Phase split / delay circuit

[rfdipper]

Hi!
A PWM signal is needed to control the speed of fans and I want to add a delay / phase shift to to have 2 signals, one is just a delayed version of the other, so that high surge current is avoided during start-up. Please give me an idea how to implement this using discrete components (op amp and transistors, resistor caps)

 

[hill]

You could feed the PWM to the 2nd motor driver's input. But connect it to a transistor's collector as well. The emitter is then connected to ground. The base of the transistor is then conneted to a RC so that during power on, the transistor is on stage, then you motor will not run. After the time constans of the RC, the transistor is off and your motor will starts to spin!

 

[rfdipper]

Hi hill

Since the fans/motors turn on at the leading edge of the PWM signal, high peak current is drawn from the supply at the leading edge every cycle. Thus, simply delaying the PWM will just delay the first leading edge but not the succeeding leading edges, thus the peak current will not be limited.