Whats wrong with this picture [Voltage comparator]

[Illum]

Or is there anything wrong with it?
Its a temperature fan controller using a LM741, I think the resistor placements are questionable but I'm not sure

Noninverting input pin 3 is divided between a 10K resistor and a 10K @ 25C thermistor. Inverting input pin 2 is divided between a 10K and a 8.2K resistor.

Since voltage comparators operate as follows:
The voltage between Pin 3(V+) is greater than Pin 2(V-), the comparator is floating.
The voltage between Pin 3(V+) is less than Pin 2(V-), comparator is negative.

How will this circuit work? the website for it says this will turn the fan on at ~31C. But the way I calculate it, at 25C [V+ > V-] the fan would be on already...wouldn't it not?

Vout = [(R2)/(R1+R2)](Vin)
Pin 3 (V+) voltage: 6V = [(10K)/(20K)](12)
Pin 2 (V-) voltage: 5.407V = [(8.2)/(18.2)](12)

Wouldn't the fan be on to begin with?
Is this his issue or my error?

the site also mentions that "As the thermistor heats up, it's resistance decreases"
IIRC thermistors work as ΔR = KΔT...where the change in resistance is directly proportional to the change in temperature...are there thermistors that are inversely proportional as the site uses?

btw, source: http://www.rason.org/Projects/fancont/fancont.htm

 

[fredb]

if in+ is > in- the output will be positive (don't believe the 741 floats its output), which won't bias the transistor and the fan will remain off.

 

[Selectron]

... are there thermistors that are inversely proportional as the site uses?

Yes, indeed there are. Thermistors may be either positive temperature coefficient (PTC) or negative temperature coefficient (NTC) - that's an NTC in the circuit to which you linked.

You'll sometimes see silicon diodes used as temperature sensors in similar circuits - the diode's P-N junction has a negative temperature coefficient in that when fed with a constant current, the forward voltage drop decreases as temperature increases; I think it's around -2mV per degree C.

 

[Illum]

hmmm... now that I looked at it, PTC thermistors only exist as circuit protectors. I wonder which idiot wrote that on the wikipedia entry for thermistors because thats where I learned it from.

fredb, would you happen to know if I can use an LM393 instead for two separate fans? the LM393 is a dual voltage comparator...which appears to work the same way as an op amp, but its not called an op amp:shrug: