Electronic circuits often have fuses on board the PCB. Fuses protect the circuitry from catching fire due to overload. Because of some fault like a short-circuit, a part of the circuit may start drawing more power than it is admissible. The additional power flow may lead to overheating and finally, a fire can break out. A fuse acts as a circuit breaker to protect against overload by interrupting the power flow. Typically, the fuse element is a thin wire with a low melting point. Higher power through the fuse means more increased current flow through it, which heats the wire and causes it to melt or blow. This interrupts the power flow.
Although the fuse wire acts as a protection, one of its drawbacks is it needs a physical replacement once it is blown. This is a problem for electronics at a remote location because the device will remain inoperative until someone fixes the problem and replaces the damaged fuse with a new one. This drawback has led to the development of PolyFuse.
There are electromechanical devices that act as self-resetting circuit breakers. However, most of such devices have a rating of 1A and above. Moreover, their physical size is not suitable for printed circuit boards. A PolyFuse is a self-resetting circuit breaker suitable for low voltage, low current electronics. Moreover, its physical size is small enough to allow its use on a small printed circuit board.
PolyFuses are similar to PTC or positive temperature coefficient resistors—initially, their resistance is low enough to allow the load current to flow unhindered. However, in case of an overload, the PolyFuse starts to heat up, and its resistance also increases. This helps in cutting down the load current through it. However, unlike PTCs, PolyFuses have a self-healing property. If the current through a PolyFuse reduces, its resistance drops back to a lower value. This is their self-resetting property.
A PolyFuse typically contains an organic polymer substance with the impregnation of carbon particles. The carbon particles are usually in close contact, as the polymer is in a crystalline state. This allows the resistance of the device to be low initially.
As current flow increases, the carbon in the PolyFuse heats up, and the polymer begins to expand in an amorphous state. This causes the carbon particles to separate, increasing the resistance of the device and a subsequent increase in the voltage drop across the PolyFuse, which leads to a decrease in the current flow through it. The residual current flow under the fault condition keeps the PolyFuse warm enough to limit the current. As soon as the cause of the overload is removed, the current reduces to allow the PolyFuse to cool down, regain its low resistance, and the correct operation to resume.
PolyFuses cannot act fast, because they need to heat up before limiting the current flow. That means they have a short but appreciable time delay before they operate. Hence, they are not very effective against fast surges and spikes. However, they are very useful because of their self-resetting property, making them effective against short-term short-circuits and overloads.