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In a circuit, a resettable fuse (or polyswitch or PPTC) is a kind of breaker component that trips when the AMPERAGE exceeds a given treshold to protect against overcurrent faults. PPTC basically trips because of heat dissipated by high current which makes its inner connection loose and opens the circuit.

The question is: If heat dissipation can be calculated using watts (amperage X volts = watts), then how can this PPTC be rated at exactly 4000mA while being suitable for between 16v and 240v? 4A*16V=64W up to 4A*240V=960W. That's 15x the power. Have you ever touched a 40W vs 100W light bulb? It's not the same heat dissipation at all. So how can this be that the same PPTC will trip at 4A no matter if the circuit is under a 16V or 240V tension?

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    \$\begingroup\$ After it is tripped it can withstand that tension, so 4A arent flowing through it anymore. \$\endgroup\$ – Wesley Lee Oct 4 '19 at 13:48
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The quick answer is that the power is not dissipated in the fuse but in the load. If, for sake of argument, the fuse has an internal resistance of 0.1 ohm then at it's full rated current it will be dissipating I^2R W = 1.6W and experience a voltage drop of IR = 0.4V. The rest of the voltage, be it 15.6V or 239.6V is across the load it is protecting.

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  • \$\begingroup\$ Ahhhh... I see! Thanks! Will be accepting the answer shortly, can't < 2 mins. \$\endgroup\$ – that-ben Oct 4 '19 at 13:53
  • \$\begingroup\$ That's only the case when the polyfuse is untripped. When it's tripped it has a much higher resistance and the majority of the voltage drop is across the polyfuse rather than the load - that's kind of the point. \$\endgroup\$ – pericynthion Oct 4 '19 at 14:04
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    \$\begingroup\$ I think Peter meant that the rest of the voltage is across the load BEFORE the polyfuse is blown. \$\endgroup\$ – that-ben Oct 4 '19 at 14:53
  • \$\begingroup\$ @pericynthion Yes, but then the current is very low and the power dissipation is still small. See the Wikipedia article. Depending on the voltage and the polyfuse's specification it may or may not reset itself. Or the load has to be removed first. \$\endgroup\$ – Peter Jennings Oct 4 '19 at 14:57
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As @PeterJennings said, the power dissipated by the polyswitch when it is conducting is a function of current. The polyswitch has its own resistance, which works against the current to generate a voltage, and, hence, heat. The switch then trips when it gets hot.

The 240V part of the rating is the voltage the switch can withstand when it is off. At a higher voltage, the switch will fail, either fully on or in some in-between state that will make it burn up (hopefully open, it would be unfortunate if they fail short).

I'm not sure why there's the 16V specification -- it's probably a guideline having to do with how much voltage drop there is across the polyswitch at full current, and the manufacturer's estimate that something running on a 16V rail will still be happy with the remaining voltage.

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