# What is the schematic symbol for resettable fuses?

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?

• I'd use the standard fuse symbol and add a text note nearby that it's re-settable, personally. – Krunal Desai May 31 '16 at 22:00
• Most PPTC regulate at 85'C which rapidly ages them with continued use...just FYI. some at a higher T. – Tony Stewart Sunnyskyguy EE75 May 17 '17 at 17:36

The symbol for a PPTC (AKA Resettable Fuse) is:

• There are a few good answers here. This answer seems the most authoritative because Bourns and TE Connectivity use this symbol in their schematics. – petEEy Jun 2 '16 at 16:20

Sparkfun has the following on their website:

Which is close to what I've seen, but NOT a standard per se.

• I like the bottom symbol the best. The symbol labled F2 is a thermistor symbol. It makes sense that the symbols are very similar since they both change resistance with temperature. – petEEy Jun 2 '16 at 16:24

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.

For example this is a PTC fuse datasheet Bel PTC Fuse datasheet, and this Murata POSISTOR datasheet is a datasheet for other types of PTCs.

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.

or

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:

or

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.

This is from one of my schematics. It's like a hard-edges version of a regular fuse.

On the level of abstraction a schematic deals with, a fuse is a fuse.

• I definitely prefer to see whether the circuit contains a one-time fuse or a resettable polyfuse. – Wouter van Ooijen May 31 '16 at 17:45
• Incorrect. To properly review a schematic, it should include enough information about what each component does so the reviewer doesn't need to reference the BOM for every item. – Fix It Until It's Broken May 17 '17 at 17:42