It seems that my earlier post did not make it to the server - I apologize if this is a duplicate (but from the archive it appears not) Hi Rob, A few things to consider: 1) Most fuses have I2t (or current squared times time) ratings. There should be a melting I2t and a clearing I2t. This is the integral of current squared over time - which for a roughly fixed fuse internal resistance represents energy. The melting one is roughly the amount needed to begin melting the fuse element (which would be permanent damage if you are trying NOT to blow the fuse) and the clearing one is the amount required to not only melt the fuse element completely but also to blow the molten metal out of the gap and result in a truly open fuse. When charging a cap through a resistor from a constant voltage source, the I2t is equal to 0.5*(1/R)*C*V^2 where R is the resistance in ohms, C is the capacitance in Farads, and V is the source voltage. For your case of charging a 100uF cap through 0.34 ohms (as Russel found) from 15V, the I2t is 0.0331 amps^2 seconds. Looking on Digikey at 750mA fast picofuses, I see two values for melting I2t: 0.041 and 0.15. Notice that both of these are higher than the value in your case. However, the 0.041 value is uncomfortably close, considering that you also have to charge the cap on the output side of the regulator (although that will contribute somewhat less than its value would suggest because of the voltage drop of the regulator). To be comfortable that the inrush is not damaging the fuse, I would want to see the actual I2t be less than 1/3rd of the melting value. 2) The current rating of a fuse is the highest current at which it is guaranteed NOT to blow at 25 deg C ambient temperature. Fuses have a "no man's land" from the rating up to about 3 times the rating. The exact factor depends on fuse design and testing and can be as low as 1.7 and as high as 3.5 from what I've seen. Current in this range should not be applied to the fuse for more than a fraction of a second (maybe longer for a slow blow fuse). It may or may not blow the fuse. It may fail to blow it but it may leave it damaged with different characteristics. It could make the fuse get so hot that it melts the fuseholder or even ignites something in the vicinity of the fuse. It may blow the fuse but not clear it completely, leaving it to act like a resistor - even as high as 10k ohms or more. 3) Fuses also have a maximum interrupting current rating - if the circuit can source more than this then the fuse may arc over or explode in a short circuit. The AC and DC values of this current may differ. The value may also depend on the maximum voltage applied. 4) The voltage rating of the fuse is often very different for AC vs DC because DC will sustain a continuous arc while AC has automatic current zero-crossings to help interrupt an arc Points number 1 and 2 are the most relevant to your case. I included 3 and 4 because I was on my soapbox ;) In summary, your design challenge is to keep your worst-case inrush I2t well below the fuse's melting I2t AND also to ensure that you are not relying on the fuse to interrupt current below about 3X the rated fuse current AND that you will not subject the fuse to current above the rating but below 3X the rating (other than inrush or other rare brief events). One more caveat I've been bitten by - resistors also have a maximum I2t which they can absorb without damage. It can be quite small for SMD resistors. If you are relying on a resistor to limit inrush current AND the peak inrush would be significantly more than the resistor could handle continuously THEN you should consult the resistor datasheet to see if there are pulse ratings (either a curve or an I2t value). If not, choose a different resistor which has pulse withstanding ratings. I have been bitten by this - it is real. Your resistor may handle 100 inrushes and fail open on the 101st. Sean On Mon, Aug 27, 2018 at 10:09 PM Rob wrote: > Hello Group, > Hate to display my ignorance but... > I have a small PIC based device and have sold a small number of them > and have not had any problems until possibly now. I received a report of > a blown fuse and that has made me want to understand why that happened > since there was not an apparent obvious reason for this, or so I > thought. My project uses about 20mA while idle and can jump up to maybe > 350mA for brief moments when an audio amplifier becomes active and set > to full volume but typically runs around 150mA under normal peak spikes. > > [SNIP] --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .