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Polyswitch fuse how they work

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My polyswitch fuse has a trip current 0f 0.5A Am I correct in saying the fuse will only trip if the fault current is 0.5A or over? If so then what happens if it's only say 0.3A
 
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Yup. It will only trip when current is 0.5A and above. It will continue as closed circuit if current is less than that
 
Unlike a conventional fuse, the actual current at which a polyfuse will trip depends on several factors.
The ambient temperature affects the fuse rating, as does the amount of 'extra' current flow, and the time a taken for the fuse to trip will depend on the temperature rise, caused by a specific excess current flow.

In the case of a 'dead short' everything happens pretty quick. In the case of just a small excess current, the fuse can take a while to 'blow'.

The datasheets show the graphs, but it can be difficult to be precise, because of the variable temperature involved.
 
The other thing with a polyswitch is that the supply voltage has an effect on the reset current. With a low supply voltage, there won't be much heating and the current will be held near the trip current.

With a large supply voltage, the polyswitch will get really hot, so its resistance will increase a lot, and the current will drop to a small fraction of the trip current. The normal load may have to be disconnected, and the polyswitch allowed to cool, before it will reset.
 
is there a better alternative then? My supply voltage is only 5v since it's an arudino type board am building.
 
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