Not something good to say, so i started with a sorry line, but thats how i see it. Perhaps i was too radical, or too pictorial, but well if i had social skills i would be with some top models now instead of posting about electronics. And please, there is no need to call names and adjectives and such. I suppose we are friends here, and better than that. If am i wrong about that, tell me and i will stop posting in your thread.
The zenner you need to clamp that must have a power rating of 12-3.3v = 8.7V @ ~40 amps = 348 Watts - not 0.5w. Because the hotend power comes from psu directly and if it shorts to thermistor leg it gets full psu power, nothing standing in between.
Sry but lets not mix overvoltage with overcurrent protection (ovp vs ocp). Or just me thinks are different.
Seriously 3.3v zenner 0.5w cant clamp a 12v, ~40amps atx psu across its terminals. This was for thermistor short to hotend block with v+, so its overcurrent. Serious solution i can think of, what some psu have is electronic fuse circuit on output, when output shorts it cuts the line, that is a low side mosfet with a big heatsink the kind there is no place onto the shield. Still doable coz it has fastest reaction time, else would need impossible heatsink for any longer. And that introduces at least one junction drop, and has its caveats, like you need to put that before the ramps board, not on it.
If 12V from psu and hotend shorts though thermistor leg and comes to the board, i can not think of something else that is gonna save it completelly. Perhaps other can and just i cant.
My point was like this. You have some current you dont want, you try to 1) dissipate it or 2) sink it to gnd.
1) You can not dissipate that 348W in realistic terms.
2) Yes one protect one pin with for that would need to redirect the current path so it doesnt go into the pin, but redirect ... where? Then it will sink some other path and something else will have to burn, thermistors leg, pcb tracks, and thats not really an overall protection is it.
So board doesnt get saved, not much. Something will burn, something will fail.
Sorry for that and please dont make it look like it is my fault.
lol (:P)
The zenner you need to clamp that must have a power rating of 12-3.3v = 8.7V @ ~40 amps = 348 Watts - not 0.5w. Because the hotend power comes from psu directly and if it shorts to thermistor leg it gets full psu power, nothing standing in between.
Sry but lets not mix overvoltage with overcurrent protection (ovp vs ocp). Or just me thinks are different.
Seriously 3.3v zenner 0.5w cant clamp a 12v, ~40amps atx psu across its terminals. This was for thermistor short to hotend block with v+, so its overcurrent. Serious solution i can think of, what some psu have is electronic fuse circuit on output, when output shorts it cuts the line, that is a low side mosfet with a big heatsink the kind there is no place onto the shield. Still doable coz it has fastest reaction time, else would need impossible heatsink for any longer. And that introduces at least one junction drop, and has its caveats, like you need to put that before the ramps board, not on it.
If 12V from psu and hotend shorts though thermistor leg and comes to the board, i can not think of something else that is gonna save it completelly. Perhaps other can and just i cant.
My point was like this. You have some current you dont want, you try to 1) dissipate it or 2) sink it to gnd.
1) You can not dissipate that 348W in realistic terms.
2) Yes one protect one pin with for that would need to redirect the current path so it doesnt go into the pin, but redirect ... where? Then it will sink some other path and something else will have to burn, thermistors leg, pcb tracks, and thats not really an overall protection is it.
So board doesnt get saved, not much. Something will burn, something will fail.
Sorry for that and please dont make it look like it is my fault.
lol (:P)