Time
Dependency Applications
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Zero Power
Sensing Applications:
Temperature Measurement:
Temperature
measurement is the
most common application
for NTC thermistors.
Such devices have
found wide acceptance
as sensors in the
-100°C to +300°C
temperature range.
A common circuit that uses NTC
thermistor elements is the Wheatstone
bridge circuit shown in Figure
# App 1. A later section covers "Thermistor
Circuit Configurations" and
discusses bridge circuits in
more detail.
The notes presented in this section
concentrate on the application
principles rather than on the
details of implementation.
Thermistor selection for a bridge
circuit is based on the temperature
range to be measured, thermal
sensitivity, working environment,
time response and dimensional
constraints. Input voltage must
be low enough to prevent self-heating
of the NTC element within the
desired temperature span. Self-heating
can cause serious errors in temperature
sensing, so it is important that
the thermistor is in Zero Power
mode. As the sensor temperature
changes, current will flow through
the indicating meter, which is
typically a sensitive current
meter.
The detection meter can be calibrated
directly in temperature for properly
designed bridge circuits. Precision
temperature measurements are
often performed by comparing
the thermistor and the adjustable
(calibrated) bridge resistance
at zero current flow through
the detection meter. Multiple
location measurements using interchangeable
thermistors are feasible using
switching or multiplexing equipment.
Signals may also be input to
amplifiers, A/D converters or
control circuitry.
Wheatstone
Bridge Circuit:
Figure
# App 1
A
revolution in circuitry
design occurred with
the introduction
of integrated circuitry.
Custom microprocessors,
A/D converters, interface
electronics and displays
are now readily available.
Inexpensive circuit
modules with built-in
thermistor Resistance-Temperature
algorithms, as depicted
in Figure # App 2,
are now available
for precision temperature
measurement.
Microprocessor
- NTC Thermistor
Circuit Schematics:
Figure
# App 2
Temperature
Alarm
The replacement of the bridge
detection meter (in figure #
App1) with a sufficiently sensitive
relay will produce a temperature
alarm circuit. The bridge output
is sufficiently small and would
not energize the relay at temperatures
below the alarm set point which
is determined by the fixed resistor
legs of the bridge circuit. At
a sufficiently high temperature,
the thermistor resistance would
be reduced causing circuit imbalance
and sufficient current to actuate
the relay. Relay selection and
circuit considerations are important
concerns for temperature alarm
design.
Temperature
Control
A simple on-off control system
utilizing a relay actuated by
a Wheatstone bridge control circuit
is depicted in Figure # App 3.
The thermistor bridge circuit
provides a large voltage output
(typically 18mV/°C) so that
signal amplification is not necessary
to energize the control relay.
When the limit point is reached
or exceeded, the heater circuit
will turn "off". A
calibrated metering relay placed
in the circuit would also provide
a means of indication and control.
Temperature
Control Circuit:
Figure
# App 3
Further
details of bridge
circuits are given
in the notes that
deal with Thermistor
Circuit Configurations.
These notes provides
some guidelines on
calculating the voltage
output from a bridge
circuit and on the
selection of resistance
values for limit
setting.
While
bridge circuits are
still very important
in the implementation
of thermistor applications,
it should be noted
that the availability
of Integrated Circuit
instrumentation has
made more sophisticated
control options economical
for temperature control
applications. However,
it is still useful
to be aware of simpler
options that may
be appropriate in
some situations.
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