Part of my problem while testing my sensors was measuring the lower resistance values with water above 200F. My sensors seem to read just a bit too high, so I donít know if they would trip the fan until after the critical temp threshold has been crossedói was it to trip before.
Water is plenty mysterious me thinks. It boils at 212 degrees (atmospheric pressure at sea level) and it won't get any hotter than 212 degrees once it reaches that boiling point. I'm no expert in any of this stuff, but it seems like water begins to vaporize before it reaches the full boiling point, as you mentioned that at about 200 degrees its latent heat and vapor pressure properties seem to begin to shift a great deal. We observe heat energy that was being absorbed (heating the water), then begins to be consumed by the water (and then removed by the gas), in its' liquid to gas conversion process. The nearer to boiling point the water gets, the more heat energy it consumes, rather than absorbs...? Observationally thats what seems to happen but I'm sure that I don't understand water very well. So yeah, I agree with ya... for some reason ya can't trust a thermister in heated water as it nears its boiling point. If you understand why/what happens, please poke me with a sharp stick until I understand too. :-)
Retro I donít know how the Op-Amp works and barely remember how to read a circuit.
I see a diode in there and many resistors. Do you have rheostats in there or is that handled by the OpAmp?
The LM741 Op-amp is configured to act as a voltage comparator device. Its no longer functioning as an Operational Amplifier in its strictest sense, in other words. It works by comparing two voltages at its two input pins (battery voltage passing thru a resistor @pin 2, versus battery voltage passing thru the thermistor/oil temp sensor @pin 3) & outputs a voltage signal to the transistor (which energizes the fan relay) when those two input voltages differ.
For instance: a 10k ohms @25c degrees thermistor @pin 3 requires a 10k ohms resistor load @pin 2 to balance the circuit @25c degrees. Whenever resistance drops below a target threshold thru the thermistor @pin 3 as it becomes heated, the LM741 responds to those two differing voltages at its' inputs by sending a voltage thru its output pin.
So we must choose our resistor load for pin 2, to meet or exceed the resistance load pin 3 will see thru the oil temp sensor in order to keep the fan turned off until the oil reaches our target temp. We put a resistor on the LM741 output pin to insure that we maintain a fully saturated transistor while the output pin is live. The three trimpots (VR1, VR2, VR3) provide us with adjustment/calibration for the circuit.
I often thot one could trick the fan relay into thinking the sensor circuit resistance was lower by wiring a smaller value resistor (or variable resistor) in parallel with the sensor. However a parallel circuit seems unlikely due to construction of the sensor itself.
Yes, I have a 200k ohms trimpot connected in parallel with the oil temp sensor on my Rancher, in fact. It works to a limited extent. My fan kicks on @205 degrees F and kicks off @150 degrees F. I don't like it because I cannot adjust for hysteresis and its temp range is static... and very limited. Can't complain really... it only cost about 50 cents for the Bournes trimpot.
So are you left with manipulating a series circuit?
Great job BTW i enjoy innovations like this.
I got no idea how this might end yet... but I'm having fun with it too, regardless. Thanks for joining in! :-)