Talking about fuses, sry for looking like im trying to defuse you.
Classic fuses arent that good either, even "ultrafast" are actually slow ones and have times much longer than whats needed, nowadays ICs are so fast that IC will burn first, and follow on that failure mode if its on short, then the fuse will burn after, if it will burn - at all.
PTC fuses are less fuses and more of current limiters of long time, can work at overcurrent for hours or half a day or sometimes more. It can effectively single handed control a short for quite some duration. PTC is the fuse you want to have in your airplane.
Zenner is rather useless there. The short that could happen on heater block is with +12v direct from psu, and clearly zenner isnt up to the job, probably will burn out faster than a fuse. Actually lets say thermistor has 2 legs and there is a 50% chance the short will go to adc terminal and otherwise will go the the ground thermistor leg. In last case is not too much of a stretch to think that pcb tracks wont handle so much instant and will raise the gnd potential and at least some of the current will choose to go from raised gnd troguh the protection zenner because this time zenner is forward biased into the uC!
To reliably clamp the voltage of a 40Amps smps psu from 12v to 3.3v, thats something better not try. Huge power clamp. Not flower-power zenner. Even for 100-500 miliseconds thats enough to reliably burn some fuse, you would need a big power device with a huge radiator. Pointless to say its not the way to do it, and not the place either. Big clamps and stuff of sorts you do it at power input, not far away into the board. And there is something else to think about. To what end? If you sink that much current you will burn the pcb tracks. A short like that isnt an instant impulse transient, its classic meltdown short, keeps coming and cant dissipate easily. There is nothing on either pcb that would deal with it or sink it. You can divert it instead, yes, but still it will burn anything else it meets, thermistor leg, pcb tracks, etc. Bottom line, the system is toasted, either way.
You can protect against low currents at 5v. But direct V+ to gnd short of a high current psu, thats a different league. You could like add electronic fuse circuits on all thermistor legs, but thats a whole lot of complication and will be a board with lots of heatsinks and probably bigger than the shield itself. Not worth as trouble/gain ratio.
So please do it like the arduino guys and rest of world did it: give fair warning - and cross your fingers, hope for the best.
And thats it. If you think it through all in all, thats a very good solution, really :)
Classic fuses arent that good either, even "ultrafast" are actually slow ones and have times much longer than whats needed, nowadays ICs are so fast that IC will burn first, and follow on that failure mode if its on short, then the fuse will burn after, if it will burn - at all.
PTC fuses are less fuses and more of current limiters of long time, can work at overcurrent for hours or half a day or sometimes more. It can effectively single handed control a short for quite some duration. PTC is the fuse you want to have in your airplane.
Zenner is rather useless there. The short that could happen on heater block is with +12v direct from psu, and clearly zenner isnt up to the job, probably will burn out faster than a fuse. Actually lets say thermistor has 2 legs and there is a 50% chance the short will go to adc terminal and otherwise will go the the ground thermistor leg. In last case is not too much of a stretch to think that pcb tracks wont handle so much instant and will raise the gnd potential and at least some of the current will choose to go from raised gnd troguh the protection zenner because this time zenner is forward biased into the uC!
To reliably clamp the voltage of a 40Amps smps psu from 12v to 3.3v, thats something better not try. Huge power clamp. Not flower-power zenner. Even for 100-500 miliseconds thats enough to reliably burn some fuse, you would need a big power device with a huge radiator. Pointless to say its not the way to do it, and not the place either. Big clamps and stuff of sorts you do it at power input, not far away into the board. And there is something else to think about. To what end? If you sink that much current you will burn the pcb tracks. A short like that isnt an instant impulse transient, its classic meltdown short, keeps coming and cant dissipate easily. There is nothing on either pcb that would deal with it or sink it. You can divert it instead, yes, but still it will burn anything else it meets, thermistor leg, pcb tracks, etc. Bottom line, the system is toasted, either way.
You can protect against low currents at 5v. But direct V+ to gnd short of a high current psu, thats a different league. You could like add electronic fuse circuits on all thermistor legs, but thats a whole lot of complication and will be a board with lots of heatsinks and probably bigger than the shield itself. Not worth as trouble/gain ratio.
So please do it like the arduino guys and rest of world did it: give fair warning - and cross your fingers, hope for the best.
And thats it. If you think it through all in all, thats a very good solution, really :)
