Reduced resistance of resistor - Page 1
Forum section: electronics / general

author: leftek
author_rank: Regular Contributor
timestamp: April 11, 2020, 04:08:13 pm
content: Hello, from one old board a have put out the resistor 56R 9W, that measured with DMM 1ohm.Can be one resistor have reduced resistance as fault?

author: floobydust
author_rank: Super Contributor
timestamp: April 11, 2020, 04:19:27 pm
content: Wirewound power resistors are infallible, and cannot go lower value.Thick film (i.e. TO-220) power resistors do go lower value if subjected to small overloads.The part you have might be something else, like 0.56R 9W or a fuse ?

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 11, 2020, 05:02:26 pm
content: FWIW, here's a very brief paper from the US Navy Sea Systems Command talking about resistor failure modes. They claim that around 9% of wirewounds can fail in short circuit mode, and 26% with "parameter change". Which I assume includes a turn-to-turn or other issue causing lowered resistance?https://www.navsea.navy.mil/Portals/103/Documents/NSWC_Crane/SD-18/ResistorsFailure.pdf

author: floobydust
author_rank: Super Contributor
timestamp: April 11, 2020, 08:27:59 pm
content: Just bizarre and a crappy report - I can only see wirewound plastic resistors deforming leaving shorted turns, but NEVER for porcelain parts. The report doesn't give any details on construction or methods.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 11, 2020, 08:32:23 pm
content: Quote from: floobydust on April 11, 2020, 08:27:59 pmAnd neither do you...

author: TimFox
author_rank: Super Contributor
timestamp: April 11, 2020, 09:13:06 pm
content: Quote from: floobydust on April 11, 2020, 04:19:27 pmA typical cheap DMM will show a fraction of an ohm with the probe tips touching, which could show roughly 1 ohm on a nominal 0.56 ohm (5%?) resistor.

author: floobydust
author_rank: Super Contributor
timestamp: April 11, 2020, 09:15:34 pm
content: Quote from: engrguy42 on April 11, 2020, 08:32:23 pmNeither did OP, but obviously 9W is ceramic league, not plastic.

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 11, 2020, 09:24:48 pm
content: Quote from: engrguy42 on April 11, 2020, 05:02:26 pmWell, you certainly didn’treadthe report before quoting a that figure from the table.QuoteAgree with Flooby, it’s a crap report.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 11, 2020, 09:26:36 pm
content: Quote from: floobydust on April 11, 2020, 09:15:34 pmDid you see the references in the document? Various resistor manufacturers' data, NASA, various MIL specs, and so on.And they say "Fixed, Wirewound (all styles)" have "Relative Failure Mode Probability" of 26% for "Parameter Change" and 9% for "Short".I'm not sure how you can decide it's a bizarre and crappy report.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 11, 2020, 09:51:48 pm
content: BTW, I assume the OP is referencing a wirewound resistor something similar to the attached. And since that is one style of wirewound resistor....

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 11, 2020, 11:01:30 pm
content: QuoteI dunno, maybe it had something to do with four or so years I spent in the QA/FA lab of what is now the worlds largest manufacturer of passive components helping to maintain the QPL for their MIL-R-55182 line of fixed resistors?Okay, so we’re not talking about MIL-R-33007 parts here in what is no doubt a consumer electronic product, but I dissected enough precision wire-wounds in competitive analysis studies and compiled the data to know that report is extremely broad in nature and would be written based on data that is exclusive to mil qualified products. While the construction of QPL (qualified product listing) parts is most times identical to their commercial counterparts, the testing that is performed on QPL parts can so significantly skew the observed failure mode data cited in the “report” that referencing that data in the context of this online discussion is downright, well, silly.For starters, we know nothing about what kind of resistor the OP is dealing with. A 9 watt device could be anything and for all we know, he could be measuring it in circuit. We know absolutely nothing about this mystery resistor and you’re hanging your hat on a very broad, generalized report that is based on components that cost a minimum ten to a hundred times what the consumer world pays. Why you think that report is somehow relevant here is beyond my comprehension.You’ve made numerous assumptions about what the mystery part is and have assumed you identified the failure mode and provided a link to a report that in your mind validates your assumptions. Whoop-dee-doo. Assume away. I’ll wait for more data from the OP before I start forming any opinions about what may be in play here.

author: wraper
author_rank: Unknown
timestamp: April 11, 2020, 11:03:51 pm
content: I suspect there is R56 written on rather than 56R.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 11, 2020, 11:09:16 pm
content: Quote from: WattsThat on April 11, 2020, 11:01:30 pmI provided data. You guys provide nothing but unsupported generalizations, and act like your word is truth.I'm just saying that the experiences of one or two guys may not reflect the OP's situation or the world in general. I tend to put more belief in people who write papers and provide data and references.Doesn't mean it's right, and I have no clue (nor do I care) what the right answer is. I'm just cautioning people to tap the breaks before you proclaim you hold the truth.Someone says wirewound power resistors are infallible and cannot go lower value. A well-referenced report says otherwise. As a minimum that should cause someone to be open to the possibility they're wrong.Basic engineering.

author: langwadt
author_rank: Super Contributor
timestamp: April 11, 2020, 11:18:18 pm
content: Quote from: engrguy42 on April 11, 2020, 11:09:16 pmI'd think something like bifilar wound for reduced inductance add alot more risk of shorts causing low resistance

author: floobydust
author_rank: Super Contributor
timestamp: April 11, 2020, 11:49:20 pm
content: Quote from: engrguy42 on April 11, 2020, 11:09:16 pmCertain safety standards where resistance decreases in value create a hazardous condition, specify using wirewound resistors which are considered "infallible" in them.The Navy report is too vague to be useful and contradicts the safety standards and decades of knowledge. Perhaps they tested plastic encapsulated wirewound resistors but even then, their core is ceramic and would not deform. I think the report is not an authority.Stick to your basic engineering, while some of us work with real world designs.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 11, 2020, 11:55:00 pm
content: Quote from: floobydust on April 11, 2020, 11:49:20 pmDo you have any actual data to support your statements?

author: bob91343
author_rank: Super Contributor
timestamp: April 12, 2020, 01:58:08 am
content: A mountain has been created from a molehill. When measuring resistors of relatively low resistance, errors appear due to dissimilar metals, oxides, lead resistance, and more.Use a four wire method to elminiate some of these errors. With my 4 wire ohmmeter I can measure the resistance of a screwdriver shaft. I made up some four wire measurement leads that I often use to remove the effect of the leads.Shorting the leads on a 2 wire ohmmeter and then subtracting that amount from your measurement is wrong due to variations in contact resistance as well as thermal effects. The four wire method is the way to go if you want accurate, consistent results.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 12, 2020, 03:20:54 am
content: You aren't dealing with milliohms here.56 ohms is well within the range of values that are commonly tested with ordinary multimeters.A short over several (or even more) turns may give a definite resistance drop, but not 5600%.A far more common fault with WW resistors is to go open circuit.If there is a reason to use a WW resistor, there is usually a bit of power involved, so in service, a short circuit would quite rapidly burn out, becoming an open circuit.My guess would either be a faulty meter, a 0.56 ohm resistor, or a strangely packaged inductor with, in both the latter cases, ambiguous markings.

author: digsys
author_rank: Unknown
timestamp: April 12, 2020, 04:17:14 am
content: Quote from: leftekLooks like you've caused a train wreck hereCan you post a pic of the actual resistor, so we can see exactly what it is ?

author: floobydust
author_rank: Super Contributor
timestamp: April 12, 2020, 04:22:49 am
content: Quote from: engrguy42 on April 11, 2020, 11:55:00 pmSafety Standard IEC 60079intrinsic safety “i”"type of protection based on the restriction of electrical energy within equipment and of interconnecting wiring exposed to the explosive atmosphere to a level below that which can cause ignition by either sparking or heating effects"8.5 Current-limiting resistorsCurrent-limiting resistors shall be one of the following types:a) film type;b) wire wound type with protection to prevent unwinding of the wire in the event of breakage;c) printed resistors as used in hybrid and similar circuits covered by a coating conforming to 6.3.9 or encapsulated in accordance with 6.6.An infallible current-limiting resistor shall be considered as failing only to an open-circuit condition which shall be considered as one countable fault.A current-limiting resistor shall be rated in accordance with the requirements of 7.1, to withstand at least 1,5 times the maximum voltage and to dissipate at least 1,5 times the maximum power that can arise in normal operation and under the fault conditions defined in Clause 5. Faults between turns of correctly rated wire wound resistors with coated windings shall not be taken into account. The coating of the winding shall be assumed to comply with the required CTI value in accordance with Table 5 at its manufacturer’s voltage rating."

author: bob91343
author_rank: Super Contributor
timestamp: April 12, 2020, 05:25:13 am
content: Don't rule out mismarking. I have a bag of resistors all marked wrong. There must be several hundred of them, marked 220 Ohms, all very different resistances. It's either a mistake or someone threw a monkey wrench into the manufacturing process.If I thought it was mismarking, I could measure them and relabel. But there is the possibility that they are unstable. It's unimportant by itself, since these parts are so cheap. But it makes suspect many similar parts.

author: leftek
author_rank: Regular Contributor
timestamp: April 12, 2020, 08:58:56 am
content: The resistor write "UTM 214-8 R56 10% z" and is a Emitter resistor from power BJT. I is from one damaged board that drive a DC motor 3A.As i have research it is 56ohm from manufacture "Vitrohm". I am wrong?I send it in attached file.

author: Twoflower
author_rank: Frequent Contributor
timestamp: April 12, 2020, 09:34:08 am
content: Everything is fine with that resistor. The letter 'R' shows the decimal point of the value:R56 = 0.56 Ohm5R6 = 5.6 Ohm56R = 56 OhmSo the value show on your meter is OK as with such low resistance values you have to take the resistance of the cables and such things into account. For example connect the cables together and you'll most likely see a value > 0.

author: leftek
author_rank: Regular Contributor
timestamp: April 12, 2020, 10:38:33 am
content: Quote from: Twoflower on April 12, 2020, 09:34:08 amThank you very much. I did not know.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 12, 2020, 10:39:24 am
content: Quote from: Twoflower on April 12, 2020, 09:34:08 amWell there's your problemAnd I'm guessing the accuracy of the meter down in those low ranges (reading of 0.7 as opposed to say 0.700) is relatively low (like maybe 1% +/- 5 digits or something?). Which also might explain the difference between 0.56 and 0.7 ? I always forget how the digits thing worksEDIT:I just checked the specs of the Hioki DT4252 and it says for resistance measurements: "Basic accuracy: ±0.7 % rdg. ±5 dgt."Doesn't +/- 5 digits with a reading of 0.7 mean the actual can be from 1.2 to 0.2 ?Also it looks like it's marked as a 10% accuracy resistor which means, hell, it can be almost anything

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 12, 2020, 11:33:00 am
content: Okay, science project time...I have a 0.5 ohm, 100 watt resistor. Applied current and voltage via my bench power supply, and got consistent values of 0.595 ohms.Checked my multimeter...leads shorted gives an ohm reading of "0.0". Measuring the resistor gives a reading that stays pretty much on "0.6" but occasionally bounces down to "0.5".So...hell, I dunno...I guess the only bit of useful info from that is if you want some accurate ohm readings at that low range try using your bench supply and a calculator since it will probably give V and I values with at least 3 digit accuracy?

author: GlennSprigg
author_rank: Super Contributor
timestamp: April 12, 2020, 12:39:52 pm
content: Quote from: Twoflower on April 12, 2020, 09:34:08 amTwoflower said it all. The 'R' replaces the decimal point, for readability !This also caused some confusion over the decades, when photo-copied "Blue-Prints" of German circuitry showed 'Commas' instead of full-stops between numbers, as the 'dot' would often be lost in such old prints. Like... 25,6 Volts...

author: Twoflower
author_rank: Frequent Contributor
timestamp: April 12, 2020, 01:13:56 pm
content: The commas in Germany are still used as decimal point. To be honest I don't know what the exact background of this difference is. TheWikipediashows Germany is not the only one :-)There's some additional potential pitfall: For German people1.000 + 2,000 = 1.002,0Solution to that: In Germany the point is optionally used as the thousands separator. In english that would translate to:1,000 + 2.000 = 1,002.0Confusing, isn't it?And with this the thread is officially derailed (but hopefully answered).

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 12, 2020, 01:19:58 pm
content: Quote from: Twoflower on April 12, 2020, 01:13:56 pmYeah, lucky guessBut how about my questions about multimeter accuracy with resistance?

author: Siwastaja
author_rank: Super Contributor
timestamp: April 12, 2020, 01:27:27 pm
content: Justnever everuse either , or . as a thousands separator and there is no problem. Even if you ignore the international aspect, both symbols look too much alike and can be mistaken ("100,000.00" and "100.000,00" visually suck even if both aretechnicallywell defined in some countries).Use spaces instead - both "100 000.00" and "100 000,00" are unambigious to anyone, regardless of language background.

author: Twoflower
author_rank: Frequent Contributor
timestamp: April 12, 2020, 01:49:37 pm
content: Oh, sorry, I missed the follow up question.To measure low resistance resistors you can use better equipment of course. For better accuracy they use 4-Wire measurement methods (one set of wires apply the current while the second pair is used to measure the voltage across).Or you do this using a bench supply as you suggested. Which comes with its own problems. As for example the power supply must be stable driving such high currents at low voltages (e.g. 1V with 1.78A). And you might do the voltage/current measurements with external meters (at least for cheaper supplies I don't trust the current readings untested). Ideal would be two meters. One for the voltage over the resistor (connected direct at the resistor leads) and one for current. This way you have the same setup like the 4-wire setup mentioned above. But again: You need to take cate about the accuracies of your meter. For example many meters lack a 2A range, so you have to use a 20A (or 10A) range. Which gets problematic with the +/-n digit error. Especially if you have only a low digit meter.In general: Low resistance measurements ain't not easy if you want to get good and reliable results. Unless you have the right tools.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 12, 2020, 01:53:23 pm
content: Thanks, but my big question was regarding multimeter resistance accuracy down in the range of the OP's resistor (ie, 0.56 ohms), and specifically what can be expected with his reading of 0.7 ohms. If the spec'd accuracy of the meter is 5 digits, does that mean the 0.7 reading can be off by +/- 0.5 ohms?

author: Twoflower
author_rank: Frequent Contributor
timestamp: April 12, 2020, 02:04:52 pm
content: Argh, that was on page one. Sorry.But your assumptions are right. Mainly the +/-n digits kill you in this case (+/-0.5Ohms in this case). And your calculations are right. The +/-m% ain't adding much (about +/-0.05Ohm). So the total value is 0.7Ohm +/-0.55Ohm. Which gives a total range of 0.15...1.25Ohm. Which is a huge error +/-467% if my math didn't fail me.One digit more in resolution would gain much in the accuracy here.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 12, 2020, 02:10:09 pm
content: Quote from: Twoflower on April 12, 2020, 02:04:52 pmCool, thanks. I was kinda addressing what you mentioned about the possible disparity between the OP's measurement of 0.7 for a 0.56 ohm resistor, and your response that it might be due to multimeter lead resistance. Seemed to me that it was more likely due to the poor meter accuracy at those levels.

author: Twoflower
author_rank: Frequent Contributor
timestamp: April 12, 2020, 02:21:41 pm
content: I haven't had looked up the accuracy of the meter itself. I should have done that, especially as I saw only one valid digit. So you had the right answer about the discrepancy of the reading.Somehow I drop such one digit measurements mentally and never think about it except how do I get a valid measurement. And then one can easily fall into the next problem. Like the mentioned contact and lead resistance.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 12, 2020, 02:42:22 pm
content: Yeah, among all the speculations surrounding thermals and resistor properties and lead resistance and 4 wire measurements and European commas and so on, we seem to miss the elephant in the room...According to mfr specs, a reading of 0.7 ohms can be off by+/- 0.5 ohmswith many/most multimeters (my Klein has about the same specs as the Hioki, 1% +/- 5 digits).

author: wraper
author_rank: Unknown
timestamp: April 12, 2020, 02:47:17 pm
content: It's not only about multimeter specs but also about lead and contact resistance.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 12, 2020, 03:07:54 pm
content: Quote from: wraper on April 12, 2020, 02:47:17 pmYes. That statement is sufficiently general and non-specific to be true and relevant in some cases.