Ohm's Law Problem
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From: life imitates life on 15 Feb 2010 15:24 On Mon, 15 Feb 2010 10:49:56 -0500, Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> wrote: >On Mon, 15 Feb 2010 07:22:37 -0800, the renowned life imitates life ><pasticcio(a)thebarattheendoftheuniverse.org> wrote: > >>On Mon, 15 Feb 2010 09:46:23 -0500, Spehro Pefhany >><speffSNIP(a)interlogDOTyou.knowwhat> wrote: >> >>>On Mon, 15 Feb 2010 06:35:19 -0500, the renowned Rich Webb >>><bbew.ar(a)mapson.nozirev.ten> wrote: >>> >>>>On Mon, 15 Feb 2010 00:48:46 -0500, Spehro Pefhany >>>><speffSNIP(a)interlogDOTyou.knowwhat> wrote: >>>> >>>>>On Sun, 14 Feb 2010 16:12:04 -0800 (PST), the renowned Bill Bowden >>>>><wrongaddress(a)att.net> wrote: >>>>> >>>>>>Ohm's Law Problem: >>>>>> >>>>>> Find the voltage at the 2 junctions of a 3 element voltage >>>>>> divider across a supply voltage of 8.4 volts. The two >>>>>> junctions of the divider both supply external current of 5mA. >>>>>> >>>>>> V1 +8.4 >>>>>> | >>>>>> R1 = 240 >>>>>> | >>>>>> V2 .---------> 5 mA >>>>>> | >>>>>> R2 = 570 >>>>>> | >>>>>> V3 .---------> 5 mA >>>>>> | >>>>>> R3 = 100 >>>>>> | >>>>>> | >>>>>> GND >>>>> >>>>> >>>>> >>>>>(V1-V2)/R1 = 5mA + (V2-V3/R2 >>>>>(V2-V3)/R2 = 5mA + V3/R3 >>>>> >>>>>V1 is known (8.40V), so this is 2 equations in two unknowns and easily >>>>>solved for V2 = 5.169230769 V, V3=0.346153846 V. >>>> >>>>The Curse of the Calculator strikes again! >>>> >>>>I3 = 3.5 mA, V3 = 0.35 V, and V2 = 5.2 V. Hard to justify more than two >>>>significant figures here. >>> >>>Hey there, we wouldn't want some question of the correct value of the >>>NINETH digit on the 'ol Agilent 3458A, now would we.. ;-) >>> >>> >>> >>>Best regards, >>>Spehro Pefhany >> >> >> Nineth? >> >> Doth thee haveth a nineth digit? > >Well, eight and a half, so not directly applicable in this particular >case, but it's not totally ridiculous calculating DC voltages to 10 or >11 places when such tools are at hand. Certainly to 7 places. > > >Best regards, >Spehro Pefhany The word is Ninth. And no, that meter doesn't have that many either.
From: Jon Kirwan on 15 Feb 2010 15:36 On Mon, 15 Feb 2010 11:43:42 -0800, "Joel Koltner" <zapwireDASHgroups(a)yahoo.com> wrote: >"Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message >news:pe7jn5pnjc9sq679eck53hsd2kbbhjned1(a)4ax.com... >[infinite resistor problem] >> It provides a very >> broadly applicable method that applies across many fields and >> provides a useful thinking tool that will serve well no >> matter where you find yourself. > >Yabbut... few practicing engineers encounter the inifnite resistor problem in >real-world problems, whereas the "three resistors with current draws" is >something much closer to a real-world problem and, in fact, it used to be a >100% real-world problem back when tube radios would have a long, multi-tapped >wirewound resistor that supplied different voltages to different parts of the >circuit (...long before just giving everything its own regulated rail was >viable). I wasn't in any way suggesting that it is a "real world problem." If you imagine I were, you missed my point. I merely used that example because it happened to be true and provides emphasis. The actual point was elsewhere. It helped to make the point, but wasn't the point. >> None of this takes away from what you are saying, either. I >> just don't think you should belittle such a powerful tool. > >Oh, I think SPICE is great; Now, I know you missed the point. The method illustrated _just happens_ to be what Spice does. In no way is that _why_ I suggested the viewpoint. Again, Spice is providing an example to emphasize the underlying point. It helps to make the case, it in no way _is_ the case. >I expect that in a given day for, e.g., Jim, well >over 90% of it is spent in SPICE with less than 10% spent doing algebra >long-hand to try to analyze or design circuits. I'm gradually becoming more sorry I even brought it up. This is way off the point I was making. >So I certainly wouldn't >suggest colleges drop coverage of the standard node voltage/loop current >matrix approach that's been taught for many decades, rather I'd add emphasis >on not-so-commonly-taught-but-useful concepts such as inverted poles and >zeroes that are quite helpful when resorting to long-hand algebra. (And I'd >find the time to teach this in that these days in a circuirt analysis class >you're not going to have to sit there and eliminate variables in a set of >simultaneous equations one by one when any decent calculator today will solve >for all the unknowns in well under a second.) Oh, well. My reason for even bothering to add more to this is _only_ because of the way you are writing about it, that seems to belittle the idea (and now appears to completely miss the point.) I don't think what others wrote is wrong, or bad, or inappropriate. I only think that there is an underlying concept here that is important and shouldn't be down played. Spice's approach to solving these problems wasn't an accident and it comes from this 'much wider' view that applies to a LOT more than electronics. It is appropriate to almost any system of diff eq with boundary conditions and that is a very wide field, indeed. But in no way am I arguing against practicality. Don't mistake me on that. In some ways, this is a continuing conflict in all of the human condition. Many people just want to know what works, right now, here, in this place. And have no interest in going beyond what is right in front of them. That is a very practical stand to take. But It takes nothing away from that practical perspective to also admit that going beyond what is directly in front of you, taking a longer view, is also of some value, too. I'll stop. I just wasn't prepared to go create a white paper or exhaustive survey on some broad topic. It's just something that has proven useful to me. If others don't see that point, I'm fine accepting it and simply moving on. My apologies. Jon
From: Joel Koltner on 15 Feb 2010 16:16 Hi Jon, My apologies for missing much of your point... "Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message news:bfajn5tapei1s9ijae10mg96kl757lpvqc(a)4ax.com... > My reason for even bothering to add more to this is _only_ > because of the way you are writing about it, that seems to > belittle the idea (and now appears to completely miss the > point.) Ah. Well, I was attempting to suggest that if one encountered such a problem in the course of daily events, the most pragmatic approach is probably not the one you used, in my opinion... but there's certainly nothing wrong with it and I recognize there's plenty of value in it in general. > In some ways, this is a continuing conflict in all of the > human condition. Many people just want to know what works, > right now, here, in this place. And have no interest in > going beyond what is right in front of them. That is a very > practical stand to take. But It takes nothing away from that > practical perspective to also admit that going beyond what is > directly in front of you, taking a longer view, is also of > some value, too. Yeah, I agree and very much recognize that conflict in that it shows up prominently when schools are trying to figure out what to teach -- particularly as technology itself advances so out of necessity some techniques have to be dropped to make up for other, newer techniques that are more in demand in industry. Often the argument does boil down to the balance between educating people for what's in demand today vs. giving them enough background they're be able to understand and create new contributions building upon what's already known. Personally I believe that too many people do lean too heavily towards just "what's needed for today" without enough emphasis on "the longer view," as you say. ---Joel
From: Joerg on 15 Feb 2010 16:29 Joel Koltner wrote: > "Jim Thompson" <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote in > message news:11sin51vdubl64ouhcgn3b2omf36q111in(a)4ax.com... >> Everyone is making it too difficult. Just write it down in sequence, >> the answer falls right out... >> http://analog-innovations.com/SED/OhmsLaw_SED_JustWriteItDown.pdf > Ohm's law? I thought that had been repealed ... :-) > That's the same as what I ended up doing (in the later part of my post), > except that you use "V3/100" rather than just "I" for the current in the > lowest resistor. :-) > >> At MIT, I was spared (*) from Guillemin's obtuseness, I had Harry B. >> Lee for passive circuit analysis ;-) > > I had a guy who was a pretty talented teacher (he'd won a couple awards > for it, and I found him quite understandable), although he had little if > any real-world design experience. > > I'd read some of Guillemin's book, and while I think the guy was pretty > darned sharp, I disagree with his notion that you have to have an > incredibly thorough understanding of network analysis down pat before > you can get useful circuit design or analysis done. ... Sadly, that's the kind of notion that drives potential EE candidates away, at least from analog. And now we have a serious shortage of those. They think they have to be a rocket scientists to be able to thrive and make money in analog. Which is wrong. I learned the majority of my skills by "winging it". IOW I built RF stuff before I knew squat about any of that. And it actually worked, some still does. Note to potential candidates: If an author or professor says something like what must have been stipulated in this book, that you must be a top notch network analyst, do not listen. It's much more important to experiment, experiment, experiment, get a "feel" for what works, _then_ dive into the theory. Not the other way around. Just my 2 cents. Whew. Now I feel better ... -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Spehro Pefhany on 15 Feb 2010 16:38
On Mon, 15 Feb 2010 12:24:59 -0800, the renowned life imitates life <pasticcio(a)thebarattheendoftheuniverse.org> wrote: >On Mon, 15 Feb 2010 10:49:56 -0500, Spehro Pefhany ><speffSNIP(a)interlogDOTyou.knowwhat> wrote: > >>On Mon, 15 Feb 2010 07:22:37 -0800, the renowned life imitates life >><pasticcio(a)thebarattheendoftheuniverse.org> wrote: >> >>>On Mon, 15 Feb 2010 09:46:23 -0500, Spehro Pefhany >>><speffSNIP(a)interlogDOTyou.knowwhat> wrote: >>> >>>>On Mon, 15 Feb 2010 06:35:19 -0500, the renowned Rich Webb >>>><bbew.ar(a)mapson.nozirev.ten> wrote: >>>> >>>>>On Mon, 15 Feb 2010 00:48:46 -0500, Spehro Pefhany >>>>><speffSNIP(a)interlogDOTyou.knowwhat> wrote: >>>>> >>>>>>On Sun, 14 Feb 2010 16:12:04 -0800 (PST), the renowned Bill Bowden >>>>>><wrongaddress(a)att.net> wrote: >>>>>> >>>>>>>Ohm's Law Problem: >>>>>>> >>>>>>> Find the voltage at the 2 junctions of a 3 element voltage >>>>>>> divider across a supply voltage of 8.4 volts. The two >>>>>>> junctions of the divider both supply external current of 5mA. >>>>>>> >>>>>>> V1 +8.4 >>>>>>> | >>>>>>> R1 = 240 >>>>>>> | >>>>>>> V2 .---------> 5 mA >>>>>>> | >>>>>>> R2 = 570 >>>>>>> | >>>>>>> V3 .---------> 5 mA >>>>>>> | >>>>>>> R3 = 100 >>>>>>> | >>>>>>> | >>>>>>> GND >>>>>> >>>>>> >>>>>> >>>>>>(V1-V2)/R1 = 5mA + (V2-V3/R2 >>>>>>(V2-V3)/R2 = 5mA + V3/R3 >>>>>> >>>>>>V1 is known (8.40V), so this is 2 equations in two unknowns and easily >>>>>>solved for V2 = 5.169230769 V, V3=0.346153846 V. >>>>> >>>>>The Curse of the Calculator strikes again! >>>>> >>>>>I3 = 3.5 mA, V3 = 0.35 V, and V2 = 5.2 V. Hard to justify more than two >>>>>significant figures here. >>>> >>>>Hey there, we wouldn't want some question of the correct value of the >>>>NINETH digit on the 'ol Agilent 3458A, now would we.. ;-) >>>> >>>> >>>> >>>>Best regards, >>>>Spehro Pefhany >>> >>> >>> Nineth? >>> >>> Doth thee haveth a nineth digit? >> >>Well, eight and a half, so not directly applicable in this particular >>case, but it's not totally ridiculous calculating DC voltages to 10 or >>11 places when such tools are at hand. Certainly to 7 places. >> >> >>Best regards, >>Spehro Pefhany > > > The word is Ninth. And no, that meter doesn't have that many either. http://www.eefocus.com/data/myspace/3/17997//blog/2a65afe9.jpg 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 |