Sounds about right with me.
Make sure the burden is fitted and has pleanty of headroom - otherwise the
open circuit  output voltage  will cause damage or hazard.

You may be able to use a mains transformer  as the CT. Just add a single
turn of heavy duty wire around the core. If you get the turns ratio &
burden value right you shouldn't need an amplifier. The 50:5 ratio is
probably not optimal for your requirement. Just make sure it is big enough
for teh power levels involved (depends on the turns ratio &  size of the
burden).

50 amps (rms) in = 5amps (rms) out. which is 25Watts into a 1 ohm resistor.
This will give you 14V pk-pk though so you really need a 0.33ohm resistor
for 5V pk-pk. Current will still be 5A so power is 8.25Watt .

If you have a 100:1 ratio then 50A = 0.5A out. R = 3.5 ohm for 5V peak to
peak and the power lost is less than a watt. You do the math for 1000:1
ratio

RS components, Farnell etc. stock CTs. If you were in NZ I could point you
at a local supplier but most instrumentation/control supply places will be
at least able to point you in the right direction. If you don't need
precision/special winding ratios/wide frequency responce they are pretty
easy to come by.

Hall effect sensors are going to be smaller & easier  to handle but may
cost more & can have offsets and low & high current non-linearities. (But
so do CTs)

Richard P




Resent with TAG.

Rgs
Ian

-----Original Message-----
From: Ian McLean [mailto:ianmm@optusnet.com.au]
Sent: Friday, 20 February 2004 4:20 am
To: 'pic microcontroller discussion list'
Subject: RE: ] Current Transformers


Hi all,

Before anyone picks me up on this, I have been doing some more research on
current transformers.  I don't need to rectify the secondary of the CT.
Just put a 'burden resistor' (high watts, low resistance) straight across
the secondary winding - then read the voltage across this resistor, op-amp,
A/D input etc.

Have I got this right ?

Anyone know where I can get a suitable 50:5 CT from ?

Or is there a "better way" ?

Rgs
Ian

>

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