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Looking at a schematic for a power regulator I found this symbol I am unfamiliar with. It looks like a variable resistor but only two connections are used. What does it mean? Symbol

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    \$\begingroup\$ Looks like a (re-settable) PTC polyfuse. \$\endgroup\$ – Steve Oct 8 '19 at 23:02
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It's a fuse - a resettable PTC.

It's resistive and when it heats up because of the current flowing through it, it gets more resistive which limits the current. When it cools down, it allows current to flow again.

The advantage is that this protects your circuit from temporary faults without having to replace the fuse because of them. A temporary fault can be caused by the user or a specific situation like a motor that is jammed.

I accidentally tested one yesterday - it got quite hot but normal operation was restored in about 30 seconds.

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  • \$\begingroup\$ It also serves as a snubber, so a seperate inrush limiter is not needed if you use a PTC. The resettable part is also relative, after tripping from a hard short it may take up to 24 hrs to reset. They can also trip from heat alone in a warm environment. \$\endgroup\$ – crasic Oct 8 '19 at 23:15
  • \$\begingroup\$ All that being said, while they are convenient, a fast-blow fuse will trip faster. \$\endgroup\$ – crasic Oct 8 '19 at 23:15
  • \$\begingroup\$ Positive Temperature Coefficient thermistor. What @le_top describes above. Here is old list of symbols by the way. \$\endgroup\$ – relayman357 Oct 8 '19 at 23:53
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The symbol is a non-linear intrinsic variable resistor by the IEC standard

In this case the part is a "polyswitch" PTC thrermistor self-resetting fuse

So the intrinsic variablility is that it's a thermistor and the non-linear feature is the sudden increase in resistance that theses parts have.

I can tell it's a polyswitch by the refdes F1 indicating a fuse and the 500mA indicating the trip current for the part.

https://en.wikipedia.org/wiki/Resettable_fuse

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