Re: how to use a current transformer

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

Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, September 01, 2003 8:13 PM
Subject: Re: how to use a current transformer


 > 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.

Almost for sure.. that 100:5 marking is the giveaway..   100 Amps through
the middle gives 5 Amps through the sense winding.. If you put a 0.2 ohm
resistor across the sense winding, then 10 Amps through the middle will give
0.5 Amps through the sense winding, and 0.1 Volt across the resistor, which
is a convenient value for running into one of those inexpensive meter
modules.



 >
 >
 > >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!!!

Or even more, if the core saturates.. hundreds of volts is not unusual...


 >
 > 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.  Th
e
 > clamp probe is all safety fixed so not much can go wrong.

I wire the burden resistor permanently across the winding (i.e. solder it)
and use a small one so it doesn't dissipate much power.


 >