From: "Sir Charles W. Shults III"
References: <firstname.lastname@example.org> <email@example.com>
Subject: Re: How to organize SMT parts?
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Sun, 24 Nov 2002 06:07:14 GMT
NNTP-Posting-Date: Sun, 24 Nov 2002 01:07:14 EST
Organization: RoadRunner - Central Florida
Yes indeed. This is most common with carbon composition resistors, but it
can happen to other types as well. Transient overvoltages can exceed the safe
current ratings of resistors, resulting in breakdown of the chemical that they
are based on. Resistors usually fail due to overheating or excessive voltage
(not counting physical damage such as impact or deformation).
Imagine a 1 kilovolt discharge through a 1kohm resistor- for that instant,
you have a current of an amp through the resistor. It can cause local hot
spots, internal arcing, or destruction of the resistive substrate itself.
Carbon composition resistors are a classic case- they can show permanent out of
tolerance changes, sometimes by as much as 25% to 50% of their imprinted value,
when exposed to 350 volts or more. This is because the carbon particles can
fuse or sinter together, changing the internal contact areas and therefore the
For metal oxide film resistors, small hot spots can burn through the oxide
film, creating small low resistance areas that cause drops in the value. For
wirewound resistors, insulation breakdown can occur, where the resistive wire
(which has a shellac or enamel insulating coat) ends up having arcs that burn
through their insulation.
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