On Sun, 04 Apr 2004 23:09:12 GMT, the renowned "James"
wrote:
I am trying to design a circuit that will vary the temperature of a heater
dependant on the temprature of an external input.
The circuit would involve two thermisters (one on the temp input and one on
the output)
The logic needs to be something like this:
Outside 20deg - Inside heated to 20 deg then switch off.
Outside 10deg - Inside heated to 50deg then switch off.
Outside 0 deg - Inside heated to 80deg then switch off.
There also need to be an adjustable "shift" on the curve so that it can be
varied downwards to something like:
Outside 20deg - Inside heated to 20 deg then switch off.
Outside 10deg - Inside heated to 30deg then switch off.
Outside 0 deg - Inside heated to 40deg then switch off.
Hopefully the ascii chart below makes it clearer!
Would an adjustable resistor on the thermister be a way to acheive this?
IIRC correctly for my design days a thermister produces a variable
resistance which does not change proptionally to temprature, but can be made
"more linear" with an appropriate resistor? Is the temp range required
(40deg - 20deg to -20 for outside, or 60deg - 20deg to 80deg) small enough
that a linear equation will work (would guess I need accurancy to about 5deg
or so)
How would people here go about acheiving this? Hopefully the solution is
simpler than it sounds!
EXT
80 x x
70 x x
60 x x
50 x x
40 x x
30 x x
20 x
xx20.....15.....10.....05.....0.....-5.....-10.....-15......-20
Heat to:
How would *I* do it? An 8-pin microcontroller, two resistors and
that's about it (plus power supply, two thermistors and the adjust
pot), assuming it was for more than just one piece to make writing the
code worthwhile.
You might find it easier to start with linear temperature sensors such
as LM50s (500mV +10mV/°C output) and use some op-amps to get the
effect you want. Then it's just op-amps 101 and pretty easy.
Best regards,
Spehro Pefhany
--
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