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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