RESETTABLE CIRCUIT PROTECTION
When it comes to Polymeric Positive Temperature
Coefficient (PPTC) circuit protection, you now have a
choice.
Polymeric fuses are made from a conductive plastic
formed into thin sheets, with electrodes attached to either
side. The conductive plastic is manufactured from a non–
conductive crystalline polymer and a highly conductive
carbon balck. The electrodes ensure even distribution of
power through the device, and provide a surface for leads
to be attached or for custom mounting.
The phenomenon that allows conductive plastic materi-
als to be used for resettable overcurrent protection de-
vices is that they exhibit a very large non–linear Positive
Temperature Coefficient (PTC) effect when heated. PTC
is a characteristic that many materials exhibit whereby re-
sistance increases with temperature. What makes the
polymeric conductive plastic material unique is the magni-
tude of its resistance increase. At a specific transition tem-
perature, the increase is resistance is so great that it is typ-
ically expressed on a log scale.
10
7
10
6
LOG R OHMS
10
5
10
4
10
3
10
2
10
1
10
0
0
20
40
60
80
TEMPERATURE
°C
100
120
140
The material will stay “hot”, remaining in this high resist-
ance state as long as the power is applied. The device will
remain latched, providing continuous protection, until the
fault is cleared and the power is removed. Reversing the
phase transformation allows the carbon chains to re–form
as the polymer re–crystallizes. The resistance quickly re-
turns to its original value.
PRODUCT SELECTION
To select the correct polymeric circuit protection device,
complete the imformation listed below for application, and
then refer to thwe resettable overcurrent protector data
sheets.
1. Determine the nromal operating current:
__________ amps
2. Determine the maximum circuit voltage (V
max
):
__________ volts
3. Determine the fault current (I
max
):
__________ amps
4. Determine the operating temperature range:
Minimum Temperature: __________
°C
Maximum Temperature: __________
°C
5. Select a product family so that the maximum rating for
V
max
and I
max
is higher than the maximum circuit volt-
age and fault current in the application.
6. Using the I
Hold
vs. Temperature Table on the product
family data sheet, select the polymeric device at the
maximum operating temperature with an I
Hold
greater
than or equal to the normal operating current.
7. Verify that the selected device will trip under fault con-
ditions by checking in the I
Trip
table that the fault cur-
rent is greater than I
Trip
for the selected device, at the
lowest operating temperature.
8. Order samples and test in application.
APPLICATIONS
The benefits of polymeric Resettable Overcurrent Pro-
tectors are being recognized by more and more design
engineers, and new applications are being discovered ev-
ery day.
The use of polymeric types of devices have been widely
accepted in the following applications and industries:
D
D
D
D
D
D
D
D
D
D
D
D
D
D
Personal computers
Laptop computers
Personal digital assistants
Transformers
Small and medium electric motors
Audio equipment and speakers
Test and measurement equipment
Security and fire alarm systems
Personal care products
Point–of–sale equipment
Industrial controls
Automotive electronics and harness protection
Marine electronics
Battery–operated toys
HOW POLYMERIC RESETTABLE
OVERCURRENT PROTECTORS WORK
The conductive carbon black filler material in the poly-
meric device is dispersed in a polymer that has a crystal-
line structure. The crystalline structure densely packs the
carbon particles into its crystalline boundry so they are
close enough together to allow current to flow through the
polymer insulator via these carbon “chains”.
When the conductive plastic material is at normal room
temperature, there are numerous carbon chains forming
conductive paths through the material.
Under fault conditions, excessive current flows through
the polymeric device. I
2
R heating causes the conductive
plastic material’s temperature to rise. As this self heating
continues, the material’s temperature continues to rise
until it exceeds its phase transformation temperature. As
the material passes through this phase transformation
temperature, the densely packed crystalline polymer ma-
trix changes to an amorphous structure. This phase
change is accompanied by a small expansion. As the con-
ductive particles move apart from each other, most of
them no longer conduct current and the resistance of the
device increases sharply.
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