Need some electronics math help

[MichaelD]

Hello, my mathematically inclined internet associates.

I need some electrical math help, please. I'm trying to figure out what size (ohms and watts) potentiometer I need.

I'm trying to control the speed of three, 12V fans with just one pot. I'd like a 7 volt minimum setting. So the range of voltage control would be 7-12V.

Each fan is rated at 9.6 watts @ 12 volts (they are high speed server fans).

Please check my math and [humility] please finish the calculations for me [/humility]

So, 9.6 watts/12 volts =.80A current per fan
Resistance is 12 volts/.80A = 15 ohms per fan
3 fans in parallel is 15/3=5 ohms
At 7V minimum voltage, current is 7 volts/5 ohms = 1.4A <---this is as far as I've gotten
???
???

Thank you!!! :beer: <--for you while you work

 

[Colt45]

You'll need a massive wire-wound rheostat. which is big, expensive, inefficient, and hard to find these days.

PWM is the way to go. something like this, if you can solder:
http://www.cpemma.co.uk/graphics/map555pwm_sch.gif

If not, you can use a small pot and a big transistor to adjust the voltage. Just as inefficient as the big pot, but cheaper, at least.

 

[Jassi]

Originally posted by: MichaelD
Hello, my mathematically inclined internet associates.

I need some electrical math help, please. I'm trying to figure out what size (ohms and watts) potentiometer I need.

I'm trying to control the speed of three, 12V fans with just one pot. I'd like a 7 volt minimum setting. So the range of voltage control would be 7-12V.

Each fan is rated at 9.6 watts @ 12 volts (they are high speed server fans).

Please check my math and [humility] please finish the calculations for me [/humility]

So, 9.6 watts/12 volts =.80A current per fan
Resistance is 12 volts/.80A = 15 ohms per fan
3 fans in parallel is 15/3=5 ohms
At 7V minimum voltage, current is 7 volts/5 ohms = 1.4A <---this is as far as I've gotten
???
???

Thank you!!! :beer: <--for you while you work

I'm pretty sure that assumption is wrong. Remember, x||y = (xy)/(x+y). Everything after that is wrong.

 

[Rubycon]

Originally posted by: Colt45
You'll need a massive wire-wound rheostat. which is big, expensive, inefficient, and hard to find these days.

PWM is the way to go. something like this, if you can solder:
http://www.cpemma.co.uk/graphics/map555pwm_sch.gif

If not, you can use a small pot and a big transistor to adjust the voltage. Just as inefficient as the big pot, but cheaper, at least.

Yep - a wirewound rheostat will also generate lots of heat. PWM can control the fan speed more accurately especially at lower ranges.

 

[Linflas]

Originally posted by: Colt45
You'll need a massive wire-wound rheostat. which is big, expensive, inefficient, and hard to find these days.

PWM is the way to go. something like this, if you can solder:
http://www.cpemma.co.uk/graphics/map555pwm_sch.gif

If not, you can use a small pot and a big transistor to adjust the voltage. Just as inefficient as the big pot, but cheaper, at least.

I bet I have some of those in my garage.

 

[MichaelD]

Thanks very much for the help. I could definitely solder that circuit up, but for what the parts would cost me, I'd be better off just buying a fan bus...which is not what I really wanted. Oh well, it was a good idea at the time.

Thanks again. :beer:

 

[Colt45]

i can probably send you the parts out of my junk heap, if you want

 

[Jeff7]

For the money, I think that this is one of the best fanbus units on the market. Full range control, and it handles 20W per fan. You might want to swap out the LEDs, or at least color over them with a black marker. They're really damn bright. After a few months, they'll dim considerably though, as Sunbeam seems to have overvolted them.

 

[Analog]

If you can find some AC fans, a simple Triac based dimmer would vary your fan speeds rather cheaply.

 

[Rubycon]

Originally posted by: Analog
If you can find some AC fans, a simple Triac based dimmer would vary your fan speeds rather cheaply.

Running lines voltages inside the case is bad. Introducing noise from a dimming circuit even worse. Not to mention the hum would be louder than the noise the OP is trying to squelch.

 

[Jeff7]

Originally posted by: Analog
If you can find some AC fans, a simple Triac based dimmer would vary your fan speeds rather cheaply.

I haven't had good luck with that combination. Some of the fans I've tried do hum, as Rubycon mentioned, others simply wouldn't spin at all.

 

[MichaelD]

Originally posted by: Jeff7
For the money, I think that this is one of the best fanbus units on the market. Full range control, and it handles 20W per fan. You might want to swap out the LEDs, or at least color over them with a black marker. They're really damn bright. After a few months, they'll dim considerably though, as Sunbeam seems to have overvolted them.

Absolutely. I was just looking at that, too. :thumbsup: We'll see how the current 3-speed Antecs do on MED (noise and cooling power). This case is brand new and I'm still learning about it's cooling properties.

@Colt45: Thanks much for the offer. I think I'll wait and see though. :beer:

 

[Gibson486]

Originally posted by: Jassi

Originally posted by: MichaelD
Hello, my mathematically inclined internet associates.

I need some electrical math help, please. I'm trying to figure out what size (ohms and watts) potentiometer I need.

I'm trying to control the speed of three, 12V fans with just one pot. I'd like a 7 volt minimum setting. So the range of voltage control would be 7-12V.

Each fan is rated at 9.6 watts @ 12 volts (they are high speed server fans).

Please check my math and [humility] please finish the calculations for me [/humility]

So, 9.6 watts/12 volts =.80A current per fan
Resistance is 12 volts/.80A = 15 ohms per fan
<<3 fans in parallel is 15/3=5 ohms
At 7V minimum voltage, current is 7 volts/5 ohms = 1.4A <---this is as far as I've gotten
???
???

Thank you!!! :beer: <--for you while you work

I'm pretty sure that assumption is wrong. Remember, x||y = (xy)/(x+y). Everything after that is wrong.

1/15+1/15+1/15 = 3/15

(3/15)^-1 = 15/3

15/3 = 5. he is right.

 

[Gibson486]

MichaelD, the problem is that you are trying to control three fans with one resistor. I do not recommend this at all. First, you would need a pot that can take that amount of current, second, that power rating is assuming that it has its own current branch. By putting a 5 ohm resistor there, you risk have all amperage go to one fan (higher ohms gives more current limiting and by putting one 5 ohm resistor, you just neglect that current limiting). I have no idea what the tolerance of fans for servers are like, but to put it in prespective, 12/5 = 2.4 Amps.

 

[Demon-Xanth]

Just put a switch to take the ground side to either ground or +5. You'll end up with a high speed setting and a low speed setting.

 

[Safeway]

Seems like this should go in Cases and Cooling forum.

 

[MichaelD]

Gibson486: Yeah, I've learned that what I orginally wanted to do was a bad idea. If I do it, I guess I'll just buy a rheobus.

I've gotten my math help...and found out thru that help that my idea stinks. LOL!

Time to close the thread. Thanks all.

 

[Gibson486]

Originally posted by: MichaelD
Gibson486: Yeah, I've learned that what I orginally wanted to do was a bad idea. If I do it, I guess I'll just buy a rheobus.

I've gotten my math help...and found out thru that help that my idea stinks. LOL!

Time to close the thread. Thanks all.

don't worry. I have an EE and that was one of my first noobie mistakes while on coop. i still remember the expression on my mentors face when I showed him the burnt board.