simple voltage drop needed: what value potentiometer?

Discussion in 'The Projects Forum' started by cazksboy, Dec 11, 2009.

  1. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
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    Sorry for the extreme newbie-level question: I need to run a small DC fan at a very low speed & I'd like to make it variable speed. The fan I have is made to run at 12VDC but I find that even 6VDC makes it run too fast for my application. If I run in with four "D" cells in series (6VDC total), what value potentiometer would make the fan receive 0 to 6VDC ? And how would it attach to the batteries - in series or in parallel?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    Using a resistor or rheostat (variable resistor) to control the speed of a fan by limiting current is wasteful, because power is dissipated in the resistor as heat.

    It is much more efficient to use PWM.

    It would be difficult to give you a range of resistances to use, since you have not supplied us with the current requirements of the fan.
     
  3. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
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    Thanks Sgt.Wookie. What I'm trying to do is just come up with a simple way to control the speed of the fan, from dead stop to full speed.

    The fan is stamped DC12V - 0.16A . I'm assuming that the 0.16A means that it requires .16 of an ampere to operate properly. Is that correct?
    Also, I don't know what PWM means. Can you clarify?
     
  4. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    cazksboy,

    Do have a spec sheet or link for your fan. 12VDC/0.16A sounds like a "boxer" fan. The type used for electronic equipment cooling. Be aware that many..most?...all? are now BLDC (BrushLess DC) and the speed is determined an internal oscillator, not by the supply voltage. I'm not sure PWM would even work on these. I think the older ones are "shaded-pole" induction and the speed is determined by line frequency.

    ken
     
  5. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
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    1
    Hello Ken,
    Yes you correctly guessed (I should have mentioned in my first post) that the fan seems to be made for electronic equipment cooling. I will try to upload a picture later today. It is 3-5/8" square x 1" thick, all black plastic construction. I do not have a spec sheet or any info about it as I picked up a dozen of these at an electronics surplus store in Los Angeles.

    So if this fan is BLDC, does that mean the speed is fixed and unchangeable?
     
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Is there a label on ths fan that gives a manufacturer and model?

    Unless others know a way around it, I'm going to say yes.

    Ken
     
  7. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
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    Yes. The whole labels reads:

    (logo: red oval with what looks like a capital "T" with a "+" superimposed on it)
    SUPERRED
    DC12V 0.16A
    CHA9212BB
    ID.EH: 7.4
    CHENG HOME ELECTRONIC
    CO. LTD. 1
    MADE IN CHINA

    Note, the "7.4" after the ID.EH: looks like it was stamped with some sort of machine stamping, while the rest of the label clearly looks like it was printed on silver metallic label stock.

    Question: no problem, if I cannot vary the speed of this fan, what sort of fan would I be able to continuously regulate the speed? It needs to be thin, flat and no larger than 6" square. That's why I thought this fan would be perfect.

    Thank you in advance...​
     
  8. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    http://www.chenghome.com.tw/web/about_chenghome/company_profile/companyprofile.htm According to this they "specialize in...brushless fans...". I couldn't get through their registration process to look at their online catalog.

    For practical purposes, what is your application? Do you truly need zero to full speed control. That range of speed control could be achieved with PWM and "brushed" type motor, or a variable frequency drive and "stepper" motor. Both have down sides and more complicated controllers than a pot.

    Ken
     
  9. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
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    Hi Ken; thanks for your input. I should have stated my goal to begin with: I'm a printing press operator by trade and I want to overcome a common problem with printing very thin paper. The press feeds the paper one sheet at a time, prints the image on it, and then deposits the sheet (again, one at a time) in a pile at the other end of the press. But frequently the sheets tend to pick up a downcurl when receiving the printed image and fail to drop straight down in the delivery pile. That's where a low-speed fan directly above the delivery pile comes in: a small thin flat-shaped fan running at a low speed provides just enough downward pressure to let the sheet fall straight down.

    This is not an invention, just an application.

    So based on the above description, my needs are:

    1. The fan needs to be thin and flat, between about 3" and 8" diameter. Round or square doesn't matter.

    2. The fan's speed needs to be low: just enough to provide slightly greater air pressure above the printed sheet than below. So the slowest speed doesn't need to be zero, just a crawl. But I would prefer the speed range to include zero. And, the top speed does not need to be high.

    If the speed of an electronic equipment cooling fan is not able to be regulated, is there a type of fan that is able to be regulated? Or, is there an electronic cooling fan that is?

    Thanks in advance,
    Doug in Overland Park KS
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    Speed control of the range you are wanting to achieve is likely to be much simpler if you use a brushed DC motor and PWM for your speed control.

    I agree with sgtwookie that using a potentiometer in series with a brushed DC motor in an effort to control its speed should be avoided.

    hgmjr
     
  11. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
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    1
    Thank you very much. I think I can take what I've learned from this thread and run with it now...there's a place near here called MicroCenter that sells computers and parts & such - now I think I know what to ask for.
    Kudos to AllAboutCircuits.com!
     
  12. BMorse

    Senior Member

    Sep 26, 2009
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  13. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
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    BMorse, wow thanks! That taught me a lot. Mainly how much I don't know.
     
  14. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    I think you may have to build you own. I would go with a stepper motor, because it can go incredibly slow. Lots of stepper motors in junked printers and scanners. Or on the surplus market.

    Fan blades are available at reasonable prices.http://www.globalindustrial.com/g/hvac/parts/fan-blades/small-aluminum-fan-blades-hubs

    Then you will need a stepper motor driver, power supply and an oscillator to control the speed. After saying this I'm thinking that it may be OK for me, but I have no idea what your electronics skills are...and this is starting to sound complicated.

    Ken
     
  15. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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  16. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
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    Kmoffat, thank you so much. Very helpful; sounds like stepper motors is the way to go for me on this project.
     
  17. Duane P Wetick

    Active Member

    Apr 23, 2009
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    Take a look at all the AC and DC fan and blower control packages on E-bay. They are very reasonaby priced from $6-$30 and even come with a potentiometer for adjusting the speed.

    Cheers, DPW [ Spent years making heaters out of op-amps.]
     
  18. Paragon

    Member

    Dec 8, 2009
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    0
    Question..
    I have some KD1208PTB1-6 fans from Sunon and they are running off 7V at the moment. They are brushless and the speed decreases fine. Those are not the only ones I have put on the fanbus. Why all the talk of the PWM? Just decrease the voltage.
     
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