Re-Hashing Auto-Fan

Thread Starter


Joined Sep 2, 2010
Hey, guys. I'm a new Electrical Engineer guy. I wanted to build a quick easy project that would be useful, so I figured I'd make a Auto-Fan with a thermistor. I found a schematic online, which lead me to this sight. And, I want to understand what exactly happens, so I was wondering if I could just explain my current knowledge, and see what's right / wrong / Needs some tweaking.

From my current knowledge, I understand that the non-inverting input (Pin 3) gets about 1.28mA. I'm not really sure the exact range of amperage the Inverting input receives because I'm not sure if this is a NTC or PTC.

From what I've gathered from hopefully my correct logic, is that the inverting input switches the voltage 180°, thus creating a negative voltage. So, if resistance goes down, the negative voltage goes up, and thus the output goes down, and vice versa.

What I couldn't find / figure out, is the voltage required to switch the transistor. I have the datasheet open, but I can't seem to figure it out.
Something tells me that it has to do with the output? But I'm not really sure. Any help would be greatly appreciated!



Joined Apr 20, 2004
In general, van Roon's circuits are shaky at best. Using a 741 op amp as a voltage comparator is not going to work. Beside the fact that the 741 is a 40 year old design, it is intended to work with supply rails of +/- 15 volts. It is not characterized to run with a single supply and it is not ever going to make an effective voltage comparator.

The circuit should have something like an LM311 (also very old, but good enough for this application) substituted for the 741. It has an open collector output, so the drive to the relay driver will need to be rejiggered.

With respect to your question about the transistor, it will take about 1/10 the collector current into the base for saturated switching. Look at the relay specs and adjust resistor values accordingly.


Joined May 6, 2009
Yes, use an actual comparator instead of ANY op amp. There are many options. The 311 is an open collector output, so it will need a pullup, but the circuit will other wise be basically the same.

Also, saying 'an input gets this or that current' is not useful, and not technically correct. The inputs themselves are very high impedance, thus only nano/pico/femto amps generally ever flow in/out of these pins. (Generally, the better the amp, the lower the input current.)

As for understanding the circuit... The output is designed to "switch" when the voltage on the '-' input crosses the voltage created by the voltage divider (R2 & R5) on the '+' input. Most of the other details pretty much go away when this is realized.

You may be misunderstanding this circuit as an amplifier circuit. It is a switching circuit, mistakenly using an op-amp to provide that function. It operates with with a small amount of positive feedback (R3 going to the '+' input) so there are only two outputs available... saturated at one or the other of the power rails. So there is generally not any 'more positive' or 'more negative' outputs. It is all or none. And it changes about where the '-' and '+' inputs are equal (with hysteresis added by R3).

This is why a comparator should really be used. They operate better in a saturated setting. Some op-amps, especially the older ones like the 741, behave badly when the output is saturated. Comparators live for this!