You could try something like theMAX17608 - other manufacturers also produce similar units. Reverse Protection, programmable Inrush limit, +/-60V max input. I'd still use a fuse though (slow blow) or fusable tracks. If the fuse fails, the unit's probably a write-off anyway. RP On Tue, 28 Aug 2018 at 14:10, 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. > Currently, the input goes directly through a 750mA fast blow soldered > pico fuse to a 1N4007 for reverse voltage protection. Then into a low > pass pi filter consisting of 2 0.1uF caps and a 2.2uH inductor to > hopefully filter out any RF. This then goes into a 100uF/50v cap for > filtering. This then is fed into an LM2940 5 volt regulator with 47uF > and 0.1 caps on the output. I typically feed this device with a > 9vDC/800mA switching wall adapter but can also be connected to a high > current 13.8vDC supply or even a car battery at times. I selected the > 750mA fuse because it is about double maximum current draw and just > under the wall wart capability so I don't know if that was the right > choice. Also, I never considered the potential for high inrush current > to charge the 100uF cap when connected to a high current power supply. > Also when I first built this, I was also wanting to keep the input > resistance low in case it was being used out in the field on a small > capacity battery so wanted to keep the input voltage drop relatively low > which of course after considering it now, increases the inrush current > capability. > I don't have a digital storage scope so can't easily measure the > inrush current and I don't know if a min/max function on my Fluke 189 is > fast enough for capturing an event like this. So if you don't mind, I > have a few questions regarding this "design". > Initially until the blown fuse report, I wanted to know if there is a > better choice for the reverse diode that would work for this circuit > with a lower forward voltage drop? Probably some schottky diode but now > realize that would also contribute to a higher inrush current so should > probably stick with the 1N4007. > The current fuse is a soldered infuse so replacement is a pain. I > tried looking into a PTC resettable type fuse with approximately the > same parameters but all I could find that could handle a sustained > current of around 500mA has an opening current rating of around 1.5 > amps. That seems a bit high for this device since the wall wart > allegedly won't even supply that much current. Plus the higher series > resistance will add to the input voltage drop. > I'm hoping I can just replace the fast fuse with an equivalent slow > blow version but I don't know if that will survive in all circumstances. > At a minimum, I'm thinking I'll make room on the PCB for a blade fuse > holder so it can be more easily replaced by the user. But a quick look > didn't turn up many automotive blade style fuses with a 750mA slow blow > rating. > Other thoughts are to use a smaller input filter cap where I was just > thinking bigger is better. Maybe use a 47uF on the input as well as what > I'm using on the output of the regulator. > I've also considered moving the fuse to the other side of the input > capacitor but don't know the wisdom of that location. > I realize not knowing the actual inrush current I'm dealing with at > this point makes this difficult to accurately design for but I'm sure > you guys have delft with this many times already and can offer some > advice as far a just better general design principles. And if > recommending any parts, they need to be through hole. > Thanks in advance, > Rob > > > > > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=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 .