Hi Ron,
You're quite right thermistors don't have hysteresis.
I was thinking of the interval between the heating
and the response from the sensor.
This gives a temperature range whilst the heating rises
and the sensor operates, and of course a similar range
whilst the temp drops and the sensor operates again.
So yes a thermistor does not have hysteresis, but the
result is that a(n) hysteresis effect still occurs.
The disadvantage with systems based on a single temp
sensor is that the system cannot rest, it will always
be either rising or falling.
Now that i have looked carefully i can see that the R2
does as you say provide hysteresis, by effectively
moving the input point a little, so this system could
if conditions allowed, come to rest between the two
settings.
I didn't really follow the bit about the relay engaging
and disengaging at intervals after the required temp
was reached.
I have re-read it a few times but i still don't get it.
My monitor is small, i am at 640 x 480, and i am fed
up with panning left and right.
I think sebi has the right approach to make the hysteresis
into a variable setting, but i would not have changed
the 100 res. as much as that, maybe to 200.
Changing that for a 1000 would push its voltage up quite
a bit, i think i would rather leave that value as it is,
or maybe a bit more like double its value.
Still, see how it goes.