>-----Original Message-----
>From: Nick Veys [mailto:nick@VEYS.COM]
>Sent: 27 February 2004 05:45
>To: PICLIST@MITVMA.MIT.EDU
>Subject: [EE]: Proper PWM Freq. for 12V DC Brushless Fan?
>
>
>Anyone have experience PWM-controlling a DC brushless 12V fan?
> Right now I have experimented with between 120Hz and 60Hz.
>I'm at 60Hz right now and I can hear knocking at mid-to-lower
>duty-cycles (40-25% area) and that's pretty undesirable of
>course.  If I up the frequency my mid-range knocking is
>eliminated, but the low end knocking (around 25%) is increased
>and there's an obvious descrease in translated power to the
>fan.  Curious if there is a sweet spot or if it varies
>fan-to-fan enough that I just have to find a best estimate.
>
>Also, how much is the speed of the fet coming into play?  Due
>to the capacitive nature if the gate, will this cause problems
>at such a low frequency or can I simply through it out?  I'm
>driving it directly w/a simple PIC18F2320, no isolation as of
>yet.  This is my first real playing w/PWM so I'm still trying
>to get used to all the variables.

Brushless fans don't tend to repond very well to raw PWM in my experience.
Unlike a brushed mtor, you have an electronic circuit insdie that was
designed for a fairly smooth supply.

I would try running your PWM as high as possible, and then filter the output
using a suitable LC filter.  The higher the fequency, the small the LC and
components need to be.

Regards

Mike




=======================================================================
This e-mail is intended for the person it is addressed to only. The
information contained in it may be confidential and/or protected by
law. If you are not the intended recipient of this message, you must
not make any use of this information, or copy or show it to any
person. Please contact us immediately to tell us that you have
received this e-mail, and return the original to us. Any use,
forwarding, printing or copying of this message is strictly prohibited.
No part of this message can be considered a request for goods or
services.
=======================================================================
Any questions about Bookham's E-Mail service should be directed to
postmaster@bookham.com.

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics