2 Watt 1% Resistors - Page 1
Forum section: electronics / general

author: SgtRock
author_rank: Unknown
timestamp: August 17, 2011, 01:29:40 am
content: Greeting EEVbees:--I need 2 watt 1% resistors in the following values: 2.2 megohms, 3.3 megohms, 4.7 megohms, 6.8 megohms and 10 megohms. I am rebuilding a couple of resistance substitution boxes so they can be used in series and parallel.--Since my best meter is a Fluke 87 (which I love), I am unable to confirm these values to 1%, considering the+- and added counts of the F87 accuracy specifications. Until I am able to afford a Fluke 87, I shall be forced to rely on the resistors themselves for accuracy.--Your ideas and comments are humbly solicited.

author: EEVblog
author_rank: Unknown
timestamp: August 17, 2011, 01:50:43 am
content: The Fluke 87 has a 0.2% basic resistance spec.You'd need 2098V to max out a 2.2M 2W resistor. I doubt you'll ever get that high!And 741V to max out a 1/4W 2.2M resistor, so I think 1/4W resistors will do you just fine.Dave.

author: SgtRock
author_rank: Unknown
timestamp: August 17, 2011, 04:15:04 am
content: Dear Dave:--Thank you for your helpful and informative response. As I under stand it the Fluke 87V does indeed have a basic resistance spec. of 0.2%. I do not as yet have a Fluke 87V and am forced to rely on (as I said in my original post) a plain old Fluke 87. I meant to say until I can afford a Fluke 87V, sorry. Looking in the manual for the F87 I read that the accuracy in the 40 megohm range (which is used for 2.2 megohms to 10 megohms) is +- 1% plus 3 counts. So you can readily see it is pretty much useless to confirm these resistances to within 1%.--I readily agree with your Ohms Law calculation that I would need "2098V to max out a 2.2M 2W resistor". Emerson said that "A foolish consistency is the hobgoblin of little minds." It would appear I was being foolishly consistent, and that indeed, I need not use 2 watt resistors for 2.2 megohm and above. I can easily obtain the resistors from Digikey in 1/4 and 1/2 watts.--One of the reasons the res. sub. boxes I am rebuilding (Eico and Heathkit), used hefty 1 watt resistors may be mechanical. The main conductor bus is supported on the resistors. Thanks again for your gentle remonstration in pointing out the obvious. Best RegardsClear Ether

author: gregariz
author_rank: Frequent Contributor
timestamp: August 17, 2011, 05:14:19 am
content: Quote from: SgtRock on August 17, 2011, 04:15:04 amI had one of the Heathkit IN series years ago. The one I had came out in the early 60's. Its worth remembering that for anyone breadboarding with tubes in those days (or still these days), a common HT supply may well be 1500Volts. So the 1Watts made sense in those days... but as was mentioned for transistor work its not really needed.

author: EEVblog
author_rank: Unknown
timestamp: August 17, 2011, 05:21:42 am
content: Quote from: SgtRock on August 17, 2011, 04:15:04 amThen you can use a lower and more accurate range to measure it.If you can measure a lower value more accurately then you can put the DUT in parallel with the known resistor and calculate it's value.Dave.

author: amspire
author_rank: Super Contributor
timestamp: August 17, 2011, 06:00:57 am
content: Usually Fluke accuracy figures are very conservative, so there is every chance the 40M range is much better then 1%, but it is a good idea to check.So you want to work out if the 4M and 40M ranges are accurate enough or need to be fixed with a correction factor.Get ten 390K 1% resistors. First just check they all match within 1% - I don't care about the accuracy, I just want the maximum variation between all the resistors to be around the 1% mark.Put all the ten 390K resistors in parallel and measure on the 0.2% accurate 40K range. Multiply this value by 100 and record the result. So say you measured 38.95K. Write down 3.895M.Next put the ten resistors in series and measure on the 4M range. Write down the value - say it is 3.902M.So to get the 4M range to match the 0.2% accuracy of the 40K range, you will have to multiple any reading on the 4M range by 3.895/3.902.Measure the same 10 resistors in series on the 40M range and do the same correction.The correction factor should correct to better then 0.3% and that should be good enough for the job.The maths is not completely correct here for the parallel resistors, but it is an approximation and the error of the approximation is much less then 0.05% so forget it. There is no need to use the exact solution for calculating parallel resistors in this case.Richard

author: ejeffrey
author_rank: Super Contributor
timestamp: August 17, 2011, 07:59:55 am
content: Are you trying to select resistors from a lower tolerance class because 2W/1% resistors are not available in those sizes? If so, I suggest three things:With 1 megaohm resistors and above, the power rating is really a indicator of the body size. You need to look at the voltage rating as well. Often the maximum voltage rating will not be enough to reach the power rating. For instance a 200 V rated 1 Mohm resistor cannot safely handle more than 40 milliwatts regardless of power rating. Maybe you only need 1/4 watt resistor for the highest range.You can construct a higher power resistor from series and parallel combinations. If you get four .5 watt resistors of the desired value and combine them in series and parallel you will get a 2 watt resistors of the same value. If you are really desperate you can combine 9 1/4 watt resistors in a 3x3 array to get 2.25 watt.Your DMM may have large error on the megaohm range, but it is very likely repeatable. Measure a low power precision resistor immediately before measuring your higher power low tolerance resistor. Assume the precision resistor is actually spot on, and and that the measured difference is accurate. This will always be slightly worse than the resistance of the reference device, so with a 1% resistor you can maybe get 1.2% accuracy.If you do need to measure such high resistances accurately, the technique of most ohm meters is not the best. For common resistances we take advantage of the near infinite input impedance of voltmeters by injecting a known constant current and measuring the voltage drop. Since the volt meter has very high resistance, almost none of the current goes through the meter and we get an accurate measurement. If the input resistance of the DMM is known to be 10 Mohm, we can apply a correction factor, but that still stops being accurate once the DUT reaches the megaohm range. For such large values it is better to apply a known voltage and measure the current. (micro)-Ammeters are not generally as perfect low impedance devices compared to the high impedance of voltmeters, but compared to megaohms they are good enough.So, take a lab power supply and connect it to your DUT in series with a 10 kohm resistor that you have measured accurately. Apply ~30 VDC and measure it with a good voltmeter. You should measure the total voltage across both resistors. Do not measure the voltage drop over only the DUT, that will cause an error current to flow through your sense resistor! Now use a millivolt meter to measure the voltage drop across your 10 kohm shunt. V/I should get you to better than 0.5%, assuming your meters have .1% accuracy on the relevant ranges. To measure a 10 Mohm resistor like this you will need 10 microvolt resolution, a higher voltage supply, or to use a larger shunt resistor. Better yet, use an amplifier similar to Dave's ucurrent adapter, or a transimpedance amplifier to allow your DMM to operate on a more comfortable range.

author: SgtRock
author_rank: Unknown
timestamp: August 17, 2011, 03:25:47 pm
content: To All:--Thanks for the helpful and informative posts. This blog is like an information waterfall. Being a beginner, it will take me a little time to digest it all, but I think I understand most of it already. There is another blog (which shall remain nameless) where half of the posts are sarcastic and derisive. They get all worked up telling me what a phool I am for worrying about precision. I always say precision is like chicken soup, it couldn't hoit. You guys just offer your best advice. I very much appreciate it. Thanks AgainClear Ether

author: ejeffrey
author_rank: Super Contributor
timestamp: August 17, 2011, 04:56:16 pm
content: Precision is its own reward

author: SgtRock
author_rank: Unknown
timestamp: August 17, 2011, 10:35:06 pm
content: To All:--Flash! This just in. If I may reproduce a post from another forum (which shall remain nameless):"Oh, and here's another fly in your very fine ointment-- Some of those metal oxide resistors don't obey Ohm's Law!As it turns out, if you talk to a physicist, they'll whisper to you that Ohm's law is not really a "law", it's just that a whole lot of materials happen to pass current proportional to the voltage. But it's not a law of the Universe.But there are some materials with really loopy voltage/current curves. It's not just tetrodes that have kinks! Sometimes for power resistors they choose materials that are scofflaws, just because they're better at handling high power."--Who knew? First the Memristor and now the Loopristor. Boy, I sure hope they are not using those loopy resistors in aircraft and other mission critical applications.--Now do not all crowd in at once. There should be plenty for everybody.Clear Ether

author: amspire
author_rank: Super Contributor
timestamp: August 17, 2011, 11:37:37 pm
content: I think you have this the wrong way round.Ohms law is perfect because it is a rule we made up defining "resistance". That is why it is a law - it is a definition we created. It didn't arrive to earth inscribed on a meteor from the "universe", and if you went from earth out into the universe, I doubt if you would find anyone who has even heard of Ohm's Law.Now it is true that there is no resistor that has an exact constant resistance as defined by Ohm's Law - that is why we have datasheets.

author: IanB
author_rank: Super Contributor
timestamp: August 18, 2011, 03:08:54 am
content: Quote from: amspire on August 17, 2011, 11:37:37 pmIn other words there is no resistor that exactly obeys Ohm's law. I think you have just reinforced what SgtRock quoted. And it's true. Ohm's law is not an exact law, it is an idealization that approximates the behavior of many real materials.There is no sense in which Ohm's law is perfect other than in the mathematical sense that it is perfectly linear. While everyone in engineering loves linear models, the real world often begs to disagree...

author: amspire
author_rank: Super Contributor
timestamp: August 18, 2011, 04:04:50 am
content: But you don't understand what a Law is.It is a made up thing so it is perfect.Resistance is Volts divided by Amps because that is what the definition of Resistance is by Ohms Law and it is absolutely perfect in every sense. Without Ohm's Law, there is no such thing as Resistance. There is no way in which Ohm's Law has any error whatsoever.The fact that the concept of Resistance happens to be useful in the real world is the reason why Ohm's Law is very useful. If we lived on a different planet where none of the available materials had a resistance constant resistance with varying voltages, then Ohm's Law along with the whole concept of resistance would probably be useless.If a material doesn't follow Ohm's Law, it doesn't have the slightest effect on the Law itself. You are not going to try and tell me Ohm's Law is in error because a diode doesn't follow Ohms law, are you? So it doesn't matter if every material at some level does not follow Ohm's law - it just means that no material has a perfectly constant resistance.If materials have the resistance varying with voltage, then that is a property of the material, and using resistance defined by Ohm's Law, you can measure the errors, put them in a data sheet, and the world is in order again.So I am sorry. Ohm's Law is still perfect.If real materials are a bit crappy, blame Nature, but don't blame Georg Ohm.

author: SgtRock
author_rank: Unknown
timestamp: August 18, 2011, 04:38:08 am
content: Dear amspire:--With regard to your comment that space aliens have probably never even have heard of Ohms Law, you are no doubt correct. They probably have never heard of the Universal Law of Gravitation or the Laws of Thermodynamics either, since they probably do not speak English. If these aliens are technologically advanced, I am pretty sure they will have all of these laws, they will just call them something else, provided of course that they have matter, atoms, electrons, protons, etc. in their corner of the Universe. Please see the below article from the Journal of the American Physical Society which can be found on the web athttp://prd.aps.org/abstract/PRD/v18/i10/p3598_1."Phys. Rev. D 18, 3598–3604 (1978)Black-hole eddy currentsAbstractReferencesCiting Articles (35)Page ImagesDownload: PDF (473 kB) Buy this article Export: BibTeX or EndNote (RIS)Thibaut DamourE. R. 176 du CNRS. Groupe d'Astrophysique Relativiste, Observatoire de Paris, 92190 Meudon, FranceReceived 19 January 1978; published in the issue dated 15 November 1978We study dissipative test electromagnetic fields in a black-hole background. Quantities such as surface velocity, tangential electric field, normal magnetic induction, total surface current, and conduction surface current are introduced and are shown to satisfy Ohm's law with a surface resistivity of 4??377 ohms. Associated with these currents there exists a "Joule heating". These currents can exist when the black hole is inserted in an external electric circuit, but they can exist even in the absence of external currents. In particular, we study the eddy currents induced by the rotation of a black hole in an oblique uniform magnetic field, and we show how the computation of the ohmic losses allows a very simple derivation of the torque exerted on the hole.© 1978 The American Physical Society"--So if Black Holes 10 billion light years away are thought conform to Ohms Law, I think it is safe to say that it is a Universal Law, and that technologically advanced aliens would be aware of it, by one name or another. Unless one wants to quibble with all of Astronomy and Cosmology. The chief bedrock principle of these sciences is that the same physical laws obtain everywhere in the Universe. Thus if we see a yellow star in the Andromeda Galaxy 2.5 million light years from Earth we can be pretty sure it produces light by exactly the same processes as our own Sol.--Of course you are also correct that there are no perfect resistors, or caps, or coils etc. Ohms Law is as far as we know is a Universal Law. Even the Law of Gravitation can get quite fiddly. If you want to get down in the weeds with Gravity you will need several years of Tensor Calculus and 10th order equations, lots of them. I highly recommend any of the Carl Sagan books. Universal Physical Laws and Alien Civilizations were his favorite topics.--Thus to a first approximation I think we are safe in saying that "All resistors obey Ohms Law" This is hardly a surprising conclusion, as otherwise they would not be resistors, would they. They would be non-linear active devises like Varistors, Thermristors or Memristors. Best RegardsClear Ether

author: IanB
author_rank: Super Contributor
timestamp: August 18, 2011, 04:50:23 am
content: Quote from: amspire on August 18, 2011, 04:04:50 amI'm afraid that I do.QuoteBut that sentence is utterly meaningless.Everything you said following is completely off base and full of misunderstanding.You can't capitalize Laws and worship them as if they are some embodiment of Perfection in an imperfect universe. That's nothing to do with science, that's religion.A physical law is nothing more than a mathematical model that is found to describe the results of experiments in a useful way. Sometimes laws have a theoretical underpinning that is based on an idealized state of matter (like the ideal gas law for example), and these idealizations can be good foundations to model more complex behavior. Therefore while a "perfect" resistor can be envisaged and can be helpful when constructing models of real devices, the models themselves are never perfect.So like I said, Ohms law is not perfect and you can't worship it. That's not its job and Herr Professor Ohm would never have intended you to think that way.

author: amspire
author_rank: Super Contributor
timestamp: August 18, 2011, 05:06:00 am
content: Ohms Law is perfect because it is a definition. Is is exactly as defined and it is nothing more or less.No matter what the real world does, Resistance equals Voltage divided by Current. If the real world behaves differently that is fine, but Ohm's Law still equals Voltage divided by Current to as many decimal places as you like.If at some level, the real world diverges from Ohm's Law, that has no effect on Ohm's Law. It just makes Ohm's Law a bit less useful.

author: IanB
author_rank: Super Contributor
timestamp: August 18, 2011, 05:31:38 am
content: Head. Wall. Bang.

author: Unknown
author_rank: Unknown
timestamp: August 18, 2011, 06:02:20 am
content: Okay I'm with you fellersBut seriously, arguing about Ohms law in terms of being "perfect" ? Wtf is this "perfect" thing anyhow?

author: SgtRock
author_rank: Unknown
timestamp: August 18, 2011, 06:49:30 am
content: To IanB:--Das ist richtig, mein Freund. Any law of science is nothing more than a useful abstraction, unless we are part of a greater Platonic Universe, where ideas have more than a virtual existence. And that would be very hard indeed to prove. Although the correspondences between Mathematics and Physics can give one a feeling somewhat akin to deja vu. For instance, two hundred years ago Leonard Euler developed a series of equations that describe the "at present putative" Gravitons very closely. Spooky perhaps, but not what one would call rigorously empirical.--Ohm's law is a logical statement, a syllogism, and as such may be considered perfectly formulated, but that really does not mean very much in the Physical Universe which can never even be completely described, let alone be perfect. So, in one way of thinking Ohm's Law is a Postulate, something which is assumed to be true, but which can never be proven, as argued by Kurt Godel (mit dem Umlauten), when he demolished Bertrand Russell and Alfred North Whitehead, and the Principia Mathematica.--In New Zealand a student one asked Richard Feynman "When we see a thing, are we really seeing the thing, or are we only seeing the light from the thing". Feynman answered "People who believe the latter soon starve to death"Mit den besten WünschenKlare Ether

author: Zero999
author_rank: Super Contributor
timestamp: August 18, 2011, 08:34:41 pm
content: I remember the Ohm's law vs non-Ohmic loads debate on another forum, the thread was hundreds of posts long!In reality Ohm's law is perfect, it's just some loads change their resistance in response to the current/voltage, for example a diode may have a resistance of 60R when the current is 10mA but 1R when the current is 1A.You could argue that a constant current load is an example which violates Ohm's law but does it? In reality it doesn't, it's resistance nearly changes with the voltage so the current stays the nearly the same. This is normally done with an error amplifier, a good analogy is someone opening and closing a tap (adjusting the resistance) to keep the flow of water constant in spite of changes in the mains pressure.

author: SgtRock
author_rank: Unknown
timestamp: August 19, 2011, 12:27:01 am
content: Dear Hero999:--I agree. Apparently some of our forum siblings did not have the benefit or High School or College Physics. Or if they did, they must have fallen asleep in class. Nowhere in the texts does it say:"Now Ohm's Law is different from all the other laws in this book. I Sometimes it is off just a little bit. It is weird that way."--Next they will be saying that the Law of Freely Falling Bodies is not perfect because meteors and gravity bombs show longer arrival times due of the effects of the atmosphere. Or, that sometimes a Volt is not really a Volt. Arrrg.Clear Ether

author: dumpystig
author_rank: Contributor
timestamp: August 30, 2011, 02:24:17 pm
content: Hmm... some odd posts/rants.Ohms Law works for me, has done for years

author: Zad
author_rank: Super Contributor
timestamp: August 30, 2011, 03:32:07 pm
content: Guess what. Ohmic conductors obey Ohm's law. Non-Ohmic conductor's don't. Isn't that a shocker?Is the particular abstract model you are using good enough for the purpose for which you are using it? Yes? Good, get on with it. We are engineers, good enough is good enough. If you want to discuss Grand Unified Theory and the ultimate detailed description of physical properties, I might suggest a physics forum.

author: SgtRock
author_rank: Unknown
timestamp: August 31, 2011, 04:27:16 am
content: Dear Zad:--You stated:"Is the particular abstract model you are using good enough for the purpose for which you are using it? Yes? Good, get on with it. We are engineers, good enough is good enough. If you want to discuss Grand Unified Theory and the ultimate detailed description of physical properties...."--You might note, Sir, that this forum is not limited to Engineers. That is why it has a "General Chat" and a "Beginners" sub forums. The reason I encouraged a discussion of Ohm's Law was to try to learn from the members of this forum. My defense of Ohm's Law, by mentioning the Laws of Physics, was to point out that the Physics texts do not consider it a lesser law.--You also stated:"If you want to discuss Grand Unified Theory and the ultimate detailed description of physical properties, I might suggest a physics forum."--No one mentioned the "Grand Unified Theory" that I can see. Some people might find some of the discussions in this thread interesting. Please note that the descriptions of topics which are appropriate for the "General Chat" are:"Anything goes. This is where everyone hangs out and rants and chats about whatever doesn't fit into the other categories on this forum."--Therefore the discussion in this forum is every bit as appropriate as, for instance, your discussions of Australian Beer, and maybe even more closely related to Electronic Engineering.--When I read the threads on this forum, I read the ones that interest me, and I do not read the those I do not find interesting. I am well aware that the forum is not "my property", therefore I do not recommend that people who post articles that do not interest me take their business elsewhere. Best RegardsClear Ether

author: IanB
author_rank: Super Contributor
timestamp: August 31, 2011, 05:06:03 am
content: Quote from: Zad on August 30, 2011, 03:32:07 pmWell said.Although, while it is of course possible to discuss GUTs and their repercussions here, the resulting discussions may not be very enlightening...

author: SgtRock
author_rank: Unknown
timestamp: August 31, 2011, 05:46:42 am
content: Dear IanB:--You stated:"Although, while it is of course possible to discuss GUTs and their repercussions here, the resulting discussions may not be very enlightening..."--Two Points:1) Where in this thread is there a discussion of GUT, or TOE?2) If there were to be such a discussion, indeed it may not be very enlightening. But that would depend on the quality of the information imparted, would it not. Best RegardsClear Ether