RE: how to use a current transformer

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

Original poster: "Mccauley, Daniel H" <daniel.h.mccauley-at-lmco-dot-com> 



Sounds like you have one of the current transformers made exclusively
for large current ammeters.
In particular, they are designed for use with standard 5A ammeters which
may have different meter readings (i.e.
0-100A, 0-200A etc...).  Your current transformer was most likely for a
0-100A ammeter.

As these current transformers are typically used for RMS current only,
they probably won't have the characteristics required to be used as a
"true" high bandwidth current transformer such as the ones made by
Pearson or Ion Physics.

The Captain



 > >Can someone give me a lesson on the use of a current transformer?  I
 > >pulled one out of some junk a few years ago and now I think
 > I can use it.
 > >It is about the size of a big donut, has a 7/8" hole, is
 > 1-1/4" thick, and
 > >has two leads (secondary?), one black and one white, 100:5
 > printed on it.
 >
 > First!!  You have to be darn sure if it needs a load
 > resistor!!  Current
 > transformers can produce giant voltages on the output if they are not
 > properly loaded with a resistor.  Your's sounds like it need
 > to see an
 > ammeter or other low resistance on the output.
 >
 >
 > >I am usiing a variable ballast in my power supply, and I am
 > ready increase
 > >my power level.  I have installed a 30 A fuse on each
 > primary leg of my
 > >transformer, and with my variable ballast I just want to observe the
 > >current.  My handheld meter is only rated for 20 amps.
 >
 > The current transformer will work fine, but you need a load
 > resistor for
 > darn sure!  Your transformer will reduce the current to
 > 1/20th, but it will
 > have an open load voltage of 20X!!!
 >
 > So if you have 100 amps on the primary lead going through the
 > hole, you
 > have 5 amps on the transformer leads.  A 2 ohm resistor will
 > provide 10
 > volts for a 10:1 ratio.
 >
 >
 > >Can I use a current transformer? Before or after the
 > ballast? Do I read
 > >current or volts at the secondary of the current transformer?
 >
 > Put a 2 ohm, 50 watt resistor on it and the voltage will be
 > 10 volts/100 amps.
 >
 > >Since I only
 > >anticipate 30 amps max, is the ct that I found going to
 > work? Do I pass
 > >one hot line through the hole?
 >
 > Yes.
 >
 >
 > >Or should I spend money on an amp clamp? If
 > >so, true rms?
 >
 > Your CT will work fine as will the fancy clamp.  The CT is
 > sort of a risky
 > thing in that it can fry things if connections and load are
 > not right.  The
 > clamp probe is all safety fixed so not much can go wrong.
 >
 >
 > >I really want to know how to use one of these, but its not
 > necessary.  I
 > >feel pretty comfortable with my calcs. and fuses and circuit
 > breakers are
 > >in place. It will help me to calibrate my homemade inductive ballast
 > >though.
 > >Once that is done, I probably won't need it in the circuit.
 > >
 > >Thanks for any insight.
 >
 > Current transformer can be darn nasty!  They reduce the
 > current 20X, but
 > they up the voltage 20X if they are not loaded by a big low value
 > resistor.  Don't hold on to anything in this circuit!
 > Current transformers
 > can kill people if stuff goes wrong!!
 >
 > I don't mean to frighten here, but CTs are pretty bad things
 > if not used
 > just right...
 >
 > Cheers,
 >
 >          Terry
 >
 >
 >
 > >Thad Howard
 >
 >
 >