Resettable Fuse Specs

[WLTerry]

Hello people

I'm designing a power supply to a friend of mine (a professional bass player) and I found out some circuit protection devices, looking at SmallBear's Small Wart design, I wanna use Resttable Fuses. The PS is based in Geofex RG's Spyder with 8 independent isolated 9V @ 180mA outputs each. Since I have no experience with Resttable Fuses, I don't know exactly the characteristics of those devices, maybe I should check at Hold Current and Trip Current. My question is: If I want to limit the current of each output up to 180mA, what kind of Resttable Fuse should I choose? what specs I should check out? Hold Current and/or Trip Current? (what are exactly Hold Current and Trip Current  )

Yes, I used the search function and Google, but results of both are still confuse to me, and I would like to read the wisdom of the people of this forum 

Any hint will be appreciated. Thanks.
Will

[gez]

It's been a while since I used them, but IIR 'Hold Current' is the current the device will happily pass without acting like a fuse.  'Trip Current' is the point at where it does act like a fuse.  Always make sure that the trip current is below the MAX current rating of whatever you're protecting.

I didn't read this (could be more DIKI than WIKI), but here's an overview:

http://en.wikipedia.org/wiki/Polyswitch

[R.G.]

Hold current is the current the device will carry forever without going high resistance.
Trip current is the current which will cause it to go high resistance within some specified short time.

You're faced with the classical problem with fuses - how do you actually size them to do what you want? This includes resettable PTCs.

Just like regular fuses, the values of hold and trip currents are different. So you have to figure out whether you design to be sure X current will NOT open the fuse and use the hold current, or whether you want to ensure that Y current will definitely open the fuse to protect something and use the trip current. Notice that if you design to the hold current, the trip current is larger and your power supply will have to source an overload equal to (or greater!) than the trip current for the time it takes to open.  And if you design to the trip current to ensure a ceiling on current, then the hold current is less, and your power supply cannot be counted on to provide more than the hold current without nuisance tripping on currents between the hold and trip currents.

Fuses are ugly business; sloppy tolerances, poor specifications, hard to design and easy for a user to misuse. I vastly prefer solid state current limiters.

[WLTerry]

Thank you guys! I'm allways learn new stuff every day in this forums