- Bad chip resistors? - 5 Updates
- Solution Manual Electronics : A Systems Approach (5th Ed., Neil Storey) - 3 Updates
| John-Del <ohger1s@gmail.com>: Jun 29 11:59AM -0700 I've run into a lot of >500K resistors open in SMPS "start" circuits - where an IC gets its start Vcc from the rectified mains and divided down to 2.5V with half a dozen high value resistors and one low value resistor at ground. The same arrangement was also used to monitor the output and use the divided voltage for feedback. I can't recall seeing a high value smd resistor open with low voltage across them. Whenever I see any inop SMPS supply with no blown output devices, I look for these first. Interestingly, I've also found these resistors can be bothered by the adhesive beneath them and read *lower* value than their printed values. When I run into a supply that drifts, I remove these resistors, clean the adhesive off the board beneath them, and reinstall them. They read on the dot after that. |
| Phil Allison <pallison49@gmail.com>: Jun 29 06:00PM -0700 legg wrote: ============ Phil Allison > >Googling tells me this is a known hazard not shared with through hole parts - has to do with the use of silver plating on top surface of the resistor being subject to sulphiding and similar air borne contaminants. > As formation of sulphides is a surface phenomenon, I doubt > it plays a role in properly reflow-soldered junctions. ** Never said it did. As the silver layer is on TOP of the SMD chip, failure occurs at the silver to resistive element interface. You would need a microscope to see evidence of it. My concern is that many more of the same resistors will fail open in time. ..... Phil |
| Jeff Urban <jurb6006@gmail.com>: Jun 29 11:47PM -0700 Well the best thing to do is probably keep it cool and don't bend it. What else ? Change them all ? |
| Phil Allison <pallison49@gmail.com>: Jun 30 04:24AM -0700 Jeff Urban wrote: ======================= > Well the best thing to do is probably keep it cool and don't bend it. ** FFS temp was never and issue nor flexing. > What else ? Change them all ? ** Soon as pigs can fly... |
| legg <legg@nospam.magma.ca>: Jun 30 08:08AM -0400 On Mon, 29 Jun 2020 11:59:40 -0700 (PDT), John-Del <ohger1s@gmail.com> wrote: >I've run into a lot of >500K resistors open in SMPS "start" circuits - where an IC gets its start Vcc from the rectified mains and divided down to 2.5V with half a dozen high value resistors and one low value resistor at ground. The same arrangement was also used to monitor the output and use the divided voltage for feedback. I can't recall seeing a high value smd resistor open with low voltage across them. >Whenever I see any inop SMPS supply with no blown output devices, I look for these first. >Interestingly, I've also found these resistors can be bothered by the adhesive beneath them and read *lower* value than their printed values. When I run into a supply that drifts, I remove these resistors, clean the adhesive off the board beneath them, and reinstall them. They read on the dot after that. Adhesive is used when SMDs are wave-soldered, or for inverted or repeated reflow. All those processes require additional skill and technical knowledge to succeed, with a reasonable service life. There are a lot of other factors influencing parasitic terminal leakage in those cases. You'd have to measure the part and the board, before and after cleaning, before and after each stage of rework, to get any idea of the leakage source. Reduction of R in higher voltage applications is also associated with tracking and bridging of the part's element and board surfaces. All of those were affected by your simple before and after comparison. RL |
| tinfaz2012@gmail.com: Jun 30 04:36AM -0700 > Solution Manual Introduction to Linear Circuit Analysis and Modelling : From DC to RF (Luis Moura & Izzat Darwazeh) > Solution Manual Embedded Systems Architecture : A Comprehensive Guide for Engineers and Programmers (Tammy Noergaard) > Solution Manual Bioimpedance and Bioelect... Please Send me Solution Manual Electronics : A Systems Approach (5th Ed., Neil Storey) or Solution Manual Electronics : A Systems Approach (6th Ed., Neil Storey) . Thank you! |
| tinfaz2012@gmail.com: Jun 30 04:37AM -0700 please Send me Solution Manual Electronics : A Systems Approach (5th Ed., Neil Storey) or Solution Manual Electronics : A Systems Approach (6th Ed., Neil Storey) if you have it. Thank you! |
| tinfaz2012@gmail.com: Jun 30 04:40AM -0700 Please send me Solution Manual Electronics : A Systems Approach (5th Ed., Neil Storey) or Solution Manual Electronics : A Systems Approach (6th Ed., Neil Storey) if you have it. Thank you in advance! |
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