Type 0ZSC Series

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Bel Fuse Inc.

300 Executive Drive, Suite 300,

West Orange, NJ 07052 USA

+1 201.432.0463

Bel.US.CS@belf.com

belfuse.com/circuit-protection

© 2023 Bel Fuse, Inc.

Rev. 0ZSC Dec2023

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 terminals 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 "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 = E^2/R. Thus the approximate r

esistance 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 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 = 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 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