Am 04.09.2016 um 19:08 schrieb Colin Law:
>
> I have it to 3.3V ok. So 4.7k is ok for 3 wire mode but for parasitic
> it should be 1.5k
>
Yes.
>
> To summarize then, for parasitic mode I should not need an external
> strong pullup and should be able to use just a 1.5k pullup and to
> specify
> dtoverlay=w1-gpio,gpiopin=4
> and
> sudo modprobe w1-gpio pullup=1
>
> With in addition the 3.3 to 5V level shifter as in your first post if
> any devices on the bus need 5V
>
Yes.
>> There are two drawbacks when you use such a strong "weak" pullup. First,
>> the power consumption. I think that's negligible. Second, heating up the
>> sensor from inside by its bus output transistor. That's negligible as
>> long as you don't measure low temperatures.
>
> My rudimentary knowledge of thermodynamics tells me that if one
> provides a certain amount of power from inside as you suggest and that
> heats the device by (for example) 0.1 degrees when the device is in
> ambient 0C, then if the device were in ambient 50C then the heating
> would still be the same (0.1 degrees). Is that wrong?
>
It's a constant heat amount. Not a constant temperature amount.
Temperature inside the sensor is a derivative value.
That said, when someone mentions the absolute temperature of something,
you can immediately say it's about radiation. Heat radiation works much
better at higher temperature. It goes with T^4.
So, at lower temperature, changes of the sensor temperature in reaction
to ambient temperature will be much slower, and internally generated
heat can make a difference when sampling the temperatures often.
For your application, you always have to check if radiation is dominant.
If your answer is yes, be aware absolute temperature matters.
Kind regards
Jan
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