How To Find The Value Of A Burnt Open Circuit Unmarked Resistor
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- Опубліковано 30 вер 2024
- Sometimes we find a faulty PCB where resistors have burnt, blackened, cracked or the markings are obliterated because of the component running hot. Often these resistors will read open circuit on a multimeter but we need to know the correct value so we can fit a replacement. So how can we we do this if we have no schematic and the value of the resistor is not marked on the board? There a number of ways we can approach this such as reverse engineering the circuit to calculate a reasonable resistor value but here is another good method we can use.
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Thank you
Richard
Ha ha very nice, Glad to see you try my method, A bit rough on making the score but after some practice you learn how to make it depending on resistor type, it is often enough to scrape it a bit with a file or knife or even just poke it off with a screwdriver. Sorry about the long name :P it is a Swedish equivalent of "Powdered Toast Man" But in Swedish you say it in one long word but more like: "Pulver-Rost-Mannen" Great video!
Hi powdered cheese man 😂 That's what I read, didn't see the R in the middle
@@stefflus08 aahaha believe me you are not the first one on that, But I actually pick this name because of how funny it is and that I like getting a good laugh from all the confusion around it lol
@@Pulverrostmannen Hi there Powdered Toast. It was really good to see your suggestion and it was a pleasure putting it out to a larger audience. Credit where credit is due and thank you. 🙂
@@LearnElectronicsRepair It was my pleasure! credits goes to the other viewers that mentioned this as well in the other video. It is a great feeling we all gather here together to learn from each other. Together we get smarter and more experienced every day :) keep up the good work!
@@Pulverrostmannen Abcoultely 100% That is the great thing when we form a community. And thanks again.
Use alligator test lead on one end connected to meter, place resistor firmly on table and use more pressure with needle tip for more stable readings, me thinks.
A diamond contactor file is a slower but more presise way to grind away the ceramic ( I use these a lot in my job to repair large expensive contactor contacts )
Film type resistors aren't necessarily physically symmetrical. The spiral groove that is cut to actually set the resistance starts at one end and finishes where it needs to, which may be moderately far from the other end. This is clearly visible with the resistor in the video starting at about 12:00
@@lurch789
Huh? If you have a point it is very far from apparent.
Fantastic. Many thanks, Richard.....and the other guys that suggested this. I never knew this was possible as a method. Great stuff.
I've done a similar thing with ceramic-coated small vertically-standing PCB modules like those used on that air conditioner board. In this case extreme care is needed so I very carefully scraped away the ceramic coating with a segmented knife. After about 5 minutes the knife went dull so I broke off the dull segment and kept going with a nice sharp edge. After about 2 hours the whole thing was uncovered then I reverse-engineered it and made a new PCB. So yeah instead of the heavy-handed approach just scrape away some of the coating _gently_ ^_^
Glad you got into electronics and not surgery on humans! Lol it was quite frustrating watching you poke all AROUND the conductive part of the resistor.
With my nitpicking out of the way, I GREATLY appreciate you sharing this idea. I will definitely put it to good use! Now if only there was a technique for SMD capacitor values. . .
Nice learned something today thanks Richard and the guys who suggested this
Nice new trick there. A big thank you to the folks who posted these suggestions.😁👍
When you started probing you got just over 7k from one end and just over 10k from the other end, so you already had your answer - 18k.
I love this. Together with the previous video on working with an unknown circuit, this really shows that there are practical methods of overcoming any challenge in electronics diagnosis and repair!
INstead of a dremel tool, Sandpaper or even a screwdriver edge I have used to remove the paint or coating to do this method, but thanks for sharing that more people can fix things, instead of them landing up in landfills
In 1967 high school electronics class we were taught to use a small triangle shape file to notch into a carbon resistor to remove carbon till we had the resistance value we needed, then use nail polish to seal the cut. I'm now stunned while watching this video that I did not think of chipping away the body of a wire wound resistor to measure the coils to estimate the value of a burn-out resistor. Thank you.
Think resistance increases when filed. I was taught, seen and did this but so long ago I cannot recall exactly. But resistors were massive devices 30 mm long and 12 mm diameter with spot colour codes. Crystals ex gov were taken apart and adjusted in a similar fashion with a blade and nail varnish. Don’t think I ever did that so cannot say how they worked afterwards, but it was a very popular way of getting ww2 equipment working.
Yep it does indeed increase and its a common trick back in the day !@@jennyearl5194
Nope, fact done it myself....get an old carbon comp and see for yourself...easy.@@krystal5887
Great untill the resistor is tottaly exploded! But then you proberbly have other problems elsewhere in the circuit!
Yeah ! no magic smoke left in at all and everywhere else is getting hot ready to realise the smoke ! @@johnrhodez6829
Those probes are also very useful for removing dirt from under the thumb nail!
8:39 You're constantly/most of the time poking the ceramic...🤣
And also, you at one time had 11 and 7.something.......Doesn't that equate to about 18???
Well the average of 11 and 7 is..... hmmm let me think this a bit.... Well it is 9 actually. So I guess double that is exactly 18. But I think you are missing the point here. When it comes to fried resistors and suchlike, you can't really hope to work out the exact original value so you have to go with what you can do, and this method will generally work well most of the time and that, to me, is a good thing. As you didn't really like it, and to me that is a fair response, then please suggest a better alternative method and I will be happy to try it on video. If you can't think of anything better, then let's face it this is the best we have, so it is this or nothing. And anything is better than nothing. Mostly. 😉
This works with many resistor types, but it could be heavily misleading with certain resistor designs, mainly when laser or grinder trimmed or with certain wear/failure modes.
Sometimes the film just gradually oxidizes, so increases its resistance. When it degrades so much it breaks, its resistance could already be way off.
Sometimes the trimming does not form such regular spiral, but a kind of mesh of parallel stripes (mainly on HF low ESL resistors). Then you get it practically only when the last stripe from the multiple parallel ones broke, then you will measure pretty bogus value (depends on the exact trimming pattern).
So you should keep in mind this and be aware about it. Good way is to look where the resistor is used and what values use to be used in such cases.
And be prepared for the "extracted value" to be wrong, so prepare a set of tests to confirm the value you came up with is really suitable (check the voltages/currents in the circuit, whether loading of the components in the area is not suspiciously off).
I agree. No method is perfect or even suitable for every occasion, but I don't think that means this method has no merit or use as it clearly works very well in some situations and that has to be a good thing yes?
Never thought of that! I suppose filing or chipping away the epoxy, albeit *más suave* we'd see the break, short the leads, and measure 99% of the resistance directly across the break. I think these are called laser-cut "thick-film" resistors, as opposed to the beefier wire-wound type.
Yeah I was a bit heavy handed in my attempt. But it did show the method is good, thanks again to the viewers who suggested it 🙂
It is unquestionably a film type resistor. A wirewound resistor in that resistance range would have a very large number of turns of very fine wire.
There are two common types of power film resistors - metal film and metal oxide film. The former generally have better stability but the latter have better transient overload handling ability. Wirewound generally have both good stability and transient overload handling ability but they are considerably more expensive and become impractical for high resistance. The inductance of wirewound can be a problem in some circuits.
@@d614gakadoug9 I'd imagine film thickness on that one might be used from 5k to 22k since on one end, the spiral cut was far from the end cap. I've seen rectangular wire-type in some applications - I think these are wound using the Ayrton-Perry format.
You would have had better luck if you had scraped off the black corrosion of the one lead which is non-conductive. Buy some alligator clips for the end of your probes.
I use alligator clips all the time. I have some really nice ones that have the plastic ends where you just insert the probe tip. And the clips are much better quality than those cheapy basic clips.
Simply a must have in the lab.
Great idea! Thanks to all who suggested it and to Richard for doing the video.
This is fantastic! Thank you for the education Sir!
I’ve been in this situation, the color codes were all browned they were not readable. Thanks for composing a video about it!
Great video....great tips
Great idea! I've tried adjusting resistance value of metal film resistors with a small triangular file. You increase resistance by removing material. If you do this, might be a good idea to cover the cut with solder mask or something similar.
this works badly for small resistors or anything over 100 ohm
Brilliant - I couldn’t resist ⚡️
Thanks for following the suggestion, Richard. I really wanted to see the outcome. I cringed a bit as I thought of the chance that the resistor could come out flying the moment that the grinding wheel touched. If you do another video on this topic, can you cut open a working resistor and measure that one as well?
Gonna make a followup to this one on the next live stream @theelectronicschannel 22nd October 5pm London time with Carlos and Detlef, let's burn some resistors and see if we can still measure the value
Interesting how the resistor still wouldn't reveal its faulty spot after having shed all of its coating.
Interesting result! As it's not burnt, I'd have immediately suspected the End caps
Think so too.
If you have no specs available you also don't know the tolerance of the resistor either. "Back in the day" when 12AX7's and 6L6's ruled the earth 20% resistors were typical - in tube circuits of hundreds of volts what mattered was that (for example) the plate voltage was "large" compared to the screen voltage. In that case, deducing 20K for a 18K value would be just fine. With many modern circuits values of 1% might be absolutely required. Hopefully you should get a feel for the required tolerance by knowing the function of the circuit and perhaps by looking at the still working components.
Heya, wel never tought of that but yes that's a possiblity. nice job well done and thanks for sharing
Nice, Definitely a nice tool in the toolbox, even if it might not work for every resistor, depending on its type, how it failed and 1/4W or less will make it harder. Maybe swabbing it off with a bit of Iso might make for less intermittent when trying to read solid meter readings.
Great diagnostic, thank you, love that stuff very much.
Sometimes, "old school" is just the best way.
I mean, it always worked before.
👍😁👍
I always just broke them in half, and then measured each half, probing around the broken cross-section...
Components addict to the circuit board, when installed or soldered onto it. Mainly you can see all this difference in chargers, smart batteries, power banks, solar phone chargers. Components have a default value, and the usage, charging calibrates diodes, resistors to support your needs. If you misuse your chargers or batteries, the components become worse, until it becomes useless, but you can still fix it again. That is caused by the smart batteries, if the user is monitored, the components become memory effected due to a satellite laser radiation. Professionals, engineers simply can’t believe this. Even mechanic simple motors can gain memory effect in the copper coil, and can be fixed by offloading the motor, and it takes 30 minutes repeated retry. This is so new, nobody even knows this. The whole problem is caused by the user’s behaviour, or money grabbing.
OK I use the blade type soldering tool.... I'm sure you know the one I mean just press on the coating and it chips off... if its a wire wound you need to be careful, the wire can come away with the coating, a dremel is too aggressive.... if its ceramic the coating just falls off but you need to measure both sides as I've had odd results with those.... but yes it can be very helpful when you have no drawings..... you can see on the example you have why the half way method won't work...
Get your fingers out of your mouth when speaking. Unbearable. l8r
If the resister has color bands thy tend to a lovely shade of brown. Look the resistor and rotate it. There may be spot where the color 'is in tact.
Suppose you find a resistor where the first band is light brown It must have been yellow. How does yellow occur? Usually with purple to for 47.
That specific component must have done something illegal in the USA. Where else could it have been instructed to stop resisting?
Great Information. You bring great information in all your Videos. Thanks!
Haha, "Pulverostmannen" is Swedish for the powder cheese man.
Cough personas. I picked up a brand new almost never used interface the problem is the power supply. When issue is that there is a quarter white resistor that gets a little too warm so I have no idea what the value was and had to pick up another unit but even that has a discolored resistor. And the company is absolutely no help. I've managed to. Use a close approximation to what the value of the second unit was only to find out there's something else wrong in the power circuit the only rears its ugly head under load. If I swap power supplies the other unit works fine.
This is great, but do you have a solution for SMD's, as all the new stuff is coming out with SMD's and no schematics. Hope there is someone out there that has an idea.
i was quite intrigued to see how........
u butchered this resistor with a dremel like a barbarian LOL
Except that some resistors are NOT Linear in their value especially carbon track jobs
wow! thanks,I've learned something, today.
Good forensic research👍👍👍👍
Dremel is a bit brute force for this, a diamond file should be more than enough. However it is a great technique.
Splendid: what a great tip!
"visibly see" is a pleonasm. You can't visibly hear, can you?
so what if its gone blown to infinity and beyond and only memories are left of that resistor no bodies left to do some autopsy and theres no marking on the board or cant find any schematic of the board for reference on the internet how do you even know whats the value of that missing resistor?
How about a simple video showing an appliance or something that does not power up and do a basic trouble shooting video to find the fault ???
Repair the resistor..!!
Buy some of that foil wrap and ceramic paint.... ;-)
Brilliant even if this is not perfect it would put you in the right ballpark. I've worked in electronics for years and this had never occurred to me. Definitely some think worth mentioning to my apprentices.
Great idea from your viewers. Thanks for sharing!
i wouldnt have thought of doing that. ! very impressive :)
Getting a reading from the middle out is a terrible idea if the wire had burnt the it's thinned before it's broken
Should have clamped the Dremel and held the resistor at both ends..more control that way....
It's also a good way to have it end up whipping away to a couple of hours of hide and seek.
@@I_Don_t_want_a_handle Resistor flying off???...... You need to lift your skills and become a Master Technician mate! lol
It's easier to control a tiny resistor than a large dremel.....
@@des2610 take them overly strong troll hands back under your bridge 😜
@@I_Don_t_want_a_handle Keep practicing son! You will reach Technician status one day! Then..... you can work to become a ******Master Technician***** like me! Then your resistors will not fly off when you try to grind them!!!
@@des2610 is it all in the wrists?
Best tool to use to grind with is a grinding pen nf V2 from eBay etc
Very interesting indeed. Thanks for sharing.
Instead of using a cutoff wheel I recommend geting a set of diamond coated Jewlers bits. One of them should look like a super tiny version of a cutoff wheel but the top is completely flat. The side works for rounded ones & the flat top works for surface mount resistors, capacitors, & the like.
i always used a regular knife to leave the resistance tracks intact,
and now i find out there is a professional option for the job🤪
I'm new to electronics, I have a board that a bad diode caused a overheat in the resistors. Three resistors grouped together all burnt completely black. I get a reading from all three but it's different on all three ones 54 ones 56 and the last one is57. No schematics for the board I have scoured the internet will it be safe to assume that I could replace these with 5.5 resistors?
Are these three resistors all connected in parallel with each other? What is the device?
Awesome. Thank you for sharing
Great content, but three years too late for me. I just guessed from the charred remains and noticed another of the same value nearby. - Didn't work but eventually found a diagram.
Without doubt there are several different and good ways to approach this problem
yes that was quite interesting and good to know. thanks!
Make it short , the video ,….
Not the resistor,…….!
Old school thinking still works today 👌
Nice! Thanks for sharing that!
That’s brilliant thanks so much.
And now SMD version. Those small, who hasn' markings on it. XD
Yeah let's try that 😁
Excellent idea, every day is a school day
But how about a carbon? It was mentioned to use a file and change the value so If you used your cutter and just uncovered the carbon somewhere near the center you should be able to accomplish the same.
I've never had luck with carbon resistors.
Usually, by the time they are totally open, they have literally overheated and the carbon turns to bits and pieces.
Sometimes, we get lucky and a burned up carbon resistor just splits in 2.
That makes finding the resistance a bit better....
*MAYBE!*
I say maybe, because again, once a carbon resistor gets that hot to fail, the entire carbon will now not measure as when it was new. If you get my meaning.
Fascinating! Great idea and demo!
That worked out pretty well :-)
You learn a new trick everyday.
Generally you can just scrape enough coating off with a knife blade. Just uncover the wire and replace with a standard value.
How much more obvious can the value be than the sum of the best readings be? 7.7k + 10.4k is a total resistance of 18.1kohms.
It's just too bad this method is not feasible, as far as I can perform it, on smd resistors.
@@Freeknickers24sighhhh... .so true. More often than not, smd resistors and caps may not even be marked at all.
So then, without a schematic, we are left analyzing the circuit it is in to figure out what the value SHOULD be. Painful... but doable.
Thanks for sharing Richard 👍
Great content, thanks ;-)
Your wesistor is bwoken.
Repair the resistor .
Demonstrate an effective way to do that.
lol not worth it
@@krystal5887 But what if you're Hans Solo or Scotty and you ship is disabled in the vasts of outter space?
The cracks look to be more on one side. If this is accurate, can you say if it was installed with that side touching the board or not. Is it better to install something with a gap or not? (Just built a Blue ESR meter, and tried to leave a gap on anything.) Thx.
U don't bother
Greetings:
If the burned/break is narrow enough! Set up for a normal resistance reading from lead to lead. Use good mechanical alligator cllips to ensure reliable contacts. Then use a conductive pick to find and bridge the actual break resulting in a reading through the break to the meter, possibly a more accurate reading? If the coils are only thin then finding the break may require removing the insulating portion further around the resistor circumference.
My guess is that the mfr process here is similar to laser trimming whereby the coils are made while measuring the lead to lead resistance while the coil separation is increased and stopping at the appropriate measurement.
Since it is wirewound, it’s open segment will likely arc across if a sufficiently high voltage is applied across the terminals, and the arc being left in place long enough should cause some charring at the location of the arc (and the break in the wire). The high voltage supply should be current limited to be on the safe side.
Absolutely love this type of content, thank you.
Btw, seems the invalid traffic bug is because of too many people running adblocker. At least Sorin thinks so after last video.
Fascinating and VERY useful! Thank you all!!!
I have some diamond coated dremel bits of many shapes .. they allow very precise and controllable work .. ideal for this .
Super useful!
👍
Join me if you thought aabout this method without opening the video
More than thought about it! Used this method many times in my old TV Repair days!
Many times a high-watt resistor had markings burned off or just clean like that one.
I did it a bit differently. I never trusted that the break would always be in the middle. It could be anywhere. So I removed the material along the length, more like he had near the end of the video. This was to expose more wire.
Sometimes, old school is still the best way.
That won't work with carbon composition resistors though, will it?
Another take on this: chip off the enamel, rather than using abrasion, starting from the caps. This way you run less of a risk of eating away at the resistive layer.
It might but you will need a very fine probe point and a steady hand. After all thats how carbon track linear variable resistors work.
@@bofor3948 I think it will work too. More live streamed experiments on this topic @theelectronicschannel 5pm Sunday 22nd October London time
I agree about working from the end caps inward. Some resistors have an extremely hard coating that is very hard to remove, but on film types its usually fairly soft. In any case, starting at an end cap gives you an idea of what you're up against.
With a carbon comp resistor that has failed due to excessive power dissipation I suspect you'd have a hard time coming up with any useful measurement. Even after gross overload carbon comp isn't too likely to go completely open-circuit.
With one in good condition cutting a tiny V down to the element in the centre of the body would work quite well since the element is a uniform "slug." But then doing that with a known-good resistor isn't really very instructive.
That's a pretty good idea. Might have to kill a resistor just so that I can open it up😊
I'd be tempted just to try and give it a little tap with a toffee hammer just to get it opened up and then pick away at it
Ingenious.
Brilliant!
The issue is when it fails to very high resistance.Lets say this one fails to MΩ.I don't think you can do anything in such case.
nice one
Thank you. Nice trick. More accurate than expected.
nice video, cheers for the shout out !
You are welcome Colin. It was an excellent idea.
What a very interesting method, love your videos 😁
Very nice ❤
Tho i find i run into this a lot more often with surface mount resistors than with through holes