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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