Would not used a wire wound resistor as this will be an inductor that my cause incorrect readings. ************************************************* Roy Hopkins :-) Tauranga New Zealand ************************************************* ----- Original Message ----- From: "Ian McLean" <ianmm@OPTUSNET.COM.AU> To: <PICLIST@MITVMA.MIT.EDU> Sent: Saturday, February 21, 2004 3:52 AM Subject: Re: [EE:] Current Transformers > Yes, thanks Bob. I am aware now of this danger. I can understand this > considering the turns ratio - 1 to, ummm... a lot. I will be sure to well > and trully overrate the burden resistor's wattage rating. If I use a 50:5 > CT, then at 5A max. on CT secondary, and using, say a 0.1 ohm burden > resistor, the dissipation is I2R = 5^2 * 0.1 = 2.5W. I'll go with a 10W > rated wire wound resistor to be safe. Sound reasonable ? I believe the TSV > diode will provide me some protection as well. Then the max. voltage across > this burden resistor = IR = 5 * 0.1 = 0.5V. If I use an op-amp with 10 > times gain, and clipping at 5V (to protect PIC A/D input), then along with > the TSV that should provide me with about as much protection as possible, > yes ? > > BTW: This is for AC mains (240VAC, 50Hz) current measurement in an AC > control box (variac controlled) to power a Tesla Coil. The mains at 12kW > max. input is powering a 15,000VAC transformer, which in turn gets stepped > up on the coil to well over 600,000VAC at about 140kHz, so ... big, > dangerous voltages everywhere here to worry about ;) > > Rgs > Ian > > > -----Original Message----- > > From: pic microcontroller discussion list > > [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Bob Ammerman > > Sent: Saturday, 21 February 2004 1:17 am > > To: PICLIST@MITVMA.MIT.EDU > > Subject: Re: [EE:] Current Transformers > > > > > > By the way, be very careful when working with a CT. If the > > secondary ever > > goes open circuit (ie: the burden resistor is disconnected) > > the voltage will > > skyrocket! > > > > Bob Ammerman > > RAm Systems > > > > ----- Original Message ----- > > From: "Ian McLean" <ianmm@OPTUSNET.COM.AU> > > To: <PICLIST@MITVMA.MIT.EDU> > > Sent: Friday, February 20, 2004 7:08 AM > > Subject: Re: [EE:] Current Transformers > > > > > > > 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 > > > > > > > > > > -- > > > http://www.piclist.com hint: To leave the PICList > > > mailto:piclist-unsubscribe-request@mitvma.mit.edu > > > > -- > > http://www.piclist.com hint: To leave the PICList > > mailto:piclist-unsubscribe-request@mitvma.mit.edu > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu > --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.592 / Virus Database: 375 - Release Date: 18/02/2004 -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu