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Re: Scoping the Coil



Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Dave,

At 06:06 PM 3/10/2002 -0800, you wrote:
>Coilers,
>
>I would like to measure the tank circuit waveforms on an oscilloscope.  My
>scope is not battery powered so any scheme involving a direct connection
>through a voltage divider puts the scope at risk.  Several people on the
>list suggest the use of an antenna or some form of capacitive coupling to
>get a signal into the scope but when I attempt to use these methods, the
>noise and trigger jitter completely obscure any detail.  On the other end
>of the scope coupling spectrum there's Finn Hammer's sophisticated fiber
>optic probe which, while it's ideal, is expensive.

The simple antenna can be used somewhat.  Try moving it around in different
places (not too close) and maybe you will pickup a better signal.

I am not sure if you want to measure voltage or current (both) in the
primary circuit.  Direct connections always carry some risk.  Very high
voltage probes (Tek6015) and wide bandwidth current transducers like the
ones Pearson makes can be used but they are expensive (unless you can find
used ones cheap like on E-Bay).  Fiber optic probes so provide safe
isolation but they are still ~$400 

So it depends on exactly what you want to measure as to what approach is
best.  You may want to see the papers below that involve measuring the
things you talk about:

http://hot-streamer-dot-com/TeslaCoils/MyPapers/MyPapers.htm

Pearson has a nice site about current monitors:

http://www.pearsonelectronics-dot-com/Pages/Monitor_instructions.html


>
>I've noticed on a project at work that signals from a high voltage pulse
>generator are coupled into oscilloscopes magnetically using a donut-shaped
>device, which I assume is just a coil of wire, encircling one of the high
>voltage leads.  Although these pulsers have a much higher frequency content
>than a Tesla coil I would think that the principle would still apply and as
>long as the coax connecting to the oscilloscope is protected from taking a
>direct strike, I think that the scope would be safe.

Those are probably current monitors like above.

http://www.pearsonelectronics-dot-com/Pages/Standard_Currrent_Monitors.html

Some have an astounding frequency range of 1Hz to 20MHz!  Someone was
asking about the frequency response of a transformer.  Pearson has it
figured out.  Apparently they are made out of pure money :o))

They are ground shielded but if you dump 1000 amps at 350Khz into them all
bets are off.  Few "grounds" can take a hit like that without the voltage
getting away.  Even the magnetically induced currents can easily cause
damage if a primary cap is shorted.  Sort of a gnarly game ;-)

>
>Is this coupling method worth pursuing?

Depends on what you need and how much money you want to spend.  Check out
the papers at the above site to get an idea of what is involved.  I have
done this stuff a lot and I can help you determine what is the best way to
go.  In many cases, you can just calculate the voltages and currents to.
Not wuite as exciting but it does work well.

http://hot-streamer-dot-com/TeslaCoils/MyPapers/modact/modact.html

Cheers,

	Terry


>
>Dave
>
>
>