Re: how to use a current transformer

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

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.


>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