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Re: [TCML] Measuring secondary current




On Sun, 2 Sep 2012, Steve Ward wrote:


and calibrated it at 2mV/mA with 14.7k ohms of resistance between the output
leads, using my signal generator to put some current into a 1 ohm resistor
via a wire through the center of the toroid.

First, 14.7k? thats wayyyyy too big of a resistance to burden
something like this with, and your maths are just wrong.
No math, at least for that value -- I arrived at 2mV/mA using a signal generator and a scope to measure the voltage across the 14.7k resistor (and arrived at the resistor value by increasing it until I got the results I wanted). Presumably there are enough parasitics loading the thing down... 


My advice, start over.  Ive made many many CTs that calibrate against
professional CTs up to a few MHZ just fine.  No tweaking, just do it
right.  Here's what i recommend:

For measuring 1A and less, i use 10 turns on a ferrite toroid (type 77
is good, anything with mu around 2000-10000 is usually good, dont use
EMI suppression types).  Burden it with 10 ohms to yield 1V/A, or 1ohm
for 100mV/A.

For medium currents (up to say, 100A), id suggest 50 turns on the
core.  With a 5 ohm burden, this is 100mV/A, or a .5 ohm burden gives
10mV/A.

For real big currents (up to say... 2000A), i wound 200 turns on a
pretty big core (1" ID), and burden it with say 2 ohms for 10mV/A, or
.2 ohms for 1mV/A.
This, along with Mr. de Queiroz's one-page guide, ought to be posted somewhere visible. I badly wish I'd had it when I started! 



Also, you MUST take care to keep leakage inductance down on the output
side of the CT.  I always terminate my CTs with the shortest
connections possible, especially between the burden resistor and your
RF connector of choice (i use BNCs so it connects straight to an
oscilloscope).
Yes, I did that (resistor right across the output leads, scope probe right across the resistor). 


 If you leave big loops of wire exposed, they can
easily pick up signal, which will contribute not only magnitude error,
but phase error to the measurement.

If you must use a loooong BNC cable, its wise to try and match the
output impedance from the CT with the 50 ohm impedance of the cable.
You can do this quite easily by putting a 50 ohm resistor in series
with the CT output before the BNC connector.  You might also need a 50
ohm terminator at the scope, which will change your CT output ratio
(should generally cut it in half).
I thought the way to do it was a 50 ohm burden resistor to get a 50 ohm output impedance, but in my case not an issue. 
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