Simulation of Thor's primary and secondary circuits

Thor's primary and secondary circuits have been simulated using MicroSim (Evaluation version 8.0). Here is how the component values have been derived:

The similarity of the measurement and the simulation results are astonishing (see below)! The simulated picture can be graphically made B/W and superimposed to the original measured and will almost perfectly overlap it. Of course, the voltage scale has been stretched to match the measurement one. The minimal differences after the end of the fourth beat are due to the spark gap open time instant, which deliberately has not been optimized to match perfectly the measured waveform.

The secondary voltage as measured by an unconnected oscilloscope probe.	The secondary voltage as predicted by the MicroSim simulation model.

The spark gap is modeled by block HS1 as a current-dependant resistor; I_IN+ and I_IN- are the current sense terminals, X_GND is meant to be connected to GND and the variable resistance is available between terminals R_OUT+ and R_OUT-. Terminal RMON is simply an aid for monitoring the spark gap resistance value and must be grounded with a resistor (any value will do). The voltage on RMON, multiplied by Rscale, gives the instantaneous value of the resistance between R_OUT+ and R_OUT-.

The spark gap parameters are:

Tsw_on	the time instant when the spark gap starts conducting
Tsw_off	the time instant when the spark gap stops conducting
Iscale	a scaling coefficient for adapting the peak value tracker to different current magnitudes
Rscale	a multiplicative coefficient that affects the spark gap output resistance

Simulation model of the spark gap.

H1 is a current-to-voltage converter, with transfer function Vout = Iin * Iscale. The following block (BLK1) takes the absolute value of the converted voltage. Next to it, the group D1, C1 and R1 tracks the peak value of the input waveform: typically, this is the decay trend of the waveform (exponential, linear, or whatever). After being divided by ten, it feeds BLK3 that contains the actual negative-resistance law, that is

V_out = - 0.3 + 0.015 / V_in³

The MIN function is used to avoid excessive overshoot when the input current is still zero. The last block of the chain is the ZX component (part of the anl_misc MicroSím library), which provides a resistance R45 = Rref * V12 between terminals 4 and 5 (that is, ZX is a voltage-to-resistance converter). By monitoring the voltage on terminal RMON and multiplying by Rref, the value of the resistance R45 can be inferred.

In the top diagram the behaviour of the spark gap model during the example simulation. The red curve is the output of the V(%IN)/10 block, i.e. the output of the peak value tracker. The green curve is the voltage on the RMON terminal. In correspondence of the fourth beat the spark gap resistance is about 2 ohms.

It is interesting to note that, increasing the value of the primary resistance R1, the decay becomes more exponential, while increasing the value of Rref makes it more straight. This corresponds to what has been usually experienced with practical Tesla Coil systems.