Topic: Voltage and current requirements of the Davis 7758 motor?

[elagache]

Dear WXForum Davis experts,

I'm switching my weather station from solar to AC power and while I'm at it, I've bought the Davis 7747 kits to provide a fan for my temperature/humidity probe.  However, I want to power that motor using an AC adapter as well.  I checked the output of the solar panel intended to power the motor, and it appears to out put about the same 2.5 Volts as the solar panel powering the ISS (the solar panels appear to be identical in fact.)

Unfortunately, 2.5 Volt AC adapters appear to be a bit rare.  1.5 Volt and 3 Volts adapters are plentiful.  Does anybody know what are the actual voltage and current requirements for this motor?  Davis does sell the replacement motor as part number 7758.  This should not be confused with the Davis 7758B kit which is a motor plus batteries for the FARS stations.  I suppose I should also ask, does anybody know if the motor is the same for both systems?

By the way, there is a review for the Davis replacement motor on Amazon that claims you can buy these motors cheaply from Chinese electronic supply houses:

http://www.amazon.com/review/R2X8WPYODT3ONA/ref=cm_cr_pr_perm?ie=UTF8&ASIN=B004XBI4DM

Does anybody know anything about that?

Cheers, Edouard

[Ian.]

Hi Edouard,

I added a fan to my wired ISS,  I ran a 24v feed as I wanted to have a heated rain cone and used a voltage regulator for the fan off Ebay, the optimal voltage I found is detailed at the lower part of the build page :

http://www.chatteris.biz/Config%20pages/ISS.html

Can't recall what the current draw is of the fan was as the regulator is rated at 2Amps and is working quite happily.

Ian

[elagache]

Dear Ian and WXForum Davis experts,

Okay thanks for the information.  That would suggest that this fan can probably run on as little as 1.5 Volts.  I haven't looked at all to see if there are AC adapters that supply exactly 2.5 Volts.  Since the solar panel is certainly providing that, that might be the best solution.

Cheers, Edouard

[moehoward4]

A 3 volt wall-wart will do the trick, with a little wiring change(connections) at the motor plug/wall-wart connection. This has been discussed A LOT here....search the forum!!!!!!!

[elagache]

Dear moehoward4 and WXForum Davis hardware experts,

A 3 volt wall-wart will do the trick, with a little wiring change(connections) at the motor plug/wall-wart connection.

Thank you!  That is exactly what I expected and wanted to hear!!

This has been discussed A LOT here....search the forum!!!!!!!

Well, I did try.  I went as far as to use the Google site search because most forum search engines are - shall we say - limited . . .

Unfortunately, it is very hard to guess what are the best keywords to use.  I tried the Davis part numbers and suppose people used more general terms - so I guessed wrong.  Oh well . . . .

Edouard

[jerryg]

Here is the replacement motor i use, it is the one Davis uses and you have to order five minimum from them, or you can order just one from Scaled Instruments. http://www.jameco.com/1/1/47812-rf-500tb-18280-r-3-volt-dc-motor-2104-rpm-voltage-3v.html

[reef]

As with the motors above, the Davis FARS fan is rated from 1-3v DC. However I found the reliability of the Davis fans to be very poor at 3 volts. I had three of them fail in the space of 6 months, with two of those lasting around 3 weeks each. In the end I got sick of it and now run it with an 80mm PC fan at 12v. The temperature response is much better than the Davis fan and its been working 24/7 for 8 months now.

I did try the Davis fan at 1.5v and it did run but airflow just wasnt enough on calm, clear nights especially.

[kobuki]

You could also try one of these for stepping down the voltage of a common 9V wall adapter for instance. You can easily set the output voltage using the trimmer potmeter.

[elagache]

Dear Jerry, Reef, kobuki, and WXForum troubleshooters,

Thanks lots of interesting ideas here.

Here is the replacement motor i use, it is the one Davis uses and you have to order five minimum from them, or you can order just one from Scaled Instruments. http://www.jameco.com/1/1/47812-rf-500tb-18280-r-3-volt-dc-motor-2104-rpm-voltage-3v.html

Okay, that's one way to avoid the problem at low cost.

In the end I got sick of it and now run it with an 80mm PC fan at 12v. The temperature response is much better than the Davis fan and its been working 24/7 for 8 months now.

That's another idea.  Those fans are plenty tough and the location I'm considering is well protected, so the fan shouldn't get too much dust or other debris.

Okay, I'll keep both those ideas in mind. 

Cheers, Edouard

[Old Tele man]

I know this is 'late-to-the-game' but while searching for info about the elusive DAVIS 3Vdc AC Power Adapter that they *USED* to (past tense) provide with their AC-powered FARS shields, I stumbled across the DAVIS #7751 AC-Power Fan-Aspirated Radiation Shield document which shows, in Figure 3. Motor Circuit, that they later simply used a 5Vdc AC-adapter with a 13-ohm "voltage-dropping" resistor in-series with one-leg of the original #7758 3-volt motor. The document also states the motor current is 175mA @ 2.0Vdc using the Figure 3 configuration. That'd be about 1/2W minimum resistor, I'd use 1W.

• source: https://www.davisinstruments.com/product_documents/weather/spec_sheets/7751_spec.pdf


Hope this helps someone else now, or in the future!

[Bobvelle]

I know this is 'late-to-the-game' but while searching for info about the elusive DAVIS 3Vdc AC Power Adapter that they *USED* to (past tense) provide with their AC-powered FARS shields, I stumbled across the DAVIS #7751 AC-Power Fan-Aspirated Radiation Shield document which shows, in Figure 3. Motor Circuit, that they later simply used a 5Vdc AC-adapter with a 13-ohm "voltage-dropping" resistor in-series with one-leg of the original #7758 3-volt motor. The document also states the motor current is 175mA @ 2.0Vdc using the Figure 3 configuration. That'd be about 1/2W minimum resistor, I'd use 1W.

• source: https://www.davisinstruments.com/product_documents/weather/spec_sheets/7751_spec.pdf


Hope this helps someone else now, or in the future!

Actually would have been a big help a few weeks ago. 
Nice research, that's some good info.

To the OP... I agree about the 3v xfmr. It worked perfectly for me for about a year and might of lasted longer but I wanted more air flow. It may be that the 50ft wire run was enough resistance to give the fan perfect voltage.

Regardless. I. HIGHLY recommend you look into the computer case fans. They come in many voltage options and move more air and a lot more reliable.

[Old Tele man]

I know this is 'late-to-the-game' but while searching for info about the elusive DAVIS 3Vdc AC Power Adapter that they *USED* to (past tense) provide with their AC-powered FARS shields, I stumbled across the DAVIS #7751 AC-Power Fan-Aspirated Radiation Shield document which shows, in Figure 3. Motor Circuit, that they later simply used a 5Vdc AC-adapter with a 13-ohm "voltage-dropping" resistor in-series with one-leg of the original #7758 3-volt motor. The document also states the motor current is 175mA @ 2.0Vdc using the Figure 3 configuration. That'd be about 1/2W minimum resistor, I'd use 1W.

• source: https://www.davisinstruments.com/product_documents/weather/spec_sheets/7751_spec.pdf


Hope this helps someone else now, or in the future!

Actually would have been a big help a few weeks ago. 
Nice research, that's some good info.

To the OP... I agree about the 3v xfmr. It worked perfectly for me for about a year and might of lasted longer but I wanted more air flow. It may be that the 50ft wire run was enough resistance to give the fan perfect voltage.

Regardless. I. HIGHLY recommend you look into the computer case fans. They come in many voltage options and move more air and a lot more reliable.

Went to the JAMECO website and downloaded the Nichibo data sheet specs for the DAVIS #7758 3Vdc motor, which match with the #7751 document:

Max-Efficiency =  0.175A @ 2104 rpm @ 60% efficiency = (0.29W.out / 0.486W.in); no-load = 0.055A @ 2700 rpm (no fan blade assembly)

[fkapp]

Questions:
From what I read the Davis 7758 draws 175 mA, is that correct?
How many amps does the 24 HR SPARS Solar Panel output?

Asking because want to figure out if 22 AWG solid wire leads of 8 ft length to the 2 x 5000 mAH are enough, or if show go thicker guage?

What i am working on is installing the batteries in weatherproof box below the radiation shield for easy access.
Just want to make sure 22 AWG is enough to avoid issues from running too thin guage.

Thanks
Frank

[davidmc36]

My real-world test of motor running on 1.26 volt fresh battery is that it draws 100ma. The 175 is likely at 3 volts and Max load.

22ga wire is more than enough for 100ma.

[fkapp]

Great news. Thanks for quick response
Frank

[azchrisf]

I believe I posted about this in the past. IIRC it will output ~2.5V at full sunlight on the panel to the motor. It drops to 1.2V at sundown running off the batteries.

You can buy an adjustable 1.5V to xV AC to DC converter board on eBay. It will have to be enclosed in something though.

If your going that far it's better just to do the conversion to a PC case fan. Much cheaper than Davis motors.

[fkapp]

Thanks. Ac would be perfect but is not feasible.
Appreciate all the input and looking forward to the 24x7 soon