nophead Wrote:
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> This is maybe what I would do.
>
> [forums.reprap.org]
> lename=clamp.jpg
>
> I would look to see if I could increase the series
> resistor enough to not need the PTC without the
> ADC input leakage current creating a significant
> error. Don't have time to look at the data sheet
> right now.
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.
-------------------------------------------------------
> This is maybe what I would do.
>
> [forums.reprap.org]
> lename=clamp.jpg
>
> I would look to see if I could increase the series
> resistor enough to not need the PTC without the
> ADC input leakage current creating a significant
> error. Don't have time to look at the data sheet
> right now.
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.