Cefiar Wrote:
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> bobc: I'm assuming that the 2x4 connector up in
> the top right corner is for 4 thermistors?
Yes.
> 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.
I was also thinking about how to protect the thermistor inputs, both for ESD and excess voltages. This is really not my field, so I'm happy to take advice. I found various things on the web e.g. [www.thebox.myzen.co.uk] but I like the idea of adding PTC fuse. I was thinking something like MF-MSMF030-2
As well or instead of zener, I am thinking of BAV99, which has fast response time. It seems like a good idea to add some series resistance and a capacitor as well, not sure how all this will fit on though :)
Last time I looked at thermocouples, the interface ics were expensive and not necessarily more accurate. However, the new MAX31855 looks cheaper and better, worth considering I think.
-------------------------------------------------------
> bobc: I'm assuming that the 2x4 connector up in
> the top right corner is for 4 thermistors?
Yes.
> 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.
I was also thinking about how to protect the thermistor inputs, both for ESD and excess voltages. This is really not my field, so I'm happy to take advice. I found various things on the web e.g. [www.thebox.myzen.co.uk] but I like the idea of adding PTC fuse. I was thinking something like MF-MSMF030-2
As well or instead of zener, I am thinking of BAV99, which has fast response time. It seems like a good idea to add some series resistance and a capacitor as well, not sure how all this will fit on though :)
Last time I looked at thermocouples, the interface ics were expensive and not necessarily more accurate. However, the new MAX31855 looks cheaper and better, worth considering I think.