Shunt ? (was Re: Ideas for inexpensive True-RMS Metering?)
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From: langwadt on 7 Jul 2010 12:56 On 7 Jul., 09:20, Fred Bartoli <myname_with_a_dot_inbetw...(a)free.fr> wrote: > John Larkin a crit : > > On Tue, 6 Jul 2010 19:06:37 -0500, "Tim Williams" > <tmoran...(a)charter.net> wrote: > > > > >> "whit3rd" <whit...(a)gmail.com> wrote in messagenews:c6d94b54-aa3a-4eb1-8810-4417e6f366cd(a)32g2000vbi.googlegroups.com... > >>> The saturation of the core makes the coupling of primary and > >>> secondary go away; only the primary side gets the fault current, > >>> the secondary current is no longer proportional (unless you choose > >>> a really odd winding scheme that stays highly flux-coupled when > >>> the core is removed). > >> Yes, which is why the energy delivered is proportional to flux as > well. It only delivers power until the transformer stops transformering. > >> > >> Say you get 1A fault current from a CT (i.e., secondary referred), > which throws your circuit into overload, so the voltage on the windings > jumps to 5V (clamped by a perfect 5V supply, assuming ideal clamp > diodes). The delivered power is evidently 1A * 5V = 5W, going into your > supply. If the winding has a saturation flux of 1mWb, this fault > current will flow for 1mWb / 5V = 0.2ms. The energy is 5W * 0.2ms = > 1mJ, or 1A * 1mWb. > >> > >> Increase the fault to 10A. The winding is clamped at 5V, so 50W is > delivered, and the fault again lasts for 0.2ms, because the flux is > 1mWb. The energy is 10mJ. > >> > >> Increase the fault to 1kA. Now you get 5kW and 1J, beyond the > capacity of a 1.5KE6. > >> > >> Strike it with a bolt of lightning. 100kA gives 500kW peak and > 100J, assuming the transformer doesn't fail first; if it has an internal > resistance of 0.01 ohm, it will drop 1kV, probably breaking down the > meager insulation in a CT. > >> > >> Tim > > > > The usual practise in electronic metering is to have the CT secondary > > drive a low-resistance wirewound or manganin strip shunt. The shunt > > resistance is considerably less than the winding resistance. > > Outrageous CT overloads don't damage the shunt... most of the power > > dissipation is in the winding. The signal conditioning opamps or > > whatever are protected by high value resistances between them and the > > shunt. > > > > I'm designing an energy metering ammeter and am looking after a > 0.1/0.2R, preferably SMT, shunt. > > On the average all of them will sum up to half a billion euro energy, so > it has to be accurate :-) > > It'll work in some harsh environment and must : > * work @ 85 C Tamb, (100 C PCB temp) > * be low tempco (preferably lower than 20ppm/K) > * real low aging for less than yearly calibration > * preferably high initial accuracy to hopefully bypass one calibration step > > None of the usual suspects fit the bill. > > Any manufacturer / part series to suggest? > > -- > Thanks, > Fred. > > -- > Thanks, > Fred. lots of stuff in this: http://www.vishay.com/docs/49789/vmn-pl03.pdf -Lasse
From: Copacetic on 7 Jul 2010 13:34 On Wed, 7 Jul 2010 10:25:23 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> wrote: > >The whole meter has to have a low tempco; making each >component so is not the easiest way to do it. Hopefully, this is where one would keep the environment which the meter sees, and that which may be proximal to the shunt, separated.
From: John Larkin on 7 Jul 2010 14:40 On Wed, 7 Jul 2010 10:25:23 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> wrote: >On Jul 7, 12:20�am, Fred Bartoli <myname_with_a_dot_inbetw...(a)free.fr> >wrote: > >> I'm designing an energy metering ammeter and am looking after a >> 0.1/0.2R, preferably SMT, shunt. > >That's odd. Why would you want to do the awkward transition >of a high current cable to a printed wiring board, then do it >again to get off the printed wiring board? >Instead of surface mount, I'd think in terms of welding/soldering the >shunt to (cable clamps/connectors). > >> * work @ 85 C Tamb, (100 C PCB temp) >> * be low tempco (preferably lower than 20ppm/K) > >The whole meter has to have a low tempco; making each >component so is not the easiest way to do it. Shunts usually self-heat, which adds a nonlinearity to the power measurement. So it's worth getting ones with a low TC. Manganin has a parabolic TC that can be made to peak at room temp or about at 50C, depending on the alloy. It will have a TC of, say, +-5 PPM/K within 10-15 K of the peak. I have a bunch of data. John
From: Winfield Hill on 7 Jul 2010 14:54 John Larkin wrote... > > Shunts usually self-heat, which adds a nonlinearity to the power > measurement. So it's worth getting ones with a low TC. > > Manganin has a parabolic TC that can be made to peak at room temp > or about at 50C, depending on the alloy. It will have a TC of, say, > +-5 PPM/K within 10-15 K of the peak. I have a bunch of data. Hah, I'd love to see some of that data. Hey, John, check your email. :-) -- Thanks, - Win
From: John Larkin on 7 Jul 2010 16:27
On 7 Jul 2010 11:54:12 -0700, Winfield Hill <Winfield_member(a)newsguy.com> wrote: >John Larkin wrote... >> >> Shunts usually self-heat, which adds a nonlinearity to the power >> measurement. So it's worth getting ones with a low TC. >> >> Manganin has a parabolic TC that can be made to peak at room temp >> or about at 50C, depending on the alloy. It will have a TC of, say, >> +-5 PPM/K within 10-15 K of the peak. I have a bunch of data. > > Hah, I'd love to see some of that data. ftp://jjlarkin.lmi.net/Manganin.zip We have developed some really weird shapes for planar shunts that can be epoxied to anodized heat sinks and have essentially no hum or eddy current problems. We punch or photoetch them from sheet manganin. A lot of power resistors and shunts have hum and eddy-current problems. Some of the Vishay heat-sunk foil resistors have vicious step responses. > > Hey, John, check your email. :-) OK, got it. John |