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

----- Original Message -----
From: "Ian McLean" <ianmm@OPTUSNET.COM.AU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, February 19, 2004 4:32 PM
Subject: Re: [EE:] Current Transformers


> > 3. The secondary should be shunted by a bi-directional TVS
> > diode (1.5KExxx
> > type works well) in case the burden resistor opens and to
> > soak up starting
> > current surges when you turn on the product. Pick a
> > comfortable voltage for
> > whatever you are driving with the CT for the TVS. I usually
> > use the 6.8V
> > version.
> >
> > 4. Rectifying the output directly is not a good idea,
> > especially with a half
> > wave rectifier. There are two reasons for this. The diode
> > will leave the
> > secondary unterminated for half of the cycle, which will
> > overvoltage zap the
> > diode if a burden resistor is not used before the rectifier. The other
> > problem is the diode offset voltage contributes a large error at low
> > currents, even with Schottky diodes. An op-amp "perfect
> > rectifier" circuit
> > is recommended for pre-pic detection.
> >
> > Regards,
> > Edward Gisske, P.E.
> > Gisske Engineering
>
> Hi Edward,
>
> I take it a by TVS you mean "Bi-Directional Transient Voltage Suppressor",
> yes ?
> I've never used one of these before.  Had to check Google to find out what
a
> "TVS" was ;)
>
> Excuse my ignorance, but how do you wire that into the secondary side of a
> CT ?
> By "shunted", I assume that just one goes in parallel with the burden
> resistor (similar to an MOV) ?  TVS come in Watt ratings as far as I can
> tell from my limited Googling so far.  How come 6.8V ?, and what watt
rating
> do I need ?  I know I will be pulling 5A on a 50:5 or what ever, but what
is
> the resistance of a TVS to calculate Watt rating ?  Or am I missing
> something here ?
>
> Would an MOV do the job ?
>
> Rgs
> Ian
>
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