Time
Dependency Applications
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ATime Dependency
Applications:
The
current-time characteristics
of NTC thermistors
provide a means of
introducing time-dependency
into electrical circuits.
There is a wide range
of applications associated
with time dependant
effects and these
represent some of
the earliest applications
of thermistors in
practical electronics.
The
current-time characteristics
are due to the thermal
heat capacity of
an energized thermistor.
It takes a certain
amount of time for
the body of the thermistor
to heat up and for
the associated drop
in resistance to
take place.
This
time associated effect
can be useful for
surge suppression,
timing delay and
other time dependent
applications. The
factors that influence
the time response
of thermistors in
an electronic circuit
include: power consumption,
thermal heat capacity,
thermal heat dissipation
to the surroundings
and thermistor material
characteristics as
discussed in previous
sections of the catalog.
Surge
Suppression
The protection of circuits from
high initial current surge during
system start-up is a significant
thermistor function. Most of
these applications require the
dissipation of high power,
requiring large thermistor elements
of the rod, disc and washer type.
BetaTHERM’s "Chip" element
products are generally not suitable
for this application.
Surge
suppression devices
are capable of withstanding
line voltages and
limit initial system
surge current for
a predetermined time
interval. The surge
protection time interval
is related to the
device thermal Time
Constant (T.C.),
base resistance and
R-T slope characteristics.
Device
construction plays
an integral role
in the thermal response
of the system function.
In general, the high
power devices have
relatively large
thermal masses. Device
T.C. values are typically
10 to 1000 seconds.
Product configurations
that thermally couple
the thermistor element
to high thermal inertia
heat sinks have been
developed by thermistor
suppliers.
Time
Delay
Time delay devices behave similarly
to componen ts used for surge
suppression. A thermistor is
placed in series with the circuit
components whose operation is
to be delayed and a fixed voltage
is applied to the network. A
timed current build-up occurs
as the thermistor element is
self-heated. The current builds
up to the level required to activate
components such as relays in
the circuit. The rate at which
the current changes is complex
and related to the thermal Time
Constant (T.C.) of the thermistor
component.
Considerations
for selection of
thermistor devices
for time delay applications
include thermistor
configuration, R
value and R-T slope,
device thermal inertia
and power dissipation.
Device characterization
for the application
is recommended and
with due care it
is possible to provide
any reasonable time
delay.
Voltage
Regulation
Thermistor devices may be used
to stabilize an output voltage
against input voltage variations
subject to the constraints of
their power handling capabilities.
A typical circuit for this application
consists of two fixed resistors
and a thermistor as depicted
in Figure # App 6. Quantitive
details of circuit design and
the selection of thermistor characteristics
for voltage regulation applications
may be found in thermistor circuit
design literature.
The sensitivity of the thermistor
to voltage gradients is directly
related to it’s thermal
mass and to it’s temperature
coefficient. Thermistors are
generally limited to voltage
regulation in circuits with low
voltages and highly resistive
loads.
Voltage
Regulation Circuit
Schematic:
Figure
# App 6
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