a capacitor, and no amount of turns will fix that.