12V Fan as windspeed indicator

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manie

Senior Member
Further to the chook-house project, I require a windspeed indicator. I have thought of using a 80 or 90mm 12V computer fan as a generator. At low speed it produces +- 80-100mV and a higher speed up to 2.8V. Just tested in front of an oscilating desk fan. You could shroud it inside a 100mm square PVC rainwater downpipe on top of the weather vane. What maximum voltage do you think this will ouput at high wind velocities, more than 5V ? If so, can one use a 5.1V Zener to protect the ADC input ?

Manie
 
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Andrew Cowan

Senior Member
manie said:
What maximum voltage do you think this will ouput at high wind velocities, more than 5V ?
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No idea. Got a compressor? Can of compressed air?

manie said:
If so, can one use a 5.1V Zener to protect the ADC input?
Click to expand...
Yes - pass the input through a 4K7 resistor, and then have a 5V1 Zener.

Alternative is to glue a magnet (or two) onto the fan (opposite each other?), and then use a reed switch to measure the speed.

If you decide to measure voltage, you may want to put a small capacitor on the fan output - I don't know how smooth the output will be. Got a scope?

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BeanieBots

Moderator
Don't clamp it with a zener, at least, not with one so close to 5v.
The 'knee' point of most zeners is very sloppy and it will skew the readings.
Maybe clamp with a 6v8 to keep very excess voltage down and (although I doubt it's needed) it will offer a load for VERY windy days.

Instead, limit current with a series resistor (10k or 22k) and clamp the input to Vcc with a schottky diode to prevent over-voltage.
 
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Dippy

Moderator
I got no idea either.
If your Missus is back from range testing then ask her if you can borrow her hairdryer :).

I, also, would scope it - into various loads.

How long would a reed switch last?
There are so many ways to skin this cat... it's time for you to play.
 
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papaof2

Senior Member
Some of the LaCrosse weather staions use a reed switch to counts tilts of the "see-saw" in the rain gauge The Whirlpool Duet clothes washer uses a reed switch to measure how much water has been run into the machine - it gives 815 pulses per gallon. Both of these units have magnet(s) that operate the reed switch as they go by. The engineers obviously expected an "acceptable" life from these items (although my definition of "acceptable" is probably 10 years longer than theirs ;-)

Still hoping to get back to me PICAXE-based irrigation controller, which uses one of the Whirlpool flowmeters to measure the amount of water delivered. I've acquired ideas for keypad, menu, realtime clock, etc, from Forum postings but haven't gotten beyond the "bits on a breadboard" yet;08M interrupts at 8MHz can keep up with the flowmeter at a flow of 4 gallons/minute and still update a serial display(4GPM * 815 pulses/gallon = 3260 pulses/minute or a little over 5 pulses/second). The bigger chips (14, 16, 28) are waiting for the extensions.

Spinal fusion surgery really slows you down - even 6 months later...

John
 
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jglenn

Senior Member
You can also use a differential pressure sensor, Freescale makes them, the MPX series, about 3/4" cube with 2 ports. One goes to a static reference, the other is the airspeed sense port. Can go to 125 MPH, or supersonic with the right range. These sensors cost about $15. No moving parts.
 
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moxhamj

New Member
"What maximum voltage do you think this will ouput at high wind velocities?"

Sounds a fun project. You would need it in PVC pipe to keep the water out - fans won't like water.

How about the standard test homebrew wind turbine makers use - strap it to the top of your car and go for a drive? Best to do this with a passenger taking the readings, but all you would need is a voltmeter and some straight road. Calibrate it with the speedometer.

If it is a 12V fan you may well get over 12V. Work out your max volts and then use a voltage divider - eg 20k and 10k in series will divide by 3 for 0-15V. And resistor values round that ballpark will also protect the picaxe so even if you did put 20V into a 20k/10k divider it wouldn't do any harm.
 
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BCJKiwi

Senior Member
Please be aware that the bearings in computer fans are not that great - ever had a noisy or stuck fan?? Also various parts of the elctronics are somewhat exposed - designed for use indoors and inside a case!

To get reasonable readings over time the bearings need to be good, free running, and well protected from dust, dirt, moisture etc etc.
 
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Colinpc

New Member
I thought that most computer fans were brushless. They are driven by electronics switching coils that then move the fan. This would then mean that it would not act as a generator with an output due to rotating the blades.

Fred
 
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Dippy

Moderator
Fred,
I've just tried it on a couple of fans and got something. Not a lot.
I've also got a headache after blowing so hard.

Always best to experiment with what you have in your hand.
 
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manie

Senior Member
A lot of info - thanks. Jglen's idea of the differential pressure "a-la-pitot-tube" as used on aircraft sounds like a good one. No moving parts, no problems later.... I will look at that for sure...

Manie
 
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Dippy

Moderator
Or a dot of Tippex on a blade and an optical sensor.
Then hang the fan and the missus out the car window.

But do it out of town or the locals might think it's a "drive by" ;)
 
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Ruzzz

Member
Dont the three pin computer fans havea tacho output on them to allow the Motherboard to monitor the fan speed ?

It probably needs to be powered, so just remove the power to the fan motor.
 
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manie

Senior Member
Not all so easy, I have a certain selection of fans, none of which have Hall's built in I think.... but I am working on it. In the meantime there are the gas sensors, the humidity sensors, the interupts etc etc, a lot to do still, see the other threads.
Manie
 
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