Catching up...
Cefiar Wrote:
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> nop: Actually you seem to be right. They are the
> same way, in which case bobc's wouldn't actually
> protect much as IMO the PTC is on the wrong side
> (it should be in-line with the 22R in his
> diagram).
Sorry, that was a bit confusing, my circuit is opposite way, thermistor on left, MPU on the right. I use the convention input on left, output on right, but should have labelled it better.
That certainly does sound simpler.
That is a good idea, I will add them.
Cefiar Wrote:
-------------------------------------------------------
> nop: Actually you seem to be right. They are the
> same way, in which case bobc's wouldn't actually
> protect much as IMO the PTC is on the wrong side
> (it should be in-line with the 22R in his
> diagram).
Sorry, that was a bit confusing, my circuit is opposite way, thermistor on left, MPU on the right. I use the convention input on left, output on right, but should have labelled it better.
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STB
It can be done with a nuch simpler circuit.
The input of the ADC behaves like a capacitor with some pico farads. No static current is flowing into this pin.
In this case, taken into account that the thermistor signals time constant is in the range of some 100ms, a simple series resistor of ~22kOhms followed by ~470nF (against noise) is enough to protect the input against voltages up to 30V. Clamping is done by the internal ESD protection diodes
That certainly does sound simpler.
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Cefiar
On the topic of protection, it would be worthwhile adding diodes on the FET outputs too, such as described in the following thread: [forums.reprap.org]
That is a good idea, I will add them.