The CDR(
Current Dependent Resistor) is a
refinement of the
PTC
thermistor. It's a
relatively new
component.
The CDR has a more
instant switch from low(a few
ohms)
to a high
resistance(several
kiloohms) than the PTC.
The CDR is
manufactured for
different 'switching'
currents,
and is used
instead of a fuse. If the current through the CDR
gets higher than the
switching current the
temperature in the component rises. This causes
the
resistance to rise rapidly, thous
protecting the rest
of the
electronics in the
circuit. When the temperature
falls again, the
resistance returns to the low state.