Created by
Created October 15, 2012
Last modified November 07, 2012
Tags 555   fan-speed-control   newbie   ntc   pwm  


A simple converter for a NTC thermal sensor to a voltage signal. The DC-Sweep simulation is used to pick the desired resistance value in series to the NTC.


This circuit is a component for a (PWM) fan speed control using a dual timer 555 and a NTC.

The circuit’s output should be from 0.5 to 4.5 Volts at 25 to 65 °C. The NTC sensor is from EPCOS, R/T characteristics “8016”, having 10 k at 25 °C, see:

I wanted to select the resistors to match the desired temperature and voltage range. Fan speed should be minimal at 25 °C (but the fan must run anyway to take hot air out of the power supply towards the NTC) and increase speed up to maximum (if necessary), when the temperature reaches about 75 °C. At design time I did not know the real temperature values but I knew it was too much! So the simulation should allow to select the right components - and I wanted to find out how close CircuitLab comes to my breadboard ;-)

To run the fan regardless of the NTC signal (might be at -5 °C …) the PWM must maintain a minimum duty cycle, this simple analog circuit can not guarantee that.

The DC - Sweep simulation needs an “ideal” voltage source, here the label “+5V”, thus my otherwise used “bench top PSU” is useless in this case.

For the sensor I got a formula to compute from temperature to resistance:


A computer fan would usually start with a PWM duty cycle of 25%, but to make sure I want to start the fan at power on with 100 % duty cycle (full speed) for about 1 second. Loading the capacitor of 100 uF (C32) at the base will achieve that and should prevent the control loop from oscillating (but could make it worse, too …).

See Part 1) for a full project description: 555 pwm for fan speed control


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