Re: Resonance

To: tesla-at-pupman-dot-com
Subject: Re: Resonance
From: Bert Hickman <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Mon, 25 Jan 1999 18:30:50 -0700
Approved: twf-at-verinet-dot-com

Tesla List wrote:
> 
> Original Poster: Philip <shadow42-at-totcon-dot-com>
> 
> I used my scope and funtion gen. Fed sine wave intp base of coil and
> hooked scope to other end of coil. I increased frequency until wave form
> grew in voltage. That I assumed was the resonance point. Is this wrong?
>   Philip

Philip,

Yes... it's wrong.  :^)

When you connected the scope probe to the top of the secondary, you
added approximately 10pF of capacitance. This significantly lowered the
measured resonant frequency from around 400 kHz to the value you
measured. The top of the secondary is extremely sensitive to any
loading. 

A more accurate method would be to tie a pair of high brightness LED's
anti-parallel (cathode to anode so that the pair will conduct in either
direction), and drive the base of the secondary through these LED's from
the function generator. You want _nothing_ connected to the top of the
secondary for the unloaded resonance measurement. Any loose wire at the
top should be wrapped around the top end of the coil form and taped in
place to reduce the parasitic capacitance from the loose wire. The same
measurement approach can be used with the toroid installed (and
connected) to find the loaded resonant frequency. 

Make sure that nothing else (including your body) is close to the
secondary coil while making these measurements. Ideally, the secondary
should be in its normal operating position relative to the primary.
BTW, the primary tank capacitor should be short-circuited to prevent
unwanted coupling effects between the primary and secondary which can
alter measurement accuracy. By carefully sweeping the test frequency
upwards, you should see a point of maximum current (maximum LED
brightness) - this will be the unloaded resonant frequency of your
secondary. Bet you'll find that your unloaded secondary's at about 400
kHz! :^)

-- Bert --