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Chance
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Could someone please tell me how long this fan here:

http://www.buyextras.com/papasi80hymu.html

...will run one of these batteries over here:

http://www.amazon.com/Saving-Combo-Li-Po......-catcorr

Basically I need to know if this would power the fan for at least 5 hours. If not, then about how long? If over, I don't really need to know.

Extra thank you points if you can send me to the proper potentiometer switch for adjusting the fan speed. What I need is a switch that will max out at about 85% of the fans maximum capacity. That way I can just set the dial all the way up and glue it into place and never have to think about it again.

I'm open to any suggestions for completeing the project in a more streamlined fashion.
rhiro
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I'm not an electrical engineer, but I'll give it a go:

I think the answer is: easily way over 5 hours

Volts x amps = Watts

(12 V) x A = 0.82 W

So A = 0.0683 amps (= 68.3 mA)

If you draw 68.3 mA, then a 2200 mah battery pack should theoretically last (2200/68.3) = 32.2 hrs.

(Yes, the 3S Li-Po is not nominally 12 V, but close enough.)

This is oversimplified (I'm sure you'll get less effective run time than this) but close enough for what you're asking for. Main thing is to not suck your Li-Pos empty. That will kill them. Most commercial devices these days (cameras, cell phones, etc.) that use Li-Pos have auto shut off features that save the battery when their voltage reaches a critical level.

As for varying speed, that fan looks to be brushless DC. Unless it has an external speed control feature (probably not; that third wire is likely a tachometer output or some kind of alarm signal) I don't think you'll have much control over dropping the speed. You might get a small drop in speed with reduced input voltage but I'm not sure if it will be the speed that you want. Brushless DC fans really want to run at the speed their electronics tell them to run at. (It would be easier to knock the speed down with reduced voltage if you used a brushed motor, but good luck finding a fan with such a motor.)

Why do you want to knock the speed down? Is this so that you can compensate later for, say, a clogged inlet or outlet, and you need more CFM? It might be helpful to know more specifically what you're trying to do.

Some brushless DC fans have a speed control feature that will control the fan speed based on an external reference resistance. (A thermistor is often used with these so that you get higher fan speed with higher sensed temperature.) I think that would work perfectly for you, if you could find one in the size you need. Unfortunately, they're not as common as the garden variety fixed speed fans.

Good luck,

Ross
Chance
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Thanks Ross, you definitely sound like you've got more experience than I do with this sort of thing.

To answer your question about usage and fan speed, I need to wear this rig under my clothing. As such, I'm thinking ahead that possibly it will cause too much wind and therefore too much distortion to my wardrobe, or in case it creates too strong of a breeze as it pushes the air out of my neck hole or the sleeves, etc. Sound might also become a factor. I won't have a definite answer to any of these issues until I'm able to try it out in real time. But I'm taking an educated guess that I'll need to tone it down some with a reducing switch.

I've already got some 7.4v/2600mAh L-Ion rechargeable batteries like these over here. Do you think these would work, or is 7.4v not powerful enough?: http://www.amazon.com/gp/product/B003U7UU78

It would be great if they could still power the fan for several hours, but at a reduced speed, and without damaging either the fan or the batteries.
rhiro
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Hi Chance,

I happen to have four identical 0.9W 12V brushless DC fans in front of me. They are quite small in size (40 mm x 40 mm x 20 mm) compared to the one you're looking at, but I thought I'd just play with them to see what voltage they can run at. I hooked them up in parallel to a variable DC power supply, and slowly upped the voltage until they started to spin:

Fan 1 started to spin at 6.42 V
Fan 2 at 6.75 V
Fan 3 at 6.96 V
Fan 4 at 7.32 V

Contrary to what I said earlier, they do speed up significantly when I up the voltage to 12 V. So, assuming your fan motor is similar, you might be able to get enough variation in speed by varying voltage for your application. If you have a variable DC power supply, or could borrow one, you might want to set up your fan rig and play around with voltages until you get the results you need.

Another thing you could do is add an adjustable voltage regulator to your battery pack so that you can adjust the voltage going to your 12V fan. You can buy an adjustable voltage regulator at Radio Shack (just so happen to have one in front of me: Radio Shack P/N 276-1778). It has a simple schematic on the back that shows you how to wire in a potentiometer. The voltage regulator with the fan will consume more power than just the fan by itself, but I think you have plenty of margin with your battery capacity. The voltage regulator will also rob some voltage even when the regulator is set to deliver max voltage, but that might be OK if you need to drop the speed down significantly. Also, you could always run the circuit with a higher voltage. Voltage regulators are great in that they can accept a wide range of voltages and still regulate the output to exactly what you want.

An issue with your 7.4V Lithium-Ion packs is that the 7.4 Volts might be too low. From my limited experiment, you might be on the hairy edge where the 12V fan won't start up.

When sizing a battery pack, you need to be wary of several things:

1. Is it at the voltage you need?
2. Can it deliver the current you need? (You fan sips current, so this is not an issue.)
3. Does it have enough capacity (that's what the mah number refers to) to last as long as you need? Excess capacity never hurts and gives you a safety margin, but too much generally means your battery pack is bigger and heavier than it needs to be.

There are, of course, many other trades. But for a first cut, here's what I'd be inclined to do:

Mock up your setup and experiment with different voltages and/or fans until you get the performance you need. Then decide what margin of safety you want to be able to increase the fan performance above and beyond what you find to be the nominal required performance. Things change over time, and having the ability to bump things could save you. (Then again, if you're absolutely sure you've got it nailed, a simple fixed setup might be the most reliable since there are less things to fail.)

As for batteries, unless you want to incorporate circuitry to protect your expensive rechargeable batteries, I'd either:

A. Run with Lithium packs that have a large margin of safety regarding capacity, and be extra careful not to overrun too long or forget to switch your system off.

or

B. Go with a different type of battery. If you have the space and can tolerate the weight, using disposable batteries could save you a lot of headache. But it's tough to beat the power density of modern day rechargeables like lithium cells.


Hope this helps,

Ross
Chance
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Ross, that was great.

I wish I could run a similar experiment, but I'm on the road and living out of my suitcase so my options are pretty limited.

Based on your input I'm pretty sure I'll know what to do if I finally choose the Lithium-Polymer route. Tenergy also has 11.1v battery packs in the 1100-1500mAh range that will work perfectly.

But... Do you know of any small rechargeable batteries in the 9-12v range that would work? Take the 12v A23 for example. Would a series of 3 of 4 do the job? Or how about a common square 9v? How many of those would I need to run the fan for 5 or 6 hours? (Just one fan, remember.) If the answer is '5 or more' of either one, then I'll fall back to the Tenergy stuff. But if it would take just 4 or less then I'd most likely go with the smaller and lighter arrangement.

So will it be...

~the Tenergy 1100mAh battery pack (powerful and rechargeable, but biggest & heaviest); or,
~a series of 12v A23's (how many will I need?); or,
~a series of square 9v radio batteries (ditto)

I am hoping you will say it could be done with just 3 or 4 9v's. That would be the easiest style for me to cobble together from where I'm sitting from a strictly mechanical point. And the lower voltage will mean that the fan is not running at full speed without even needing a reducing switch.

With this last bit of info I'll know which way I have to go with this. Thanks again!

Chance
rhiro
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Hi Chance,

OK, it helps to know you're on the road. Sounds like you need a Macgyver-ish solution. Or at least something you can solve with a trip to Radio Shack.

Let's start with capacity... If you're pulling 68.3 mA, and need this to last 5 hours, you need a minimum battery capacity of (68.3 mA x 5 hrs) = 342 mah.

Most 9V rechargeable nickel metal hydride (nimh) batteries I've seen are nominally only 7.2 V because they contain six 1.2V cells. Some have an extra cell to get the voltage closer to 9V (7 x 1.2V = 8.4V). If you put two of the 6-cell variety 9V rechargeable batteries in series, you'll get 14.4 V nominal. If you put two of these assemblies together in parallel (i.e. four 9V rechargeable batteries total), you'll now have essentially a single battery pack that has twice the nominal voltage (14.4 V) and twice the rated capacity. Regarding the capacity, the 9V nimh batteries I've seen have advertised capacities of 150-175 mah, so such a 2 x 2 setup of 9V batteries would theoretically give you 300-350 mah. I think this is too low for your application. It might sound close to the 342 mah figure from above, but in my experience you'll likely need a bigger margin than that in actual practice. Also, 14.4 V means you'll be overvolting your fan. Even if your fan can take this without smoking (I just checked an old 12V brushless DC fan of mine, and it survived), the current draw is probably going to be higher than nominal because of the higher voltage. That means you'll deplete your batteries even faster.

The next step up in common rechargeable batteries are AAA nimh batteries. The ones I've seen are typically 700-800 mah, which is sounding closer to what you need. You will need ten of these to get 12V nominal but you could probably run with fewer cells if your fan has excess performance. In fact, this could be a way to play around with different speeds -- just fiddle with the total number of cells until you get the performance you need. Radio Shack sells battery holders in 2 and 3 cell varieties, and you can use different combinations of these in series to vary the voltage. If you can tolerate the size and weight of six to ten AAA cells in battery holders, and have the charger(s) to deal with these, this might be your easiest road-doable solution.

BTW, if you wind up buying rechargeable nimh batteries, I recommend you stick with the recognizable brands, like Sanyo, Energizer, Duracell, or eneloop. I've consistently had very bad results with bargain brand rechargeables.

I would not bother with the A23 batteries. These only have around 55 mah, and you'd need at least SEVEN of these in parallel to deliver the minimum capacity I think you need, and they're disposable to boot. That will get expensive really fast!

A caveat: If your setup is sensitive to fan performance and you really need the fan to run consistently at the same speed throughout the 5 hours, you might have an issue with the performance changing as the battery voltage starts to drop during discharge. That's another reason why I like the idea of using a voltage regulator in conjunction with a large capacity battery pack -- the voltage delivered to the fan remains a constant, so the fan performance will remain consistent over time.

Needless to say, whatever setup you go with, be sure to test it thoroughly to make sure it does what you need it to do for the full 5 hours.

Good luck!

Ross
Chance
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Thanks again Ross. The "smaller" options don't seem so small once you put it that way. So it looks like I'll be going with the Tenergy 11.1v pack. I don't need 35hrs running time, so I'll look in 1100-1500mAh range. I'll also look for the Radio Shack P/N 276-1778 regulator.

Final question. Based on what you know now, which potentiometer would you recommend? A Shack model would be perfect since I'm going there for the regulator anyway. Make it easy on yourself and just shoot me the number; I trust that you know your stuff and that you wouldn't mislead me.

Best,

Chance
rhiro
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Hi Chance,

A schematic is on the back of the Radio Shack packaging for the 276-1778 regulator that shows a circuit that would work for you. You can leave off the capacitors, in which case the only other components you would need is a 5K Ohm potentiometer (Radio Shack #271-1714 would work), and a 240 Ohm resistor (a 220 Ohm resistor is close enough, Radio Shack #271-1313). If you really tax the regulator you need to add a heat sink (Radio Shack #276-1368) but I doubt you'll need this because your fans draws so little current. If you find the regulator getting really hot, add the heat sink.

One thing some of the Radio Shack reviewers complain about is confusion with the pinout on the regulator. The diagram on the back of the packaging for the regulator is a bit confusing. (It's correct if you have the markings on the regulator facing you.) Page 3 of the following document shows the pinouts more clearly:

http://www.st.com/internet/com/TECHNICAL......0455.pdf

Another caveat: With an input voltage of 11.1V and the above regulator circuit, I can dial anything from 1.25V to 9.5V to the fan. (Yes, I put this circuit together. I'm feeling particularly charitable today.) You can't get a voltage above this unless you get rid of the voltage regulator and run off 11.1 V directly, or up the input voltage by going with a higher voltage battery pack (like a 4 cell Li-Po). That's a reality with voltage regulators -- they need to regulate down a certain minimum voltage from the input voltage.

I hope I've answered all of your questions. FYI, I hit the road shortly and won't be having 'net access until probably Monday night.

Regards,

Ross
Chance
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Ross, pls check your PM's...
Chance
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I want to bring this back up to the top and give a really big shout out to Ross. He's really quiet about it around these parts, but it turns out that he is a magical builder and specializes in electrical work. The piece he just made for me is an absolute work of art, and the price was very fair. I would definitely use Ross again for this type of work!

Chance
Magnus Eisengrim
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Quote:
On 2011-06-10 16:17, rhiro wrote:
Hi Chance,

I happen to have four identical 0.9W 12V brushless DC fans in front of me. They are quite small in size (40 mm x 40 mm x 20 mm) compared to the one you're looking at, but I thought I'd just play with them to see what voltage they can run at. I hooked them up in parallel to a variable DC power supply, and slowly upped the voltage until they started to spin:

Fan 1 started to spin at 6.42 V
Fan 2 at 6.75 V
Fan 3 at 6.96 V
Fan 4 at 7.32 V

Contrary to what I said earlier, they do speed up significantly when I up the voltage to 12 V. So, assuming your fan motor is similar, you might be able to get enough variation in speed by varying voltage for your application. If you have a variable DC power supply, or could borrow one, you might want to set up your fan rig and play around with voltages until you get the results you need.

Another thing you could do is add an adjustable voltage regulator to your battery pack so that you can adjust the voltage going to your 12V fan. You can buy an adjustable voltage regulator at Radio Shack (just so happen to have one in front of me: Radio Shack P/N 276-1778). It has a simple schematic on the back that shows you how to wire in a potentiometer. The voltage regulator with the fan will consume more power than just the fan by itself, but I think you have plenty of margin with your battery capacity. The voltage regulator will also rob some voltage even when the regulator is set to deliver max voltage, but that might be OK if you need to drop the speed down significantly. Also, you could always run the circuit with a higher voltage. Voltage regulators are great in that they can accept a wide range of voltages and still regulate the output to exactly what you want.

An issue with your 7.4V Lithium-Ion packs is that the 7.4 Volts might be too low. From my limited experiment, you might be on the hairy edge where the 12V fan won't start up.

When sizing a battery pack, you need to be wary of several things:

1. Is it at the voltage you need?
2. Can it deliver the current you need? (You fan sips current, so this is not an issue.)
3. Does it have enough capacity (that's what the mah number refers to) to last as long as you need? Excess capacity never hurts and gives you a safety margin, but too much generally means your battery pack is bigger and heavier than it needs to be.

There are, of course, many other trades. But for a first cut, here's what I'd be inclined to do:

Mock up your setup and experiment with different voltages and/or fans until you get the performance you need. Then decide what margin of safety you want to be able to increase the fan performance above and beyond what you find to be the nominal required performance. Things change over time, and having the ability to bump things could save you. (Then again, if you're absolutely sure you've got it nailed, a simple fixed setup might be the most reliable since there are less things to fail.)

As for batteries, unless you want to incorporate circuitry to protect your expensive rechargeable batteries, I'd either:

A. Run with Lithium packs that have a large margin of safety regarding capacity, and be extra careful not to overrun too long or forget to switch your system off.

or

B. Go with a different type of battery. If you have the space and can tolerate the weight, using disposable batteries could save you a lot of headache. But it's tough to beat the power density of modern day rechargeables like lithium cells.


Hope this helps,

Ross


Very nice, Ross. Your method is correct his time and, as you noted, was not the first time. Nice to see good advice coupled with humility. Props to Ross.

John
The blood-dimmed tide is loosed, and everywhere
The ceremony of innocence is drowned;
The best lack all conviction, while the worst
Are full of passionate intensity.--Yeats
en2oh
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Hi there,
THis might have already been suggested to you but an easy way to control the speed of brushless motors such as you find in these muffen fans is by using Pulse WIdth Modulation. There are many "kits" that can be purchased that will do the job for you. I would use the LiPo battery packs. They are easily charged, have predictable discharge curves and ultimately, a great bang for the buck in terms of cost. Again, specific charger circuits are needed but they are available quite cheaply from various vendors including eBay.

I don't know that you've actually said what you want to use this for but you can avoid the issues with the heat loss associated with controlling speed via potentiometers, through PWM.

Hope that helps.
doug
rhiro
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John: Thanks for the nice comment. I wrote my first bonehead email early in the morning, and clearly wasn't thinking. Later I thought, "I know that fans in computers run at different speeds, so maybe I better test this out.."

Doug: I have one of those PWM kits (mine is Kit #67 from kitsrus.com), and it does work with my brushless DC 12 V fan. It produces a bit more audible noise, though.

Regards,

Ross
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