# PTC fuse resistance characteristic?

I'm currently working on a project where I have to take into account the resistance of the in series PTC fuse, however after reading the datasheet I found two specs for resistance Rmin and R1max. I did a little more research and I came across this application note which states the following: "Figure 4 shows typical behavior for a PolySwitch device that is tripped and then allowed to cool. In this figure, we can clearly see that even after a number of hours the device resistance is still greater than the initial resistance. Over an extended period of time, the resistance will continue to fall and will eventually approach the initial resistance.However, since this time can be days, months, or years, it is not practical to expect that the device resistance will reach the original value for operational purposes" So my question basically my question is the use of R1max as the resistance of the PTC fuse the general consensus for engineering desing involving PTC fuses? Thanks

• +1 for interesting info on PTC fuse. But, seriously, you want general consensus to dictate how you design around two very clear parameters? Its pretty clear, if you want your project to behave well after a PTC trip, design for the R1max. If you want it to be fickle and maybe do something weird (I don't know, turn on the 'check engine' light or something), design with Rmin. – Bobbi Bennett Mar 25 '13 at 15:30
• The value will be something between Rmin and Rmax, so if you're depending on its exact value you're going to have trouble. – pjc50 Mar 25 '13 at 15:34

One should design a device to work adequately (and safely) with any resistance between RMin and RMax. In most cases, one would want the resistance to be as low as possible, and so RMax is the most relevant specification. There are, however, some cases where knowing that the resistance will not fall below RMin may be very important. Among other things, the RMin will limit peak fault current to (E/R) in the event of a short-circuit, even the supply could output much more. If RMin weren't specified, it might be necessary to add series resistance to ensure that peak currents stay within allowable limits [even with RMin specified, resistors might still sometimes be necessary, but specifying RMin will allow them to be eliminated in many cases where they're not].
Yep, short circuit current can be up to $V_{in}/R_{min}$ initially and as low as $V_{in}/R_{1max}$ an hour after a trip.
So it would be prudent to design so that the device protected by the PTC operates with resistance between $R_{min}$ and $R_{1max}$. The longer you can tolerate down time after a fusing incident, the closer you can design for between $R_{min}$ and $R_{max}$ instead.