Re: how to use a current transformer

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

Original poster: Crow Leader <tesla-at-lists.symmetric-dot-net> 

On Mon, 1 Sep 2003, Tesla list wrote:

 > Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>
 >
 > Hi Thad,
 >
 > At 07:45 PM 9/1/2003 -0700, you wrote:
 > >Hi List,
 > >
 > >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.

It's probably best to stick to a shunt, not random resistors. Most small
instrument current transformers are rated several VA. A 2 Ohm resistor =
10 volts at 5A output from the current transformer's output terminals.
This is 50watts, and a large overload of the current transformer.

There are 10 volt current transfomers out there, but this it's one of
them.

If you want a different ratio than the transformer is made for, you can
always decrease the ratio by adding another pass of wire. One "turn" on
wire though a current transformer is single pass of the load you are
measuring though the hole. A 100:5 transforer can be 10A full scale by
passing the load wire though the hole 10 times, or 20A full scale (again
the output terminals will make 5A) with 5 passes of wire.

I measured the resistance of my Simpson 35086 4.5" panel AC meter. It's
rated 5 amps AC full scale, and has an internal shunt. The DC resistance
is under 0.01 Ohms when I measure it, and the Simpson datasheet says 0.008
Ohms. At full scale, we can see it uses I^2 R which is 25 * 0.01 or
0.25watts

Ohmite "lo-mite" current sense resistors are around $1.75 each in 5 watt
ratings from Digikey and come in milliohm ratings and 3% tolerance, the
same as most current transformers are themselves.

KEN

 > >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
 >
 >
 >