Created by
Created October 12, 2012
Last modified November 07, 2012
Tags 555   behavioral-sources   fan-speed-control   newbie   ntc   pwm  


A 25 kHz clock circuit, set to about 75 % duty cycle, using a 555 timer.


This circuit is a component for a (PWM) fan speed control using a dual timer 555 and a NTC. The clock output is used to trigger the variable PWM timer.

It is important to have about 75 % duty cycle because the 25 % off-time determines the minimum PWM signal to keep the fan running and to maintain an air flow, regardless of the NTC signal.

This circuit could be used to directly set the speed to a fixed value, but I wanted to “hear” the temperature of my Time Capsule (Apple) - and to test the CircuitLab simulator ;-)

In this simulation I use my own Power Supply Unit to supply the circuit because I want to simulate a power ramping up in 10 usec with RI = 100 mOhm. Using CL’s voltage sources in TDS is tricky as these are “ideal” sources.

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


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