Batteryless current clamps?
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From: John Larkin on 19 Nov 2009 11:40 On Thu, 19 Nov 2009 07:29:26 -0800, Fester Bestertester <fbt(a)fbt.net> wrote: >So, for a millivolt output probe, this might be as simple as 2 windings (or a >tapped single winding) with a range switch to select the winding? Current transformers are usually dumped into a load resistor aka burden resistor, to convert their output current into voltage. I'm sure the Fluke clamp-on has an internal burden resistor, and they may switch that to change ranges. Without a burden resistor, the output voltage will be proportional to frequency and very dependent on core reluctance, which would be fatal for a clamp-on meter with a hinge and a non-repeatable air gap. Coreless Rogowsky coils are used unloaded, but need a downstream integrator to accurately measure current. http://en.wikipedia.org/wiki/Rogowski_coil The coolest current transformer is a second-harmonic DCCT, accurate to parts-per-million from DC to many kilohertz. http://www.gmw.com/electric_current/Danfysik/866_867/867.html John
From: George Herold on 19 Nov 2009 13:27 On Nov 19, 11:40 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 19 Nov 2009 07:29:26 -0800, Fester Bestertester <f...(a)fbt.net> > wrote: > > >So, for a millivolt output probe, this might be as simple as 2 windings (or a > >tapped single winding) with a range switch to select the winding? > > Current transformers are usually dumped into a load resistor aka > burden resistor, to convert their output current into voltage. I'm > sure the Fluke clamp-on has an internal burden resistor, and they may > switch that to change ranges. > > Without a burden resistor, the output voltage will be proportional to > frequency and very dependent on core reluctance, which would be fatal > for a clamp-on meter with a hinge and a non-repeatable air gap. > > Coreless Rogowsky coils are used unloaded, but need a downstream > integrator to accurately measure current. > > http://en.wikipedia.org/wiki/Rogowski_coil > > The coolest current transformer is a second-harmonic DCCT, accurate to > parts-per-million from DC to many kilohertz. > > http://www.gmw.com/electric_current/Danfysik/866_867/867.html > > John Wiki's great thanks John. That second link didn't have much info on how the device works. Is the following the same thing? http://adweb.desy.de/mdi/CARE/Lyon/Lyon%20DCCT_Technology_Review.pdf (I googled second-harmonic DCCT) George H.
From: John Larkin on 19 Nov 2009 15:46 On Thu, 19 Nov 2009 10:27:34 -0800 (PST), George Herold <ggherold(a)gmail.com> wrote: >On Nov 19, 11:40�am, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Thu, 19 Nov 2009 07:29:26 -0800, Fester Bestertester <f...(a)fbt.net> >> wrote: >> >> >So, for a millivolt output probe, this might be as simple as 2 windings (or a >> >tapped single winding) with a range switch to select the winding? >> >> Current transformers are usually dumped into a load resistor aka >> burden resistor, to convert their output current into voltage. I'm >> sure the Fluke clamp-on has an internal burden resistor, and they may >> switch that to change ranges. >> >> Without a burden resistor, the output voltage will be proportional to >> frequency and very dependent on core reluctance, which would be fatal >> for a clamp-on meter with a hinge and a non-repeatable air gap. >> >> Coreless Rogowsky coils are used unloaded, but need a downstream >> integrator to accurately measure current. >> >> http://en.wikipedia.org/wiki/Rogowski_coil >> >> The coolest current transformer is a second-harmonic DCCT, accurate to >> parts-per-million from DC to many kilohertz. >> >> http://www.gmw.com/electric_current/Danfysik/866_867/867.html >> >> John > > >Wiki's great thanks John. That second link didn't have much info on >how the device works. Is the following the same thing? > >http://adweb.desy.de/mdi/CARE/Lyon/Lyon%20DCCT_Technology_Review.pdf > >(I googled second-harmonic DCCT) > >George H. Fig 3 is about right. The green feedback path is usually an N-turn winding to net an N-to-1 current transformer. There are usually two physical toroids. Ib, If, and T3 are wound on both, as if they were a single core. T1 and T2 are each wound on one of the cores, in opposite directions so that there's no net coupling of the carrier frequency into T3 or the customer's Ib circuit. The AC path (sense winding T3 and its amplifier, driving If) fight to keep the flux zero at higher frequencies. The second-harmonic system works at low frequencies, again to keep net core flux zero, which happens when Ib = N * If. I designed one of these once. It was fun, and not all that easy. John
From: ChrisQ on 20 Nov 2009 07:54 John Larkin wrote: > > The coolest current transformer is a second-harmonic DCCT, accurate to > parts-per-million from DC to many kilohertz. > Enquiring minds etc :-). I thought at first that sounded like a variation on the old fluxgate compass idea from ww2 and earlier, where the earth's magnetic field varies the saturation of a toroidal ring and the second harmonic amplitude recovered in 3 levels in 120 degree segments, but it's quite different. Here's a good article on the principle: cdsweb.cern.ch/record/1183400/files/CERN-BE-2009-019.pdf Page 3 of 4 for the block diagram. Using a feedback loop to cancel the induced magnetic field. Very neat. Something new to learn in electronics every day :-)... Regards, Chris
From: ChrisQ on 20 Nov 2009 08:01
ChrisQ wrote: > > cdsweb.cern.ch/record/1183400/files/CERN-BE-2009-019.pdf > > Page 3 of 4 for the block diagram. > > Using a feedback loop to cancel the induced magnetic field. Very neat. > The above last line is rubbish. Engage brain and analyse the schematic properly before posting :-( !. In fact, it is the second harmonic that is detected... Regards, Chris |