Disconnect DC fan when voltage drops below

Thread Starter

phil w

Joined Oct 14, 2014
3
Im looking at an example of a DC fan circuit that regulates voltage according to temperature. Its an online example that i have modified slightly.

R1 is a 10k thermistor.
Q1 and Q2 are NPN and PNP transistors.

Id like to modify the circuit so that the fan will only come on when at least 3v is present. The fan is not shown on the diagram but i would put in it parallel with R7/D2. The fans I have have a minimum operating voltage of 6v and so running them at anything less will probably stall them.

thus if the output is less than 3 volts then the output should be 0
if the output is >=3 then the output passed should be 6
else the output should be passed unadjusted.

any help is appreciated.
 

Attachments

wayneh

Joined Sep 9, 2010
17,155
Yeah, sorry, I misunderstood the problem. You're talking about the situation when the controller sends, say, 2.9V to the fan.

There are plenty of ways to accomplish the cutoff (such as comparator, voltage reference, MOSFET switch), but by the time you implement all that you might as well switch to using PWM of fan speed in the first place. It'll give you smoother control and allow the fan to run to a lower rpm if needed.
 

wayneh

Joined Sep 9, 2010
17,155
Not predictably. You're way off the chart for that MOSFET, so only an experiment will tell you how much current it will pass with 3V on the gate.
 

wayneh

Joined Sep 9, 2010
17,155
He wants the fan to cut out when its being supplied <3V. Or is it 6V?
Slows down under temp control, then stops completely.
 
Top