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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