Variable speed DC pancake fan?

[Ritchie200]

Did not know where else to put this...  Hey, has anyone ever played around with varying the voltage to a DC pancake fan?  The reason I ask is that I have a salt fish tank the uses a real cool custom shaped venturi fan to cool the water.  Unfortunately it sounds like a small prop plane taking off.  I also have to point it somewhat away from the water because if I blow it directly on, I get too much cooling and evaporation.  If I could slow it down or ideally vary the speed, I could blow direct, at a slower QUIETER speed.  I would experiment with this, but it cost me too much $$ to burn this thing up.  It uses a wall wart.  Fan is 12vDC@.5A, 3,000 rpm. 

I guess I'm just getting lazy...  Twenty years ago, I would have run down to Radio Shack, bought a fan, and proceed to torture it for the sake of furthering my edjumacation!

Thanks!
Jim

[jjasilli]

It should run OK at down to 8VDC.  Use a dropping reisitor on the +leg of the supply.  I assume you can do the math for the voltage drop, and wattage rating.  Don't go lower or the fan might not be able to start, and could burn out.

[EL34]

Kind of hard to find resistors that you can use for this.
A few of the 1 ohm 2 watt resistors I sell in series may do the trick.

My calulations while holding a Sam Adams are this:
12 volts - 8 volts = 4 volts dropped
4 volts / .5 AMPS = 8 OHMS

That's assuming your fan is drawing 500ma
Does not have to be 4 volts dropped or 8 ohms.

Add 1 ohms in series until the noise level drops to a level you can stand and the fan still works properly.

Then again, this calcualtion was done quickly by Sam Adams

[Ritchie200]

Thanks guys!  I've got some 1ohm 10watt resistors that are laying around collecting dust - overkill, I know.  But I don't know where the heck I would ever need them.  I used them for my daughter's Science Fair project on "What should I feed my iDog?"  I had her find the current draw, She went to Wiki and looked up Electricity, then Ohm's law, calculate resistance needed (3ohms), found out Radio Hack only had the 1ohms, looked up series and parallel resistance to figure out how to connect them, soldered the resistors to a battery holder, and then we plotted voltage drop over time to find out which AA batteries are the best.  She did it all - I was one proud papa!   She was in 8th grade and was already a pretty good hand with the soldering iron.  She also soldered the 1ohms together. 

Blah, blah, blah - you want to see some baby pictures?  Sorry about that...

I'll give this a shot and will report back!

Hmm, I'll have my daughter go over Sam's figures!

Thanks!
Jim

[PRR]

> My calulations while holding a Sam Adams are ... assuming your fan is drawing 500ma

It likely draws less at lower voltage.

An alternative approximation: 12V at 0.5A is 24 ohms. Pretend it is about 24 ohms at any useful voltage (it may go way high or low when it stops turning, but that's not a useful condition).

jjasilli suggests 8V and I agree this is a good trial target. 8V is 2/3rd of 12V. In other words, we want two parts fan, one part resistor. Which means the resistor is half of the fan. If fan is 24 ohms, trial resistor is 12 ohms.

-Some- such fans will run at half voltage. That's obviously 24 ohms, although at half voltage the current may not be exactly "half". No matter; you want enough ohms to drop the noise, not so many that it could fail to start or not cool well; you don't need any specific voltage.

Doug> 4 volts / .5 AMPS

Note that 4V*0.5A is 2 Watts.

The alternate guesstimate, 4V on 12 ohms, is 1.33 Watts in the resistor. Or 6V at 0.25A would be 1.5W.

> 1ohm 10watt ... - overkill, I know.

Yes. You should probably tie 4 or 8 onto a board as a Dummy Speaker Load. Until you get around to it, eight or twelve of them are perfect test-rig to find a happy fan condition. They will NOT overheat (worst-case, 0.5A fan and 0.5A in 1 ohm is 0.25W, in 10W resistor, is hardly-warm).

If this large lump of parts is too big for the fish shelf, you can then count the actual ohms and watts, order something neater.

[Ritchie200]

Thanks PRR!

I've been very busy with my biz the last few days and have not had a minute to play.  I will hit it this weekend.  Yeah, the problem with the evaporation is that it screws up the salinity.  Also, it only takes a little water loss and the pump starts blowing bubbles.  I've wanted a marine tank for about 40 years and decided to pull the trigger this summer - wow, it's a lot of work.  But wow, it is a living, breathing biosphere.  I'm even using macro algae in the filter that has it's own grow light system.  It's a delicate balance that makes you appreciate mother nature!

Thanks again guys!
Jim

[PRR]

> point it somewhat away from the water because if I blow it directly on, I get too much cooling and evaporation.

Why are you blowing the water? Blowing the glass will remove essentially the same dry heat, but zero wet heat (evaporation). Relative to air, even moving air, glass (or thin plastic) is a "perfect thermal conductor". If you wanted to maximize heat removal, or cool below room temperature, you would blow the water. But it sounds like you shouldn't be.

[billcreller]

  Why couldn't you use a 2N3055 transistor on a small heat sink, with a pot to bias the transistor?  It works fine for 12 volt panel lights.  Just an idea.......

[Ritchie200]

PRR,
Hmm, that's a nice thought, so I blew it down the backside of the filter box.  This is essentially the width and depth of the tank and is plastic.  We keep the house at about 75-78.  Just the heat generated by the pumps will raise the water temp above 80.  Unfortunately, it did not make enough of a diference.  I am hoping that in the winter when we keep the house around 70-72, I can turn the fan off!

I did find an old 500mA 9v wall wart for an old rechargable flashlight - it works perfect!  It is now quiet and I can blow it direct on the water in the filter box.  No problems starting the fan and no excessive motor heat.  I will keep an eye on it.

Bill,
I may try this if I need to tweak fan speed with the indoor temp - great idea!

Thanks again everyone!
Jim