I have seen symbols for one shot fuses and circuit breakers, but not for resettable fuses which reset themselves when the fault goes away. What is the recommended schematic symbol to use?
The symbol for a PPTC (AKA Resettable Fuse) is:
This is actually a more difficult question than it would seem. This is because there are a variety of PTC (positive temperature coefficient) resistor devices.
- A fuse; heating causes the resistor to go open circuit.
- An overload limiter; similar to the fuse but does not go quite as high impedance when hot.
- A current limiter; increasing resistance is intended to limit current in the circuit but keep the circuit functional
- A temperature sensor a.k.a. a thermistor; resistance of the PTC device is measured to infer its temperature.
I believe it is important for the schematic to show as much about the circuit functionality as possible. The difference between fusing and current limiting is big. In the first case the circuit stops operating in the second it does not.
My understanding is that using the rectangle or the zig-zag for a resistor depends on which side of the Atlantic you are on. Putting a diagonal line across the resistor seems to be a common way to indicate a value change. A line with an arrow across a resistor is the common way to show an adjustable resistor -- typically mechanically adjustable.
The lines at the end of the diagonal appear to be used to indicate achieving an end state. Hence the other answer has a comment pointing out the difference between a PTC thermistor and PTC fuse symbol. The thermistor is not supposed to reach an end state of open circuit; the fuse does.
While the other PTCs that do not go to an end state but rather limit current should the use a diagional line with only one horizontal line:
I like the indicating the direction of the temperature coefficient without using an initialism, which is tied to English, but I think it should be "\$+T^\circ\$" rather than with a lower case "t" to differentiate temperature from time.
I suggest that the difference between a current limiting function and thermistor be noted with text on the symbol. Something "cur. lim." or just "limit" on the current limiting devices.
I don't use these often so please let me know if I am incorrect.