Selecting a resettable fuse

[alec_t]

I'm designing a circuit with a resettable fuse (for economy/simplicity) for overcurrent protection and would like advice on fuse selection. The fuse must NOT trip if the current profile is as per the first waveform in the attached, but MUST trip, within a 0.4 to ~1 second period, if the current profile is as per the second waveform. To get the trip speed of response I think an SMD fuse will be needed, but manufacturers datasheets only seem to specify maximum trip times at a steady ~5 times the fuse holding current (Ihold). Any suggestions?

 

[ericgibbs]

hi alec.

pdf link for a range of FRT PPTC resettable fuse, may be helpful for selecting.

https://www.google.co.uk/url?sa=t&r...GDQsbk0Bh3yp-m-8g&sig2=uuPJaNi8kcYWy8i-2gNh3Q

 

[alec_t]

Thanks, Eric. Gives me a few candidates, but alas I still can't track down much (any?) info on things like minimum response time, how response time is affected by heat-sinking, does device respond to average (I assume) or peak current, etc. I don't suppose any manufacturer will be forthcoming with those details, so it may boil down to suck it and see.

 

[JimB]

Can I offer this:


or this:
http://www.cooperindustries.com/content/public/en/bussmann/products.html

Trying to define fuses for overload conditions can be tricky, especially if you have to avoid nuisance trips during startup.

You do not state what the load is, I am guessing a motor driven pump from your graphs.
Also, you don't tell the supply voltage and whether AC or DC.

Depending on your application, it may be worth considering an MCB (Miniature Circuit Breaker).
MCBs are available with different tripping characteristics to suit the type of load.

Have a look at this:
https://www.electro-tech-online.com...rcuit_Breaker_Application_Guide_Technical.pdf

JimB