Re: Output Characteristics of SIMPSON 200:5 Current Transformer

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

Original poster: David Speck <dave-at-davidspeckmd-dot-org> 

Dan
I agree fully with Jim's response below.  Only problem is that you end up 
with low voltage AC across your sensing resistor, which may be hard to 
input to your uP, unless you construct a precision rectifier or RMS 
converter circuit.  Doable, but a hassle.

As an alternative, check out:

http://cgi.ebay-dot-com/ws/eBayISAPI.dll?ViewItem&item=2594675143&category=26215

This transducer takes 20A full scale AC and converts it nicely to 1 mA DC 
isolated.  If you feed it with the output of your 200:5 CT, it will go 
0-250 uA for your full scale range of whatever CT you connect to it.

Might save you a bit of hassle.

Also might want to check out:

http://cgi.ebay-dot-com/ws/eBayISAPI.dll?ViewItem&item=2593705989&category=4665

These LEM current transducers work much like CT, but have the huge 
advantage of being able to sense DC as well as AC with a respectable 
frequency response..  The output it as an isolated analog reflection of the 
combined DC and AC through the sensing opening.  They require +/- 15 volts 
DC for excitation, but have the distinct advantage of not putting out any 
dangerous voltages if you pass primary current through them without a load 
on them, or if the drive voltage is absent.

The tech support people at LEM are very helpful, but I have spec sheets for 
these units if you need them.  They make some models that include a 
precision rectifier, for DC only output, but I've never seen these on eBay.

These units may take on a bias if you pass primary current through them for 
long periods without drive power applied, but you can demagnetize them like 
an old tape recorder head with a strong AC magnet to cancel the bias.

HTH,
Dave

>The usual x:5 current transformer is a current output device, designed for a
>rated current in the sense winding of 5Amps, corresponding to X amps through
>the hole.  Under no circumstances should you run one of these open circuit
>(the core saturates, and that makes big voltage spikes on the secondary).
>If you're feeding into a A/D, put a small burden resistor across the
>secondary (say, 0.1 ohms) and measure the voltage across the resistor. Even
>at 5A, that 0.1 ohm resistor will dissipate 2.5 Watts, so make sure it's big
>enough.  You could also put a 0.01 ohm resistor, which will have lower
>voltage (50 mV at 5A), but will also dissipate less power.