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Re: [Fwd: secondary waveforms]



At 04:25 PM 2/15/97 -0700, you wrote:
>Subscriber: jim.fosse-at-bdt-dot-com Sat Feb 15 15:43:34 1997
>Date: Sat, 15 Feb 1997 02:23:00 GMT
>From: Jim Fosse <jim.fosse-at-bdt-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: Re: [Fwd: secondary waveforms]
>
>
>>Subscriber: sgreiner-at-mail.wwnet-dot-com Thu Feb 13 22:10:08 1997
>>Date: Thu, 13 Feb 1997 16:04:24 -0800
>>From: Skip Greiner <sgreiner-at-mail.wwnet-dot-com>
>>To: tesla list <tesla-at-pupman-dot-com>
>>Subject: [Fwd: secondary waveforms]
>>
>Hi Skip,
>>
>>I have wound a small toroid with a few turns of wire which are connected
>>to an oscope. The grounded lead from the TC secondary passes thru the
>>toroid and then to ground. Purpose, I thought, was to look at the
>>wavfeform of the current in the secondary during operation. I have
>>waveforms but not quite what I expected.
>>
>>The waveform does indeed exhibit a decreasing envelope which is
>>synchronized with spark gap. The oscillations within the envelope are
>>not pure sine waves. They appear to be a mixture of probably two sine
>>waves of about equal amplitude, one being about 2x the frequency of the
>>other.
>
>You don't mention a termination resistor on your current transformer.
>The ferrite core current transformers used in switch mode power
>supplies are terminated with a 100 ohm resistor. With that
>termination, they give 1 volt out per amp in. (this is only 1 type,
>but illustrates my point. other ratios are available.) Without the
>termination resistor, the Inductance of the current transformer's
>secondary will ring with the capacitance of your scope probe. Try
>switching your scope's input impedance to the 50 ohm setting, or add a
>50 or 100 ohms dampening resistor.
>
>>The question: What am I seeing? I assumed that the ring down in the
>>secondary would be sinusoidal at a frequency somewhat less than the
>>natural resonant frequency in the unpowered state. Should the ring down
>>be a single frequency sinusoid in a correctly operating TC? Most
>>write-ups seem to imply this. If this is the case, where is the second
>>frequency coming from and how does one get rid of it?
>>
>You are ringing the LC circuit formed by the inductance of the current
>transformer, the capacitance of the scope probe, stray inductance and
>stray capacitance. You must terminate this circuit.
>
>	Cheers,
>	
>	jim
>
>
Skip,

What jim suggests as the cause for the "other frequency" is exactly what I
mentioned earlier.  The Q of any CT for wideband work must approach zero as
closely as possible inorder to avoid ringing.  I use a .01 ohm carbon comp
resistor shunted across my rogowski coil in the water arc discharge system.

Richard Hull, TCBOR