Re: HV probe for primary measurements

["Tesla list" <tesla-at-pupman-dot-com>]

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

Hi Greg,

At 02:31 PM 9/27/2002 +1000, you wrote:
>Hello TCML,
>
>
>I would like to use my CRO to examine the waveforms in my primary 
>circuit during operation. Can someone point me in the direction of 
>some suitable schematics for either a HV probe or some kind of 
>current transformer metering device? 

You say "primary currents" so I assume that is exactly what you are
interested in.  Different parts of a coil have different ways of measuring
them.  A lot also depends on how accurate you want to be.  If you just want
to see ringdowns and quenching practically anything will work.


>I saw Terry's setup using the 
>optocouplers, but I can't find any schematics or theory of operation.

It's there buried in a few Gig of files :-))

http://hot-streamer-dot-com/TeslaCoils/Misc/Project_color.pdf

I am afraid to ever change anything at my site now since "I" know where
everything is :o))

>Can I just use a big string of resistors? 

The problem is shielding the resistors so they don't pick up stray
fields...  "Almost" impossible...

>The idea of directly 
>measuring the primary voltages and currents, quite frankly, scares 
>the utter crap out of me. 

My scope is worth about $4000 (my money), tell me about it :o)))

>But I am determined to investigate them. I 
>will NOT be touching the CRO during operation as a result. I just 
>don't want to go anywhere near it. 

Ground the thing well.  That removes 95% of the risk.

>
>What about making a current transformer by winding some wire around a 
>ferrite core?

So it's a big subject....  See these papers first:

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

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

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

http://hot-streamer-dot-com/TeslaCoils/MyPapers/planant/waveant3.html

Especially see the last one if you are interested in secondary voltages.


The "poor man's" equipment is free and probably will work fine for 95% of
what you want to do.  Just a scope probe hooked to about 1 foot of wire as
an antenna.  Placed well away from the coil, it will easily pick up nice
clean signals that show a lot!  Not NIST standard accuracy, but excellent
data for being free ;-))  If you loop the probe's ground wire back to the
tip, you have a nice current probe.  Just a small single turn loop.  Since
the primary is hundreds of amps at least, it works very well and you can
place it near but safely next to the primary coil.  Both are excellent ways
to basically see the secondary voltage or primary current.

If you want to make something, consider the plane wave antenna in the paper
above.  I use mine a lot!  Basically like the wire antenna but it can be
calibrated and it has excellent response and characteristics so you see
what is really there.  The fiber-optic probes are good for measuring
currents to streamers and such.  Lots of possibilities there.  I played
with Rogowski coils once buy they are pretty flaky and not much use when
you have 5 Pearsons laying around ;-)  You "can" make you own current
transformer but it would have to be like 100:1 at 1000 amps peak and stuff
that makes it hard...

If you want to buy something.  EBAY!!!  EBAY!!!  EBAY!!!
Pearson current transformers go for about $50 to $100  Big 40kV TEK probes
go for practically nothing (but try to get the ones will silicon oil
instead of freon).  I even found a 60kV Jennings vacuum probe there.  I
never had a problem with ebay but buying current probes is probably safer
than buying Rolex watches there ;-))  I do note that once school started
up, probes and technical stuff got more scarce and the prices jumped as
everyone tools up for the next school season.

If you can be more specific about what you want to do, I can probably
answer any question or help you.  I have been there ;-))

Cheers,

	Terry



>
>Cheers,
>
>
>Greg.
>