Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Cooling Fan speed control Questions

  • Please log in to reply
25 replies to this topic

#1 John Miele

John Miele

    Aurora

  • *****
  • topic starter
  • Posts: 4,827
  • Joined: 29 May 2005
  • Loc: North Alabama

Posted 17 September 2021 - 10:17 AM

Hi,

 

I'm building a 10" mirror cell and plan to have a cooling fan blowing on the back of the mirror. I read the article by Brian Greer here

 

https://www.fpi-prot...r/fanselect.htm

 

about airflow requirements and 15 to 25 CFM is recommended for a 10" mirror. I want to be able to control (reduce) the fan speed in case there are vibration issues or if the fan is just plain too noisy. Based on the fans I'm seeing on amazon, I'm a bit stuck with two questions:

 

1. Can I control the speed of a fan by simply changing the input voltage using a variable potentiometer between the fan and the battery?

 

2. Do I need a 3 pin fan to do this or will a 2 pin fan work just as well?

 

My short list for fans contains these two...any thoughts of which is better or are there other proven fan choices to consider?

 

https://www.amazon.c...Y3YGSS6XQ&psc=1

 

https://www.amazon.c...2dDbGljaz10cnVl

 

I'm leaning to the second fan because it is rated much quieter. I'm assuming low noise also indicates low vibration?

 

Thanks for any advice you can offer!

 

cs...John



#2 LuscombeFlyer

LuscombeFlyer

    Vostok 1

  • *****
  • Posts: 137
  • Joined: 11 Nov 2018
  • Loc: Lakeland, FL

Posted 17 September 2021 - 10:47 AM

You may be able to just use a potentiometer, but most fan speed controllers use PWM. I haven't used this device, but it looks like it should do what you're asking.

 

https://www.amazon.c...38&sr=8-50&th=1

 

Or for a simple 2-wire fan, you could get away with this:

https://www.amazon.c...31893694&sr=8-5


Edited by LuscombeFlyer, 17 September 2021 - 10:49 AM.


#3 Couder

Couder

    Apollo

  • *****
  • Posts: 1,291
  • Joined: 24 Feb 2014
  • Loc: Mansfield, Missouri in the Ozarks

Posted 17 September 2021 - 10:55 AM

Short answer = dc motors run slower at lower voltage, so if you can reduce the voltage the fan will slow down. This can be done with a pot, electronically, or even by using lower voltage (maybe two 6 volt batteries, use 1 for slow, 2 in series for fast, depending on how involved you want to get.) 



#4 Chucky

Chucky

    Surveyor 1

  • *****
  • Posts: 1,509
  • Joined: 16 Apr 2010

Posted 17 September 2021 - 11:06 AM

Just buy a Noctua NF-A8 5V 3-pin USB fan and pop on a small device which supports USB in/out ports which can turn on/off and then regulate speed - NOYITO 5V USB Fan Stepless Speed Controller Regulator with Switch Speed Module DC 4-12V to 2.5-8V 5W.  Power with a small inexpensive USB portable battery.  This setup is fully plug and play.  Simple.  I have this exact setup on my 8 and 10 tube Dobs. Couldn't be happier!! 


Edited by Chucky, 17 September 2021 - 11:47 AM.


#5 rhetfield

rhetfield

    Surveyor 1

  • *****
  • Posts: 1,978
  • Joined: 14 Aug 2019
  • Loc: Suburban Chicago, IL, USA

Posted 17 September 2021 - 11:34 AM

You may be able to just use a potentiometer, but most fan speed controllers use PWM. I haven't used this device, but it looks like it should do what you're asking.

 

https://www.amazon.c...38&sr=8-50&th=1

 

Or for a simple 2-wire fan, you could get away with this:

https://www.amazon.c...31893694&sr=8-5

The PWM will use less power than a pot.  With a pot, anything not going into the fan will dissipate as heat in the pot.  The PWM works by using a solid state switch to turn the voltage on/off at a high frequency for different periods of time.  That way, the only excess power consumption is in the solid state switch.



#6 chemistorge

chemistorge

    Lift Off

  • -----
  • Posts: 10
  • Joined: 11 Sep 2021

Posted 17 September 2021 - 11:37 AM

If you experience vibration at low speeds with PWM control put a 10-100 uF cap across the fan power lines. It reduced the noise of a fan system on my bench power supply. If you a decent low noise fan this should not be a problem though.

#7 John Miele

John Miele

    Aurora

  • *****
  • topic starter
  • Posts: 4,827
  • Joined: 29 May 2005
  • Loc: North Alabama

Posted 17 September 2021 - 12:39 PM

Great ideas! I'll get a PWM type of controller as suggested. It's more efficient than my old school "pot" idea! 

 

I like your setup a lot Chucky. May just go that same route. Is something like this the right interface between 3 pin molex and USB...?

 

https://www.amazon.c...31900001&sr=8-3

 

 

Thanks!

 

JM



#8 Chucky

Chucky

    Surveyor 1

  • *****
  • Posts: 1,509
  • Joined: 16 Apr 2010

Posted 17 September 2021 - 01:35 PM

 All I know is what I recommended is all you would need. That fan comes packaged with an extra cable to allow the fan to connect to any USB female input.


Edited by Chucky, 17 September 2021 - 01:40 PM.


#9 John Miele

John Miele

    Aurora

  • *****
  • topic starter
  • Posts: 4,827
  • Joined: 29 May 2005
  • Loc: North Alabama

Posted 17 September 2021 - 02:41 PM

Roger that! Thanks!



#10 Chucky

Chucky

    Surveyor 1

  • *****
  • Posts: 1,509
  • Joined: 16 Apr 2010

Posted 17 September 2021 - 02:51 PM

Good luck John whatever you end up doing. I'm sure it will blow up a storm!

#11 bokemon

bokemon

    Viking 1

  • *****
  • Posts: 761
  • Joined: 28 Jul 2020
  • Loc: Silicon Valley, California

Posted 17 September 2021 - 09:51 PM

Warning is that some PWM controllers will cause the fan to emit a high-pitched screeching noise, due to the PWM frequency.  Maybe that capacitor mentioned will help.

I recommend a Noctua fan 80mm, 3 pin version with the low speed adapters.



#12 yeldahtron

yeldahtron

    Vostok 1

  • -----
  • Posts: 127
  • Joined: 16 Jan 2014
  • Loc: Rideau Ferry, ON

Posted 17 September 2021 - 09:52 PM

I use an off-the-shelf PWM controller on my 12" Dob. It works great. See 

https://www.cloudyni...s-slowing-fans/

for details.



#13 BGRE

BGRE

    Soyuz

  • *****
  • Posts: 3,543
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 18 September 2021 - 02:37 AM

3 wire computer fans have a speed output.

4 wire fans have a speed output as well as an input for controlling the fan speed,

The speed control input is a low power 5V open collector or open drain signal with internal pullup built into the fan.

The PWM frequency has to be within a range specified in the fan datasheet.

The fan datasheet specifies the PWM duty cycle range to which the fan responds.

There are 3 wire and 4 wire computer fan standards.



#14 Bob4BVM

Bob4BVM

    Mercury-Atlas

  • *****
  • Posts: 2,960
  • Joined: 23 Mar 2015
  • Loc: W. Oregon

Posted 19 September 2021 - 04:57 PM

On my 2-wire fans i use a simple hi/low speed approach-

12V on high, switchable to 8V for low speed.

Control is via a 50-cent LM7808 regulator ckt.

 

CS

Bob



#15 Chucky

Chucky

    Surveyor 1

  • *****
  • Posts: 1,509
  • Joined: 16 Apr 2010

Posted 19 September 2021 - 05:44 PM

All this talk resulted in me ordering my 3rd Noctua fan today. This one will go with my 7 inch Starmaster. I'll close off the end with foam core board.

#16 John Miele

John Miele

    Aurora

  • *****
  • topic starter
  • Posts: 4,827
  • Joined: 29 May 2005
  • Loc: North Alabama

Posted 19 September 2021 - 06:38 PM

I ordered mine yesterday. Will arrive Wednesday! Of course, the mirror cell is still in construction so I can't even use it right now!



#17 Chucky

Chucky

    Surveyor 1

  • *****
  • Posts: 1,509
  • Joined: 16 Apr 2010

Posted 20 September 2021 - 06:53 AM

Man Amazon is quick on my fan order.  Placed it yesterday at Noon, got notice later in the day that it was shipped.  Now says Expected Arrival Tomorrow.  That's 48 hours from the click of my mouse to when I'll install the fan.  Amazing. 



#18 Tim C

Tim C

    Surveyor 1

  • *****
  • Posts: 1,948
  • Joined: 11 Nov 2007
  • Loc: Marietta, GA

Posted 20 September 2021 - 09:45 AM

I bought a 3 pin Noctua 92mm for my 10 inch build last week and one of the el cheapo PWM motor speed controllers from Amazon. It seems to work very well. Only gripe is that the PWM has a red light on it, wonder if I can cover that up or put a drop of black paint on it.

#19 ad701xx

ad701xx

    Surveyor 1

  • *****
  • Posts: 1,679
  • Joined: 22 Nov 2003
  • Loc: Oregon, USA

Posted 20 September 2021 - 10:00 AM

I bought a 3 pin Noctua 92mm for my 10 inch build last week and one of the el cheapo PWM motor speed controllers from Amazon. It seems to work very well. Only gripe is that the PWM has a red light on it, wonder if I can cover that up or put a drop of black paint on it.

I ended up with several annoying lights on my 16" Dob. I just put a blob of black RTV over the offending LEDs. When I used paint in the past I've had to use more than one coat.


  • Tim C likes this

#20 skywolf856

skywolf856

    Viking 1

  • -----
  • Posts: 674
  • Joined: 25 Jan 2008
  • Loc: SE Michigan

Posted 20 September 2021 - 09:06 PM

This is what I used for my Noctua's.

 

https://www.amazon.c...24-135149aa9081

 

My setup on my 18" Obsession.

 

 

Attached Thumbnails

  • 20210208_192146.jpg

Edited by skywolf856, 20 September 2021 - 09:09 PM.

  • Tim C likes this

#21 Tim C

Tim C

    Surveyor 1

  • *****
  • Posts: 1,948
  • Joined: 11 Nov 2007
  • Loc: Marietta, GA

Posted 20 September 2021 - 09:20 PM

So, you have a couple of fans in the corner (or do you have 4 total with two on the other side)? I've been curious about that set up since there aren't any intake (or exhaust) holes in the mirror box.  Does it work well and do you leave on while observing?

 

-Tim

 

 

This is what I used for my Noctua's.

 

https://www.amazon.c...24-135149aa9081

 

My setup on my 18" Obsession.



#22 skywolf856

skywolf856

    Viking 1

  • -----
  • Posts: 674
  • Joined: 25 Jan 2008
  • Loc: SE Michigan

Posted 20 September 2021 - 09:40 PM

I only have two on the bottom of the mirror box to scrub the surface, but I also have one blowing on the back of the mirror.


Edited by skywolf856, 20 September 2021 - 09:40 PM.


#23 skywolf856

skywolf856

    Viking 1

  • -----
  • Posts: 674
  • Joined: 25 Jan 2008
  • Loc: SE Michigan

Posted 20 September 2021 - 09:42 PM

It seems to work well with no vibrations apparent. I can leave it running continuously.


  • Tim C likes this

#24 John Miele

John Miele

    Aurora

  • *****
  • topic starter
  • Posts: 4,827
  • Joined: 29 May 2005
  • Loc: North Alabama

Posted 22 September 2021 - 09:07 PM

So, you have a couple of fans in the corner (or do you have 4 total with two on the other side)? I've been curious about that set up since there aren't any intake (or exhaust) holes in the mirror box.  Does it work well and do you leave on while observing?

 

-Tim

Hey Tim,

 

Before you go with corner fans and cutting cross holes for ventilation check out this thread

 

https://www.cloudyni...r/#entry2986931

 

I'm going to try this baffle idea for boundary layer removal with my cooling fan blowing on the mirror up the tube. Much simpler. However, my tube is solid and you have an open truss so not sure if it still works in a truss design.

 

John


  • Tim C likes this

#25 Tim C

Tim C

    Surveyor 1

  • *****
  • Posts: 1,948
  • Joined: 11 Nov 2007
  • Loc: Marietta, GA

Posted 23 September 2021 - 06:47 AM

thanks John - what would be the diameter of the baffle for your 10 inch and how high above the face of the mirror?

 

 

Hey Tim,

 

Before you go with corner fans and cutting cross holes for ventilation check out this thread

 

https://www.cloudyni...r/#entry2986931

 

I'm going to try this baffle idea for boundary layer removal with my cooling fan blowing on the mirror up the tube. Much simpler. However, my tube is solid and you have an open truss so not sure if it still works in a truss design.

 

John




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics