LITT TECHNICAL DATA

Design notes

Article written 9/6/2026 by Asher H, LITT coil designer.

Schematic driver version presented: 1.5.4

How a Tesla Coil works:

A Tesla coil is a type of transformer that achieves extraordinarily high voltages (and currents!) by taking advantage of something called LC resonance. LC resonance occurs when a capacitor is connected to an inductor and the resulting circuit is energized. The result is a sinusoidal oscillation with voltages and currents often exceeding the inputs. The Tesla coil’s secondary is the inductor and the interwinding capacitance and top load capacitance completes the circuit forming an LC resonator. The primary’s job is to energize the resonator at the frequency of resonance (f0) via magnetic coupling to the secondary. Additionally, some voltage boosting occurs because of the turn ratio between primary and secondary. When the voltage across the secondary resonant network is high enough to exceed the breakdown voltage of air, a spark forms.

Dual resonance:

The LITT DRSSTC is a dual resonant coil, meaning the primary coil is part of a resonant network (C17, C18, C15, and C19 being the tank capacitors, series with the primary) with a resonant frequency close to that of the secondary. This allows for larger sparks than would normally be possible because the reactance created by the primary inductance is cancelled. This particular arrangement however, can be extremely stressful on the switching transistors because the primary current is even higher than it would be without primary resonance. Additionally, IGBTs have a particularly annoying characteristic: tail current. Tail current produces more losses and results in slower switching in general non-resonant applications. As such, hard switching must be avoided.

Soft switching:

We avoid hard switching by using a current transformer (T4) to measure primary current and watch for every primary resonant current zero crossing. As soon as a zero crossing is detected, the transistors switch which drastically reduces switching losses and stress on the transistors while also making tail current inconsequential (there is no current to begin with at the time of switching). This enables the IGBTs to be used in this circuit despite them not being designed for such high frequencies. One issue that arises however is an overcurrent event.

OCP and over-pulse detection:

Overcurrent events typically happen when the primary and secondary resonant frequency drift apart because of environmental changes like a nearby metal object or person. This can cause the secondary to stop absorbing energy from the primary and, as a result, cause primary current to skyrocket. This is fatal for the IGBTs even with soft switching. We prevent this by having an additional current transformer that monitors primary current. If it exceeds a pre-set value (determined by RV1) it will wait for the next zero crossing and shut down the coil and latch off - causing the PROTECTION MODE lights to turn on. This significantly improves the odds of the transistors surviving. Additionally, there is another critical circuit that monitors interrupter pulses and ensures they aren’t too long (like external light leaking into the optical sensor). The maximum pulse time the circuit permits is determined by a voltage bias set by RV2 which is compared to the voltage slope of an RC low pass filter connected to the optical input.