1/3W resistor failure mode?

:Was in a valve /tube amp HT line as a dropper carying about 250V , no :obvious heating of the body or surround of this 10K resistor and no obvious :burning of the resistor track, microscope view :

:B is the original blue body colour, C is part of the ceramic coating that :got left after my scraping. My scraping was axial so not the cause of the :tapered loss of metal oxide in the spiral between * and *, actual break just :off pic and a slight trace of this loss on the spiral above *-*. What caused :this tapering loss ? electrochemical? manufacturing fault ? All other 1/3 W :resistors in the amp look the same manufacture so wonder if being in the HT :line is significant or could any/all of the other 1/3W resistors fail in :similar fashion

Could it be the voltage rating of the resistor and the possibility of higher than normal transient peaks?

I had a similar case where I used a Beyschlag 0.25W/70C MF resistor in a 240Vac mains driven circuit and after about 6 months the device stopped working. I discovered this resistor, which looked perfectly normal, was open circuit. I replaced it with an identical resistor and some months later, similar fault. I decided to replace it this time with a hi-stab 0.5W carbon film which was physically larger and it hasn't failed since. Checking the specs for the MF resistor showed it had a max voltage rating of only 250V, and it would have been exposed to peaks of more than 300V from inductive components during operation. The carbon film resistor replacing it was rated at 350V.

Perhaps your MO resistor has suffered a similar failure mode. But unless you know the manf specs for the resistor in question it may be difficult to determine.

On Sun, 15 Mar 2009 20:58:36 -0000, "N_Cook" put finger to keyboard and composed:

I recently had a 1/4W 100K resistor go O/C in a Pioneer SX-202R receiver. It straddled the +/-45 rails (in series with two diodes) and saw 90V for its entire life. That's only 81mW. The coating was only slightly discoloured. There was no other damage to the amp.

I've also seen resistors go open in 5V circuits without good cause.

Then there are the high value startup resistors that go open in PSUs despite been operated well within their ratings ...

- Franc Zabkar

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That was part of my thinking. Whatever the rating, they were of a type used throughout the amp so presumably whatever is cheapest so not HV rating. What makes the difference in construction or whatever , between HV and LV use , afterall the value of resistance and wattage sets the volts across the length of the resistor. I've always observed the policy for HV ones as droppers for the supply for zero-crossing ICs etc for triac circuits etc, but have never known the physiscal reason, other than the wrong , ie LV ones, will fail in such circumstances.

Anyone know the chemical/physical reasoin for the tapering failure, I suspect some very sophisticated magnetic field distribution effect, but just a guess.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

I should have said there is a waveyness to the terminator between surviving MO track and the erroded section of the taper on the original full resolution pic for this

shown here

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

What was the voltage rating of the resistor? Even though it is dissipating less than the max. power rating, things like electromigration may causing the open.

Al

I've no idea but it was the same 5 band, 1/3W, type used all over the amp , I assume 5 band does not necessarily mean suitable for > 200 volt use. Electromigration relative to a nearby ground plane if that was the case (not here. polyester pcb and chassis 20 mm away) but why a problem with a standing DC of 250V and only a few volts drop along the resistor.

"N_Cook" wrote in news:gpjq5b$khr$ snipped-for-privacy@news.motzarella.org:

I used to design and oversee the making of resistors and capacitors for Sprague in the late 60's and early 70's.

We 'adjusted' the value of our resistors and capacitors by abrading the material.

In the late 60's, we used sand blasting to remove material. In the 70's, we used lasers on many resistors (and still used sand for capacitors).

It looks to me as if the spiral cut, that adjusted the resistance, varies in width. It should be UNIFORM all along the length of the cut.

It looks like poor quality control allowed the conductive track to be too thin in one area.

The thin track had an area of high power dissipation because of the high resistance in that area. Most of the voltage drop would ALSO be along that region and lead to failure.

Good design and good quality control would have rejected the resistor.

Of course, it survived many hours of use, so the design wasn't all THAT bad.

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bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

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All the MO resistors I've had a close look at seem to have the spirals formed by a milling process (ends are semicircles) rather than some ablating process as used in precision Rs. But some sort of runout error in the milling could cause a taper error. But I would expect a straight, but misplaced cut, rather than that wavy edge. Reducing any part of the track sets the resistance value and I would have thought a taper down to 20 percent of intended would push it up from 10K to 50K or more and early failure of the resistor in circuit.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

On 3/15/2009 11:38 PM N_Cook spake thus:

Oh, come on; there's a very simple explanation: the resistor was *made* that way. Shit happens.

Sheesh.

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Made From Pears: Pretty good chance that the product is at least
mostly pears.
Made With Pears: Pretty good chance that pears will be detectable in
the product.
Contains Pears:  One pear seed per multiple tons of product.

(with apologies to Dorothy L. Sayers)

"N_Cook" wrote in news:gpltpd$nvr$ snipped-for-privacy@news.motzarella.org:

....

And this one may be 'an early failure', compared to similar resistors correctly milled.

-------bonus information section--------------- Take a tubular resistor that is uncut and measures 1000 ohms.

Take one with circumference is equal to length. This makes it equivalent to a 'square' flat resistor.

Resistance is proportional to length and inversely proportional to width.

Designers use 'ohms per square' in designing resistors. It doesn't matter if it is a square inch or a square cm or a square mile, the resistance will be the same. (the size, however, would effect the POWER rating)

The above resistor would have 1000 ohms per square.

If you cut a single narrow grove straight from one end to the other, you have not changed the resistance significantly.

Cut a single spiral around the resistor along the length. You have just made the resistor LONGER and Narrower.** You have NOT significantly changed the amount of power it can dissipate. You have raised its value. The effective length will be sqrt(2) x original length. The effective width will be sqrt(1-sqrt(2)/2) x original length. So the ratio of length to width will be ~ 2.613, or the resistance will be

2613 ohms.

Change the pitch of the spiral until you have as many turns as needed to get the desired value.

Again, if the cut is narrow, you have not changed the power rating but you have increased the resistance.

Now, imagine that you made a mistake and the resistor material had too low an ohm per square value. You didn't notice this until after the spiral groove had already been cut. The resistor is too low in value. You can throw it in the trash or you can go back and raise the resistance by grinding away and making the groove wider.

If the heat conductivity of the substrate is good then you will probably get away with this.

It looks like that is what they did, until now.

** Note, the situation is NOT quite as simple as given above. It assumes all current follow equal length paths. If you only cut a single spiral, some current paths are MUCH shorter than others. Several turns are required to ensure that the current flow is uniform across the width of the spiral.

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

:I should have said there is a waveyness to the terminator between surviving :MO track and the erroded section of the taper on the original full :resolution pic for this :

:shown here :

It is difficult to know whether or not that this particular resistor suffered from a manufacturing defect which produced a thinner than normal track at the point of failure.

There is some literature related to metal film resistor failures due to single shot high voltage pulses but it is not known if a manufacturing defect may have contributed to such failures.

There has been a case where an ineffective cleaning process left small traces of chlorine on the resistor body which caused etching after the the encapsulation or coating process.

While not on the same subject there is also a x-ray study of a metal film resistor which exhibited a drop in resistance due to foreign matter shorting adjacent tracks. It highlights the possibility of manufacturing defects though.

Would you be able to confirm my interpretation of fusible resistor construction, from the analysis of a failed one (reason known that time). As

1 minute timing does not ring true to me, unless under 1 minute can mean 2 seconds. From a makers data (not for the one in question) and 5 band colour coding Markings are to be shown on the resistor body by color coding. 1st, 2nd and 3rd color codes : nominal resistance value. 4th color code: resistance tolerance-J (±5%) - gold color. 5 th color code : fusing characteristics green color -8 times blue color -12 times white color -16 times violet color -32 times if 1W then white band means subjecting to 16W will fuse in under 1 minute Surely any normal 1W resistor subjected to that sort of overload will have burnt up quicker than in 1 minute or have I misread something ? Removed the grey coating and remnant parts of the metalisation/ MO sum to about 1.5 ohm. The joining section is about 1 x 1 mm with a neat hair-line crack across it and no sign of any heating there. So mechanical breakage, probably a torsional failure due to one end of the resistor swinging one way and the other end swinging the other way, when the amp was dropped. Before removing coating, was definitely 2.2 ohm markings with white band (if) for x16 presumably referring to that 1 x 1 mm section, so x32 violet banded one would perhaps have a something like 0.7 x 0.7mm intercept region and a green band x8 would have more like 1.4 x 1.4 mm fusing area. Is a microscope view of the crack. Viewing magnified it can now be seen the fault developement. The just left of centre part of the crack must be part of a crack that formed initially but enough current flowed in the other parts making touching contact until there was too much localised fusing. 1 to 1 marks the 1mm or so of conductor between the 2 straight line etching/millings ? into the conductor to form part spiral paths. The C marks the end of one of these formed breaks and there is another one on the other track , off the top left of the image, giving about 1 x 1mm central fusible conductor. Nowhere on the remaining conductor is there any discolouration due to overheating.

"N_Cook" wrote in news:gpnlds$33t$ snipped-for-privacy@news.motzarella.org:

Not an expert of fusible resistors. Sorry.

I would think it means multiples of the length of time it will sustain the overload rather than multiple of the power.

The vaporized material may have gone with the cover coating.

Not an expert on fusable resistors. Sorry.

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap