droftarts: Thanks for letting us know about that.
Even if it's not the firmware that gets used in the long run, that could prove very useful for testing in the short term to ensure that everything works appropriately.
bobc: I'm assuming that the 2x4 connector up in the top right corner is for 4 thermistors?
Something we might want to consider here is putting a 1N746A (3.3V, 0.5W Zener diode) across each. This will limit the voltage into the Due to a max of 3.3v, if the hot end voltage comes in contact with the thermistor. I just checked, and the diodes are about 4 cents AUD each. To protect the Zener & the Due from long-term over-voltage, we should probably put a 100-150mA fuse (0 ohm resistor style - I've seen these for about 35 cents AUD) in series with the thermistor (between the Due input/pull-up resistor/filter cap end and the thermistor input). This way, if the thermistor input does short to a >3.3V source somehow, the 0 ohm fuse will blow if the current dissipated by the Zener is too high, protecting the Due. Another alternative is to put a small resistance in, that will dissipate that voltage, but that is not as good a solution, as it will definitely affect the reading out of the thermistor (though only slightly), and may not provide protection against larger voltages.
There are also PTC fuses like the Bourns MF-R005 (100mA trip current), which "may" be useful. As I've stated I don't trust PTC fuses much, but at least these are rated to 60V (well above any voltage we'd expect on the input). They are about 41 cents each AUD (cheapest I've seen). I don't know if the PTC will trip fast enough (5s to trip at 100mA, less at higher currents). It also adds 10 Ohms into the circuit, which is unlikely to be an issue but you never know. You can see the spec sheet at: [www.farnell.com] (top entry).
All up, for 4 thermistor inputs, we're adding a total cost of about $1.50-2.00 AUD in components to the board with 0 ohm or PTC fuses. Either way, a worthwhile cost for protection IMO (assuming it will work).
That said, I haven't compared this cost against how much it would add if we were to use Thermocouple inputs instead, though I assume it would cost more, simply for the support circuity. I also don't know if that would provide any protection from higher voltages reaching the Due. If a chip is involved then I suspect it might take the burden, rather than the Due, but even then it depends how the chip will fail.
Notes:
1. Since the Zener 3.3V, 0.5W, this gives 151mA at full rating. So that's the max we can expect the Zener to bypass if there is a fault. Outside of faults, we need to make sure the thermistor itself doesn't blow the fuse in normal operation. The thermistor will probably get down to 100 ohms at 300 Deg C (the EPCOS axial ones are about this from the spec sheet). Add in the minimum voltage divider of 1k (RAMPS is usually 4.7K, but 1K gives better resolution at higher temps, so some people may change it), and you get 1.1k, across 3.3V, which is 3mA of current pulled through the fuse. Even if we assume the thermistor input shorts to ground, that still leaves 1K in circuit, which is 3.3mA - well within the 100-150mA rating of the fuse.
2. I chose the 3.3V Zener entirely on it's 3.3V rating only. If the Due can take slightly higher voltages, then perhaps we should use a 3.6V or 3.9V Zener to avoid the Zener tripping too close to 3.3V (eg: due to tolerances). I also have no idea if having the Zener in parallel with the input will skew the results from the thermistor at all, due to any extra current it might pass below the threshold voltage.
Even if it's not the firmware that gets used in the long run, that could prove very useful for testing in the short term to ensure that everything works appropriately.
bobc: I'm assuming that the 2x4 connector up in the top right corner is for 4 thermistors?
Something we might want to consider here is putting a 1N746A (3.3V, 0.5W Zener diode) across each. This will limit the voltage into the Due to a max of 3.3v, if the hot end voltage comes in contact with the thermistor. I just checked, and the diodes are about 4 cents AUD each. To protect the Zener & the Due from long-term over-voltage, we should probably put a 100-150mA fuse (0 ohm resistor style - I've seen these for about 35 cents AUD) in series with the thermistor (between the Due input/pull-up resistor/filter cap end and the thermistor input). This way, if the thermistor input does short to a >3.3V source somehow, the 0 ohm fuse will blow if the current dissipated by the Zener is too high, protecting the Due. Another alternative is to put a small resistance in, that will dissipate that voltage, but that is not as good a solution, as it will definitely affect the reading out of the thermistor (though only slightly), and may not provide protection against larger voltages.
There are also PTC fuses like the Bourns MF-R005 (100mA trip current), which "may" be useful. As I've stated I don't trust PTC fuses much, but at least these are rated to 60V (well above any voltage we'd expect on the input). They are about 41 cents each AUD (cheapest I've seen). I don't know if the PTC will trip fast enough (5s to trip at 100mA, less at higher currents). It also adds 10 Ohms into the circuit, which is unlikely to be an issue but you never know. You can see the spec sheet at: [www.farnell.com] (top entry).
All up, for 4 thermistor inputs, we're adding a total cost of about $1.50-2.00 AUD in components to the board with 0 ohm or PTC fuses. Either way, a worthwhile cost for protection IMO (assuming it will work).
That said, I haven't compared this cost against how much it would add if we were to use Thermocouple inputs instead, though I assume it would cost more, simply for the support circuity. I also don't know if that would provide any protection from higher voltages reaching the Due. If a chip is involved then I suspect it might take the burden, rather than the Due, but even then it depends how the chip will fail.
Notes:
1. Since the Zener 3.3V, 0.5W, this gives 151mA at full rating. So that's the max we can expect the Zener to bypass if there is a fault. Outside of faults, we need to make sure the thermistor itself doesn't blow the fuse in normal operation. The thermistor will probably get down to 100 ohms at 300 Deg C (the EPCOS axial ones are about this from the spec sheet). Add in the minimum voltage divider of 1k (RAMPS is usually 4.7K, but 1K gives better resolution at higher temps, so some people may change it), and you get 1.1k, across 3.3V, which is 3mA of current pulled through the fuse. Even if we assume the thermistor input shorts to ground, that still leaves 1K in circuit, which is 3.3mA - well within the 100-150mA rating of the fuse.
2. I chose the 3.3V Zener entirely on it's 3.3V rating only. If the Due can take slightly higher voltages, then perhaps we should use a 3.6V or 3.9V Zener to avoid the Zener tripping too close to 3.3V (eg: due to tolerances). I also have no idea if having the Zener in parallel with the input will skew the results from the thermistor at all, due to any extra current it might pass below the threshold voltage.