Fan current vs. Audio

[Dragon Tamer]

I have been making a fan speed controller using a 555 and an op-amp on the control pin said 555. I was reading an article about fan speed control and the advantages of each. It says that for PWM it is best to use a frequency between 20Hz and 160Hz. The problem with that is it will put the PWM frequency right in the audible range and would likely be amplified by the amplifier circuit. Right now I have the PWM frequency set to 100kHz and a duty cycle between 15% and 85%. The article said that a lower frequency will increase the life of the fan itself, but I don't want to hear the noise the fan makes through the power bus.

How long should the fan last with a frequency of 100kHz? (if I asked the company they would have just asked me to buy one of their fan control chips)

**broken link removed**

 

[crutschow]

The only one that can answer that question is the fan manufacturer.

 

[Reloadron]

I can tell you this. I did some experimenting with fan speed control a few years ago. That Maxim article is quite good by the way. I did 0lenty of PWM experiments at 30 Hz. and never once did I hear the PWM in any audio, never. These were all home PC experiments. I would suggest the lower frequency and then see if you have any audio interference. Your call though.

Ron

 

[grim]

you certainly don't need to be up at 100kHz to avoid audio problems. unless you're a dog

 

[Dragon Tamer]

Thanks for your help, I'll try a lower frequency and see if that has any effect on the circuit.

 

[Sceadwian]

What reload said makes a lot of sense, the fan is going to have plenty of rotational energy to keep it moving even under 30 cycles per second and that shouldn't be audible at all. Most PC fans are brushless and run from a controller I'm sure the controller will like the lower frequency much better. Higher PWM frequencies might be good if you're driving a load directly, but in this case you're actually driving another circuit which is driving the load.