Help me understand this failure-- Cruisair relay trigger.

[danhaun]

Over past 5 years I have had two relay trigger failures where the lead on the 16K ohm resistor has parted (photos attached). i replaced these with the new versions.


I don't see evidence of excessive heat and the resistors ohm out properly. So what happened? Can I repair these with new resistors? Or is this a symptom rather than the cause of the failure?

Any help is appreciated.

Dan

[GordMay]

When current passes through a resistor element, heat is generated, and the temperature change causes mechanical changes by expansion or contraction in each of the materials involved in the component. Tension is often applied to axial leads, during the mounting process, and pressure can be exerted on the package by mechanically induced forces. I suspect that your resistor may have been installed under tension.
Bending forms are available for around $5. ➥

https://www.youtube.com/watch?v=fVI1YasH76A

[jimbunyard]

Or you could give it a longer bend to allow for the lead to absorb the movement...

[danhaun]

Thanks Gord for the tip and to Jim for "fixing" the board. Very clever photoshop I guess.

I ordered new resistors and will work on improved bends before replacing.

Dan

[transmitterdan]

The break is due to vibration. A longer bend is a good idea but it won’t fix the problem. It has to do with the vibration frequency and the distance along the wire from where it is soldered to the board and the break. There is a standing wave at the vibration frequency that determines where it will break.

Try this trick. After you install the new resistor with the longer bend get some clear RTV (bathtub caulk is ok). Squeeze out a blob of RTV between the resistor and the wire. Use enough to fill the gap between the wire and the resistor body from about 1/2 way up all the way down to the circuit board. It doesn’t have to be pretty. That will provide “dampening” and hopefully change the vibration pattern to prevent the issue in the future.

[danhaun]

Got it, thanks.

[Emmalina]

16Kohm unlikely to get hot. I think it was nipped with side cutters a while ago and has just parted. in 35 years of electronic repairs I have never seen this !

[jimbunyard]

Except if's happened at least twice, on the same resistor. Wouldn't the heating involve the amount of current that was being resisted, not the ohm rating? Perhaps the resistor is mis-sized?

[Lake-Effect]

I really don't understand it either. It can't be heating or current, because the resistance of the lead is a few orders of magnitude less than the resistor itself, and the resistor does not look like it was ever really hot. And it doesn't look to me like the lead was overstressed by the installation.

To me it looks like errant cutters, too. Maybe a rushed tech used cutters to grasp the lead to bend it over, and that created a weak spot that became a failure from vibration?

I would just replace the resistor, or (more likely cos I'm cheap) use a short piece of wire to replace the broken lead. The short piece could be a different gauge, or a piece of stranded wire even, if vibration is the culprit. Tape a couple spare resistors inside the case, and carry on.

(btw it looks like an 18k resistor - brown-gray-orange. 16 isn't a std value)

[transmitterdan]

I’ve seen it many times on these vertical mount resistors. It’s vibration....