NTC
Thermistor Theory
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Voltage
- Current Characteristics
-(E.I. behaviour)
As
stated previously,
thermistors are devices
that obey Ohm’s
law at temperature
points within their
useful range. Since
Ohm’s law relates
Voltage and Current
of a component, (V
= I x R) it is
useful to consider
the voltage versus
current characteristics
of thermistor components.
Typical
voltage-current characteristics
for selected BetaTHERM
thermistors are illustrated
in Graph # 9,
below. The unique
characteristic of
thermistors where
the body temperature
increases as current
passes through it
can be seen by considering
the power levels
in the thermistor.
This is the "self-heat" mode,
and it is indicated
by the inflection
of the graph at higher
power levels.
Voltage-Current
Characteristics
for Selected BetaTHERM
Thermistors:
Graph # 9
For
thermistors being
used for temperature
sensing, control
or compensation,
it is required that
very low current
levels be utilized.
Typical values are
less than 100 microamps.
These are generally "zero-power" sensing
applications.
Applications
where thermistors
are used in self-heat
mode include
Liquid Level Control,
Air Flow Measurements,
Voltage Control,
Gas Chromatography
and Time Delay. These
applications are
based on detecting
changes in the Dissipation
Constant (D.C.) of
the system being
measured.
The next section of the catalog
discusses the specification of
thermistors in terms of precision
relative to nominal R/T characteristics,
this relates to tolerance
of thermistors.
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