[Prev][Next][Index][Thread]

A simple test system



----------
From:  Antonio C. M. de Queiroz [SMTP:acmq-at-compuland-dot-com.br]
Sent:  Wednesday, March 11, 1998 11:04 PM
To:  Tesla List
Subject:  A simple test system

Hi:

I want to describe a very simple method for the testing and tuning of
Tesla coils at low power, in perfectly safe conditions:

Disconnect the power supply, short-circuit the spark gap, and apply
across the primary coil a high-impedance current source made with
two PNP transistors in cascode configuration (please use a fixed width
font when viewing or drawing ascii art):

     +-----+--+--+---o +Vcc
     |     |  |  |
     R1    R2 C2 R3
     |     |  |  |
  (A)|   |<   +--+  
  +--+---|       |
  |  |   |\      |
  C1 R5    >|    |
  |  |      |----+-R4-gnd
  +--+     /|   (B)
     |    |
     o    0-----
    Vin         To Lprimary
          0-----
          |
         gnd 

Use for example Vcc=12 V, biasing (A) at 10.4 V (With Vin=0) and (B)
at 7.4 V using the resistive dividers. C2 is a "large" capacitor, and
C1 a small one to compensate for the input capacitance of the transistor
circuit (use about 100 pF). 
This will produce a current of 1/R2 A (use R2=100 Ohms for 10 mA)
being switched on and off as Vin changes between 0 and 12 V. Use
a signal generator or a 555 oscillator to generate Vin as a square
wave of a frequency in the order of 1/100 of the resonance frequency.
Observe the waveforms in the circuit with an oscilloscope. They are
the same, except for the first cycle, as would appear with the power
supply connected and a lossless spark gap, before quenching, scaled
down. 
Proper working is indicated by the observation of two identical
transients, in inverted versions, as the current is switched on and
off.
To search for the tuning point, see what is the condition that creates
a voltage waveform with beats at the lowest possible frequency and maximum
amplitude over the primary coil.
Try this interesting experiment: Reduce the primary capacitance until
you observe again a beating waveform, that is the second resonance
of the secondary. Slide a finger along the secondary, and see how this
interferes in the waveform. You will find a point where the interference
is small, that is the first voltage node. Reduce still more the capacitance
and you will find other resonances, with more voltage nodes.
If someone tries this (I did), I would be interested in what where
the resonance frequencies found, along with the other parameters of the
system.

Have fun,
  
Antonio Carlos M. de Queiroz
http://www.coe.ufrj.br/~acmq