The Circuit & Capacitive Discharge Theory:

The circuit's theory is quite simple although it's needed some understanding of capacitive discharge circuits:

Two medium power NPN transistors (TIP 41A - 6A/60volts) auto-oscillate alternatively switching half of the primary coil of T1 (step up transformer) thus inducing a higher voltage on the secondary side, usually between 400 to 1000 volts (but this depends on the magnetic properties of T1's ferrite core, the number of turns of the secondary,the NPN transistor characteristics and the values of base resistors, and the supply voltage ).

Stepped up supply voltage given by T1's output is then rectified and used to charge a bank of capacitors C2 (paper/oil/mylar type- 2x470nF/630volts) through a charging resistor R2 (1 kohm).

This bank is discharged over the primary of a car ignition coil, producing a high voltage dampened wave with a ringing frequency given by the ignition coil primary inductance (L2p) and the total capacity of the bank (C2/2). The process of charging/discharging the bank is repeated several times per second (pulse repetion rate or PRR).

The PRR is given by a relaxation oscillator used to control a TRIAC which discharges the capacitors on T2's primary coil (L2p). This oscillator built around a 2N2646 UJT (unijunction) transistor has an adjustable pulse repetition period (PRP).

UJT's emitter senses the voltage on capacitor C1 (100 nF) which is being charged through R1 and P1 (10 kohm and 1 Mohm); when the charging voltage on C1 reaches the UJT emitter's peak-point value Vp (Vp=k*Vbb+0.6volt; where Vbb=voltage between UJT's bases and k=intrinsic stand-off ratio: aprox 0,7), the UJT turns on and a voltage pulse is sent to the TRIAC control gate turning it on, thus discharging the capacitors. The TRIAC will remain in on state until the current flowing through drops below the holding current value Ih (typically 20 to 60 mA).

This discharging of C2 on L2p produces a primary ringing wave which is further stepped up in the high voltage secondary of T2.


Now let's analyze current values:

• The capacitor bank C2 (235 nF) is charged through R2 (1 kohm), this will give a total charging time (at 5RC) of Tch = 1.175 ms.

• Ignition coil primary ringing frequency, assuming 8 mH for L2p is 3.7 kHz, which gives 0.27 ms for the first dampened wave period.

• The relaxation oscillator capacitor C1 (100nF) is charged through R1 and P1. Assuming P1=0 then the UJT's emitter's peak-point value Vp will be 21.6 volt, for (Vp=k*Vbb+0.6volt; where Vbb=30v k=0.7)
  
  How long does it take then to charge C1 to Vp, thus firing the TRIAC? Well that depends on R1 value, so let's calculate how big R1 should be to have a minimum PRP (pulse repetition period) equal to the capacitor bank charging time Tch; doing some math we have:
  
  R1 * C1 = Tch / ln { [ Vbb / (Vbb-Vp) ] }
  
  and replacing values we conclude that R1 should be greater than 9.2 kohm (let's adopt 10 kohm) in order the PRP to be greater than 1.175 ms (PRR=1/PRP or 850 hertz).
  
  Thus if the PRR (pulse repetition rate) exceeds 850 Hz, the capacitor bank will not recharge between pulses and the HV output voltage will decrease with the increased PRR.

• With the power supply at 30 volts and P1 set to zero, current circuit values will give HV output pulses of 40 to 60 kv @ 4 khz at a base frequency of 850 Hz (max). Increasing P1 will increment the PRP thus lowering the base frequency.

T1 step up transformer construction tips:


Transformer T1 is hand-made using an old flyback ferrite core (see drawing) with aprox. 1 centimeter diameter and with all original windings removed.

A new primary coil is built with 50+50 turns (enamel wire AWG #18 for example).

A new secondary is also built with 1000 to 2000 turns (AWG #28 or 32) built in several layers (for example 20 layers of 100 turns each).

Each layer must be properly isolated from the other, specially the last ones (outer layers). (For example the secondary could be 4 cm long which will give a coil of less than 5 cm diameter for 20 layers)

This transformer will rise 12 volt to 350, self-oscillating at 5 kHz whereas at 30v will step up to 1kv @ 7 kHz (values may vary).