Type 0ZRS Series
PTC’s – Basic Theory of Operation / “Tripped” Resistance Explanation
A Bel PTC consists of a block of polymeric material containing conductive carbon granules which is
sandwiched between two conductive metal plates. When this polymer block reaches approximately 125C,
either due to current passing through it via conductive chains of carbon particles or due to an external heat
source; it swells volumetrically. This expansion breaks apart a majority of the chains of carbon granules that
run randomly between the two conductive plates. This behavior results in a sharp increase in resistance
across the two plates which all but eliminates current flow through the device, allowing just enough residual
current flow to maintain the block’s internal temperature at 125C. Once this “tripped” state current is cut off,
the polymer brick cools and shrinks to its original size, thereby allowing its broken carbon chains to
reestablish themselves and permit the part to return to its low resistance state. Once cooled to room
ambient, the PTC will once again exhibit a resistance less than its “R1max” rating.
At currents below the device IHOLD rating, AND at temperatures below 100C, the PTC maintains a
resistance value below its R1 MAX rating.
The catalog data for each device specifies a "Typical 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 = E²/R. Thus the approximate resistance of a “Tripped” PTC can be determined by: R = E²/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 will 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 = 60²/1 = 3,600 ohms
This same tripped device when used to protect a 12V circuit would now present an apparent resistance of:
R = 12²/1 = 144 ohms
The value for Typical Power is "typical" because any physical factors that affect heat loss (such as ambient
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.
Average Time Current Characteristic Curve at 23°C
The Average Time Current Characteristic Curve and Temperature Rerating Curve are affected by a number of variables and these curves are
provided for guidance only.
Specifications subject to change without notice
Bel Fuse Inc.
206 Van Vorst Street
Jersey City, NJ 07302 USA
© 2019 Bel Fuse, Inc.
Rev. 0ZRS Sep2019