Interestingly, Metal Oxide Semiconductor (MOS)
CO (carbon monoxide) detectors use heated tin oxide.
When CO is present, the heated tin oxide reacts with
it and sounds an alarm. There are three basic types
of CO sensors:
•The most sensitive and (unfortunately) most
expensive is the electrolytic sensor.
•A colorimetric sensor measures the build-up of CO
over time but takes 48 hours to re-set once activated.
•The least expensive and most common CO sensor
is the MOS detector. This can also detect other gases
including chlorine bleach and certain silicones. This
additional detection capability explains some cases
of false alarms.
The follow-through current from a lightning strike
may generate excessive current that
permanently damages a varistor. The primary
varistor breakdown is localized heating caused
by thermal runaway. This is due to a lack of
conformity in individual grain-boundary
junctions, which leads to the failure of
dominant current paths under thermal stress.
Varistors can absorb part of a surge; however,
they do not absorb a significant percentage
of a lightning strike.
Figure 17. The static resistance of a metal oxide
varistor versus its protection level.
Varistor Useful Life
Life expectancy of a varistor is virtually
synonymous with its energy rating. As MOV
joules increase, the number of transient pulses
increase and the “clamping voltage” during
each transient decreases.
Figure 18. The maximum non-repetitive surge current is defined
by an 8/20 µs waveform (rise time 8 µs/decay time to half value 20 µs)
according to IEC 60060.
A joule is a unit of electrical energy equal
to the work done when a current of one
ampere passes through a resistance of one
ohm for a one second duration.
Now that you know all about the various technologies
and a few applications associated with resistors, what if
you need a different resistance once the resistor is already
within your planned or prototype circuits? This inability to
adjust resistance can be exasperating. This is why there
are variable resistors called potentiometers (pots, for
short). Pots tend to be larger and often panel mounted,
Most modern circuits are smaller, draw less current,
and generally use variable resistors commonly called
Figure 19. Surge currents of relatively long duration
are required for testing maximum energy absorption
capability. A rectangular wave of 2 ms is shown here as
required by IEC 60060.
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