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’s (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:
•
Telecommunication Products
•
Electronic Power Supplies
•
Automotive Motor Protection
•
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’s temperature. To maintain the low resistance
“on” 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 “on” state while operating at rated ambient
temperature (typ 25°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°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°C. Five curie materials
illustrate the design flexibility offered by
ceramic PTCR’s.
Fig P-2
PTC Thermistor Overcurrent Protectors
Ambient Temperature Derating of Hold and
Trip Currents
140%
120%
100%
80%
60%
40%
20%
70°C Curie Material
80°C Curie Material
90°C Curie Material
105°C Curie Material
120°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°C
Document Number: 23089
Revision 14-May-02
V
AC
or
DC
Ambient Temperature (°C)
LOAD
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11
307C Overcurrent Thermistors
Vishay Cera-Mite
PTCR Overcurrent Protection
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 (I
H
) is conducted, I
2
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.
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.
The current required to trip (I
T
) is typically specified as two
times the hold current (2 x I
H
). I
T
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.
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 I
2
R ) is required to reach the trip temperature. Low
ambients require greater energy input. Approximate derating
effects are shown in Fig P- 2.
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(T
SW
- T
A
)
Trip Time (t) =
2
I R - D(T
SW
- T
A
)
Where: k = coefficient of heat absorption = 0.603 J/g/ °C
M = mass of PTCR = volume x 5.27x10
- 3
g/mm
3
R = zero power resistance of PTCR at 25°C
Fig P-3
PTC
RESISTANCE
100000
10000
Resistance (log scale)
1000
REGION 1
BASE
RESISTANCE
REGION 2
HIGH
RESISTANCE
100
CERAMIC MATERIALS
The temperature at which the PTCR changes from the
base resistance to high resistance region is determined by
the PTCR ceramic material. Switching temperature (T
SW
)
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 (R
SW
= 2
x R 25 ). Design flexibility is enhanced by Cera-Mite’s wide
selection of ceramic PTCR materials with different switching
temperatures
(Fig P- 4).
Fig P-4
100K
R
SW
=
2 x R
25
R
25
10
25°C
T
SW
R vs. T Operating Characteristics
PTC
Temperature
PHYSICAL DESIGN CONSIDERATIONS
Diameter (D) - Common diameters range from 4 to 22mm.
Thickness (T) - Typical thickness ranges from 1 to 5mm.
Curie (Switching) Temperature (T
SW
) -
See Fig P-4.
Resistivity (ρ) -
Determined during sintering process; combined
with pellet geometry results in final resistance
based on:
ρT
R
25
= zero power resistance at 25°C =
Area
Table 2
How Various Physical Parameters Influence a
PARAMETER
VOLTAGE & CURRENT CAPABILITY
Disc Diameter (D) Increased diameter will increase voltage
and current ratings.
Disc Thickness (T) Increased thickness will increase
voltage rating; may or may not
increase current rating.
Curie (Switch) (T
SW
) Typically, lower switch temperature
Temperature
materials have higher voltage/
current capability.
Resistance (R
25
) Higher resistance will increase
voltage capability.
Thermal Loading Increased thermal loading typically
(Heat Sink)
reduces the maximum interrupting current.
Wire Leads
Wire leads added to a PTCR pellet act as
a thermal load resulting in reduced
maximum interrupting current.
Coating Material
Applying coating to a leaded PTCR has
minimal effect on voltage/current ratings.
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.
Higher switch temperature
materials increase hold current
and lengthen trip time.
Lower resistance will increase hold
current and lengthen trip times.
Increased thermal loading increases
hold current and lengthens trip times.
Depends on thermal conductivity of
wire used. Copper will increase
hold current and trip time.
Applying coating to a leaded PTCR
increases hold current/trip time 10-20%.
Document Number: 23089
Revision 14-May-02
10K
Resistance Ratio
1K
100
10
2.0
1.0
0.1
Vishay Cera-Mite offers
a wide selection of
ceramic PTC materials
providing flexibility for
different ambient
temperatures. Close
protection levels are
possible by designing
resistance and physical
size to meet specific
hold current and trip
current requirements.
55°C
80°C
90°C
105°C
120°C
70°C
Curie Temperature °C (±5°)
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.
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307C Overcurrent Thermistors
PTCR Overcurrent Protection
Vishay Cera-Mite
PTC THERMISTORS FOR
TELECOMMUNICATIONS
PTC Thermistors provide protection
for large digital switches. Vishay Cera-
Mite has pioneered this field with
ceramic PTC thermistors working
closely with major telephone equip-
ment and telephone protection
manufacturers. The requirements are
dynamic, as switch makers continually
strive to protect at lower levels. Vishay
Cera-Mite participates with industry
standard technical committees to
establish common definitions and
understanding of this new tech-
nology.
Fig P-8
Time-Trip Curves for Popular Telecom Pellets
100
R
L
R
SHUNT
50 - 100 mV
3
18
20
PTCR
Fig P-5
C.O. Ground
Tip
Line
Tip
C.O. Ring
Over-Current
Pressure
Contact
Fig P-6
Leaded PTCR
Pressure
Contact
Over-voltage
C.O. Tip
PTCR
Line Tip
OV
PTCR
Line Ring
C.O. Ring
To Battery
C.O. Tip
C.O. Ring
PTCR
OV
PTCR
Main Switch
PTC THERMISTOR PELLETS FOR TELECOMMUNICATIONS
Table 2
VISHAY
Fig P-7
HOLD (I
H
) TRIP (I
T
) RESISTANCE SWITCH SIZE (D) CERA-MITE
Solid Ceramic
CURRENT CURRENT
R
25
TEMP. NOMINAL
PART
Disc
mA
mA
Ohms
°C
mm
NUMBER
110
220
30
105
6.5
307C1127
Base
100
200
15
70
8
307C1128
Electrode
D
100
200
20
80
8
307C1126
Silver
110
220
18
80
8
307C1268
Electrode
120
240
15
80
8
307C1129
140
280
15
105
8
307C1435
T
110
220
15
70
9.5
307C1134
2.5mm
130
260
15
80
9.5
307C1130
140
280
9
70
9.5
307C1436
Rated Voltage = 60 V
DC
150
300
10
80
9.5
307C1437
Rated Current = 3A
Maximum Voltage = 220 Vr ms
Note 1
Note 2
Note 3
1/2
Time
To
Scope
Time (Seconds)
10
307C1130
307C1129
307C1128
307C1126
307C1127
1
0.1
0
.05
1
1.5
2
2.5
Current (Amperes)
Operating Time to 50% Current
Note 1
Hold and trip currents
are specified at
25°C ambient.
Note 2
R
25
is nominal zero power
resistance at 25°C with
tolerance of ± 20%.
Note 3
All pellets have silver elec-
trodes suitable for pressure
contact mounting.
Fig P-10
INTERRUPTING CAPACITY ESTIMATES
Under unusual circumstances, telecommunication lines may
be subjected to high surge currents as might occur from
lightning effects or accidental crossing with power lines or
transformer primaries.
Fig P-10
shows trip time curves for higher currents. Estimated
interrupting capability data is also shown in Table 3 and
is expressed as “I
2
t Let Through” based on test data
conducted in accordance with UL 497A and CSA 22.2 No.
0.7-M1985.
The data shown is for reference. Specific short circuit data or
interrupting capability is partially determined by the mounting
means and circuit application.
Fig P-9
15
85
275
A
B
C
PTCR
Current
Limiter
A = 1.5 sec.
B = 5 sec.
C = 30 min.
Time VS. Current Curves for High Current Surges (25°C)
10,000
Time (Milliseconds)
1,000
10 ohm, 80°C Pellets for
Diameters (mm
14.5mm
12.5mm
11.0mm
9.5mm
8.0mm
6.5mm
100
Variable
Voltage
Source
0-600V
60Hz
Timed Interrupter
2
Current
Probe or
26 AWG
Pair
10
2
4
6
8
10
12
14
16
t Let Through
Current (Amperes)
Document Number: 23089
Revision 14-May-02
ceramite.support@vishay.com
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13
307C Overcurrent Thermistors
Vishay Cera-Mite
CUSTOM PTCR PELLET
DESIGN CAPABILITY
• Vishay Cera-Mite will customize
solid state overcurrent protector
PTCRs to your exact requirements
for telecommunication, power
supply, or general electronic
use. Providing great flexibility to
establish specific voltage, hold
current, time-trip characteristic, and
ambient temperature values.
• Each device must be evaluated
and ratings established per appli-
cation. Mechanical packaging
influences performance ratings.
PTCR Overcurrent Protection
Table 3
RATING CHART FOR CUSTOM PELLETS
DISC DIAMETER (2.5mm THICK) 6.5mm
8mm
9.5mm
11mm
12.5mm
14.5mm
Continuous Voltage Rating (rms)
(proportional to resistance)
100 – 300 100 – 300100 – 300 100 – 300 100 – 300 50 - 300
Resistance Range @ 25°C (ohms) 10 to 35
Continuous Carry Current (mA)
Ambient 25° to 50°C (inversely
proportional to resistance)
Approximate Minimum Power
to Trip or Reset (watts)
Interrupting Capability
A. Repetitive (25 to 300 V
RMS
)
Peak power in watts
B. Non-repetitive (for 10 ohm
pellet) I
2
t Let Through
Maximum Safe Interrupting
Voltage (rms) (voltage rating
is proportional to resistance)
600
2.5
300
700
4.0
350
800
7.5
400
900
15
450
1000
20
500
1100
30
600
7 to 25
5 to 20
4 to 17
2 to 15
1 to 10
60 – 120 75 – 175 100 – 200 110 – 250 130 – 400 150 – 600
0.4
0.5
0.6
0.7
0.8
0.9
Rating applies to pellets with silver electrodes and pressure connections.
TRANSIENT VOLTAGE &
CURRENT
Because of the thermal storage
capacity of the ceramic PTCR,
transient surges do not cause tripping.
The PTCR is considered to be
transparent to these low energy
transients.
Fig P-11
shows a typical
test circuit for such transients.
Fig P-11
1000
10 x 1000µ sec.
± 1000V peak
PTCR
Pass-Thru Pulse
WIRE LEADED PTC
TELECOM THERMISTORS
Resettable current limiters featuring
hold current and voltage ratings for
telecommunication applications.
Fig P-12
Tinned Copper Wire
22 AWG Standard
20 AWG on D=14.5mm
5mm
max
32mm min
4.5mm
max
D
LS = 5mm
Table 4
TELECOM CURRENT LIMITERS
VISHAY
Note 1
HOLD (I
H
) TRIP (I
T
) RESISTANCE SWITCH SIZE (D) MAX. CERA-MITE
Hold and trip
CURRENT CURRENT R
25
TOL. TEMP. NOMINAL VOLTAGE
PART
V
RMS
mA
mA
Ohms %
°C
mm
NUMBER
currents specified at
70
100
100
110
110
120
120
120
130
120
120
150
120
125
135
150
170
110
125
140
200
200
220
220
240
240
240
260
240
240
300
240
250
270
300
340
220
250
Note 1
100
20
30
18
25
15
20
25
13
39
25
12
15
20
10
10
10
23
18
Note
25
20
20
20
20
20
20
20
20
30
25
20
25
20
25
20
20
20
25
2
120
80
105
80
105
80
105
120
80
120
105
90
80
105
80
105
105
80
80
6.5
8
8
8
8
8
8
8
8
8.7
8.7
8.7
9.5
9.5
9.5
9.5
11.2
14.5
14.5
265
220
220
220
220
220
220
220
120
250
250
110
220
220
220
220
220
300
265
307C1418
307C1305
307C1506
307C1354
307C1514
307C1129
307C1296
307C1470
307C1421
307C1505
307C1501
307C1439
307C1465
307C1507
307C1469
307C1233
307C1234
307C1262
307C1254
Note 3
25°C ambient.
Note 2
R25 is nominal zero
power resistance
(± 25%) at 25°C.
Note 3
P/N suffix describes
options including:
Tape & Reel
Wire Size
Wire Style &
Length
Lead Spacing
Coating Material
Rated Voltage = 60Vdc; Rated Current = 3A at rated voltage.
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Document Number: 23089
Revision 14-May-02
307C Overcurrent Thermistors
PTCR Overcurrent Protection
Table 5
Vishay Cera-Mite
GENERAL PURPOSE
PTC CURRENT LIMITERS
GENERAL PURPOSE PTC THERMISTORS OVERCURRENT PROTECTORS
RATED
VOLTAGE
VRMS
12
12
12
24
24
24
50
50
50
50
50
120
120
120
120
120
120
240
240
240
240
240
240
240
240
240
MAX.
HOLD (I
H
)
VOLTAGE CURRENT
VRMS
mA
15
15
15
30
30
30
60
60
60
60
60
140
140
140
140
140
140
375
340
310
265
265
320
320
265
265
130
170
600
130
175
600
60
120
150
325
475
60
85
95
115
105
350
20
28
31
34
40
45
55
65
90
Note 1
TRIP (I
T
)
MAX.
CURRENT CURRENT
mA
A
260
340
1200
260
350
1200
120
240
300
650
950
120
170
190
230
210
700
40
56
62
68
80
90
110
130
180
1.1
2.4
10
2.3
3.4
11
0.8
2
2.6
10
12
0.6
0.8
1.5
2
1
5
0.2
0.3
0.33
0.34
0.45
0.4
0.5
0.6
1
RES
R
25
Ohms
13
6
1.2
10
6
1.3
50
12
10
3.5
2
50
30
39
27
20
4.5
600
300
240
200
125
150
100
70
45
Note 2
SWITCH
TEMP
°C
120
105
105
105
105
105
105
105
105
105
105
105
105
105
105
105
105
105
105
105
105
105
105
105
105
105
D
MAX.
mm
5.5
8
16
8
9.5
17.5
6.5
8
9.5
14.5
17.5
6.5
8
11
12.5
9.5
19
6.5
6.5
6.5
6.5
6.5
9.5
11
9.5
11
VISHAY
CERA-MITE
PART
NUMBER
307C1455
307C1308
307C1311
307C1315
307C1429
307C1318
307C1321
307C1323
307C1548
307C1325
307C1326
307C1329
307C1330
307C1302
307C1303
307C1331
307C1333
307C1335
307C1336
307C1337
307C1338
307C1340
307C1339
307C1341
307C1342
307C1343
Note 3
•
Designed as resettable current
limiters, PTC thermistors offer an
alternative to conventional over-
current protection devices such as
fuses or circuit breakers.
•
A wide variety of sizes and current
ranges are available for many
electronic, industrial and automotive
applications. Both standard parts and
custom designs are offered.
Fig P-13
Tinned Copper Clad Steel Wire
24 AWG when D < 9.5mm
22 AWG when D > 11mm to < 19mm
20 AWG when D > 21mm
5mm
max
4.5mm
max
D
CL
LS
CL - Cut Leads are Standard 4.75± 0.5mm
LS - Standard Lead Spacings:
5mm when
D
≤11mm
7.5mm when D = 12.5 to 17.5mm
10mm when
D
≥19mm
CUSTOM CURRENT LIMITER GUIDELINES
Table 6
APPLICATION CONSIDERATIONS:
• PTC current limiters are intended for service on
telecom systems, automobiles, or the secondary of
control transformers 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.
• Fuses and circuit breakers result in total circuit
isolation after tripping. PTC thermistors provide a
current limiting function by switching to a high
resistance mode. Safety consideration must be given
to the potential shock hazard caused by the steady
state leakage current and voltage potential remaining in
the circuit.
• Wire leaded PTC current limiting thermistors are intended
for applications which expect a limited number of tripping
operations. Actual life is a function of operating parameters.
For high duty cycle applications, ceramic PTC pellets
mounted in spring contact mechanical housings are
preferred.
• Wire size, wire type and coating material can be used to
precisely tailor required operating characteristics.
• Options Include: Tape & Reel; Wire Forms; Lead
Spacings.
RANGE CHART FOR CUSTOM WIRE LEADED DESIGN
MAX.
D (mm)
COATED
5.5
6.5
8
9.5
11
12.5
14.5
16
DESIGN LIMITS (APPROX.)
V
RMS
600
15
600
15
600
15
600
15
600
15
600
15
600
15
600
15
I
HOLD
OHMS
5 mA 2000
150 mA 13
7 mA 1200
200 mA
8
10 mA 850
275 mA
6
13 mA 500
350 mA
4
20 mA 350
450 mA 2.5
22 mA 250
500 mA 2.0
30 mA 200
650 mA 1.5
35 mA 150
800 mA 1.2
MAX.
D (mm)
COATED
17.5
DESIGN LIMITS (APPROX.)
V
RMS
I
HOLD
OHMS
600
40 mA 125
15
950 mA 0.8
19
600
45 mA 100
15
1.1 A
0.7
21
600
55 mA
80
15
1.2 A
0.6
22.5
600
60 mA
70
15
1.3 A
0.5
23.5
600
70 mA
60
15
1.4 A 0.45
25
600
80 mA
50
15
1.5 A
0.4
Resistance is propor tional
to voltage and inversely
propor tional to hold
current (I
H
)
Conformal coating adds 1.5mm
Document Number: 23089
Revision 14-May-02
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15
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