Fundamentally, a Bel Fuse 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 100°C, the PTC maintains a resistance value below its R1 MAX rating.

As the device's temperature approaches 130°C, 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 125°C) 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 23°C. By Ohm's Law, power can be stated as W = EA2/R. Thus, the approximate resistance of a "tripped" PTC can be determined by R = EA2/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 W protecting a circuit using a 60 V supply will demonstrate an apparent, tripped resistance "R" of
  • R = 6QA2/1 = 3,600 ohms 
• This same tripped device when used to protect a 12 V circuit would now present an apparent resistance of
  • R = 12A2/1 = 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.

Features		Specifications
Matte tin-plated, copper radial leads RoHS6 compliant Pb-free Halogen-free Bulk pack and tape-and-reel, auto-insertable		240 VDC voltage rating  50 mA to 2 A IHOLD ampere range UL, TUV agency approvals Wave soldering peak temp of 260°C -40°C to ±85°C operating temperature