307C Overcurrent Thermistors
PTCR Overcurrent Protection
Vishay Cera-Mite
APPLICATION DATA
TRIPPING ACTION DUE TO OVERCURRENT
During normal operation, the PTCR remains in a low base
resistance state (Fig P-3, Region 1). However, if current
in excess of hold current (IH) is conducted, I2 R losses
produce internal self heating. If the magnitude and time of
the overcurrent event develops an energy input in excess of
the device’s ability to dissipate heat, the PTCR temperature
will increase, thus reducing the current and protecting the
circuit.
Since the tripping operation is due to thermal change, there
is a time-trip curve associated with each device. At relatively
low magnitudes of overcurrent, it may take minutes for the
device to trip. Higher current levels can result in millisecond
response time. Trip time (t) can be calculated as follows
kM(TSW -TA)
Trip Time (t) =
I2 R - D(TSW-TA)
Where: k = coefficient of heat absorption = 0.603 J/g/°C
M = mass of PTCR = volume x 5.27x10 - 3 g/mm3
R = zero power resistance of PTCR at 25°C
PTC current limiters are intended for service on telecom
systems, automobiles, or the secondary of control transform-
ers or in similar applications where energy available is limited
by source impedance. They are not intended for application
on AC line voltages where source energy may be high and
source impedance low.
Fig P-3
PTC
RESISTANCE
100000
The current required to trip (IT) is typically specified as two
times the hold current (2 x IH). IT is defined as the minimum
rms conduction current required to guarantee thermistor
switching into a high resistance state (Fig P-3, Region 2) at
a 25°C ambient temperature.
10000
REGION 1
BASE
REGION 2
HIGH
Ambient temperature influences the ability of the PTCR to
transfer heat via surface radiation and thermal conduction at
the wire leads. At high ambient temperatures, less energy
input (via I2R) is required to reach the trip temperature. Low
ambients require greater energy input. Approximate derating
effects are shown in Fig P-2.
1000
100
RESISTANCE
RESISTANCE
RSW
=
2 x R25
CERAMIC MATERIALS
The temperature at which the PTCR changes from the
base resistance to high resistance region is determined by
R25
10
TSW
R vs. T Operating Characteristics
PTC
Temperature
25°C
the PTCR ceramic material. Switching temperature (TSW
)
described by the boundary between regions 1 & 2 (Fig P-3),
is the temperature point at which the PTCR has increased
to two times its base resistance at 25°C ambient (RSW = 2
PHYSICAL DESIGN CONSIDERATIONS
Diameter (D) - Common diameters range from 4 to 22mm.
Thickness (T) - Typical thickness ranges from 1 to 5mm.
Curie (Switching) Temperature (TSW) - See Fig P-4.
Resistivity (ρ) -
x R ). Design flexibility is enhanced by Cera-Mite’s wide
25
selection of ceramic PTCR materials with different switching
temperatures (Fig P-4).
Fig P-4
100K
Determined during sintering process; combined
with pellet geometry results in final resistance
based on:
Vishay Cera-Mite offers
a wide selection of
10K
1K
ceramic PTC materials
providing flexibility for
different ambient
ρT
R25 = zero power resistance at 25°C =
Area
temperatures. Close
protection levels are
possible by designing
resistance and physical
size to meet specific
hold current and trip
current requirements.
Table 2
100
10
How Various Physical Parameters Influence
a
PTCs:
HOLD CURRENT & TRIP TIME
Increased diameter will increase
hold current and lengthen trip time.
Increased thickness will increase
hold current and lengthen trip time.
PARAMETER
Disc Diameter (D)
VOLTAGE & CURRENT CAPABILITY
Increased diameter will increase voltage
and current ratings.
Increased thickness will increase
voltage rating; may or may not
increase current rating.
Disc Thickness (T)
2.0
1.0
Curie (Switch) (TSW
Temperature
)
Typically, lower switch temperature
materials have higher voltage/
current capability.
Higher switch temperature
materials increase hold current
and lengthen trip time.
0.1
Curie Temperature °C (±5°)
Resistance (R25
)
Higher resistance will increase
voltage capability.
Increased thermal loading typically
reduces the maximum interrupting current. hold current and lengthens trip times.
Wire leads added to a PTCR pellet act as Depends on thermal conductivity of
a thermal load resulting in reduced
maximum interrupting current.
Lower resistance will increase hold
current and lengthen trip times.
Increased thermal loading increases
SELF RESETTING - NON CYCLING - REPEATABLE
After tripping, the PTCR will remain latched in its high
resistance state as long as voltage remains applied and
sufficient trickle current is maintained to keep the device
above the switching temperature. After voltage is removed,
the PTCR resets (cools) back to its low resistance state and
is again ready to provide protection.
Thermal Loading
(Heat Sink)
Wire Leads
wire used. Copper will increase
hold current and trip time.
Applying coating to a leaded PTCR
increases hold current/trip time 10-20%.
Coating Material
Applying coating to a leaded PTCR has
minimal effect on voltage/current ratings.
www.vishay.com
12
Document Number: 23089
Revision 14-May-02
ceramite.support@vishay.com
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