Thanks Edward,

That clears things up nicely.  I am sure I can get this to work now ;-)

Thanks to everyone else who responded as well, and for the links to CT
suppliers.

Rgs
Ian

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Edward Gisske
> Sent: Friday, 20 February 2004 6:01 pm
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE:] Current Transformers
>
>
> Ian,
>
> Typically, you run the current carrying wire through the core of the
> transformer one time (sometimes a few turns). That is the
> primary. The wires
> coming off the CT toroid are the secondary.
>
> You are correct about the TVS (also known as a "Bi-Zener").
> Please pardon my
> acronyms. The voltage rating is selected to be low enough so
> you don't blow
> up whatever is downstream of the CT. A 1.5KExxx part will
> soak up 1.5KW
> surges without blowing up. It is unlikely you will have to
> dissipate any
> more than that under transient conditions, so it should
> work,. The TVS is
> only in action in starting or overload conditions. It is an
> open circuit
> under normal conditions. Think of it as two Zener diodes
> wired  back-to-back
> that limit AC waveforms to the breakdown voltage of the TVS.
>
> MOV's usually don't have a low enough turn-on voltage to be
> effective for
> PIC-style circuits (under 5V) so the TVS works better.
>
> Just put it in parallel with the burden resistor, which is in
> parallel with
> the CT secondary. It is a belt-and-suspenders sort of back-up
> to protect the
> rest of the circuit under fault conditions.
>
> Regards,
> Edward Gisske, P.E.
> Gisske Engineering
> 608-523-1900
> gisske@offex.com
>

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