A Bel PTC consists of a block of polymeric material containing conductive carbon particles, sandwiched between two conductive metal plates. When the polymer block reaches approximately 125°C — whether due to current flowing through the conductive carbon particle chains or an external heat source — it undergoes volumetric expansion. This expansion disrupts most of the randomly distributed carbon particle chains between the two conductive plates. This behavior causes a dramatic increase in resistance between the two plates, nearly eliminating current flow through the device, allowing only enough residual current to maintain the internal temperature of the block at 125°C. Once current is interrupted in this "tripped" state, the polymer block cools and contracts back to its original dimensions, allowing the broken carbon chains to reconnect and restoring the component to its low-resistance state. After cooling to room temperature, the PTC's resistance will once again fall below its "R1max" rated value.
At currents below the device's IHOLD rating and temperatures below 100°C, the PTC's resistance remains below its R1 MAX rated value.
The catalog data for each device specifies a "Typical Power" value. This is the power required for the device in its tripped state to dissipate heat to the surrounding environment at 23°C. By Ohm's Law, power can be expressed as: W = E² / R. Therefore, the approximate resistance of a PTC in its tripped state can be determined by: R = E² / W, where "E" is the voltage across the PTC (typically the open-circuit voltage of the power supply) and "W" is the Typical Power value for the specific PTC.
Since the function of a PPTC is to maintain a constant internal temperature, its apparent resistance will vary depending on the applied voltage and, to a lesser extent, ambient conditions. Consider the following examples...
A PTC with a Typical Power of 1 watt protecting a circuit powered by a 60V supply will have an apparent resistance "R" in the tripped state of: R = 60² / 1 = 3600 ohms
When this same tripped device is used to protect a 12V circuit, its apparent resistance will be: R = 12² / 1 = 144 ohms
The Typical Power value is "typical" because any physical factor affecting heat dissipation — such as ambient temperature or air convection — will to some degree alter the power level required for the PTC to maintain its internal temperature. In short, a PTC does not exhibit a constant, quantifiable tripped-state resistance value.
Bel Fuse 0ZRE0075FF1A