Len Lundy
Product Marketing Manager
Littlefuse Inc.
Des Plaines, Ill.
Circuit breakers and traditional fuses are no longer the only devices available for electronic-circuit protection. The choices now include polymer- based positive-temperature coefficient (PTC) devices.
Typical fuses are one-time devices because they protect circuits by opening once, then they need to be replaced. PTC devices, on the other hand, are reusable or resettable. They may be used repeatedly in a circuit and usually need not be replaced for the life of the product.
The process by which PTC devices operate is much different than the principles of operation for fuses that have been well documented over the years and are widely understood. The polymer material used in PTC devices contains particles of carbon black as the conductive media. The amount of carbon black introduced into the mixture controls the resistance. Heat produces some expansion of the polymer, shifts the carbon black, and then the material becomes less conductive or increases in resistance.
The PTC device clips a potentially damaging overcurrent to a safe level. Specifically, the excessive current through the device produces internal heating (I2R) which raises the temperature of the PTC and increases its resistance. The resistance of the PTC is generally a small part of the total circuit impedance until heated. The increase in resistance for a polymer- based PTC is nonlinear as shown in the graph and the relatively large increase in resistance reduces or limits the circuit current to a safe level. The transition point from low to high resistance is referred to as the trip point.
Heat generated by this limited current keeps the temperature of the PTC high which, in turn, forces the resistance to remain high This thermal equilibrium continues until power is removed from the circuit, which lets the PTC cool down and decrease its resistance.
The reversible action of resistance with temperature explains the resettable feature of a PTC. The PTC resets or returns to a lower resistance state when the power is removed from the circuit. This lets the device cool down. The PTC is then ready to protect further overloads. The resistance remains low when the fault has been removed. But if the overcurrent recurs, the device again switches to the high resistance state.
One critical parameter of PTC devices is leakage current. Although PTCs trip when transitioning from the low to the high-resistance state, the high current does not completely shut off as does current in a fuse when it opens. A small leakage current continues to flow. Leakage current is part of the device specification and typically ranges from about 100 mA at rated voltage to several hundred milliamps at lower voltages.
PTCs also have an interrupting rating, a maximum shortcircuit current at rated voltage. This fault current level is the maximum current that the device can withstand. A typical PTC short-circuit rating is about 40 A. Fuses, on the other hand, have almost limitless ratings from microamps to kiloamps at rated voltage.