Data Sheet

Type 0ZRE Series
2
/
4
Bel Fuse Inc.
206 Van Vorst Street
Jersey City, NJ
07302 USA
+1
201.432.0463
[e-mail]
belfuse.com/circuit
-
protection
© 2019
Bel Fuse, Inc.
Rev. 0ZRE
J
an
2019
PTC’s
–
Basic Theory of Operation / “Tripped” Resistance Explanation
Fundamentally, a Bel PTC
consists of a block of polymeric material containing conductive filler and bonded
between two conductive, planar terminations.
At currents below the device IHOLD rating, AND at temperatures below 100C, the PTC maintains a
resistance value below its R1 MAX
rating.
As the device's temperature approaches 130C, either due to an increase in ambient temperature or a
current exceeding its I TRIP rating, volumetric expansion of the filled polymer breaks apart the majority of
conductive pathways across the terminal
s created by chain contact of adjacent filler particles or device
resistance increases sharply by several orders of magnitude.
At the much higher “Tripped” resistance, there is just enough leakage current to allow internal heating to
“hold” the device in
its tripped state (around 125C) until power is interrupted. Once power is removed, the
PTC's core cools and contracts allowing conductive chains to reform and return the device to its low
resistance state.
The catalog data for each device specifies a "Typ
ical Power" value. This is the power required to exactly
match the heat lost by the tripped device to its ambient surroundings at 23C. By Ohm's Law, power can be
stated as: W = E^2/R. Thus the approximate resistance of a “Tripped” PTC can be determined by:
R =
E^2/W, where "E" is the voltage appearing across the PTC (usually the supply's open circuit voltage), and
"W" is the Typical Power value for the particular PTC.
Since the PPTC acts to maintain a constant internal temperature, its apparent resistance w
ill change based
upon applied voltage and, to a lesser degree, ambient conditions. Consider the following example....
A PTC with a Typical Power of 1 watt protecting a circuit using a 60V supply will demonstrate an apparent,
tripped resistance "R" of:
R
= 60^2/1 = 3,600 ohms
This same tripped device when used to protect a 12V circuit would now present an apparent resistance of:
R = 12^2/1 = 144 ohms
The value for Typical Power is "typical" because any physical factors that affect heat loss (such as a
mbient
temperature or air convection) will somewhat alter the level of power that the PTC needs to maintain its
internal temperature. In short, PTCs do not exhibit a constant, quantifiable tripped resistance value.
Type Time
–
To
–
Trip at 23
℃
.
Spe
cifications subject to change without notice