Many loudspeaker systems incorporate PolySwitch devices for overcurrent protection. By tripping during high-current conditions and resetting when no longer needed, PolySwitch devices provide reliable protection without the nuisance and replacement costs associated with fuses.
The Problem
Today’s speakers are generally designed and sold independently of amplifiers. Thus, mismatches may occur, which can lead to damage. Also, the advent of digital recordings and compact discs places extra burdens on sound systems. Speaker damage can result from a number of factors, including the following situations:
• High-power amplifiers used with low-power speakers may simply overdrive the speaker coils with excessive power during sustained
high volume.
• Low-power amplifiers may be overdriven so that clipping occurs. This causes an upward frequency shift of power that can overload the tweeter. This problem is especially common with the wide dynamic ranges found on compact discs.
• Digital recordings, including compact discs, with their ability to reproduce high-frequency material, place extra strain on tweeters. The protection choices for loudspeaker systems are fairly limited. Fuses will protect the speaker, but a blown fuse will be a source of frustration for the user and may result in field returns for the manufacturer. Also, the addition of a fuse holder and wire so that the fuse is accessible will increase material costs. Because the fuse must be accessible, it can be defeated or replaced with the wrong fuse.
Circuit breakers are an alternative method. However, they can arc as they start to open and cause disturbing noise until they are fully open. PolySwitch resettable devices are typically used to solve these problems.
Shunting Lightbulb
A third method is to use a shunting lightbulb in parallel with the PolySwitch device as shown schematic shows the load power characteristics for a 0.5Ω and 1.5Ω lightbulb.
As with the shunting resistor, the PolySwitch device normally carries most of the current. When the PolySwitch device trips, most of the current now passes through the lightbulb. As the bulb filament lights and heats it exhibits a PTC
effect (about 1 decade of resistance increase). As with the fixed shunt resistor, increases in drive voltage will increase load power. However, the PTC effect of the lightbulb causes this increase to be much flatter than the increase seen with the fixed-shunt resistor. The result is less of an increase in speaker power as the volume is increased.
The lightbulb is typically used only for its PTC effect, but it can also be used as an overload indication to the user. An LED in series with a resistor can also be used as an overload indication, but it does not have any PTC effect.
Source: Circuit protection Databook , Raychem