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legg legg is offline
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Default Bad chip resistors?

On Mon, 29 Jun 2020 11:59:40 -0700 (PDT), John-Del
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