Fuses are commonly used to help protect an electronic circuit from damage caused by excessive current. However, in many applications resettable devices such as polymeric positive temperature coefficient (PPTC) devices, ceramic PTC devices, bimetallic breakers, and thermostats are the preferred solution.
Despite the inherent advantages of resettable devices, there are circumstances where a fuse may be the preferred form of circuit protection. Under conditions where restoration of normal operation poses a potential safety hazard and/or where service on the equipment should be performed after a fault condition has occurred, a fuse or circuit breaker is appropriate.
For example, in a device with no inputs or outputs, where high fault current could only be caused by a shorted capacitor, fuse protection might be preferred. A fuse or a manual push-button resettable circuit breaker would also be appropriate for a garbage disposal since the blades could cause serious harm if the motor were to suddenly resume operation.
On the other hand, the resettable PPTC device is a logical solution for helping protect loudspeaker coils that might be damaged by excessive power during sustained high-volumes. In this case, although a fuse could be used to protect the speaker, a blown fuse could be a source of frustration for the user and might result in a warranty return. Circuit breakers are an alternative solution, but they can arc as they open and create disturbing noises.
PPTC devices are often used in this application because they provide soft switching into a high-resistance tripped state and automatically reset to a low-resistance state when the source voltage is reduced. In most applications, power must be removed and the fault condition cleared in order to reset the PPTC device and restore the circuit to normal operation. However, if the voltage can be varied during operation, automatic reset can be designed into the application.
A typical circuit protection method is to place a PPTC device in series with the speaker. The PPTC device must be sized so that its time-to-trip at any particular current is less than the time required to damage the driver at that current. As the source power is increased, current through the PPTC device increases, eventually causing it to trip and limit the power the speaker experiences (Figure 1). As the source power is decreased to normal levels, the voltage drop across the PPTC decreases. If the voltage drops to a sufficiently low level such that the PPTC cannot dissipate enough power to stay warm it will return to a low resistance state and will "automatically reset."
When the following condition is met the device will automatically reset: