Anyone ever experience flaky low value chip resistors? - Page 1
Forum section: electronics / projects

author: Konkedout
author_rank: Frequent Contributor
timestamp: February 01, 2025, 01:44:30 am
content: Today I was working on a boost converter. I have two parallel 0805 current sense resistor locations at the MOSFET source.A 68 mOhm resistor has stayed in one location. No serious problem with that one but I needed to tweak the value down. The converter became unstable when I placed 150 mOhms in the other parallel location.The yellow trace is the MOSFET drain, the blue trace is the MOSFET gate, and the purple-pink trace is the MOSFET source voltage developed across the two current sense resistors. The converter is running in Discontinuous Conduction Mode.Note the varying slope of the current sense waveform. It is like my 150 mOhm resistor is intermittent!I tried increasing the input voltage filtering. I tried cleaning the board around the controller chip. I tried removing and re-soldering the 150 mOhm resistor. I also re-soldered my inductor. None of these helped.Then I got this oscilloscope photo which was more informative than my previous ones. Finally when I replaced the 150 mOhm resistor with a 100 mOhm resistor everything worked more normally.Has anyone seen a resistor that was intermittent this way? Same behavior even after removing and resoldering it?

author: langwadt
author_rank: Super Contributor
timestamp: February 01, 2025, 01:48:11 am
content: maybe you cracked the resistor when you first soldered it in

author: Konkedout
author_rank: Frequent Contributor
timestamp: February 01, 2025, 02:01:44 am
content: Thanks yes of course that is possible. But I have soldered a lot of chips and never had similar happen. SFAIK I did not abuse this one more than most.I do wonder whether there is something different with these low value resistors.

author: coppercone2
author_rank: Super Contributor
timestamp: February 01, 2025, 02:12:57 am
content: I noticed sometimes when parts break its just like what?i think it just happened to you. Its rare, but possible to have a cracked part.Its kind of like stepping into a hole in the dirt, no effort requiredI notice it on other parts too, like potentiometer bends. I do it the same every time. I must have opened a hundred of those things. Once it just cracked right off.And it reminds me of the one time I had a glass on the countertop. It was literarly sitting there, on a granite surface, and suddenly a ring fracture occurred right in front of my eyes. I did not handle it recently, it was just sitting there. Makes me wonder if it would still crack if I was not looking at it.And then there is the people that ran into some bad luck, and you see them do some operation you do 100's of times like its a anti tamper switch on a land mine, moving in slow motion LOL (and it still can break if you do this)Its kind of luck of the draw I think, your probobly better having it break on you, because if you gotta be that gentle its gonna go. I am almost convinced that being too careful actually some how makes the situation worse when it comes to fragile objects. At least some of them.

author: mawyatt
author_rank: Super Contributor
timestamp: February 01, 2025, 03:57:29 pm
content: We've had issues a few times with small valued SMD resistors and wanted a means to evaluate these SMD resistors with good repeatability. This lead to the development of the Split-Kelvin Fixture mentioned here.https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/A few folks have utilized this Split-Kelvin Fixture/Technique with very good results, even down to milli Ω levels.Best

author: jmelson
author_rank: Super Contributor
timestamp: February 01, 2025, 05:37:19 pm
content: It is not an intermittent resistor, it is a fluctuation in the circuit. Notice there are multiple yellow traces starting at different times. If you take a single shot with the scope, it may make it easier to understand.Jon

author: BennoG
author_rank: Frequent Contributor
timestamp: February 01, 2025, 06:39:11 pm
content: I had this reproducible with 1.0 ohm resistors.But to be fair I was putting 1ms 5-15A spikes in them 2 times a second.The total power was well below the rated 0.125W but it was the spikes that killed them.There are special resistors for this, i found out later after a lot of try and error.Benno

author: Konkedout
author_rank: Frequent Contributor
timestamp: February 01, 2025, 11:01:48 pm
content: I think I have 99% proven that the resistor is the issue.The voltage across resistor is R*I. I is a ramp. In a boost converter that ramp slope = V/L. I did try resoldering the brand spanking new inductors, but that changed nothing. I also added bypassing to the +12 input but that did not also had no obvious effect.Basically with that resistor in circuit I saw a varying slope (I do not mean curvature in the slope although there may have been abitof that due to inductor core saturation). Rather my arrow points to one spot in the capture where at least two ramps clearly show different slopes.With that resistor out of the circuit the problem was gone. I replaced that 150 mOhm with a 100 mOhm and that result was also OK.

author: Phoenix
author_rank: Frequent Contributor
timestamp: February 02, 2025, 05:19:12 am
content: Looks more like instability. Changing the sense resistance changes the loop gain. You can also see the output voltage varying which would explain the di/dt variation.

author: Konkedout
author_rank: Frequent Contributor
timestamp: February 02, 2025, 06:39:50 am
content: Quote from: Phoenix on February 02, 2025, 05:19:12 am1) Instability is can happen but that does not account for the slope of the current sense ramp changing between pulses. I think THAT needs to be caused by a change in input voltage or boost inductance or current sense resistance.2) It was stable with one 0.068 ohm resistor. It was also stable with (0.068 in parallel with 0.1) = 0.0405 ohms. But it was unstable with (0.068 in parallel with 0.15) = 0.0468 ohms. The instability argument also does not make sense in this context.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 02, 2025, 04:55:39 pm
content: So let's look at all possible explanations of the varying slope:1. Current waveform is unchanging, but the sense resistor is changing2. Current waveform is changing due to fluctuation of boost inductance3. Current waveform is changing due to fluctuation of Vin3 can easily be ruled out by monitoring Vin with the scope, I'm assuming the OP has already done so1 and 2 are equally plausible and difficult to rule out. Either could be due to crack in the resistor lead/ferrite core, combined with some mechanical stress (from vibration or thermal expansion).Agreed that control loop instability (i.e. subharmonic oscillation) isn't the issue. If it's only operating in DCM then inductor saturation also can't explain it.Simply replacing the suspicious component and seeing if it has an effect is the most straightforward means of determining if it's 1 or 2. If replacing one of the sense resistors does the trick, then yeah I would also conclude the resistor was the problem.I have seen intermittent resistors in the past, no reason to think that it couldn't happen with a current sense resistor. Though if I soldered the resistor(s) by hand my first assumption would be that I damaged it.

author: Konkedout
author_rank: Frequent Contributor
timestamp: February 02, 2025, 07:29:53 pm
content: Quote from: mtwieg on February 02, 2025, 04:55:39 pm++++++++++++++++++++++++++++++++++++++++++++++++++++++Thanks yes I pretty much agreed with all that you said above.