Author Topic: Reduced resistance of resistor  (Read 931 times)

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Offline leftek

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Reduced resistance of resistor
« on: April 11, 2020, 04:08:13 pm »
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?
 

Offline floobydust

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Re: Reduced resistance of resistor
« Reply #1 on: April 11, 2020, 04:19:27 pm »
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 ?
« Last Edit: April 11, 2020, 04:21:23 pm by floobydust »
 

Offline engrguy42

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Re: Reduced resistance of resistor
« Reply #2 on: April 11, 2020, 05:02:26 pm »
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
« Last Edit: April 11, 2020, 05:10:35 pm by engrguy42 »
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Offline floobydust

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Re: Reduced resistance of resistor
« Reply #3 on: April 11, 2020, 08:27:59 pm »
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.
 

Offline engrguy42

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Re: Reduced resistance of resistor
« Reply #4 on: April 11, 2020, 08:32:23 pm »
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.

And neither do you...  :D
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Offline TimFox

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Re: Reduced resistance of resistor
« Reply #5 on: April 11, 2020, 09:13:06 pm »
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 ?

A 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.
 

Offline floobydust

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Re: Reduced resistance of resistor
« Reply #6 on: April 11, 2020, 09:15:34 pm »
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.

And neither do you...  :D

Neither did OP, but obviously 9W is ceramic league, not plastic.
 

Online WattsThat

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Re: Reduced resistance of resistor
« Reply #7 on: April 11, 2020, 09:24:48 pm »
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


Well, you certainly didn’t read the report before quoting a that figure from the table.

Quote
As a rule, film styles are most susceptible to resistance drift while wirewounds usually fail in the open circuit mode. Resistors failing in the short circuit mode are rare and only accounting for 3 to 9% of all resistor failures.

Agree with Flooby, it’s a crap report.
 

Offline engrguy42

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Re: Reduced resistance of resistor
« Reply #8 on: April 11, 2020, 09:26:36 pm »
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.

And neither do you...  :D

Neither did OP, but obviously 9W is ceramic league, not plastic.

Did 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. 
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Offline engrguy42

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Re: Reduced resistance of resistor
« Reply #9 on: April 11, 2020, 09:51:48 pm »
BTW, I assume the OP is referencing a wirewound resistor something similar to the attached. And since that is one style of wirewound resistor....
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Online WattsThat

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Re: Reduced resistance of resistor
« Reply #10 on: April 11, 2020, 11:01:30 pm »
Quote
I'm not sure how you can decide it's a bizarre and crappy report.

I 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.

 

Online wraper

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Re: Reduced resistance of resistor
« Reply #11 on: April 11, 2020, 11:03:51 pm »
I suspect there is R56 written on rather than 56R.
 

Offline engrguy42

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Re: Reduced resistance of resistor
« Reply #12 on: April 11, 2020, 11:09:16 pm »
Quote
I'm not sure how you can decide it's a bizarre and crappy report.

I 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.

I 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.
« Last Edit: April 11, 2020, 11:27:16 pm by engrguy42 »
- The best engineers know enough to realize they don't know nuthin'...
- Those who agree with you can do no wrong. Those who disagree can do no right.
- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 

Online langwadt

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Re: Reduced resistance of resistor
« Reply #13 on: April 11, 2020, 11:18:18 pm »
Quote
I'm not sure how you can decide it's a bizarre and crappy report.

I 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.

I 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, I'm just cautioning people to tap the breaks before you proclaim you hold the truth.

Someone says wirewound power resistors are infallible. A well-referenced report says otherwise. As a minimum that should cause someone to be open to the possibility they're wrong.

Basic engineering.

I'd think something like bifilar wound for reduced inductance add alot more risk of shorts causing low resistance

 

Offline floobydust

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Re: Reduced resistance of resistor
« Reply #14 on: April 11, 2020, 11:49:20 pm »
[...] 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.

Certain 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.
 

Offline engrguy42

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Re: Reduced resistance of resistor
« Reply #15 on: April 11, 2020, 11:55:00 pm »
[...] 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.

Certain 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.

Do you have any actual data to support your statements?

- The best engineers know enough to realize they don't know nuthin'...
- Those who agree with you can do no wrong. Those who disagree can do no right.
- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 

Offline bob91343

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Re: Reduced resistance of resistor
« Reply #16 on: April 12, 2020, 01:58:08 am »
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.
 

Offline vk6zgo

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Re: Reduced resistance of resistor
« Reply #17 on: April 12, 2020, 03:20:54 am »
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.
 

Offline digsys

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Re: Reduced resistance of resistor
« Reply #18 on: April 12, 2020, 04:17:14 am »
Quote from: leftek
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?
Looks like you've caused a train wreck here  :)  Can you post a pic of the actual resistor, so we can see exactly what it is ?
Hello <tap> <tap> .. is this thing on?
 
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Offline floobydust

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Re: Reduced resistance of resistor
« Reply #19 on: April 12, 2020, 04:22:49 am »
[...] Do you have any actual data to support your statements?

Safety Standard IEC 60079
intrinsic 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 resistors
Current-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."
 
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Offline bob91343

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Re: Reduced resistance of resistor
« Reply #20 on: April 12, 2020, 05:25:13 am »
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.
 

Offline leftek

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Re: Reduced resistance of resistor
« Reply #21 on: April 12, 2020, 08:58:56 am »
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.
 

Online Twoflower

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Re: Reduced resistance of resistor
« Reply #22 on: April 12, 2020, 09:34:08 am »
Everything is fine with that resistor. The letter 'R' shows the decimal point of the value:
R56 = 0.56 Ohm
5R6 = 5.6 Ohm
56R = 56 Ohm

So 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.
 
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Offline leftek

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Re: Reduced resistance of resistor
« Reply #23 on: April 12, 2020, 10:38:33 am »
Everything is fine with that resistor. The letter 'R' shows the decimal point of the value:
R56 = 0.56 Ohm
5R6 = 5.6 Ohm
56R = 56 Ohm

So 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.

Thank you very much. I did not know.
 

Offline engrguy42

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Re: Reduced resistance of resistor
« Reply #24 on: April 12, 2020, 10:39:24 am »
Everything is fine with that resistor. The letter 'R' shows the decimal point of the value:
R56 = 0.56 Ohm
5R6 = 5.6 Ohm
56R = 56 Ohm

So 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.

Well there's your problem  :D

And 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 works  :D

EDIT: 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  :D
« Last Edit: April 12, 2020, 11:01:43 am by engrguy42 »
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