Thanks Donald. I never ran across that Wiki article even though I was looking for some answers on this. Still seems like a long time though to get back to normal status. Made me think of "muscle wires" although they seem to work very quickly.
Dave
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On 11/10/2019 5:29 AM, Donald H Locker wrote:
The mechanism of the resettable fuse PTC is thermo-mechanical. When heated, the resistive elastomeric elements expand, increasing the length of the resistive elements. It can take a very long time, even after cooling down for those elements to settle back to their pre-heated state. (c.f. )
Donald.
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On 2019-11-09 6:54 p.m., Dave wrote:
Thanks Jan,
?? That is a good way of looking at it. I just wonder why it takes hours/days at room temp too come back down to/towards 1.5 ohms. That said, my circuit will likely never see that fuse trip anyway, I am just adding it to cover me as it will be a device that plugs into a USB port and only controls LED's. I am pretty sure USB hubs and pc motherboards also use ptc fuses to protect the motherboard.
Dave
On 11/9/2019 12:52 AM, Jan Kok wrote:
I would treat the PTC as a very loose-tolerance resistor. Make sure your circuit will work properly if the PTC resistance is anywhere between 1.5 and 10 ohms. It's probably close to 10 ohms soon after tripping or being soldered, and may drift back down toward 1.5 ohms after hours or days at room temperature.
On Thu, Nov 7, 2019 at 5:46 PM Dave <theschemer@... <mailto:theschemer@...>> wrote:
??? In reference to the part number 1206L010/60 (second one down in top
??? list), I have a question.
??? Resistance R (min) = 1.5 ohms
??? Resistance R (1max) = 10 ohms
??? R min = Minimum resistance of device in initial (un-soldered) state.
??? Rtyp = No spec given.
??? R 1max =? Maximum resistance of device at 20°C measured one hour
??? after
??? tripping or reflow soldering of 260°C for 20 sec.
??? Question: If I were to figure in the resistance for the PTC fuse
??? as part
??? of my equation for selecting the correct resitors further down the
??? circuit...What number in ohms should I use? In other words, when the
??? circuit is in normal operation, what will the resistance of the PTC
??? actually be?
??? Thanks,
??? Dave
