307C Overcurrent Thermistors
Vishay Cera-Mite
PTCR Overcurrent Protection
FEATURES:
Sizes For Your Application -
Hold currents from 5 mA to 1.5 A are available
in sizes from 4 to 22mm.
Better Protection, Maintenance Free -
PTCRs reset after an overcurrent
situation. Protection levels may be set lower than possible with fuses, without
worrying about nuisance trips.
Resetting, Non Cycling -
Functioning as a manual reset device, PTCR
overcurrent protectors remain latched in the tripped state and automatically
reset only after voltage has been removed. This prevents continuous cycling,
and protects against reclosing into a fault condition.
Simplified Mounting -
PTCRs may be mounted directly inside end use
equipment. Unlike fuses, no bulky fuseholder or access for user replacement
is required.
Ceramic Material Selection -
Various curie materials are available to tailor
hold and trip current operating points.
Repeatable, No Hysteresis -
After resetting, ceramic PTCRs return to the
initial resistance value, providing repeatable, consistent protection levels.
Unlike polymer type PTCRs, Vishay Cera-Mite devices exhibit no resistance
hysteresis application problems.
Telecom Line Balance -
In telecom circuits matched pairs are used to
maintain line balance. Unlike polymer PTCRs, ceramic devices maintain
balance after resetting.
A NEW DIMENSION
The Positive Temperature Coefficient
Resistor鈥檚 (PTC thermistor) unique property
of dramatically increasing its resistance
above the curie temperature makes it
an excellent candidate for overcurrent
protection applications. Overcurrent
situations in electronic devices occur
due to voltage fluctuations, changes in
load impedance, or problems with system
wiring. PTC thermistors monitor current in
series connected loads, trip in the event
of excess current, and reset after the
overload situation is removed, creating a
new dimension of flexibility for designers.
APPLICATIONS:
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Telecommunication Products
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Electronic Power Supplies
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Automotive Motor Protection
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Industrial Control Systems
APPLICATION DATA
In a typical current limiter application, the PTC device is connected in series
with a load impedance
(Fig P-1).
When current (I) flows, internal I
2
R losses
attempt to increase the PTCR鈥檚 temperature. To maintain the low resistance
鈥渙n鈥?state, stabilization must occur below the switching temperature, where
the heat generated (I
2
R) is balanced by heat lost due to radiation and
conduction.
Hold current (I
H
) is the maximum continuous current at which a PTCR can
be maintained in a low resistance 鈥渙n鈥?state while operating at rated ambient
temperature (typ 25擄C). To prevent nuisance tripping, choose the rated hold
current to be greater than the normal current expected.
Since heat dissipated by the device is proportional to the ambient temperature,
hold current must be derated for ambients higher than 25擄C according to
the following relationship:
Percent Derating
This relationship is shown in
Fig P-2,
which provides hold current (I
H
) derating
estimates for ambient temperatures in
excess of 25擄C. Five curie materials
illustrate the design flexibility offered by
ceramic PTCR鈥檚.
Fig P-2
PTC Thermistor Overcurrent Protectors
Ambient Temperature Derating of Hold and
Trip Currents
140%
120%
100%
80%
60%
40%
20%
70擄C Curie Material
80擄C Curie Material
90擄C Curie Material
105擄C Curie Material
120擄C Curie Material
Hold Current (I
H
) =
Where:
D
D( T
SW
- T
A
)
R
PTC
Fig P-1
= Dissipation Constant
(varies based on disc
size, wire type, &
coating material)
Typical PTC Current Limiter Application
PTC
0%
-10 0
10
20 30
40
50 60
70 80
90 100 110 120
T
SW
= Switching (Curie)
Temperature of
PTCR Material
T
A
= Ambient Temperature
R
PTC
= Resistance of PTCR
at 25擄C
Document Number: 23089
Revision 14-May-02
V
AC
or
DC
Ambient Temperature (擄C)
LOAD
ceramite.support@vishay.com
www.vishay.com
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