Current Monitors Vs. Current Transformers

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

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

Hi Matt,

There are two "types" of "current transformers" CTs that are really very
different.

The first type is a simple current transformer usally refered to as a "CT"
that is optimized for 60Hz use to monitor power lines and electrical
equipment that runs at 60Hz.  These NEED a load put on their output
terminals to prevent very high voltages from being generated.  A 50:1
transformer open load on say a 480 VAC line will have 24000 volts on the
output!!  So you always must be sure they are connected to a "burden"
resistor or ammeter to basically short the outputs out so you only see the
current and little voltage.  These current transformers are made by
Simpson, GE and others in the AC equipment industry.  Very common and run
about $50 new.  Ohm out used ones to be sure they have not been burned up
from the open load event.  Your GE and Simpson CTs are of this type.  They
can be used with a suitable ammeter or resistor voltmeter to see the input
current to a Tesla coil system.  Note that these CTs usually have poor
bandwidth outside 60Hz so harmonics and odd signals my not be represented
well.  The uses they see typically are pretty close to pure 60Hz sine
waves.  These typically last forever but the newer ones are getting CE,
CSA, TUV, UL, and a zillion other safely ratings on them and have increased
protection against fire if they open load and are harder to electrocute
yourself with.  There are many smaller PC mount versions too for using
inside equipment.


The second type is the "current monitor" as sold by Pearson and a few other
odd companies.  They are much the same "basic" design as the above but
these are laboratory-grade, high-accuracy, and high-bandwidth types.  They
are often called CTs too, but that is really not right.  They have a built
in 50 ohm burden resistor made to impedance match 50 ohm coax cable (BNC or
N output connectors).  For high frequency work, they should be run into a
50 ohm termination.  They can be run open load without worry.  These are
used for calibration and engineering uses where the signals are far from
60Hz.  The 410 and 110 you saw are the same but the 110 has a much larger
center hole.  The specs on the 110 are: 

Hole diameter = 2 inches
1 amp = 0.1 volt  (note the output in current monitors is in volts)
Max peak current = 5000 amps
RMS current = 65 amps
rise time = 20nS
Bandwidth = 1 Hz to 20MHz
Accuracy = +1% -0%
 
The last two specs are what account for the $500+ difference in price :-))
The -3dB bandwidth is 1Hz (yes, that is just one cycle per second!) all the
way up to 20,000,000 Hz with a flat frequency response (+1 -0 percent in
the pass band which implies <0.05 dB error!)  So very high bandwidth and
very high precision are what set these "instruments" apart.  Prices new are
$450 for the smaller popular types, $850 for the rarer larger types, and a
few thousand for the big 10 inch hole 500,000 amps types (nice for
lightning strikes and transmission line fault study which IS what they are
used for!)  Used prices vary but about $75 for the popular ones like the
410 and 110.  Then they jump to about the $100 TO $200 dollar range.  Some
odd versions will be much higher.  Some versions split in half for clamping
over existing cables which adds a lot to the price but hurts precision.  In
general, they tend to be hard to find used when you want them ;-)  Pearson
has them all one the self and they take credit cards and can ship next day
(they keep reminding me of this :-))

For Tesla coil work, here are a few added tips:

The 1 amp = 1 volt versions usually have a lower frequency limit of 100 to
300Hz which makes them not too useful for 60Hz AC stuff but the upper limit
is higher at 20MHz and one goes to 200MHz! 500 amps peak and 5 amps RMS is
also a little limited.  Note that a 500 amp pulse will present an
oscilloscope with 500 volts on the input which some are not designed for.
Pearson does sell a 10:1 voltage divider for a few hundred bucks
(high-precision over a wide-bandwidth...).  These 1:1 monitors are great
for all kinds of electronic works and they are very common.  Far better for
electronics (amplifiers, SMPS, etc.) rather than AC line stuff.

The 1 amp = 0.1 volt version have 1 HZ to 20MHz response and a 1000 amp
pulse only hits the scope with 100 volts.  Great for AC work with 50 RMS
amps.  These have a wide variety of uses and are also very popular for
higher current work.  If you want one, these are the ones to get.  Both the
411 and 110 are perfect but the larger hole in the 110 is probably a bit
better.  Especially with heavy insulation on the wire or for wires with
terminals that don't like to fit through 1/2 inch holes.  Perfect for many
uses and toob coil stuff.  They are only limited for tiny signals or
primary tank stuff.  In general, they do everything.

For serious primary tank current measurement, the 1 amp = 0.01 volt version
is best.  They take maybe 50,000 amps peak and 200 amps RMS so a tank
circuit is nothing to them :-))  Many still have the 1Hz to 20MHz response
but upper bandwidth does start to go down now.  Best to get 2 inch holes
here so you can pack a ton of insulation over the cables.

Some have double EMI shielding but I don't think that matters much for
Tesla coil stuff where the signal are pretty strong :-))  However, the
grounding may be better if it arcs over depending on exactly how they are made.

Of course, to justify the use of a current monitor pretty much assumes you
have a scope or fancy wideband meter to plug it into.  But for AC use, you
could use any old voltmeter too.  But, a fancy RMS AC clamp meter is still
cheaper.

If you like to crush quarters or cans then to 1A = 1000V versions at
500,000 amps peak are in range but the upper bandwidth really starts to go
down at about 1MHz.

I have the Pearson specs on-line at:

http://hot-streamer-dot-com/temp/PearsonCTs.gif

Pearson's site is at:

http://www.pearsonelectronics-dot-com/

If anyone hears of Pearson current monitors going for a good price, let us
know!!


Cheers,

	Terry





At 12:32 PM 3/23/2002 -0500, you wrote: 
>Hi Terry, Paul, All,
>I have an opportunity to buy a couple of NOS current transformers: GE 200:5 
>type JAK-O and Simpson 50:5 #01293. I just saw two Pearsons (410 & 110) go 
>for $152.50 for the pair, but I don't have a feel for the TC suitability or 
>reasonable price for these other two. Any knowledgeable input appreciated.
>Thanks,
>Matt D.
>