[chbot] Snubber circuits

[Peter Harris]

A little experience with snubbers.
Short version: never use metal film resistors in snubber circuits.

Long version: Way back in the day I worked on the production of control
boards for remote control gates. These used relays to control the motors
and had RC snubbers as described by Mark protecting the contacts. For
reasons unknown 1W metal film resistors were used in the design. We had a
number of units fail in the field, inspection showed that the resistors
were open circuit and the relay contacts were burning out.
It turned out that there was a spiral cut in the metal film at one end of
the resistor presumably to trim the value and when a voltage spike hit the
snubber is would spark across the film at the end of the spiral burning
some film away. We manage to replicate the issue in the lab and watched as
the sparks cutting a hairline spiral along the resistor body until it
finally went open circuit.
Changing the BOM to use carbon resistors fixed the issue.

Experience: What you have right after you needed it.

  P
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