Hi,
A PTC fuse has an initial resistance. (Here 1.6 Ohms)
When current flows caused by th load, then there is a voltage drop across the fuse .. causing power dissipation .. causing temperature rise.
The temperature rise increases resistance.
At as certain load current this causes an avalance and the fuse "trips".
But the resistance doesn´t become infinite.
There is a certain fuse resistance where a balance exists:
* increasing resistance cuses the current to drop and thus causing the power dissipation to drop
* decreasing resistance causes the voltage to drop and thus causing the power dissipation to drop
When the power dissipation drops below "tripped power dissipation" (here 0.8W) then the avalanche stops.
I made a chart, I hope it helps to understand the working principle of a resettable PTC fuse.
5V power supply, 4 Ohms load, PTC fuse with 0.8W tripped_power_dissipation.
Please see:
* The cyan line. It is the current of a 4 Ohms load related to the voltage drop of the fuse.
(With 0V fuse_drop the full 5V are at the 4 Ohms load causing 1.25A to flow. With 5V across the fuse there is no voltage remaining for the load. 0V --> 0A)
* The blue line. This shows the (fuse) current related to the voltage drop of the fuse, where the fuse dissipates 0.8W.
(Some points: 1V x 0.8A = 0.8W; 2V x 0.4A = 0.8W; 4V x 0.2A = 0.8W)
* The pink line: This shows the power dissipation of the fuse in series with the load (related to the voltage drop of the fuse)
Now if you follow the pink line (beginning from left) you see with increasing current the power dissipation increases. Somewhere above 0.3W (Depending on fuse characteristic, ambient temperature, fuse thermal resistance...) the avalance starts.
Expect a voltage drop of about 0.5V (Means only 4.5V at your load !!) when the fuse starts to trip.
Then follow the dotted line "avalanche". During the avalanche the power dissipation in the fuse is higher than the 0.8W making the time during the trip to become short. (Partially overheat).
Then there is the point where the avalanche stops. This is where the power dissipation becomes exactely 0.8W and the fuse comes into a balance.
What current (fuse resistance) this means depends mainly on the supply voltage and the load resistance.
In my example you see it somewhere at 185mA. Not close to zero like a standard fuse. (Remember: fuse hold current is 0.1A!!)
The voltage at the 4 Ohms load still is somewhere at about 0.75V.
The fuse_tripped_resistance in my 5V/4Ohms example is somewhere at 23 Ohms.
So you see the behaviour of a PTC fuse is very different to a standard fuse.
Klaus