Why did the resistor fail? - Page 1
Forum section: electronics / projects

author: mtamas91
author_rank: Newbie
timestamp: January 27, 2024, 04:59:20 pm
content: Hello, I have a circuit that worked for months but a couple of days ago suddenly stopped working. It turned out, that R8 resistor failed, went into an open circuit. I shorted out R8 and the circuit works again but I can not not understand why it happened. 5V_IN comes from a regular USB phone charger. Current to the 3.3V regulator (through R8) is minimal cause it supplies a microcontroller and 2 transistors so it should not overheat. So I do not see any reason how it could fail... Do you have any idea? Is there a design mistake somewhere? Schematic is attached. Thank you.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 27, 2024, 05:09:02 pm
content: Maybe a bad resistors, maybe it was fractured by mechanical stress.Add a picture of the PCB + a close-up or a microscope picture with the failed resistor.Is there any visible damage to the resistor, cold solder joints, etc.?

author: Siwastaja
author_rank: Super Contributor
timestamp: January 27, 2024, 05:12:47 pm
content: Sounds like whoever thought using RC filter on power line was a good idea did not calculate worst-case power dissipation correctly.Note that now with the resistor shorted out, the linear regulator is dissipating more power than before. Make sure it doesn't overheat next.

author: mtamas91
author_rank: Newbie
timestamp: January 27, 2024, 05:39:44 pm
content: "Sounds like whoever thought using RC filter on power line was a good idea did not calculate worst-case power dissipation correctly.Note that now with the resistor shorted out, the linear regulator is dissipating more power than before. Make sure it doesn't overheat next."What is the worst-case power dissipation? Current is less than 3 mA, with a 2R2 resistor is it 20 mW by my calculations. Not enough to fail.

author: mtamas91
author_rank: Newbie
timestamp: January 27, 2024, 05:41:02 pm
content: "Maybe a bad resistors, maybe it was fractured by mechanical stress.Add a picture of the PCB + a close-up or a microscope picture with the failed resistor.Is there any visible damage to the resistor, cold solder joints, etc.?"Unfortunately I can not provide a microscope picture, but there is not any visible damage.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 27, 2024, 07:07:41 pm
content: Pulse power is a possibility, if this is a very small general-purpose part, and especially if C7 is an MLCC (assuming from schematic symbol, it is?).Assuming far smaller than 2.2ohm ESR for C7 and whatever supplies the 5V, initial pulse power dissipated by R8 is (5V)^2 / 2.21ohm = 11.3W. The duration of the pulse could be ballpark approximated as R*C = 22uF * 2.2ohm = 50µs.If you used a pulse-withstanding resistor type, e.g. random datasheet off the web:https://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F9-1773463-9%7FB%7Fpdf%7FEnglish%7FENG_DS_9-1773463-9_B.pdf%7F2176331-4(page 5, one pulse),then you could see even the 0402 size could take the pulse, but just barely. Problem with standard general purpose parts is, this data is almost never available, so how much worse it is going to be compared to pulse withstand types, I don't know.Could be something else, but I wouldn't rule out pulse power.

author: floobydust
author_rank: Super Contributor
timestamp: January 27, 2024, 09:05:02 pm
content: TheMIC5504has "auto-discharge" feature but I think it should not activate unless EN goes low? Not sure if it latches either. Would get triggered by incoming power doing a sag possibly.Check the relay power is really coming from before the resistor, +5V_IN net looks up in the air on the schematic.TheATtiny402MCU will run off 5V.I don't like PA6 being hard grounded, if your code ever tries to turn on the pin it will cause an overload.

author: mtamas91
author_rank: Newbie
timestamp: January 28, 2024, 08:11:54 am
content: Quote from: Siwastaja on January 27, 2024, 07:07:41 pmThank you. I used the 0603 version of this resistor:https://hu.mouser.com/datasheet/2/210/kamaya_watc_s_a0005256878_1-1991085.pdfThough I cannot find any relevant information in the datasheet.And yes, C7 is an MLCC, this one:https://hu.mouser.com/datasheet/2/281/1/GRM188R61A226ME15_02A-1985787.pdfDo you think that replacing R8 with a pulse-withstanding type could help? Or what if I just short out R8? I will not have the input RC filter in this case, just an LC, but would it cause any problem? As I mentioned, 5V comes from a regular USB phone charger.

author: mtamas91
author_rank: Newbie
timestamp: January 28, 2024, 08:13:57 am
content: Quote from: floobydust on January 27, 2024, 09:05:02 pmThank you. Yes, the relay power is coming from before the resistor, I double checked it.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 28, 2024, 08:30:23 am
content: Do I understand that R8 is a 0603 package? These can typically handle 100mW.You have 33µF of capacitance total right after R8. A capacitor basically acts as a short when discharged. So whenever you power the circuit on, up to about 2.3A can get through this resistor peak (which is nearly 12W of dissipation). Shouldn't be for long, sure (RC ~ 73 µs), but possibly enough to end up vaporizing the poor chap. Other capacitors after the regulator and on the power rails will also increase the peak current further when powering the circuit on.The inrush current due to capacitors is very, and much too often overlooked. Not sure what you can do to be safe, short of using a larger package for the resistor, and/or adding some inductor in series to limit the inrush current too. Possibly the inductor can actually replace the resistor entirely in this circuit.

author: ArdWar
author_rank: Frequent Contributor
timestamp: January 28, 2024, 08:53:11 am
content: Quote from: SiliconWizard on January 28, 2024, 08:30:23 amNot all chip resistors are equal. While resistors usually have similarsteady statepower dissipation capacity for a given physical size, different resistance element pattern and/or trimming method can give different behavior to a sudden surge of current. The common thick film resistors historically perform on the worse end of spectrum due to their manufacturing process while thin films perform better. Of course these days there also thick films that are surge resistant, but they're usually from specific series.For a short reference:https://www.vishay.com/docs/48516/_ms9702509-2401-vishaychecklistpulseload.pdf

author: KerimF
author_rank: Frequent Contributor
timestamp: January 28, 2024, 09:24:35 am
content: Quote from: mtamas91 on January 27, 2024, 05:39:44 pmIf the current is less than 3 mA or (I) and you can find out the voltage's drop (V) which is needed by the 3V3 regulator for this current, you can calculate the maximum resistance of this resistor in series:R=(5-V-3.3)/IFor example, if I=3 mA and V=1.5 VR= (5-1.5-3.3)/3 = 0.2/3 ~= 66R (66 Ohm)I mean you don't have to replace the defective resistor 2R2 with a similar one having the same value.Added:Just a note, your 20 mW (above) is actually 20 uW.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 28, 2024, 09:58:02 am
content: Quote from: mtamas91 on January 28, 2024, 08:11:54 amMaybe/possibly; no pulse ratings for such general purpose parts. Replace by pulse withstand type with rated peak power of significantly more than 11W for significantly longer than 100µs or so.I would question your filter design anyway. Why so many parts? Why create the resonance mess by paralleling different MLCC values? Do you have some specific reason to use LC filter or is it "just in case"?My recommendation: remove R8 and FB2. Replace C7 by electrolytic type (or add series resistor to the capacitor branch). Remove C3 and C8, i.e., use the recommended circuit on MIC5504 datasheet, just 1uF on input and output.

author: mtamas91
author_rank: Newbie
timestamp: January 28, 2024, 11:40:09 am
content: Quote from: Siwastaja on January 28, 2024, 09:58:02 amThank you for your answer. Well, 5V is coming from a USB phone charger which is probably quite noisy. That's why I wanted to use a "good" filter. Would it be better to remove everything (except the two 1uF caps as you mentioned)? Will the noise of the charger not cause any issue? Thank you.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 28, 2024, 12:19:34 pm
content: USB power supplies are specifically designed to power circuits like this. Extra filtration is rarely needed. Some bulk high-ESR capacitance (such as electrolytic capacitor) is usually a good idea, for damping. Otherwise, linear regulator plus the MLCCs do very good job at power filtration as-is. Avoid paralleling different sizes of MLCCs because in some unlucky cases, they could amplify some unlucky noise frequency. Plus having more parts in your bill-of-materials is unnecessary cost and complexity.

author: mtamas91
author_rank: Newbie
timestamp: January 28, 2024, 12:32:08 pm
content: Thank you so much!