Self
Heating Effect Thermistors
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Self
Heating effect
of Thermistors:
To
fully avail of
the information
in the published
tables of Resistance
versus temperature
for thermistors
and to optimize
the accuracy of
the Steinhart-Hart
Equation it is
essential to consider
the electrical
power levels in
the thermistor
during measurements.
When the resistance
of a thermistor
is being measured
there is a voltage
across it and a
current passing
through it (from
Ohm’s Law
).
Ohms Law states
that for
a resistive
component: V
= IxR where,
V is
the voltage
across
the component
in Volts,
I is
the current
through
the component
in Amps
R is
the resistance
of the
component
in Ohms.
The power in the component
is defined as the product
of the current and voltage:
P = I x V
where P is the power
in Watts, I is in Amps,V
is in Volts.
This
power is dissipated
in the component,
and for a thermistor
the power causes
heating of the
thermistor. The
heating effect
in turn causes
the resistance
of the thermistor
to decrease. This
power dissipation
is known as self-heating
of the thermistor.
If
the power levels
are moderate (of
the order of several
milli-Watts (mW),
the self-heating
will not continue
indefinitely, because
the thermistor
will reach thermal
equilibrium with
it’s environment.
The stage at which
this equilibrium
is reached depends
on the thermal
characteristics
of the system.
It
should be noted
that when this "steady" state
is reached, the
resistance of the
thermistor will
not accurately
represent the temperature
of it’s environment.
Instead, the resistance
of the thermistor
will be lower than
expected, because
of the self heating
effect. To obtain
a resistance reading
from the thermistor
that accurately
represents the
temperature of
it’s environment
it is critical
that the power
levels (essentially
the current levels)
associated with
the measurement
are low enough
not to cause appreciable
self heating.
The
self heating effect
should be considered
in all thermistor
applications and
even in basic resistance
measurements using
a digital ohm-meter
(multi-meter).
Most manufactures
of measuring instruments
specify the magnitude
of current used
on the various
resistance measuring
ranges and it is
important to be
aware of these
values in performing
resistance measurements
on thermistors.
The
self-heating effect
is a disadvantage
in attempting to
make accurate resistance
measurements, but
it is the basis
of other applications.
These applications
are discussed in
a later section
of the catalog.
Because
the self-heating
effect can influence
the measured resistance
value of a thermistor
it is important
to quantify it
in some manner.
This is done by
using the concept
of "Zero-power
resistance characteristic."
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