H O W P O LY M E R I C R E S E T TA B L E
O V E R C U R R E N T P R O T E C TO R S W O R K
The conductive carbon black filler material in the
polymeric device is dispersed in a polymer that has a
crystalline structure. The crystalline structure densely
packs the carbon particles into its crystalline
boundary so they are close enough
together to allow current to flow
through the polymer insulator
via these carbon Òchains.Ó
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 materialÕs temperature to rise. As this self heating
continues, the materialÕs 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 matrix changes to an
amorphous structure. This phase change is
accompanied by a small expansion. As the
conductive particles move apart from each other, most of
them no longer conduct current and the resistance of the
device increases sharply.
The material will stay Òhot,Ó remaining in this high
resistance 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 returns to its original value.
ST 3
50
RA
090
RA
075
RA
060
RA
600
RA
400
RA
300
R E S E T TA B L E C I R C U I T P R O T E C T I O N
When it comes to Polymeric Positive Temperature
Coefficient (PPTC) circuit protection, you now have a
choice. If you need a reliable source, look to polymeric
resettable fuses from SCHURTER.
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 black. 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 materials
to be used for resettable overcurrent protection devices 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
resistance increases with temperature. What makes the
polymeric conductive plastic material unique is the
magnitude of its resistance increase. At a specific
transition temperature, the increase in resistance is so
great that it is typically expressed on a log scale.
10
7
10
6
10
5
LOG R OHMS
10
4
10
3
10
2
10
1
10
0
0
20
40
60
80
TEMPERATURE
°C
100
120
140
104
Schurter, Inc. ¥ Phone 707-778-6311 ¥ Fax 707-778-6401 ¥ E-mail info@schurter.com ¥ Website http://www.schurter.com
ETC [ ETC ]
