Today we'll have a look inside my little fuse storage As you can see:Several different types And you might want to deploy these if you're not planning to burn down your house This is a classic fuse as you can see there is a wire inside which will melt on fault conditions this causes to circuit to be interrupted and protects the device There is one catch: You can use this fuses only a single time Once it's blown you have to replace it Thats somewhat easy on this automotive type and also glass fuses like 6x30 or 5x20 are usually no problem to replace This however will get a bit tiresome Thse 315mA-guses are soldered onto a printed circuit board If these blow you can only replace them using your soldering iron - you may want to prevent this from happening And lets not start to talk about SMD-fuses like this 1206-type Replacing this type requires a quite steady hand So how can we prevent replacing fuses on every fault? We can use circuit breakers I showed their inner workings in a previous video If they trigger you just have to switch it back on using the leaver But on comparison: There is a obvious size difference There is however a device in between - and that's what we'll have a look today The so called "PPTC" You might have noticed another device name in this abbreviation: PTC "Positive Temperature Coefficient" A device which will increase its resistance when the temperature rises PPTC means Polymeric Positive Temperature Coefficient Same concept but optimized to serve as fuse replacement As such the resistance features a very steep rise at the target temperature Depending on your supplier the devices are sold using different names: PFRA, Polyfuse, Polyswitch, Multifuse, Everfuse, etc The advantage? It is temperature-driven So there is no wire to melt but a heat-sensitive polymer With increaing current the device heats up and will reach a very high resistance - the fuse is "blown" However: If you clear the fault the device will slowly cool down and reduce its resistance - the fuse turns itself back on A big advantage in places where faults are likely to occur prominent example: USB-hubs. Broken or improperly plugged in devices may short ports If you remove the device the port will however resume operation once its PPTC has cooled down There are however several limitations if you campare against a classic fuse A melted wire leaves a gap which insulates the circuits It can break high currents in a short period of time, a PPTC is inferior looking at these parameters They are usually rated for 40V or 60V max And this 900mA fuse can propably not break more than 30-40A fault current furthermore the breaking is based on temperature so the device has to heat up first a 900mA-fuse might take 10 to 15 seconds to reach it's target temperature and breaking the circuit this time of course varies depending on the applied load we also should not forget: Its a resistor even if it's cold there is - in this...
