PTC Thermistors Application Notes

PTC Thermistors Application Notes
PTC THERMISTORS
Commercial PTC thermistors fall into two major
categories. The first category consists of thermally
sensitive silicon resistors, sometimes referred to as
“silistors”. These devices exhibit a fairly uniform
positive temperature coefficient (about +0.77% /°C)
through most of their operational range, but can also
exhibit a negative temperature coefficient region at
temperatures in excess of 150°C. These devices are
most often used for temperature compensation of
silicon semiconducting devices in the range of -60°C
to +150°C.
Figure<22> illustrates the differences between
silistor and switching PTC thermistors.
Types of Switching PTC Thermistors
Commercial switching PTC thermistors are manufactured much like the NTC disk and chip type
thermistors with metallized surface contacts.
Connection into an electrical circuit is accomplished
by means of a spring contact system or through lead
wires soldered to the device. The most common
body style is the disk, however, recent innovations
have led to chip, lozenge and surface mount
packages for these devices.
Fabrication of Switching PTC Thermistors
The material composition is blended, milled,
dried and then crushed into a powder. Following
this, the material is calcined and then milled again
with suitable binders. Next, binders are blended into
the mix and the material is spray dried in towers. A
mixture of particles with an outer coating of binders
is thus formed into a fine powder with carefully
controlled particle sizes. This powder is then
compacted to form the desired geometry in a die
using very high pressures.
Figure 22: R-T Characteristics of PTC Thermistors.
The other major category, and the one that we
shall concentrate on in this section, are referred to
as switching PTC thermistors. These devices are
polycrystalline ceramic materials that are normally
highly resistive but are made semiconductive by the
addition of dopants. They are most often manufactured using compositions of barium, lead and strontium titanates with additives such as yttrium, manganese, tantalum and silica.
These devices have a resistance-temperature
characteristic that exhibits a very small negative
temperature coefficient until the device reaches
a critical temperature, that is referred to as its
“Curie”, switch or transition temperature. As this
critical temperature is approached, the devices
begin to exhibit a rising, positive temperature coefficient of resistance as well as a large increase in
resistance. The resistance change can be as much
as several orders of magnitude within a temperature
span of a few degrees.
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The PTC thermistors must then be sintered to
form the ceramic body. Metallized contacts are
applied to the surface of the device by painting,
dipping, sputtering or flame spraying.
The processing of PTC thermistors requires very
careful control of materials and particle size in order
to produce a device with the proper switching
characteristics and voltage ratings. Contaminations
on the order of a few parts per million will cause
major changes in the thermal and electrical
properties of the PTC thermistor.
Most PTC thermistors are designed to operate
with a transition temperature somewhere between
60°C and 120°C, however, devices can be
manufactured that can switch as low as 0°C or
as high as 200°C.
The majority of switching PTC thermistors are
intended to operate without an insulation coating.
The devices are available, however, with insulation coatings when required. Also, small PTC
disks / pellets can be sealed into glass envelopes for
operation as fluid level detectors.
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Surface mount devices represent a recent technological advance in the manufacture of PTC
devices. Both power and sensing devices are available in surface mount styles.
PROPERTIES OF PTC THERMISTORS
As with the NTC devices, the PTC thermistors
have specific thermal and electrical properties which
are important considerations in each application.
These properties are also a function of the geometry
of the thermistor and of the particular “material
system” that is being used.
Thermal Properties
Electrical Properties
The electrical characteristics used to describe
PTC devices are the following:
•
•
•
•
•
•
•
•
•
Current-Time Characteristic
Resistance-Temperature Characteristic
Power and Minimum Resistance
Minimum Resistor
Temperature Coefficient of Resistance
Transition Temperature
Voltage Dependence
Voltage Rating
Voltage-Current Characteristic
The thermal characteristics of PTC devices are
similar to those of the NTC devices. Those characteristics can be described by the following terms:
Current-Time Characteristic
Any change in the amount of power applied to the
PTC thermistor will result in a change of its body
temperature. The time that it takes for the device to
either heat-up or cool-down is an important consideration in applications that involve resettable fusing,
time delay, motor start and degaussing.
• Heat Capacity
• Dissipation Constant
• Thermal Time Constant
Heat Capacity
The product of the specific heat and mass of the
thermistor, it is the amount of heat required to
produce a change in the body temperature of the
thermistor by 1°C.
Dissipation Constant
The ratio of the change in the power applied to
the thermistor to the resulting change in body
temperature due to self-heating. The factors that
affect the dissipation constant will include such
things as: leadwire materials, method of mounting,
ambient temperature, conduction or convection
paths between the device and its surroundings, and
even the shape of the device itself.
Resistance-Temperature Characteristic
Although PTC thermistors can be used for
temperature measurement and control applications
in a zero-power mode, they are not usually operated
that way. Data is not usually presented in the form
of R-T tables or interpolation equations. However,
there are some important resistance-temperature
characteristic terms that require understanding by
the designer / user of the devices.
Power and Minimum Resistance
The zero-power resistance of the PTC thermistor is
usually specified at a standard reference temperature (normally 25°C).
Thermal Time Constant
Minimum Resistance
The amount of time required for the thermistor to
change 63.2% of the difference between the selfheated temperature and the ambient after power is
disconnected. The thermal time constant is also
influenced by the same environmental factors as
those that affect the dissipation constant.
The minimum resistance of the PTC device is the
lowest value of the R-T curve that the thermistor can
achieve. It is the point just below the transition
temperature where the slope of the R-T characteristic goes to zero as the device changes from the
slight negative value for the temperature coefficient
of resistance to a large positive value.
The discussions of the thermal properties of
PTC thermistors are based upon simple device
structures. The thermal time constant and dissipation constant data which is given in thermistor
product literature is only valid for the test methods
and mounting methods employed. For information
on test methods contact Thermometrics
Engineering.
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Temperature Coefficient of Resistance
The slope of the R-T characteristic changes from
a slight negative value at temperatures below the
transition point to a positive value above the
transition point. The maximum positive value for the
temperature coefficient of resistance (maximum rate
of change) occurs within a few degrees above the
transition point. As the device gets hotter, the
change in the positive value of the coefficient begins
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to decrease gradually. Eventually the value can go
negative again, however, this usually occurs at an
extremely high temperature beyond the normal
operating range for which the device has been
designed or specified.
Transition Temperature
The transition temperature is the point at which
the resistance-temperature characteristic begins to
increase sharply. This corresponds roughly with the
Curie point of the material, however, it is difficult to
assign an exact value to this temperature. PTC
manufacturers will define this temperature as the
point where a specified ratio exists between the minimum resistance (or the 25°C zero-power resistance) and the transition temperature resistance.
For example, Thermometrics specifys the point
where the resistance is twice (2x) the
minimum value. Whereas other manufacturers
might use a figure of ten (10x).
Figure 24: Voltage Dependence of PTC’s.
Voltage Rating
The voltage rating of a PTC thermistor is the
maximum normal steady state voltage that guarantees long-term stability and service life for the
device. This value is determined by the properties of
the basic PTC ceramic structure.
Exceeding the maximum voltage rating of a device
can lead to catastrophic failure. The device can selfheat to the point beyond the maximum resistance
value shown in the R-T characteristic where the
slope of the temperature coefficient goes into a
negative region. At this point the device will go into
a thermal runaway condition until the self-heating
causes physical destruction of the device.
Figure 23: R-T Characteristic
Voltage Dependence
Voltage dependence of PTC thermistors must be
considered in any discussion of the resistance-temperature characteristic. Figure <24> shows that
for a PTC maintained at a constant temperature, the
resistance decreases as the voltage across the
device is increased. Thus, any measurements of the
resistance-temperature characteristic must specify
the voltage applied during test in order to be
meaningful.
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Voltage-Current Characteristic
The volt-amp (V/I) curve defines the relationship
between current and voltage at any point of thermal
equilibrium. It is clear from Figure 25 that the temperature and the resistance of the PTC are affected
by both power dissipation (self-heating) and ambient
conditions. Any factor that changes the dissipation
constant also changes the shape of the V/I curve.
and specifications that permit the designer or user to
create an ideal model of the device. This greatly
simplifies the design process, and is adequate for
most applications involving self-heated PTC
thermistors.
The ideal model of a PTC thermistor assumes the
following conditions:
• The resistance of the device is equal to the
minimum resistance for all temperatures below
the transition temperature.
• The resistance of the device is infinite at all temperatures above the transition temperature.
• The dissipation constant does not change over
the temperature range of interest.
• The voltage dependency of the device is not
considered.
Similar to NTC devices, the steady state voltagecurrent characteristic of PTC devices will be affected
by changes in the ambient temperature, radiation,
dissipation constant and electrical parameters in the
circuit.
Applications for PTC Thermistors
Figure 25: Voltage-Current Characteristic.
There are very few commercial applications
involving PTC thermistors that are based upon the
resistance-temperature characteristic. Most PTC
thermistor applications are based upon either the
steady state self-heated condition (voltage-current
characteristic) or upon the dynamic self-heated
condition (current-time characteristic) or a combination of both. We shall now list some of the most
popular uses for PTC thermistors.
Resettable Fuses
Figure 26: Voltage-Current Characteristic.
Figures <25> and <26> illustrate the two types
of voltage-current characteristic plots used in PTC
thermistor design applications.
The dramatic rise in resistance of a PTC at and
above the transition temperature makes it ideal for
over current protection. A circuit is designed such
that for all currents below the desired limiting current, the power dissipated in the thermistor is not
sufficient to self-heat the device to its transition
temperature. However, should an over-current
condition occur, the thermistor will self-heat beyond
the transition temperature and its resistance rises
dramatically. This causes the current in the overall
circuit to be reduced.
The voltage-current characteristic for most PTC
thermistors is usually not plotted from exact data.
Rather, the manufacturer provides certain key data
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Time Delay, Motor Starting, Degaussing
These three applications are somewhat similar in
that they all rely on the dynamic operation (CurrentTime Characteristic) of a self-heated PTC thermistor.
In each case, current is allowed to pass through a
series circuit for a prescribed amount of time before
the thermistor self-heats into a high resistance
condition.
+tº
Time Delay
+tº
Motor Starting
Figure 27: Resettable Fuse.
Figure <27> illustrates the principle of operation
behind a PTC thermistor designed to operate as a
resettable fuse. The region indicated as “A” in the
figure represents the normal range of current operation. When current exceeds the value of IMAX , the
device will self-heat, increase its resistance and
cause the circuit to operate now in the region
indicated as “B”.
The position of the load line for the circuit can be
designed such that the over-current protection is
either automatically reset, or requires a manual
reset. In the automatic reset mode, the load line
intercepts the V-I characteristic at the point labeled
“F”. Stable operation can only occur at this point
for normal loads.
In the manual reset mode, the load line
intercepts the V-I characteristic at three points in the
figure; “C”, “D” and “E”. Point “D” is in an unstable
region of the curve and so, in practice, steady
stable operation will only occur at points
“C” and “E”.
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+tº
Degaussing
Figure 28: Time Delay, Motor Start, Degaussing.
In the time delay circuit of Figure <28>, as the
thermistor self-heats and increases its resistance,
the voltage drop on the fixed resistor decreases to a
minimum.
In the motor start circuit of Figure <28> the PTC
has a low resistance at turn-on so that a significant
amount of current is permitted to flow in the starting winding of the motor. After the thermistor has
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self-heated into a high resistance state, the current
through the starting winding becomes negligible.
In the degaussing circuit of Figure <28>, the
current through the demagnetizing coil is initially
large and decreases to a negligible value as the
thermistor self-heats.
In all of these cases, the length of time required
for the switching action to occur depends upon the
amount of power applied to the PTC and its thermal
characteristics.
Figure <30> shows the V-I plots for two
modes of operating PTC thermistors as a liquid level
or flow sensors. In the top figure, the load line represents a series resistor with a fairly high value. This
causes the thermistor to cool below its transition
temperature in the high dissipation medium. The
lower figure shows the effect of a low value of series
resistor.
Heaters and Thermostats
The PTC thermistor can provide a combination of
heater and thermostat in one device. As depicted in
Figure <29>, the voltage-current characteristic
curve is very nearly a constant power locus. Voltage
fluctuations are compensated for by a corresponding
change in current. As the ambient temperature
increases, the PTC resistance also rises so that the
effect is a power reduction to the circuit.
Figure 29: Heater / Thermostat.
Liquid Level and Flow Sensing
Figure 30: Flow Sensor / Liquid Level.
These applications rely upon a change in the
dissipation constant and the result it has upon the
steady state V-I Characteristic. Operation is similar
to that of the NTC thermistors previously described.
An increase in the dissipation constant, such
as placing the device into a liquid or an air flow
condition, will lower the PTC operating
temperature and increase the amount of power
needed to maintain a given body temperature.
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