Batteryless current clamps?
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From: JosephKK on 20 Nov 2009 12:00 On Tue, 17 Nov 2009 07:20:32 -0800 (PST), Tim Williams <tmoranwms(a)gmail.com> wrote: >On Nov 17, 2:18 am, Fester Bestertester <f...(a)fbt.net> wrote: >> I'm curious how the Fluke i200s current clamp probe can give mV output >> without the use of batteries. > >Is that the one with the 10/100 switch and a green LED? > >I'm pretty sure most of the weight is not ferrite, it's a battery >somewhere. Actually about equally ferrite and plastic with copper coming at about 1/3 either of those. No battery whatsoever and AC only. > >They also read DC, and have an offset knob to account for the >ferrite's hysteresis. Only active probes do that. > >The passive probes only read AC, and as I recall, are 1 or 10 mV/A. > >Tim I can make any output ratio i want, i know how they work.
From: John Fields on 20 Nov 2009 19:38 On Fri, 20 Nov 2009 10:43:14 -0800 (PST), Bill Sloman <bill.sloman(a)ieee.org> wrote: >On Nov 20, 1:31�pm, John Fields <jfie...(a)austininstruments.com> wrote: >> What Joel _actually_ said was that energy could be extracted from the >> varying magnetic field surrounding a power line by wrapping turns around >> it. >> >> Since you pointed out that energy can't be had by wrapping turns around >> bundled conductors carrying charge flowing in opposite directions, >> that's something that, obviously, every dunce realizes. >> >> Knowing that, my take on Koltner's lighthearted comment was that he was >> referring to a single conductor, such as the ones used in high voltage >> distribution systems which are called, by the way, "power lines". > >Wrapping turns around a high voltage power line probably wouldn't be a >good idea. --- So, then, you agree that, aside from the obvious danger of wrapping turns around a high voltage line, no appreciable power can be drawn from the line by a coaxial solenoid surrounding the line? JF
From: JosephKK on 21 Nov 2009 07:21 On Fri, 20 Nov 2009 12:54:00 +0000, ChrisQ <meru(a)devnull.com> wrote: >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 No, more like an inside out mag-amp making a detector down to DC.
From: JosephKK on 21 Nov 2009 07:29 On Fri, 20 Nov 2009 08:44:19 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Fri, 20 Nov 2009 10:53:45 -0500, daestrom <daestrom(a)twcny.rr.com> >wrote: > >>Tzortzakakis Dimitrios wrote: >>> ? "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> ?????? ??? >>> ?????? news:dnsag51jfmub8nppj2kfj3nt07l3jv6djg(a)4ax.com... >>>> 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 >>> Anyway, current transformers must always be operated with the secondary >>> shorted. In the generating facilities in Kozani, West Macedonia, where 400 >>> kV current transformers were involved, the operators of the plant had a >>> special indicator whether the secondary was shorted. >>> >> >>Some old switchboard CT's I worked on in the Navy had very thin >>insulator between two spring clips. Whenever we wanted to remove a >>meter for cal, we slip the insulator out so the two clips would short >>together, shorting the CT. Then we could open circuit the meter and >>remove it from the panel. I don't remember exactly what the blade was >>made of, but it's surface wasn't perfectly smooth like polished >>material, more porous like unglazed ceramic (of course it wasn't any metal). >> >>The reason they built the insulator so thin was that if one accidentally >>open-circuited the CT without removing the wafer first, the high voltage >>developed by the CT would just 'punch thru' the wafer and safely short >>the CT. Then all you had to do to repair things was make sure you >>closed the circuit and replace the wafer-thin insulator blade. >> >>Was kind of surprised when I moved to commercial power systems that they >>didn't use something similar. Just has to have a breakdown voltage that >>is low enough to avoid damaging the CT. >> >>daestrom > >Lower-power CT, like residential-metering size, 100 amps or so, will >generally tolerate being unloaded. They will saturate and make two >not-too-huge voltage spikes per cycle and not get very warm. The nasty >part is that, once the burden is reconnected, they are very likely to >wind up magnetized, which will mess up low-current accuracy. > >John There is no CT in residential metering, the energy meter is connected directly. Even commercial / light industrial you do not see CTs in the meter circuit until 600 A, and before that you are typically at 480 V 3-phase (in the US).
From: John Larkin on 21 Nov 2009 10:42
On Sat, 21 Nov 2009 04:29:36 -0800, "JosephKK"<quiettechblue(a)yahoo.com> wrote: >On Fri, 20 Nov 2009 08:44:19 -0800, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Fri, 20 Nov 2009 10:53:45 -0500, daestrom <daestrom(a)twcny.rr.com> >>wrote: >> >>>Tzortzakakis Dimitrios wrote: >>>> ? "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> ?????? ??? >>>> ?????? news:dnsag51jfmub8nppj2kfj3nt07l3jv6djg(a)4ax.com... >>>>> 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 >>>> Anyway, current transformers must always be operated with the secondary >>>> shorted. In the generating facilities in Kozani, West Macedonia, where 400 >>>> kV current transformers were involved, the operators of the plant had a >>>> special indicator whether the secondary was shorted. >>>> >>> >>>Some old switchboard CT's I worked on in the Navy had very thin >>>insulator between two spring clips. Whenever we wanted to remove a >>>meter for cal, we slip the insulator out so the two clips would short >>>together, shorting the CT. Then we could open circuit the meter and >>>remove it from the panel. I don't remember exactly what the blade was >>>made of, but it's surface wasn't perfectly smooth like polished >>>material, more porous like unglazed ceramic (of course it wasn't any metal). >>> >>>The reason they built the insulator so thin was that if one accidentally >>>open-circuited the CT without removing the wafer first, the high voltage >>>developed by the CT would just 'punch thru' the wafer and safely short >>>the CT. Then all you had to do to repair things was make sure you >>>closed the circuit and replace the wafer-thin insulator blade. >>> >>>Was kind of surprised when I moved to commercial power systems that they >>>didn't use something similar. Just has to have a breakdown voltage that >>>is low enough to avoid damaging the CT. >>> >>>daestrom >> >>Lower-power CT, like residential-metering size, 100 amps or so, will >>generally tolerate being unloaded. They will saturate and make two >>not-too-huge voltage spikes per cycle and not get very warm. The nasty >>part is that, once the burden is reconnected, they are very likely to >>wind up magnetized, which will mess up low-current accuracy. >> >>John > >There is no CT in residential metering, the energy meter is connected >directly. Even commercial / light industrial you do not see CTs in >the meter circuit until 600 A, and before that you are typically at >480 V 3-phase (in the US). Of course there is. I delivered over 1800 electronic submetering units for the Battery Park City apartment complex, and, trust me, we used CTs. Lots of electronic meters use CTs. You should get out more. John |