Shunt ? (was Re: Ideas for inexpensive True-RMS Metering?)
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From: John Larkin on 9 Jul 2010 17:25 On Fri, 9 Jul 2010 11:16:28 -0700, dplatt(a)radagast.org (Dave Platt) wrote: >In article <lkfe36hr1od5sgs7npfig7tbs9894ir2ql(a)4ax.com>, >John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>We glue the planar shunts to anodized aluminum blocks with thermal >>epoxy. A surface-mount pcb (amps, dacs) gets stuck to the block too, >>and a Minco or similar heater, and we stabilize the whole mess at 60C. >>It doesn't appear to me that the TCE differential between the block >>and the manganin is affecting the overall TC much. >> >>The big problem turns out to be the eddy currents induced into the >>block, messing up the transient response of the shunt. > >Could you avoid that by using a ceramic block of some sort (perhaps >natural marble) instead of aluminum? The thermal conductivity would >be somewhat lower, but might be sufficient? Yes, the eddy effects would go away but the thermal conductivity would be much worse. Self-heating changes shunt resistance and can cause transient thermocouple effects. Aluminum nitride is really tempting. It's an insulating ceramic that's almost as good as aluminum thermally. Solid diamond would be even better. John
From: krw on 9 Jul 2010 18:44 On Fri, 09 Jul 2010 11:16:39 -0400, Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> wrote: >On Thu, 08 Jul 2010 12:12:35 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Thu, 8 Jul 2010 11:29:10 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> >>wrote: >> >>>On Jul 8, 9:24�am, AM <thisthatandtheot...(a)beherenow.org> wrote: >>>> On Thu, 08 Jul 2010 08:56:21 -0700, John Larkin >>> >>>> >> �That's what lasers are for. �Worst case, send them out to be custom >>>> >>trimmed. >>> >>>> >We used to make planar manganin shunts using dies and a punch press, >>>> >but that was expensive. We tried CO2 laser machining, but the laser >>>> >mostly bounced off (manganin is mostly copper, an excellent IR >>>> >reflector) and the edges were awful anyhow. Lately we have them >>>> >photo-etched, which works beautifully. ... >>>> >We don't trim our shunts, we just trim gain electronically downstream. >>> >>>> �Get a micro cleaver setup like the chemical/bio labs use to cleave off >>>> pieces of frozen sample media for analysis. >>> >>>I've seen shunts that were trimmed by holding 'em up to a grinding >>>wheel. >>>The problem is, mechanical stress raises the resistance (until heat or >>>age >>>anneals the metal). Chemical etching leaves a raw-metal surface, >>>which >>>will in time oxidize (changing the resistance). Laser trimming is >>>fine for SMALL resistors, removing serious metal from a 200A >>>shunt is a frightening prospect. >>> >>>Make the shunt, anneal and passivate the surface, then use some >>>other gain trim downstream. >> >>We do anneal our shunts. Then dip the relevant parts in a little >>dilute HCl to clean them up for soldering. >> >>John > >I find it amusing that manganin is also used to make pressure >sensors... a nice linear piezoresistive response, and you can get a >percent or three change in resistance with (large) pressure. I imagine >you must have to consider the mounting method carefully to get >ppm-level stability, considering the heat generated and connections >required. Wouldn't an identical reference slab not under pressure, mounted (but isolated) from the test slab be the way to go? The stuff is pretty cheap so duplicating it wouldn't be costly.
From: Spehro Pefhany on 9 Jul 2010 23:34 On Fri, 09 Jul 2010 17:44:46 -0500, the renowned "krw(a)att.bizzzzzzzzzzzz" <krw(a)att.bizzzzzzzzzzzz> wrote: >On Fri, 09 Jul 2010 11:16:39 -0400, Spehro Pefhany ><speffSNIP(a)interlogDOTyou.knowwhat> wrote: > >>On Thu, 08 Jul 2010 12:12:35 -0700, John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>On Thu, 8 Jul 2010 11:29:10 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> >>>wrote: >>> >>>>On Jul 8, 9:24�am, AM <thisthatandtheot...(a)beherenow.org> wrote: >>>>> On Thu, 08 Jul 2010 08:56:21 -0700, John Larkin >>>> >>>>> >> �That's what lasers are for. �Worst case, send them out to be custom >>>>> >>trimmed. >>>> >>>>> >We used to make planar manganin shunts using dies and a punch press, >>>>> >but that was expensive. We tried CO2 laser machining, but the laser >>>>> >mostly bounced off (manganin is mostly copper, an excellent IR >>>>> >reflector) and the edges were awful anyhow. Lately we have them >>>>> >photo-etched, which works beautifully. ... >>>>> >We don't trim our shunts, we just trim gain electronically downstream. >>>> >>>>> �Get a micro cleaver setup like the chemical/bio labs use to cleave off >>>>> pieces of frozen sample media for analysis. >>>> >>>>I've seen shunts that were trimmed by holding 'em up to a grinding >>>>wheel. >>>>The problem is, mechanical stress raises the resistance (until heat or >>>>age >>>>anneals the metal). Chemical etching leaves a raw-metal surface, >>>>which >>>>will in time oxidize (changing the resistance). Laser trimming is >>>>fine for SMALL resistors, removing serious metal from a 200A >>>>shunt is a frightening prospect. >>>> >>>>Make the shunt, anneal and passivate the surface, then use some >>>>other gain trim downstream. >>> >>>We do anneal our shunts. Then dip the relevant parts in a little >>>dilute HCl to clean them up for soldering. >>> >>>John >> >>I find it amusing that manganin is also used to make pressure >>sensors... a nice linear piezoresistive response, and you can get a >>percent or three change in resistance with (large) pressure. I imagine >>you must have to consider the mounting method carefully to get >>ppm-level stability, considering the heat generated and connections >>required. > >Wouldn't an identical reference slab not under pressure, mounted (but >isolated) from the test slab be the way to go? The stuff is pretty cheap so >duplicating it wouldn't be costly. What would be the purpose of the reference slab? Manganin doesn't have much in the way of tempco- I guess you could partially cancel out what there is, particularly if you made it from the same batch of alloy. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff(a)interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
From: krw on 9 Jul 2010 23:49
On Fri, 09 Jul 2010 23:34:37 -0400, Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> wrote: >On Fri, 09 Jul 2010 17:44:46 -0500, the renowned >"krw(a)att.bizzzzzzzzzzzz" <krw(a)att.bizzzzzzzzzzzz> wrote: > >>On Fri, 09 Jul 2010 11:16:39 -0400, Spehro Pefhany >><speffSNIP(a)interlogDOTyou.knowwhat> wrote: >> >>>On Thu, 08 Jul 2010 12:12:35 -0700, John Larkin >>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>> >>>>On Thu, 8 Jul 2010 11:29:10 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> >>>>wrote: >>>> >>>>>On Jul 8, 9:24�am, AM <thisthatandtheot...(a)beherenow.org> wrote: >>>>>> On Thu, 08 Jul 2010 08:56:21 -0700, John Larkin >>>>> >>>>>> >> �That's what lasers are for. �Worst case, send them out to be custom >>>>>> >>trimmed. >>>>> >>>>>> >We used to make planar manganin shunts using dies and a punch press, >>>>>> >but that was expensive. We tried CO2 laser machining, but the laser >>>>>> >mostly bounced off (manganin is mostly copper, an excellent IR >>>>>> >reflector) and the edges were awful anyhow. Lately we have them >>>>>> >photo-etched, which works beautifully. ... >>>>>> >We don't trim our shunts, we just trim gain electronically downstream. >>>>> >>>>>> �Get a micro cleaver setup like the chemical/bio labs use to cleave off >>>>>> pieces of frozen sample media for analysis. >>>>> >>>>>I've seen shunts that were trimmed by holding 'em up to a grinding >>>>>wheel. >>>>>The problem is, mechanical stress raises the resistance (until heat or >>>>>age >>>>>anneals the metal). Chemical etching leaves a raw-metal surface, >>>>>which >>>>>will in time oxidize (changing the resistance). Laser trimming is >>>>>fine for SMALL resistors, removing serious metal from a 200A >>>>>shunt is a frightening prospect. >>>>> >>>>>Make the shunt, anneal and passivate the surface, then use some >>>>>other gain trim downstream. >>>> >>>>We do anneal our shunts. Then dip the relevant parts in a little >>>>dilute HCl to clean them up for soldering. >>>> >>>>John >>> >>>I find it amusing that manganin is also used to make pressure >>>sensors... a nice linear piezoresistive response, and you can get a >>>percent or three change in resistance with (large) pressure. I imagine >>>you must have to consider the mounting method carefully to get >>>ppm-level stability, considering the heat generated and connections >>>required. >> >>Wouldn't an identical reference slab not under pressure, mounted (but >>isolated) from the test slab be the way to go? The stuff is pretty cheap so >>duplicating it wouldn't be costly. > >What would be the purpose of the reference slab? Manganin doesn't have >much in the way of tempco- I guess you could partially cancel out what >there is, particularly if you made it from the same batch of alloy. Yes, made at the same time with the same geometry (having designed chips one starts to think this way ;). Yeah, I was thinking about thermocouples. Nevermind. |