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Almost done my creation - help w 12VDC circuit

  1. Oct 18, 2008 #1

    DaveC426913

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    OK, I've almost finished the hardware for my micro-aquarium.

    See diagram
    P=pump, H=heater, L=light (20W halogen), F=fan(CPU cooler)

    The 12V system is my last tweak. The basis of it is one of those 3x20W puck light sets from IKEA. I've ditched two of the lights and wired up the fan instead. The transformer is rated at 12VAC 0.5A. The fan wants 12VDC 0.13A so there's a rectifier in there.

    When wired up and running, I get 15V across the fan connectors.

    My problem: the fan is running too fast and too loud for my liking. So I disconnected the capacitor. This way, I get only about 5.5V.

    It drops the fan RPMs fine; the problem comes when the timer cycles the light/fan combo off and on again - there isn't enough juice to get the fan rotating again. It just vibrates.


    What I'd like is something halfway between the two outputs. Quiet but with enough kick to get it started. 10VDC? Would it simply be a matter of putting a resistor in the fan circuit somewhere?



    Also, the light/fan circuit has a faint, high-pitched whine when the timer's off. Is this bad/avoidable?
     

    Attached Files:

  2. jcsd
  3. Oct 18, 2008 #2
    Voltage readings with a meter on DC of half or full wave rectified AC are likely to be wrong... Form factor.. RMS and all that.

    Simple way to drop a DC volatge is with a diode (rectifier). Produces are fairly constant drop of 0.7 to 1.0 volts depending on current. A high power rectifier 3 or 6 Amp will have a lower voltage drop than a 1 A. Have to try 2, 3 or 4 diodes in series.
     
  4. Oct 18, 2008 #3

    Redbelly98

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    There are 4 simple ways to do this I can think of.

    1. A single 5V Zener diode. What is the fan current at 10V? You can get 5.1V Zeners that handle nearly 200 mA at either DigiKey or Radio Shack:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=1N4733ADICT-ND
    http://www.radioshack.com/product/i...&sr=1&kw=zener&origkw=zener&parentPage=search

    2. Several standard diodes as Pumblechook said. As many as 7 of them if for example it's a 0.7V drop for each. The 1N400x series are rated for 1A current.

    3. A resistor will work, you just need to know what current the fan draws at 10V in order to calculate the correct resistance and wattage.

    [EDIT] Oh, just saw you said it's 0.13A at 12V. Good enough. Calculates to 40-45 ohms, 0.6W to get a 5V drop. Hmmm, closest common size is 47 ohms. Or combine resistors in series/parallel. And get at least 1W power rating. [/EDIT]

    4. A smaller capacitor than the current one. Best way to find a suitable value is probably trial and error.

    edit:
    The 15Vdc bridge output sounds about right. The transformer's 12 Vac would have a 17V amplitude. Subtract from that two diode drops for the bridge, and you're close to 15Vdc.

    edit #2:

    Hmmm. Can you disconnect one item at a time, to figure out which is causing the noise?

    Goog luck! If you get this working post back to let us know.
     
    Last edited: Oct 18, 2008
  5. Oct 18, 2008 #4

    DaveC426913

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    HM. OK. I'll try a reg. And maybe some some smaller capacitors.

    And maybe a breadboard. Or at least some alligator clips.

    Thanks. I'll let you know what worked.
     
  6. Oct 18, 2008 #5

    DaveC426913

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    It's the light not the fan. I can hear it.

    It's not enough to be a problem for me, I'm just concerned if it's damaging or otherwise bad for the parts.
     
  7. Oct 18, 2008 #6
    Large cap-started motors use a centrifugal switch to take the cap out.
    What about using a timer to take the cap out of the circuit after it gets the fan moving.


    Sorry I don't have any suggestions as to a source of suitable timers.
     
  8. Oct 18, 2008 #7

    Redbelly98

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    If it's an incandescent light, it should be fine. Could be a problem for gas discharge lamps.

    If it's not bothering you, I'd leave it alone but be alert to whether the light needs frequent replacement. A 12V lamp will last much longer than a 120 or 240 V one.
     
  9. Oct 18, 2008 #8

    Redbelly98

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    I thought that was for single-phase AC motors? This one is a DC motor.
     
  10. Oct 18, 2008 #9

    DaveC426913

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    Bah. Haven't made any changes yet. Turns out it's noisy even at this low RPM. I'm going to need to get something much quieter. Part of it may be the fan, but I'll bet part of it is a very noisy waveform.
     
  11. Oct 19, 2008 #10
    i don't know what kind of aquarium you're planning, but one thing you should think about is water temperature, especially in a micro tank. if the lights are adding too much heat to the system, spending more money on a low-noise fan may be a better solution than reducing fan output.
     
  12. Oct 19, 2008 #11

    DaveC426913

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    Where would I get one of these low-noise fans? It has to be only about 1" diameter.
     
  13. Oct 19, 2008 #12
    digi-key catalogs used to have fans listed by CFM and dB rating. not sure about your 1" spec, though. maybe someone else would know a better/cheaper source.
     
  14. Oct 19, 2008 #13
    You are correct...

    That was just what spawned the idea. I am thinking he could leave the cap in only for a start. Then take it out with a timer
    to run quietly.
     
    Last edited: Oct 19, 2008
  15. Oct 20, 2008 #14

    Redbelly98

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    Digi-Key has 1" fans. Here are links to some catalog pages. Just look for "25 x ???" in the left-hand column (dimensions are in mm). Noise ratings are shown as well.
    http://digi-key.dirxion.com/WebProject.asp?BookCode=dus08flx&SectionIndex=0&PageIndex=2303#
    http://digi-key.dirxion.com/WebProject.asp?BookCode=dus08flx&SectionIndex=0&PageIndex=2306#
    http://digi-key.dirxion.com/WebProject.asp?BookCode=dus08flx&SectionIndex=0&PageIndex=2308#


    Ah. Understood.
     
  16. Oct 20, 2008 #15

    DaveC426913

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    Yeah but I've got nothing to compare the noise ratings or CFM to for the fan I've got now. Futile to swap one of these in without knowing what I'm gaining.
     
  17. Oct 20, 2008 #16

    DaveC426913

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    Saaay, rectifiers sizzle!

    (Transformers do not.)
     
  18. Oct 20, 2008 #17
    hmm, why are you using a fan again? i assumed heat, but maybe i'm wrong.

    but before adding critters, i'd suggest putting the whole thing together in the environment you expect to run it, and with the highest expected ambient temp. things like powerheads can actually add a lot of heat to the water. and acrylic tanks tend to hold in more heat than glass ones.

    if the idea of a fan was to promote gas exchange at the water surface, that may not be the best strategy. what works best is something that skims or otherwise keeps the surface agitated so a film doesn't form on top.

    sorry for the tangent, but i used to keep fish so i don't just see an electrical problem.
     
  19. Oct 20, 2008 #18

    DaveC426913

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    The fan is to
    1] pull heat away from the halogen light. I prefer halogen to fluorescent or even incandescent.
    2] circulate air in an otherwise mostly-sealed lid.

    A have a powerhead to ensure good air exchange with the water column.

    A heater is to ensure a consistent temp - important. (Didn't skimp on the heater. It's a Hagen.)

    I'll ensure I can get a consistent temp of ~24-25C. I little warm for neons, but juuuust right for a Betta.

    This is not my first microtank.
     
  20. Oct 20, 2008 #19
    ahh, i see. that should be one happy betta, then.
     
  21. Oct 21, 2008 #20

    Redbelly98

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    Hello Dave,

    Just had a couple of ideas to throw at you about the noisy fan you have:

    1. You said you suspect a noisy waveform. Try running the fan from a 9V battery and see what it sounds like. If that improves things, build a 9V regulator circuit to run it.

    2. You might consider removing one of the fans from your computer and trying it, just to see if it's much quieter than what you have. Fairly simple with a desktop computer, but probably impractical if it's a laptop. But if it works, google the fan's model number and try to buy another one just like it.

    Mark
     
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