[time-nuts] temperature sensor

EWKehren at aol.com EWKehren at aol.com
Tue Jun 24 20:19:40 EDT 2014


We use NTC 10K with the FRK. Precision is not important.We have to play  
with the settings in order to have fan starting voltage over the full temp.  
range.
Bert Kehren
 
 
In a message dated 6/24/2014 8:12:45 P.M. Eastern Daylight Time,  
shalimr9 at gmail.com writes:

After  having tried just about every solution under the sun, my opinion is 
that  within the ambient temperature range (up to at least 100°C) and 
homebrew  budgets, nothing beats an NTC thermistor. 

They are inexpensive, have a  large output and interface most easily with a 
microcontroller's ADC in  ratiometric mode requiring a single precision 
resistor. Even cheap ones have a  1% tolerance which is more precision than you 
will ever need.

My  favorite is a 10k at 25°C with a B factor of 3380 at 25/85 that costs 
$1.25 at  Digikey. The math to derive the temperature from the ADC reading is 
simple  (you do need the log function) and a mundane 12 bit ADC gives you 
temperature  with a fraction of a degree resolution and much better than a 
degree absolute  precision if you actually needed it. With a 2.5V reference 
(like those in my  favorite Silabs uC), self heating is negligible, even in 
open air (a fraction  of a degree).

This is implemented in my Thunderbolt Monitor kit  software to measure 
ambient temperature.

If you need better resolution  than what you can get by directly measuring 
the voltage at the junction of the  resistor and thermistor (due to limited 
resolution of the ADC), add one op-amp  and three resistors, or use the PGA 
if your microcontroller has  one.

Didier KO4BB


On June 24, 2014 6:43:07 PM CDT, Chris  Albertson 
<albertson.chris at gmail.com> wrote:
>On Tue, Jun 24, 2014  at 3:24 PM, Mark Sims <holrum at hotmail.com> wrote:
>
>>  ...
>> However it is disappointing that no one has stepped up to  tackle the
>> temperature problem.
>
>
>I can post  what I have.  It's a uP based PWM fan controller.  It is  a
>stand
>alone device that does not know anything about the  FE5680 but it would
>be
>easy to add into any GPSDO using the  GPSDO's existing uP provided there
>were enough extra analog pins  available.  It uses the Arduino software
>environment but I built  it using a bare 8-pin DIP.  The problem with it
>is
>I do not  have a good temperature sensor, The ones I tried are noisy.  
>SO
>I'm looking forward to your sensor info.
>
>My  first controller used a comparator chip.  Then I figured the uP  was
>the
>same cost and same 8-pin package but could do things  like PID, data
>logging, led status blinking and "whatever".  This  is a start on the
>code
>that works just like the comparator and  uses a pot for the user to
>adjust a
>set point.
>
>If  you see a way to make this better post the changes.
>Some things I will  do are (1) use better sensor, (2) use PID library,
>(3)
>measure  ambient air  temp.
>FanController.ino
><https://dl.dropboxusercontent.com/u/28915695/FanController.ino>
>
>BTW  is there any design info on the FE5680 controller.  Schematics  of
>source code?
>
>
>
>-- 
>
>Chris  Albertson
>Redondo Beach,  California
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-- 
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