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On Mon, 11 Nov 2019, 9:06 pm Dave, <theschemer@...> wrote:

Hi Harvey,

?? In my case I am just using the USB port for handy readily available
power to run IR Led's so it is the simplest circuit. They have a .100 mA
max spec for amperage and I can run them at 50% and they still work fine
so I have plenty of leeway on the design. The USB 2.x ports have 5V and
maximum 0.500 mA available. The newer USB 3.x goes up to 0.9 mA but I
won't be using a cable that fits that one. So if my PTC kicks out at
0.200 mA or even a little later, it won't be a problem. Now protecting a
transistor may be a different story.

Thanks,

Dave

On 11/10/2019 1:58 PM, Harvey White wrote:
> Yep, from what I read, the trip current vs time to trip is a lot
> longer than for fuses, the advantage being only that they reset. From
> what I can see, they're best used in catastrophic failure cases where
> the battery (supply/circuit) can stand an overload for a bit of time,
> then the fuse trips.? They're not the kind of thing I'd think of using
> to protect an FET unless the trip current were substantially below the
> rating of the FET.? Just me, of course.
>
> Harvey
>
>
> On 11/10/2019 1:27 PM, Dave wrote:
>> Hi Harvey,
>>
>> ? According to the LittleFuse datasheet, PTC fuses are used to protect:
>>
>> USB peripherals
>> Disk drives? CD-ROMs
>> Plug and play protection for motherboards and peripherals
>> Mobile phones - battery and port protection
>> Disk drives
>> PDAs / digital cameras
>> Game console port protection
>>
>>
>> But in reality they may be used more in batteries. I just read about
>> them in relation to protecting USB ports and in order to have an
>> option other than a battery pack, I wanted to protect me from any
>> liability. :) Probably never even gonna trip the PTC fuse except for
>> when it is first soldered in. Just covering my you know what. I have
>> thoroughly thought out my plan and everything will work fine. Just
>> trying to learn as I go.
>>
>> Thanks for the information,
>>
>> Dave
>>
>> On 11/8/2019 6:35 PM, Harvey White wrote:
>>>
>>> I spent a little time looking at PTC fuses.
>>>
>>> Made the following observations:
>>>
>>> 1) their resistance is higher than a regular fuse (and a fuse *must*
>>> have a resistance)
>>>
>>> 2) the best use of the fuses is to protect from a short circuit.?
>>> The more current through the fuse, the faster it blows. If you would
>>> expect a normal 1 amp fuse to blow at 2 amps, the PTC won't, not for
>>> a while
>>>
>>> 3) the normal use for a PTC fuse is to protect a high current
>>> battery.? It's generally installed inside the pack, not user
>>> replaceable.
>>>
>>> 4) the more you use them, slowly, the resistance creeps up.
>>>
>>> 5) if you're going to use them in a circuit where you're watching
>>> voltage drops, then very carefully think the voltages and currents.?
>>> They? have a higher resistance than you'd think.
>>>
>>> Harvey
>>>
>>>
>>> On 11/8/2019 5:43 PM, Dave wrote:
>>>> Brad,
>>>> ? Thanks for the test procedure. The PTC's I am going to have to
>>>> test are .100mA hold and .250mA trip. I should be able to figure it
>>>> out but if not, I will ask for some more help. :)
>>>> Dave
>>