Resistor calc. needed to reduce 4-cell Lipo pack Voltage for 12 V fan

[BVH]

Question one:

I need help! I want to power a 4 CFM, 12 Volt fan from a 4-cell, 16.8 max Volt, 12 AH Lipo. The fan's operating Voltage range is 10.2 to 13.8V. The fan's current consumption is 170 ma/2.04 Watts. Under initial ballast load, (estimated 6-10 seconds) I'm predicting the battery to sag to maybe 15.5V but as the ballast starting cycle winds down, it may rise back to 16V within 15 seconds of initial start. Final pack voltage at depletion will be 12 Volts. The fan needs to run full-time until battery depletion. I realize rpm will fall a bit as Voltage falls but it will move enough air at 11.5 Volts.

Could one of the electronic experts point me to the proper resistor and show me the calculation that was used to bring down the Voltage range? In my crude understanding, I think I need to reduce a pack range of 16.8 V - 12.0V down to 13.8V - 11.5V. If needed, I think the final range could go down to 11.0V if it had to.

Question 2:

Is there a small electronic delay-off timer (maybe up to 1"x1"x 1/4" thick) that would sit electrically after the main light power switch and before the fan, that would allow for immediate activation of the fan upon activation of the power switch and would allow the fan to run for about 3-5 minutes after the power switch is turned off?

 

[spencer]

This will give you 12V until your battery pack drops below 12V. After that the voltage going to the fan will be equal to the voltage of your pack. This is a linear voltage regulator so voltage dissipated will be given off as heat.
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For example, your pack at 16V. It has to dissipate 4V to go down to 12V. 4V*0.17A is 0.68Watts. This is when it will be most inefficient. As the pack drops in voltage it will become more efficient. This part should be heatsinked.

 

[Kremer]

Yep, a voltage regulator would probably be best, but since you specifically asked for it:

16.8V max of battery - 13.8V max of fan = 3V

desired current in circuit = .170A

V=IR or V/I=R we'll use the 2nd version.

R needed = 3V/.170mA = 17.6 Ohm needed.

Now to check the power rating needed for that resistor
Watts = VI = 3*.17 = .51watt.

So an 18ohm resistor rated 1/2 watt or better will drop the voltage the 3V you need at the high end. Unfortunately a resistor would drop the voltage roughly the same 3V for the whole operating range of the battery. i.e. if the fan is passing .17A then the resistor is dropping the voltage 3V. so at your low end of 12.0V the fan will only see 9V.

The fan current I'm sure isn't linear with voltage either, I'm sure it draws more at higher voltage and less at lower. which helps smooth the resistor calc, since at higher current the resistor would drop the voltage more and at lower current it would drop less.

 

[mikespike2]

Kremer nailed it.

The voltage regulator will be better because you get 12V until the pack gets below that.

The resistor will keep dropping the pack voltage at a linearly decreasing rate as the voltage of the pack decreases.

 

[Mr Happy]

The voltage regulator will be better because you get 12V until the pack gets below that.

I don't think that's true. Most voltage regulators need excess voltage to regulate, typically 2-3 V of headroom. So the 7812 will not reliably regulate to 12 V output unless the input voltage is greater than about 15 V (maybe 14 V if you are lucky). So the 7812 won't necessarily work any better than a simple resistor in this application.

 

[Conte]

By lipos are you referring to Lifepo4 batteries ? Or li-polymer ?

It sound to me like you are describing the same kind of DC fans used in a computer. I know these fans well, I have a box of them sitting beside me. What size fan is it ?

I'm very confident you could direct drive that fan without a problem, unless you want to maintain a lower CFM.

DC motors are pretty tolerant of a mild overdrive.
The ratings on the fan are simply to specify how it will perform under the most common conditions in which it will be used, and not necessarily the upper limits of its tolerance.


Yeah, I just hit a spare 80mm computer rated for 12v 0.15a with 20volts. It's spinning alot faster, but not complaining otherwise.
I've heard of people overdriving such fans with as much as 24 volts in the computer modder / overclocker circles.

The wonderfull this about a fan is, it has lots of air moving past it, cooling its motor, allowing it to tolerate such punishment.

I don't think that's true. Most voltage regulators need excess voltage to regulate, typically 2-3 V of headroom. So the 7812 will not reliably regulate to 12 V output unless the input voltage is greater than about 15 V (maybe 14 V if you are lucky).

I've also worked with alot of 78 series regulators, I can confirm this. If you do regulate and it falls short, its not big deal, it'll still work just regulator will regulate a few volts less and the fan will spin a tad slower.

 

[DynoMoHum]

Recently I was in the market for a voltage regulator, I had ideas of making one using Texas Instruments Powermodules. My application was for a relatively high power application (powering a camera speedlight that would draw around 6 amps at 5 volts).

Well long story shorter, I found a off the shelf product that is almost as cheap as it would cost to buy the components to make one myself. For your fan application you may not need this high power, but for anyone looking for a relatively affordable and versatile voltage regulator, here's something to consider....

In the RC world, they have need to regulate voltage to receiver packs and servo electronics, apparently is some planes and/or lager RC vehicles they need large amounts of current capacity...

Castle Creations makes a product called "CC BEC Pro", it sells for $45 or less ($39 at tower hobbies). This thing will take input voltages up to 50 volts, and regulate the output anywhere from 4.8 to 12.5 volts. It's current handling is rated at 20 amps peak (with some air blowing across it, and depending on what voltage input you have, other variables... ) should easily handle 10 amps continuous at input voltages in the 16 volt or less range..

The one part that is not so very appealing about this (well it depends on how you look at things) is the output voltage is selectable only via a USB interface and/or computer software. Now, once it's set it will retain that setting, but you will need to get it set initially, and the interface to do that will cost you a bit more. However Castle Creations is a relatively small American company, out of Kansas, you may be able to buy one and request that it be set up to a specific output voltage, if you were to contact them directly and ask for that. Or possibly you could find a local hobby store and/or a RC guy who already has the 'Castle link' device who would let you borrow it to set the output voltage for your needs.

Either way, if anyone's in the market for a relatively cheap and versatile voltage regulator, you might want to look into this product. They also sell a cheaper one that has lower power ratings and has a smaller voltage range that it can operate at...

Oh, and according to the info I read about this product, when the input voltage drops bellow the set ouput voltage, the output voltage will be equal to the input voltage, so it seems this device does not require a few volts of headroom. I personally have not tested yet. I own the smaller 10 amp rated version, but haven't yet had time to do any testing on it.

http://www.castlecreations.com/products/ccbec.html

 

[BVH]

Thank you all for the great info! I've copied the formula's so I'll have them for future. Well, I have three of these fans. These are a little different than typical propeller-type PC fans. They're squirrel cage style. I got brave and used my variable PS to gradually work up to a sustained 16.2 Volts for 10 minutes. No sign of damage and it runs very cool. I don't think I'd see more than 16.2 Volts for 10 minutes in my application so I'm gonna wing it!

Thanks again!

 

[65535]

90% of 12VDC fans/blowers use nearly identical brush-less motors. They underdrive well and take 2-3x the voltage no problem.

I've run fans at 24VDC and they love it, at 36VDC they burned out rather fast.

 

[HKJ]

Thank you all for the great info! I've copied the formula's so I'll have them for future. Well, I have three of these fans. These are a little different than typical propeller-type PC fans. They're squirrel cage style. I got brave and used my variable PS to gradually work up to a sustained 16.2 Volts for 10 minutes. No sign of damage and it runs very cool. I don't think I'd see more than 16.2 Volts for 10 minutes in my application so I'm gonna wing it!

The best way to drop voltage might be a few diodes (Like 1N4004), they are about 0.6 volt each. The best solution is 12 volt a LDO regulator.

If you need to calculate electronic stuff, maybe my calculator can help you (but it is a bit on the technical side).

 

[Lips]

Question 2:

Is there a small electronic delay-off timer (maybe up to 1"x1"x 1/4" thick) that would sit electrically after the main light power switch and before the fan, that would allow for immediate activation of the fan upon activation of the power switch and would allow the fan to run for about 3-5 minutes after the power switch is turned off?

Would a manual overide be good for this so you could turn on and off at will. How would you wire that...

Any delayed off in-inline switches out there that would allow the fan to continue to run a little for cooling after power is switched off...



.

 

[BVH]

Lips...Manual switch would not be a problem - just another switch sticking out on top.