Selecting ptc resettables for home made arduino

[amateur_25]

Hi
I've decided I'll put ptc fuse before the regulator that way it should  protect my entire circuit right?
I know the genuine arudino use a 500ma fuse. So do I just get one for 500ma tripping current?

[djsb]

In my experience you need to choose one by actually testing it with a power supply and an ammeter. I have resettable fuses fitted to some boards at work which are rated at 500mA but they actually trip at around 750mA. They do not go from fully open circuit to fully closed either. There is a gradual increase in resistance as the fuse gets hotter and it can hold a steady temperature. At a certain point (which you need to measure) the resistance increases exponentially and the fuse goes open circuit. You need to measure the HOLDING current (at your specified voltage) and the TRIP current. There are values mentioned in the data sheet but I wanted to test things for myself (better safe than sorry).

David.

[westfw]

A fuse (and especially a PTC resettable) is generally too slow to protect your circuit, but most definitions.  Hopefully it will be enough to protect the device (eg your laptop) powering your circuit, or at least the environment around the two of them from the fire that might otherwise result.

(for instance, AFAIR, the PTC in the official Arduino boards was added because shorts on a protoboard were causing "unprotected" laptops to reset when the 5V voltage dropped.)